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Title: Soils, their formation, properties, composition, and relations to climate and plant growth in the humid and arid regions

Author: Eugene W. Hilgard

Release date: July 5, 2024 [eBook #73975]

Language: English

Original publication: New York: The MacMillan Company

Credits: Charlene Taylor and the Online Distributed Proofreading Team at https://www.pgdp.net (This file was produced from images generously made available by The Internet Archive/Canadian Libraries)


*** START OF THE PROJECT GUTENBERG EBOOK SOILS, THEIR FORMATION, PROPERTIES, COMPOSITION, AND RELATIONS TO CLIMATE AND PLANT GROWTH IN THE HUMID AND ARID REGIONS ***





Transcriber’s Notes:

  Underscores “_” before and after a word or phrase indicate _italics_
    in the original text.
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                                 SOILS

                      [Illustration: ·The MM Co·]

               THEIR FORMATION, PROPERTIES, COMPOSITION,
               AND RELATIONS TO CLIMATE AND PLANT GROWTH
                     IN THE HUMID AND ARID REGIONS

                                  BY
                     E. W. HILGARD, PH.D., LL.D.,

                    PROFESSOR OF AGRICULTURE IN THE
                UNIVERSITY OF CALIFORNIA, AND DIRECTOR
           OF THE CALIFORNIA AGRICULTURAL EXPERIMENT STATION

                               New York
                         THE MACMILLAN COMPANY
                     LONDON: MACMILLAN & CO., LTD.
                                 1921

                           COPYRIGHT, 1906,
                       BY THE MACMILLAN COMPANY.

            Set up and electrotyped. Published July, 1906.

                            Norwood Press:
              Berwick & Smith Co., Norwood, Mass., U.S.A.




SUMMARY OF CHAPTERS.


    1. ORIGIN AND FORMATION OF SOILS.
      Introduction.
      Chapter   I. Physical Processes of Soil Formation.
         “     II. Chemical Processes of Soil Formation.
         “    III. Chief Soil-forming Minerals.
         “     IV. The Various Rocks as Soil-Formers.
         “      V. Minor Mineral Ingredients of Soils. Mineral
                   Fertilizers. Minerals Injurious to Agriculture.

    2. PHYSICS OF SOILS.
      Chapter   VI. Physical Composition of Soils.
         “     VII. Density, Pore Space, and Volume-Weight of Soils.
         “    VIII. Soil and Subsoil; Causes and Processes of
                    Differentiation; Humus.
         “      IX. Soil and Subsoil; Organisms Influencing
                    Soil-Conditions. Bacteria.
         “       X. Soil and Subsoil in their Relations to Vegetation.
         “      XI. Water of Soils; Hygroscopic and Capillary Moisture.
         “     XII. Water of Soils; Surface, Hydrostatic, and Ground
                    water; Percolation.
         “    XIII. Water of Soils; Conservation and Regulation of
                    Soil Moisture. Irrigation.
         “     XIV. Absorption by Soils of Solids from Solutions.
                    Absorption of Gases. The Air of Soils.
         “      XV. Colors of Soils.
         “     XVI. Climate.
         “    XVII. Relations of Soils and Plant-Growth to Heat.

    3. CHEMISTRY OF SOILS.
      Chapter XVIII. Physico-Chemical Investigation of Soils in
                     Relation to Crop Production.
         “      XIX. Analysis of Virgin Soils by Extraction with
                     Strong Acids, and its Interpretation.
         “       XX. Soils of Arid and Humid Regions.
         “      XXI. Soils of Arid and Humid Regions continued.
         “     XXII. Alkali Soils, their Nature and Composition.
         “    XXIII. Utilization and Reclamation of Alkali Lands.

    4. SOILS AND NATIVE VEGETATION.
      Chapter XXIV.  Recognition of the Character of Soils from their
                     Native Vegetation. Mississippi.
         “     XXV.  Recognition of the Character of Soils from their
                     Native Vegetation. United States at large, Europe.
         “    XXVI.  Vegetation of Saline and Alkali Lands.




TABLE OF CONTENTS.


    PREFACE xiii

    INTRODUCTION, xxiii.—Definition of Soils, xxiii.—Elements
        Constituting the Earth’s Crust, xxiii.—Average Quantitative
        Composition of the Earth’s Crust, xxiii.—Clarke’s Table,
        xxiv.—Oxids Constitute Earth’s Crust, xxiv.—Elements
        Important to Agriculture; Table, xxv.—The Volatile Part of
        Plants, xxvi.

                              CHAPTER I.
    AGENCIES OF SOIL FORMATION, 1.—1. _Physical Agencies_,
        1.—Effects of Heat and Cold on Rocks, 1.—Unequal
        Expansion of Crystals, 2.—Cleavage of Rocks, 3.—Effects
        of Freezing Water, 3.—Glaciers; Figure, 3.—Glacier Flour
        and Mud, 4.—“Green” and “White” Rivers, 4.—Moraines,
        5.—Action of Flowing Water, 5.—Enormous Result of
        Corrasion and Denudation, 6.—Effects of Winds, 8.—Dunes,
        8.—Sand and Dust Storms in Deserts, Continental
        Plateaus and Plains, 8.—Loess of China, 9.—Migration
        of Gobi Lakes, 9.—_Classification of Soils_, 10.—Their
        Physical Constituents, 10.—Sedentary or Residual Soils,
        11.—Colluvial Soils, 12.—Alluvial Soils. Diagram,
        12.—Character of these Soil Classes, 13.—Richness of
        Flood-plain and Delta Lands, 14.—Lowering of the Land
        Surface by Soil Formation, 15.

                              CHAPTER II.
     CHEMICAL PROCESSES OF SOIL FORMATION, 16.—2. _Chemical
        Disintegrations or Decomposition_, 16.—Ingredients of
        the Atmosphere, 16.—Effects of Water; of Carbonic Acid,
        17.—Carbonated water a universal solvent, 17.—Ammonic
        carbonate, effect on silicates, 18.—Action of oxygen;
        on ferrous compounds, 18.—_Action of Plants and their
        Remnants_, 19.—_A._ Mechanical; Force of Root Penetration,
        19.—_B._ Chemical; Action of Root Secretions, 19.—Bacterial
        Action, 20.—Humification, 20.—_Causes Influencing Chemical
        Action and Decomposition_, 21.—Heat and Moisture,
        21.—Influence of Rainfall on Soil-Formation, 22.—Leaching
        of the Land, 22.—Residual Soils, 22.—Drain Waters; River
        Waters. Tables of Solid Contents, 22.—Amount of Dissolved
        Matters Carried into the Sea; Amount of Sediment, 24.—Sea
        Water, Composition of; Waters of Landlocked Lakes,
        25.—Results of Insufficient Rainfall; Alkali Lands, 28.

                             CHAPTER III.
    ROCK-AND SOIL-FORMING MINERALS, 29.—Quartz, quartzite,
        jasper, hornstone, flint, 29.—Solubility of silica in
        water; absorption by plants, 30.—Silicate Minerals,
        31.—Feldspars, their Kaolinization, 31.—Formation of Clays,
        33.—Hornblende or Amphibole, Pyroxene or Augite, 33.—Their
        Weathering and its Products, 33.—Mica, Muscovite and
        Biotite, 35.—Hydromica, Chlorite, 35.—Talc and Serpentine;
        “Soapstone”, 36. The Zeolites; Exchange of Bases in
        Solutions, 36.—Importance in Soils, in Rocks, 38.—Calcite,
        Marble, Limestones; their Origin, 39.—Impure Limestones
        as Soil-Formers, 40.—Caves, Sinkholes, Stalactites, Tufa,
        41.—Dolomite; Magnesian Limestones as Soil-Formers,
        42.—Selenite, Gypsum, Land Plaster; Agricultural Uses,
        42.—Iron Spar, Limonite, Hematite, Magnetite, 44.—Reduction
        of Ferric Hydrate in Ill-drained Soils, 45.

                              CHAPTER IV.
    THE VARIOUS ROCKS AS SOIL-FORMERS, 47.—General Classification,
        47.—Sedimentary, Metamorphic, Eruptive, 47.—Sedimentary
        Rocks; Limestones, Sandstones, Clays, Claystones, Shales,
        47.—Metamorphic Rocks: Formed from Sedimentary, 48.—Igneous
        or Eruptive Rocks, Basic and Acidic, 49.—_Generalities
        Regarding Soils Derived from Various Rocks_, 49.—Variations
        in Rocks themselves. Accessory Minerals, 50.—Granites;
        not always True to Name; Sierra Granites, 51.—Gneiss.
        Mica-schist, 51.—Diorites, 51.—Diabases, 51.—Eruptive
        Rocks; Glassy ones Weather Slowly; Basaltic Oxidize
        Rapidly, 52.—Red Soils of Hawaii, Pacific Northwest,
        52.—Trachyte Soils; Light-colored, rich in Potash.
        Rhyolites generally make Poor Soils, 53.—_Sedimentary
        Rocks_, 53.—Limestones, 53.—“A Limestone Country is a
        Rich Country,” 53.—Residual Limestone Soils; from “Rotten
        Limestone” of Mississippi; Table, 54.—Shrinkage of Surface,
        55.—Sandstone Soils, 55.—Vary According to Cement, and
        Nature of Sand, 55.—Calcareous, Dolomitic, Ferruginous,
        Zeolitic, 56.—Clay-sandstones, Claystones, 57.—_Natural
        Clays_, 57.—Great Variety, Enumeration and Definition,
        58.—Colors of Clays, 58.—_Colloidal Clay_, Nature and
        Properties, 59.—Plasticity; Kaolinite Non-plastic,
        59.—Causes of Plasticity, 60.—Separation of Colloidal Clay,
        its Properties, 61.—Effects of Alkali Carbonates on Clay,
        62.

                              CHAPTER V.
     THE MINOR MINERAL INGREDIENTS OF SOILS; MINERAL
        FERTILIZERS, 63.—Minerals Injurious to Agriculture,
        63.—_Minerals used as Fertilizers_, 63.—Apatite;
        Phosphorites of the U. S., Antilles, Africa, Europe,
        63.—Phosphatic Iron Ores, “Thomas Slag,” 64.—Animal
        Bones; Composition and Agricultural Use, 64.—Vivianite,
        Dufrenite, 65.—Chile Saltpeter, 66. Occurrence in Nevada,
        California, 66.—Origin of Nitrate Deposits, 67.—Intensity
        of Nitrification in Arid Climates, 68.—Potash Minerals,
        68.—Feldspars not Available, 68.—Depletion of Lands by
        Manufacture of Potashes, 69.—Discovery of Stassfurt
        Salts, 69.—Origin of these Deposits, 70.—Nature of the
        Salts, 71.—Kainit, 71.—Potash Salts in Alkali Soils,
        72.—Farmyard or Stable Manure; Chemical Composition,
        Table, 72.—Efficacy largely due to Physical Effects
        in Soils, 73.—Green-manuring a Substitute for Stable
        Manure, 74.—Application of Stable Manure in Humid and
        Arid Climates, 74.—_Minerals Unessential or Injurious to
        Soils_, 75.—Iron Pyrite, Sulphur Balls, 75.—Occurrence
        and Recognition. Remedies 75.—Halite or Common Salt,
        76.—Recognition of Common Salt, 76.—Mirabilite or Glauber’s
        Salt; in Alkali Lands; not very Injurious, 77.—Trona
        or Urao; Carbonate of Soda, “Black Alkali,” 77.—Injury
        Caused in Soils, 78.—Epsomite or Epsom Salt, 78.—Borax,
        79.—Soluble Salts in Irrigation Waters, 79.

                              CHAPTER VI.
    PHYSICAL COMPOSITION OF SOILS, 83.—Clay as a Soil Ingredient,
        83.—Amounts of Colloidal Clay in Soils, 84.—Influence of
        Fine Powders on Plasticity, 85.—Rock Powder; Sand, Silt
        and Dust, 86.—_Weathering in Humid and Arid Regions_,
        86.—Sands of the Humid Regions, 86.—Sands of Arid Regions
        not Sterile, 86.—_Physical Analysis of Soils_, 88.—Use of
        Sieves. Limits, 88.—Use of Water for Separating Finest
        Grain-Sizes, 89.—Elimination of Clay by Subsidence and
        Centrifugal Method, Hydraulic Elutriation, 90.—Schöne’s
        Instrument, 90.—Churn Elutriator with Cylindrical Tube,
        91.—Figures of Same, 91.—Yoder’s Centrifugal Elutriator,
        92.—Number of Grain-sizes Desirable, 93.—Results of such
        Analyses, 93.—Physical Composition Corresponding to
        Popular Designations of Soil-Quality. Table, 96.—Number
        of soil-grains per Gram, 99.—Surface Offered by various
        Grain-sizes, 99.—Influence of the several Grain-sizes
        on Soil Texture, 100.—Ferric Hydrate, its Effects on
        Clay, 100.—Other Substances, 101.—Aluminic Hydrate,
        101.—Influence of Granular Sediments upon the Tilling
        Qualities of Soils, 102.—“Physical” Hardpan, 103.—Putty
        Soils, 103.—Dust Soils of Washington; Table, Physical
        Analyses of Fine Earth, 104.—Slow Penetration of Water,
        105.—Effects of Coarse Sand, 105.

                             CHAPTER VII.
     DENSITY, PORE-SPACE AND VOLUME-WEIGHT OF SOILS,
        107.—Density of Soil Minerals, 107.—No Great Variation,
        107.—Volume-weight most Important, 107.—Weight per
        Acre-foot, 107.—Air-space in Dry Natural Soils. Figure,
        108.—May be Filled with Water, 108.—Effects of Tillage.
        Figures, 109.—Crumb or Flocculated Structure; Cements,
        109.—How Nature Tills, 111.—Soils of the Arid Regions;
        do not Crust, 112.—Changes of Soil-Volume in Wetting and
        Drying, 112.—Extent of Shrinkage, 113.—Expansion and
        Contraction of Heavy Clay Soils. Figure, 113.—Contraction
        of Alkali Soils on Wetting, 114.—“Hog Wallows,”
        114.—Physical Analyses of such Soils. Table, 115.—Crumbling
        of Calcareous Clay Soils on Drying, 116.—Yazoo Bottom, Port
        Hudson Bluff, 116.—Loamy and Sandy Soils, 117.—Formation of
        Surface Crusts, Physical Analyses, 117.—Effects of Frost on
        the Soil; Heaving; Ice-flowers, 118.

                             CHAPTER VIII.
    SOIL AND SUBSOIL; CAUSES AND PROCESSES OF DIFFERENTIATION
        HUMUS, 120.—Soil and Subsoil ill-defined, 120.—_The
        Organic and Organized Constituents of Soils_, 120.—Humus
        in the Surface Soil, 120.—Soil and Subsoil; Causes
        of their Differentiation, 121.—Ulmin Substances or
        Sour Humus, 122.—Sour Soils, 122.—Cultivation Induces
        Acidity, 123.—Humin Substances, 123.—Porosity of Humus,
        124.—Physical and Chemical Nature of the Humus Substances.
        Table, 124.—Chemical Nature, 125.—Progressive Changes and
        Effect on Soils, 126.—The Phases of Humification, Wood to
        Anthracite; Table, 127.—Amounts of Humus and Coal formed
        from Vegetable Matter, 128.—Figure, From Port Hudson Bluff,
        128.—Conditions of Normal Humification, 129.—Eremacausis in
        the Arid Regions, 129.—Black Earth of Russia; Kosticheff’s
        Table, 130.—Losses of Humus from Cultivation and Fallowing,
        131.—Estimation of Humus in Soils; Unreliability of
        Combustion Methods, 132.—Grandeau Method, “Matière Noire,”
        132.—Amounts of Humus in Soils, 133.—Humates and Ulmates,
        134.—Mineral Ingredients in the Humus, 134.—Functions
        of the Unhumified Organic Matter, 135.—The Nitrogen
        Content of Humus, 135.—Table for Arid and Humid Soils,
        136.—Decrease of Nitrogen Content in Humus with Depth,
        138.—Table, Russian River Soils, 139.—Influence of the
        Original Material upon the Composition of Humus, 139.—Table
        of Snyder, 139.—Effect of Humus in rendering Mineral Plant
        Food Available, 140.

                              CHAPTER IX.
    SOIL AND SUBSOIL (_continued_), 142.—ORGANISMS INFLUENCING
        SOIL-CONDITIONS. BACTERIA, 142.—_Micro-organisms of the
        Soil._ Bacteria, Moulds, Ferments, 142.—Numbers at Various
        Depths, given by Early Observers, 142.—Investigations of
        Hohl; Mayo and Kinsley. Tables, 143.—Multiplication of the
        Bacteria, 144.—Aerobic and Anaerobic Bacteria, 144.—Food
        Materials required, 145.—Functions of the Bacteria,
        145.—Nitrifying Bacteria. Figures, 146.—Conditions of
        their Activity. Table, 146.—Effects of Aeration and
        Reduction, 147.—Unhumified Organic Matter does not Nitrify,
        148.—Unhumified Vegetable Matter, Functions in Soils,
        148.—Denitrifying Bacteria. Figures, 148.—Ammonia-forming
        Bacteria. Figures, 149.—Alinit, 149.—Effects of Bacterial
        Life on Physical Soil Conditions, 149.—Root-bacteria, or
        Rhizobia of Legumes, 150.—Figures of Root Excrescences
        and Corresponding Bacteroids, 152.—Varieties of Forms,
        154.—Mode of Infection, 154.—Cultural Results, 155.—Table
        Showing Increased Production by Soil Inoculation,
        155.—Other Nitrogen-absorbing Bacteria, 156.—Distribution
        of Humus in the Surface Soil, 157.—Fungi, Moulds and Algae,
        157.—_Animal Agencies_—Earthworms, Insects, Burrowing
        Quadrupeds, 158.

                              CHAPTER X.
     SOIL AND SUBSOIL IN THEIR RELATIONS TO VEGETATION,
        161.—Physical Effects of the Percolation of Surface
        Waters, 161.—Chemical Effects; Calcareous Subsoils and
        Hardpans, 161.—“Rawness” of Subsoils in Humid Climates,
        162.—Subsoils in the Arid Region, 163.—Deep Plowing and
        Subsoiling in the Arid Region; examples of Plant growth on
        Subsoils, 164.—Resistance to Drought, 167.—Root System in
        the Humid Region, 168.—Figures of the Root System of an
        Eastern (Wisconsin) Fruit Tree, 168.—_Comparison of Root
        Development in the Arid and Humid Regions_, 169.—Prune
        on Peach Root, 169.—Adaptation of Humid Species to Arid
        Conditions, 169.—Grapes, 170.—Kentucky and California
        Maize, 175, 176.—Hops, 172.—Deep-Rooting in the Arid
        Region, 174.—Goose Foot and Figwort, 174.—_Importance
        of Proper Substrata in the Arid Region_, 173.—Injury
        from Impervious Substrata. Figure, 177.—Faulty Lands of
        California. Figure, 178.—Shattering of Dense Substrata
        by Dynamite, 181.—Leachy Substrata, 182.—“Going-back”
        of Orchards, 182.—Hardpan, Formation and Varieties,
        183.—Nature of the Hardpan Cements, 184.—Bog Ore,
        Moorbedpan and Ortstein; Calcareous and Alkali Hardpan,
        184.—The Causes of Hardpan, 185.—“Plowsole,” 186.—Marly
        Substrata, 186.

                              CHAPTER XI.
    THE WATER OF SOILS. HYGROSCOPIC AND CAPILLARY MOISTURE,
        188.—General Properties, 188.—Physical Factors
        of Water compared with other Substances. Table,
        188.—Capillarity or Surface Tension, 189.—Heat Relations,
        190.—Density, 190.—Specific Heat and its Effects,
        190. Ice, 191.—Vaporization, 191.—Solvent Power,
        191.—Water-requirements of Growing Plants, 192.—Evaporation
        from Plants in Different Climates, 192.—Relations between
        Evaporation and Plant Growth. Table, 193.—Fortier’s
        Experiments. Figure, 194.—Different Conditions of
        Soil Water, 196.—_Hygroscopic Water in Soils; Table_,
        196.—Influence of Temperature and Air-Saturation,
        197.—Utility of Hygroscopic Water to Plant Growth,
        199.—Mayer’s Experiments, 200.—Summary, 200.—_Capillary
        Water_, 201.—Ascent of Water in Soil-Columns. Table,
        202.—Ascent in Uniform Sediments. Figure, 204.—Maximum and
        Minimum Water-holding Power, 207.—Capillary Water held at
        different Heights in a Soil Column. Table, 208.—Capillary
        Action in Moist Soils, 210.—Proportion of Soil Moisture
        Available to Plants, 211.—Moisture Requirements of Crops in
        the Arid Region, 211.—Tables of Observations in California,
        214.

                             CHAPTER XII.
     SURFACE, HYDROSTATIC AND GROUND WATER. PERCOLATION,
        215.—Amount of Rainfall, 215.—Natural Disposition of Rain
        Water, 216.—The Surface Runoff, 216.—Washing-away and
        Gullying in the Cotton States, 217.—Injury in the Arid
        Regions, 219.—Deforestation, 219.—Prevention of Injury to
        Cultivated Lands from Excessive Runoff, 220.— _Absorption
        and Movements of Water in Soils_, 221.—Determination of
        Rate of Percolation. Diagram, 221.—Summary, 224.—Influence
        of Variety of Grain-sizes, 224.—Table of King’s
        Experiments, 224.—Percolation in Natural Soils. Figure,
        225.—Ground or Bottom Water, 227.—Lysimeters, Surface of
        Ground Water; Variations, 227.—Depth of Ground Water most
        Favorable to Crops, 228.—Moisture Supplied by Tap Roots,
        229.—Reserve of Capillary Water, 229.—Injurious Rise of
        Bottom Water from Irrigation, 230.—Consequences of the
        Swamping of Irrigated Lands; Prevention, 231.—Permanent
        Injury to certain Lands, 231.—Reduction of Sulfates,
        232.—Ferruginous or Red Lands, 233.

                             CHAPTER XIII.
    WATER OF SOILS; THE REGULATION AND CONSERVATION OF SOIL,
        MOISTURE; IRRIGATION, 234.—Loosening of the Surface,
        234.—Effects of Underdrains; Rain on Clay Soils,
        235.—Winter Irrigation, 236.—_Methods of Irrigation_,
        236.—Surface Sprinkling, 237.—Flooding, 237.—Check
        Flooding. Furrow Irrigation, 237, 238.—Figure Showing
        Penetration, 239.—Figure Showing Faulty Irrigation
        in Sandy Lands, 239.—Distance between Furrows and
        Ditches, 241.—Irrigation by Lateral Seepage, 242.—Basin
        Irrigation of Trees and Vines; Advantages and Objections,
        243.—Irrigation from Underground Pipes, 245.—_Quality of
        Irrigation Waters_, 246.—Saline Waters; Figures of Effects
        on Orange Trees, 246.—Limits of Salinity, 246.—Mode of
        Using Saline Irrigation Waters; Apparent Paradox, 249.—Use
        of Drainage Waters for Irrigation, 250.—“Black Alkali”
        Waters, 250.—Variations in the Salinity of Deep and Shallow
        Wells, 250.—Muddy Waters, 251.—_The Duty of Irrigation
        Waters_, 251.—Causes of Losses, 252.—Loss by Percolation.
        Figure, 252.—_Evaporation_, 253.—Tables Showing same
        at California Stations, 255.—Evaporation in Different
        Climates; Table, 255.—Evaporation from Reservoirs and
        Ditches, 257.—Prevention of Evaporation; Protective Surface
        Layer, 257.—Illustrations of Effects of Tillage; Table,
        258.—Evaporation through Roots and Leaves, 262.—Weeds
        waste Moisture, 264.—Distribution of Moisture in Soils
        as Affected by Vegetation, 264.—Forests and Steppes,
        265.—Eucalyptus for Drying Wet Lands, 265.—Mulching;
        Effects on Temperature and Moisture, 266.

                             CHAPTER XIV.
     ABSORPTION BY SOILS OF SOLIDS FROM SOLUTIONS. ABSORPTION
        OF GASES, AIR OF SOILS, 267.—_Absorption of Solids_,
        267.—Desalination, 267.—Decolorization, 267.—Complexity
        of Soil-Action, Physical and Chemical, 268.—“Purifying”
        Action of Soils on Gases and Liquids, 269.—Waste of
        Fertilizers, 269.—Variation of Absorptive Power,
        270.—Generalities Regarding Chemical Action and Exchange,
        270.—Drain Waters, 271.—Distinctions not Absolute,
        272.—_Absorption or Condensation of Gases by the Soil_,
        272.—Proof of Presence of Carbonic and Ammonia Gases in
        Soils, 273.—Absorption of Gases by Dry Soils. Figure,
        274.—Composition of Gases Absorbed by Various Bodies from
        the Air. Table, 275.—Discussion of Table, 277.—_The Air of
        Soils_, 279.—Empty Space in Dry Soils, 279.—Functions of
        Air in Soils, 279.—Insufficient and Excessive Aeration,
        280.—Composition of the Free Air of Soils, 280.—Carbonic
        Dioxid vs. Oxygen, 281.—Relation to Bacterial and Fungous
        Activity, 281.—Putrefactive Processes, 282.

                              CHAPTER XV.
    COLORS OF SOILS, 283.—Black Soils, 283.—“Red” Soils,
        284.—Origin of Red Tints, 285.—White Soils,
        285.—Differences in Arid and Humid Regions, 286.—White
        Alkali Spots, 286.

                             CHAPTER XVI.
    CLIMATE, 287.—Heat and Moisture Control Climates, 287.—Climatic
        Conditions, 287.—_Ascertainment and Presentation of
        Temperature Conditions_, 288.—Annual Mean not a Good
        Criterion, 289.—Extremes of Temperature are most
        Important, 289.—Seasonal and Monthly Means, 289.—Daily
        Variations, 290.—_The Rainfall_, 290.—Annual Rainfall
        not a Good Criterion, 290.—Distribution most Important,
        290.—_Winds_, 291.—Heat the Cause of Winds, 291.—Trade
        Winds, 291.—Cyclones, 292.— Influence of the Topography
        on Winds; Rains to Windward of Mountains, Arid Climates
        to Leeward, 293.—General Distribution of Rainfall on the
        Globe. Figure, 294.—Ocean Currents, 295.—The Gulf Stream,
        295.—The Japan Stream, 296.—Contrast of Climates of N. W.
        America, 297.—Continental, Coast and Insular Climates,
        297.—Subtropic. Arid Belts, 298.—Utilization of the Arid
        Belts, 299.

                             CHAPTER XVII.
    RELATIONS OF SOILS AND PLANT GROWTH TO HEAT, 301.—Temperature
        of Soils, 301.—Water Exerts Controlling Influence,
        301.—Cold and Warm Rains, 302.—Solar Radiation,
        302.—Penetration of the Sun’s Heat into the Soil,
        302.—Change of Temperature with Depth, 303.—Surface
        Conditions that Influence Soil Temperature, 303.—Heat of
        High and Low Intensity, 304.—Reflection vs. Dispersion
        of Heat, 304.—Influence of Vegetation, and of Mulches,
        305.—Influence of the Nature of the Soil-Material,
        306.—Influence of Evaporation, 307.—Formation of
        Dew, 307.—Dew rarely adds Moisture, 308.—Dew within
        the Soil, 308.—Plant Development under Different
        Temperature-Conditions, 309.—Germination of Seeds; Optimum
        Temperature for each Kind, 309.—Artificial Heating of
        Soils; by Steam Pipes or Water, 310.

                            CHAPTER XVIII.
     PHYSICO-CHEMICAL INVESTIGATION OF SOILS IN RELATION TO CROP
        PRODUCTION, 313.—_Historical Review of Soil Investigation_,
        313.—Popular Forecasts of Soil Values, 313.—Cogency of
        Conclusions Based upon Native Growth, 314.—Ecological
        Studies, 315.—Early Soil Surveys of Kentucky, Arkansas
        and Mississippi, 316.—Investigation of Cultivated Soils,
        316.—Change of Views, 317.—Advantages for Soil Study
        offered by Virgin Lands, 318.—Practical Utility of Soil
        Analysis; Permanent Value _vs._ Immediate Productiveness,
        319.—_Physical and Chemical Conditions of Plant Growth_,
        319.—Condition of Plant-food Ingredients, in the Soil,
        319.—Water-soluble, Reserve, and Insoluble Part,
        320.—Hydrous or “Zeolitic” Silicates, 321.—Recognition of
        the Prominent Chemical Character of Soils, 322.—Acidity,
        Neutrality and Alkalinity, 322.—Chemical Analysis,
        323.—Water-Soluble and Acid-Soluble Portions most
        Important, 324.—We cannot Imitate Plant-root Action, 324
        Cultural Experience the Final Test, 324.—Analysis of
        Cultivated Soils, 325.—Methods of Analysis, 325.—_The
        Solvent Action of Water upon Soils_, 327.—Extraction of
        Soils with Pure Water, 327.—Continuous Solubility of
        Soil Ingredients. Tables, 328.—King’s Results. Table,
        329.—Composition and Analysis of Janesville Loam,
        331.—Solubility of Soil Phosphates in Water, 332.—Practical
        Conclusions from Water Extraction, 332.—_Ascertainment
        of the Immediate Plant-food Requirements of Cultivated
        Soils by Physiological Tests_, 333.—Plot Tests; their
        uncertainties. Diagram, 334.—Crop Analysis as a Test
        of Soil Character, 337.—Chemical Tests of immediately
        Available Plant Food; Dyer’s Method, 338.

                             CHAPTER XIX.
    ANALYSIS OF VIRGIN SOILS BY EXTRACTION WITH STRONG ACIDS AND
        ITS INTERPRETATION, 340.—Loughridge’s Investigation on
        Strength of Acid and Time of Digestion, 340.—_Writer’s
        Method_, 342.—Virgin Soils with High Plant-food Percentages
        are always Productive. Table, 343.—Discussion of Table,
        343.—Low Plant-food Percentages not always Indication of
        Sterility, 346.—What are “Adequate” Percentages of Potash,
        Lime, Phosphoric Acid and Nitrogen, 347.—Soil-Dilution
        Experiments, 347.—Table of Compositions, 350.—Figures of
        Plants and their Root-Development, 351.—Limitation of Root
        Action, 351.—Lowest Limits of Plant-food Percentages and
        Productiveness Found in Virgin Soils, 353.— _Limits of
        Adequacy of the Several Plant-food Percentages in Virgin
        Soils_, 353.—Lime a Dominant Factor in Interpretation,
        353.—Potash, 354.—Phosphoric Acid, 355.—Action of Lime and
        Ferric Oxid, 355.—Table of Hawaiian Ferruginous Soils,
        356.—Unavailability of Ferric Phosphate, 356.—Nitrogen,
        357.—Nitrification of the Organic Matter of the Soil,
        358.—Analysis of Soil from the Ten-Acre Tract at Chino,
        Cal., 358.—Experiments and Results; Matière Noire the
        Only Guide, 360.—What are Adequate Nitrogen Percentages
        in the Humus? 360.—Table of Humus and Nitrogen-Content
        of Californian and Hawaiian Soils, 361.—Confirmatory
        Experiment. Figure, 362.—Data for Nitrogen-Adequacy. Table,
        363.—_Influence of Lime upon Soil Fertility_, 365.—“A
        Lime Country is a Rich Country,” 365.—Effects of High
        Lime-Content in Soils, 365.—Table of Soils showing Low
        Phosphoric Acid with High and Low Lime-Content, 366.—What
        are Adequate Lime-percentages? Differ for Light and Heavy
        Soils, 367.—Table Showing Need of High Lime Percentages
        in Heavy Clay Soils, 368.—European Standards for Land
        Estimates, 369.—Maercker’s Table, 369.

                              CHAPTER XX.
     SOILS OF THE ARID AND HUMID REGIONS, 371.—Composition of
        Good Medium Soils; Table, 371.—Criteria of Lands of
        the Two Regions, 371.—Tables of Soil-Composition in
        Both Regions, 372.—Soils of the Humid Region governed
        by Time, 374.—Soils of the Arid Region Governed by
        Moisture, 374.—Lime and Magnesia Uniformly High in Arid
        Soils, Despite Scarcity of Limestone Formations; Potash
        also High, 374.—_General Comparison of the Soils of the
        Arid and Temperate Humid Regions_, 375.—Basis of Same,
        376.—New Mexico and Analysis of Soil, 376.—General Table,
        377.—Discussion of the Table, 378.—_Lime_; Summary of
        Physical and Chemical Effects of Lime Carbonate in Soils,
        378.— Discussion of Summary, 379.—_Magnesia_: Its role
        in Plant Nutrition, 381.—_Manganese_: Its Stimulant
        Action, 383.—The “_Insoluble Residue_” or Silicates,
        384.—_Soluble Silica and Alumina_, 384.—Analysis of Clay
        from Soil, 385.—Difference in Sand of Arid and Humid
        Regions. Table, 386.—Soluble Silica or Hydrous Silicates
        more Abundant in Arid than in Humid Soils, 388.—_Aluminic
        Hydrate._ Table, 389.—Retention of Soluble Silica in Alkali
        Soils, 391.—_Ferric Hydrate_, 392.—_Phosphoric Acid_,
        392.—Sulfuric Acid, 394.—_Potash_ and _Soda_, Retained
        more in Arid Soils, 394.—Arid Soils Rich in Potash,
        395.—_Humus_, Low in Arid Soils, but Rich in Nitrogen,
        396.—The Transition Region, 397.

                             CHAPTER XXI.
    SOILS OF ARID AND HUMID REGIONS CONTINUED, 398.—_Soils of the
        Tropics_, 398.—Humus in Tropical Soils, 399.—Investigations
        of Tropical Soils, 401.—_Soils of Samoa and Kamerun_,
        402.—Soils of the Samoan Islands, 403.—Soils of Kamerun,
        404.—_Soils of Madagascar_, 405.—_Soils of India_, 410.—The
        Indo-Gangetic Plain, 411.—The Brahmaputra Alluvium in
        Assam, 413.—Black Soils of Deccan, 414.—Red Soils of the
        Madras Region, 415.—Laterite Soils, 416.—_Influence of
        Aridity upon Civilization_, 417.—Preference of Ancient
        Civilizations for Arid Countries, 417.—Irrigation
        Necessitates Co-operation, 419.—High and Permanent
        Productiveness of Arid Soils Induces Permanence of Civil
        Organization, 419.

                             CHAPTER XXII.
     ALKALI SOILS, THEIR NATURE AND COMPOSITION, 422.—Alkali
        Lands _vs._ Seashore Lands, 422.—Origin, 422.—Deficient
        Rainfall, 423.—Predominant Salts, 423.—Geographical
        Distribution, 424.—Their Utilization of World-wide
        Importance, 424.—Repellent Aspect, Plate, 424.—Effects
        of Alkali upon Culture Plants. Figures of Apricot Trees,
        426.—Nature of the Injury, External and Internal,
        426.—Effects of Irrigation, 428.—Leaky Irrigation Ditches,
        429.—Surface and Substrata of Alkali Lands, 429.—Vertical
        Distribution of the Salts in Alkali Soils, 429.—How
        Native Plants Live, 430.—Figures of various Phases of
        Reclamation, 431.—Upward Translocation from Irrigation,
        433.—Distribution of Alkali in Sandy Lands, 433.—In Heavier
        Lands, 436.—Salton Basin or Colorado Delta, 436.—Diagram of
        Alkali Distribution in Same, 438.—Horizontal Distribution
        of Alkali Salts in Arid Lands, 439.—Alkali in Hill
        Lands, 439.—Usar Lands of India, 440.—“Szek” Lands of
        Hungary, 440.—Alkali Lands of Turkestan, 441.—_Composition
        and Quantity of Salts Present_, 441.—Nutritive Salts,
        441.—Black and White Alkali. Tables, 442.—Estimation of
        Total Alkali in Land, 444.—_Composition of Alkali Soils
        as a whole._ Tables, 445.—Presence of much Carbonate
        of Soda, 448.—Cross Section of an Alkali Spot. Table,
        448.—_Reactions between the Carbonates and Sulfates of
        Earths and Alkalies._ Figure of Curve, 449.—Inverse
        Ratios of Alkali Sulfates and Carbonates. Diagrams,
        451.—Exceptional Conditions, 453.—Summary of Conclusions,
        453.

                            CHAPTER XXIII.
    UTILIZATION AND RECLAMATION OF ALKALI LAND,
        455.—Alkali-resistant Crops, 455.—Counteracting
        Evaporation, 455.—Turning-under of Surface Alkali,
        456.—Shading, 457.—Neutralizing Black Alkali,
        457.—Removing the Salts from the Soils, 458.—Scraping
        off, 458.—Leaching-Down. Figure, 459.—Underdrainage, the
        Final and Universal Remedy for Alkali, 460.—Possible
        Injury to Land by Excessive Leaching, 462.—Difficulty in
        Draining “Black” Alkali Land, 462.—Swamping of Alkali
        Land, 463.—Removal of Alkali Salts by Certain Crops,
        463.—Tolerance of Alkali by Culture Plants, 463.—Relative
        Injuriousness of the several Salts. Effects on Sugar Beets,
        464.—Table of Tolerances; Comments on same, 467.—Saltbushes
        and Native Grasses. Australian Saltbushes, 469.—Modiola;
        Native and Cultivated Grasses, 469.—Other Herbaceous Crops,
        472.—Legumes, 472.—Mustard Family, 473.—Sunflower Family,
        473.—Root Crops, 474.—Stem Crops, 475.—Textile Plants,
        475.—Shrubs and Trees, 475.—Vine, Olive, Date, Citrus
        Trees. Deciduous Orchard Trees. Timber and Shade Trees,
        475.—Inducements toward the Reclamation of Alkali Lands,
        481.—Wheat on Reclaimed Land at Tulare; Figure, 482.—Need
        of Constant Vigilance, 484.

                             CHAPTER XXIV.
     THE RECOGNITION OF SOIL CHARACTER FROM THE NATIVE
        VEGETATION; MISSISSIPPI, 487.—Climatic and Soil Conditions,
        487.—Natural Vegetation the Basis of Land Values in the
        United States, 488.—Investigation of Causes Governing
        Distribution of Native Vegetation, 488.—Investigations
        in Mississippi, 489.—Vegetative Belts in Northern
        Mississippi, 490.—Sketch Map of Same, with Tabulation of
        Lime Content and Native Vegetation, 490.—Lime Apparently a
        Governing Factor, 492.—Soil Belts in Southern Mississippi,
        493.—Vegetative and Soil Features of Coast Belts. Diagram,
        495.—Table of Plant-Food percentages and Native Growth,
        496.—Definition of Calcareous Soils, 496.—_Differences in
        the Form and Development of Trees_, 498.—Forms of the Post
        Oak. Figures, 498.—Forms of the Black Jack Oak. Figures,
        500.—Characteristic Forms of other Oaks, 502.—Sturdy Growth
        on Calcareous Lands, 502.—Growth of Cotton, 503.—Lime
        Favors Fruiting, and compact Growth, 504.—Physical vs.
        Chemical Causes of Vegetative Features, 505.—Lowland Tree
        Growth, 506.—Contrast between “First” and “Second” Bottoms,
        506.—Tree Growth of the First Bottoms. The Cypress,
        507.—Figures of Swamp and Upland Cypress, 508.—Other
        Lowland Trees, 509.—General Forecasts of Soil Quality in
        Forest Lands, 509.

                             CHAPTER XXV.
    RECOGNITION OF THE CHARACTER OF SOILS FROM THEIR NATIVE
        VEGETATION. UNITED STATES AT LARGE, EUROPE, 511.—Forest
        Growths outside of Mississippi; Alabama, Louisiana, Western
        Tennessee, and Western Kentucky, 511.—North Central States
        East of the Mississippi River, 513.—Upland and Lowland
        Vegetation in the Arid and Humid Region, 515.—Forms of
        Deciduous Trees in the Arid Region, 516.—Tall Growth of
        Conifers, 517.—Herbaceous Plants as Soil Indicators,
        517.—Leguminous Plants Usually Indicate Rich or Calcareous
        Lands, 518.—European Observations and Views on Plant
        Distribution and its Controlling Causes, 519.—Composition
        of Pine Ashes on Calcareous and Non-calcareous Lands.
        Table, 520.—Calciphile, Calcifuge, and Indifferent Plants,
        521.—Silicophile _vs._ Calciphile Flora, 523.—What is a
        Calcareous Soil? 524.—Predominance of Calcareous Formations
        in Europe, 525.

                             CHAPTER XXVI.
    THE VEGETATION OF SALINE AND ALKALI LANDS, 527.—_Marine Saline
        Lands_, 527.—General Character of Saline Vegetation,
        527.—Structural and Functional Differences Caused by Saline
        Solutions, 528.—Absorption of the Salts. Table, 529.—Injury
        from the Various Salts, 531.—Reclamation of Marine Saline
        Lands for Culture, 533.—_The Vegetation of Alkali Lands_,
        534.—_Reclaimable_ and _Irreclaimable Alkali Lands_ as
        Distinguished by their Natural Vegetation, 534.—Plants
        Indicating Irreclaimable Lands, 535.—Tussock Grass; Bushy
        Samphire; Dwarf Samphire; Saltwort; Greasewood; Alkali
        Heath; Cressa; Salt Grass, 536.—Relative Tolerances of the
        different Species; Table, 549.

                              APPENDICES.
     A.—Directions for taking Soil Samples, issued by the
        California Experiment Station, 553.

     B.—Summary Directions for Soil-Examination in the Field
        or Farm, 556.

     C.—Short Approximate Methods of Chemical Soil-Examination
        Used at the California Experiment Station, 560.

     General Index, 565.
     Index of Authors referred to, 591.




PREFACE.


This volume was originally designed to serve as a text and reference
book for the students attending the writer’s course on soils, given
annually at the University of California, who complained of their
inability to find in any connected treatise a large portion of
the subject matter brought before them. As all these students had
preliminary training in physics, chemistry and botany, no introductory
chapters on these general subjects were necessary or contemplated; the
more so as good elementary treatises embracing the needful preparation
are now numerous.

As time progressed, however, outside demands for a book embodying the
writer’s soil studies in the humid and arid regions, especially the
latter, became so numerous and pressing that the scope of the work has
gradually been much enlarged to conform to these demands; and this,
rather than completeness of detail, when such detail can be found well
given elsewhere, has been the guide in the necessary condensation of
the whole. To give the entire subject matter full elucidation, would
require several more volumes.

It may not be unnecessary to explain at the outset why and how this
treatise deviates in many respects from previous publications on the
same general topic. From boyhood up it has fallen to the writer’s lot
to be almost continuously in more or less direct contact with the
conditions and requirements of newly settled regions, as well as with
those hardly yet invaded even by the pioneer farmer; where the question
of cultural adaptation was yet undetermined or wholly in the dark.
Being during his active life constantly called upon in his official
capacity to give information and advice to pioneer farmers or intending
settlers in regard to the merits and adaptations of virgin soils, the
writer’s attention was naturally and forcibly directed toward soil
investigation as a possible means of determining, beforehand, the
general prospects and special features of agriculture in regions where
actual experience was either non-existent or very brief and partial.
In the pursuit of these studies he has been favored by exceptional
opportunities, extending over a varied climatic area reaching on the
south from the Gulf of Mexico to the Ohio, across to the Pacific coast,
and to British Columbia on the north. That a systematic investigation
of soils over so large an area, covering both humid and arid regions,
should lead to some unexpected and novel results, is but natural; and
it is the discussion of these results in connection with those obtained
elsewhere, and with some of the prevailing views based thereon, that
must serve as the justification for the present addition to an already
well-stocked branch of literature.

From the very beginning of the scientific study of agriculture, the
investigation of soils with a view to the _à priori_ determination
of their adaptation, permanent value, and best means of cultural
improvement, has formed the subject of continuous effort. It is not
easy to imagine a subject of higher direct importance to the physical
welfare of mankind, whose very existence depends on the yearly returns
drawn by cultural labor from the soil.

It is certainly remarkable that after all this long-continued effort,
even the fundamental principles, and still more the methods by which
the object in view is to be attained, are still so far in dispute that
a unification of opinion in this respect is not yet in view; and a
return to pure empiricism is from time to time brought forward to cut
the Gordian knot.

While this state of things is primarily due to the intrinsic complexity
and difficulty of the subject itself, it has unquestionably been
materially aggravated by accidental, partly historic conditions.
Foremost among these is the fact that until within recent times, soil
studies have borne almost entirely on lands long cultivated and in most
cases fertilized: thus changing them from their natural condition to a
more or less artificial one, which obscures the natural relations of
each soil to vegetation.

The importance of these relations is obvious, both from the theoretical
and from the practical standpoint. From the former, it is clear that
the native vegetation represents, within the climatic limits of the
regional flora, the result of a secular process of adaptation of plants
to climates and soils, by natural selection and the survival of the
fittest. The natural floras and sylvas are thus the expression of
secular, or rather, millennial experience, which if rightly interpreted
must convey to the cultivator of the soil the same information that
otherwise he must acquire by long and costly personal experience.

The general correctness of this axiom is almost self-evident; it is
explicitly recognized in the universal practice of settlers in new
regions, of selecting lands in accordance with the character of the
forest growth thereon; it is even legally recognized by the valuation
of lands upon the same basis, for purposes of assessment, as is
practiced in a number of States.

The accuracy with which experienced farmers judge of the quality of
timbered lands by their forest growth, has justly excited the wonder
and envy of agricultural investigators, whose researches, based upon
incomplete theoretical assumptions, failed to convey to them any such
practical insight. It was doubtless this state of the case that led a
distinguished writer on agriculture to remark, nearly half a century
ago, that he “would rather trust an old farmer for his judgment of land
than the best chemist alive.”[1]

[1] “The Soil Analyses of the Geological Surveys of Kentucky and
Arkansas.” S. W. Johnson in _Am. Jour. Sci._, Sept. 1861.

It is certainly true that mere physico-chemical analyses, unassisted
by other data, will frequently lead to a wholly erroneous estimate of
a soil’s agricultural value, when applied to cultivated lands. But
the matter assumes a very different aspect when, with the natural
vegetation and the corresponding cultural experience as guides, we seek
for the factors upon which the observed natural selection of plants
depends, by the physical and chemical examination of the respective
soils. It is further obvious that, these factors being once known, we
shall be justified in applying them to those cases in which the guiding
mark of native vegetation is absent, as the result of causes that have
not materially altered the natural condition of the soil.

It is probable that, had agricultural science been first developed
in regions where the external conditions permitted the carrying-out
of such a course of investigation, instead of in the abnormally
temperate, even and humid climate of middle Europe, with its
long-cropped, worn fields, and very predominantly calcareous soils,
the present condition of this science might differ not immaterially
from that actually existing. As a matter of fact, it has attained
its present state under very disadvantageous external conditions,
which frequently necessitated a recourse to highly complex and
laborious methods and artificial appliances, for the establishment and
maintenance of the conditions which elsewhere might have been found
abundantly realized in nature; thus permitting, by the multiplication
of observations over extended and widely varied areas, the elimination
and control of accidental errors of experiment and observation.

Just as in historical geology the subdivisions of formations observed
and accepted in Europe formed for many years a procrustean bed upon
which the facts observed elsewhere had to be stretched, so in the
domain of soil physics and chemistry, and even in vegetable physiology,
the observations made in the really exceptional climates and soils
of middle Western Europe, have often erroneously been construed as
constituting a general basis for unalterable deductions.

The rapid extension of civilization and the carrying of minute
scientific research into other regions, now rendered possible by the
improved means of communication, has shown the one-sidedness of some of
the views prevailing heretofore, inasmuch as they are really applicable
only to accidental and rather exceptional conditions.

It is therefore one object of this volume to present and discuss
summarily the facts of physical and chemical soil constitution and
functions with reference to the additional light afforded on the wider
basis, embracing both the humid and the arid regions; of which the
latter has, as such, received but scant and desultory attention thus
far, to the detriment of both the work of the agricultural experiment
stations and of agricultural practice. The book therefore includes the
discussion both of the methods and results of direct physical, chemical
and botanical soil investigation, as well as the subject matter
relating to the origin, formation, classification and physical as well
as chemical nature of soil, usually included in works on scientific
agriculture.

In the presentation of these subjects, it has been the writer’s aim
to reach both the students in his own classes and in the agricultural
colleges generally, as well as the fast increasing class of farmers of
both regions who are willing and even anxious to avail themselves of
the results and principles of scientific investigation, without “shying
off” from the new or unfamiliar words necessary to embody new ideas. It
would seem to be time that the latter class, and more especially those
constituting farmers’ clubs, should learn to understand and appreciate
both the terms and methods of scientific reasoning, which are likely to
form, increasingly, the subjects of instruction in the public schools.
But in order to segregate to some extent the generally intelligible
matter from that which requires more scientific preparation than can
now be generally expected, it has been thought best to use in the text
two kinds of type; the larger one embodying the matter presumed to be
interesting and intelligible to the general reader, while the smaller
type carries the illustrative detail and discussion which will be
sought chiefly by the student.

As regards the chemical nomenclature used in this volume, the writer
has not thought it advisable to follow the example set by some late
authors in substituting for the well-known names of the bases and
acids, those of the elements, and still less, those of the intangible
ions. Any one who has taught classes in agricultural chemistry will
have experienced the difficulty and loss of time unnecessarily incurred
in the incessantly recurring transposition of terms, and complication
of formulæ, serving no useful purpose save that of academic
consistency. It is of at least doubtful utility to present to the
farmer, _e. g._, the inflammable and dangerous elements phosphorus and
potassium as prime factors in the success of his crops, and of healthy
nutrition.

Inasmuch as all the elements are presented to and contained in the
plant in compounds only, and these compounds are themselves, in the
dilute solutions used by plants, known to be largely dissociated into
their basic and acid groups, it seems to be most natural to present
them under the corresponding, even if not absolutely theoretically
correct names of acids and bases, to which the farmer and the trade
have been accustomed for half a century. Upon these considerations the
long-used designations of potash, soda, lime, phosphoric, sulfuric,
nitric and other acids and bases have been retained in this volume,
adding the chemical formula where, as in analytical statements, a doubt
as to their meaning might arise. Assuredly, the diffusion of scientific
knowledge should not be needlessly hindered by the adoption of a
pedantic mode of presentation.

The great breadth of the subject of this volume has rendered
inadvisable any extended bibliography, such as it has of late become
customary to add to works of this kind. References have therefore been
restricted to publications specially discussed, and to such as are not
widely known on account of limited circulation.

The author’s warmest acknowledgments are due to Professor R. H.
Loughridge, of the University of California, for efficient and
sympathetic assistance, both in the revision of the manuscript, and
active personal help in the preparation of the illustrations. Without
his coöperation the preparation and publication of the volume would
have been much longer delayed.

Acknowledgments are also due for helpful suggestions and criticism
to Professors L. H. Bailey, of Cornell University, F. H. King, of
Wisconsin, and Jacques Loeb of the University of California.

                                               E. W. HILGARD.
    BERKELEY, CALIFORNIA,
        _November 15, 1905_.




INTRODUCTION.


_Definition of Soils._—In the most general meaning of the term, a soil
is the more or less loose and friable material in which, by means of
their roots, plants may or do find a foothold and nourishment, as well
as other conditions of growth. Soils form the uppermost layer of the
earth’s crust; but the term does not indicate any such definite average
texture as is sometimes implied by its popular use to designate certain
loose, loamy materials found in older geological formations. We do find
in these, not unfrequently, layers that in the past have served to
support vegetation, as evidenced by remains of plants found therein.
But as a rule, such ancient soils are much compacted and otherwise
changed, and would not now be capable of performing the office of
plant nutrition without previous, long-continued exposure to the same
agencies by which all soils were originally formed from pre-existing
rocks. Within the latter category must be included, in scientific
parlance, not only the hard rocks known as such in daily life, but also
such soft materials as clay, sand, marls, etc., which often compose,
partially or wholly, the bodies of wide-spread geological formations.

_Elements Constituting the Earth’s Crust._—More than seventy elementary
substances have been found within the portion of the earth accessible
to man; most of these are present only in very minute proportions; of
those occurring in relatively considerable quantities, a list showing
their approximate proportions is given below.

_Average quantitative composition of the Earth’s Crust._—The total
thickness of the outer shell of the earth, thus far known to us, does
not exceed about 95,000 feet, as observed in the accessible rock
deposits. Estimates of the proportions in which the more abundant
elements contribute to the composition of these constituent rocks, have
repeatedly been made. The latest and most widely accepted of these, by
F. W. Clarke, of the U. S. Geological Survey, is given herewith. It
includes the constituents of the sea and atmosphere as well; these two
constitute about 7 per cent of the whole, 93 per cent being solid rocks.

    RELATIVE ABUNDANCE OF THE ELEMENTS TO A DEPTH OF TEN KILOMETERS.

                 SOLID CRUST       OCEAN           MEAN,
                (93 PER CENT).  (7 PER CENT).  INCLUDING AIR.

    Oxygen         47.29           85.79           49.98
    Silicon        27.21                           25.30
    Aluminum        7.81                            7.26
    Iron            5.46                            5.08
    Calcium         3.77            0.05            3.51
    Magnesium       2.68            0.14            2.50
    Sodium          2.36            1.14            2.28
    Potassium       2.40            0.04            2.23
    Hydrogen        0.21           10.67            0.94
    Titanium        0.33                            0.30
    Carbon          0.22            0.002           0.21
    Chlorin         0.01            2.07            0.15
    Phosphorus      0.10                            0.09
    Manganese       0.08                            0.07
    Sulphur         0.03            0.09            0.04
    Barium          0.03                            0.03
    Nitrogen                                        0.02
    Fluorin         0.02                            0.02
    Chromium        0.01                            0.01

It will be noted that one-half of the total consists of oxygen, and
that nearly 86% (or 47.29% of the 49.98%) of this amount is contained
in the solid rocks; nearly 2.50% of the remainder in sea and other
water; and .41% in the atmosphere, in the free condition, in which it
serves for the respiration of animals and plants, and for the various
processes of slow and rapid combustion, or “oxidation.” This relatively
small proportion of the whole, is, nevertheless, the most directly
important for the maintenance of organic life.

_Oxids Constitute Earth’s Crust._—The vast predominance of oxygen in
the above list suggests at once that most of the other elements must
exist in combination with it, _i. e._, as “oxids.” H. S. Washington[2]
has lately revised the estimates heretofore made, on the basis of a
very large number of analyses made by him and others, of rocks within
the United States, and gives the following table; alongside of which
is placed a revised estimate by Clarke, which also includes rocks from
abroad; both being given in terms of oxids of the several elements.

[2] U. S. Geol. Survey, Professional Paper No. 14, p. 108.

                                     WASHINGTON.   CLARKE.

    Silica                  SiO₂         57.78      59.89
    Alumina                 Al₂O₃        15.67      15.45
    Peroxid of Iron         Fe₂O₃         3.31       2.64
    Protoxid of Iron        FeO           3.84       3.53
    Magnesia                MgO           3.81       4.37
    Lime                    CaO           5.18       4.91
    Soda                    Na₂O          3.88       3.56
    Potash                  K₂O           3.13       2.81
    Water, basic            H₂O⁺          1.42       1.52
    Water, acid             H₂O⁻           .36        .40
    Ferric Sulphid          FeS₂          1.03        .60
    Phosphoric acid         P₂O₅           .37        .22
    Manganese Protoxid      MnO            .22        .10

The salient point which at once attracts attention in these tables is
the great predominance of the oxid of silicon—silica, silicic acid,
quartz, etc.,—over all other substances. While quartz occurs alone
in enormous masses, as will be shown later, probably the greater
proportion is found in combination with other oxids, notably those of
aluminum, calcium, iron, magnesium, and the alkali metals potassium and
sodium. Chlorin and fluorin, however, do not occur as oxids.[3]

[3] A trifling amount of chlorin is found oxidized in the form of
sodium perchlorate, in the nitre deposits of Chile.

_The Chemical Elements Important to Agriculture._—Of the numerous
elements known to chemists, only eighteen require mention in connection
with either soil formation or plant growth; and of these only thirteen
or fourteen participate in normal plant growth. They are the following:

    METALLIC ELEMENTS.    NON-METALLIC ELEMENTS.

    Potassium               Carbon
    Sodium                  Hydrogen
    Calcium                 Oxygen
    Magnesium               Nitrogen
    Iron                    Phosphorus
    Manganese               Sulphur
    Aluminum                Chlorin
    Titanium                Fluorin
                            Iodin
                            Silicon.

Of this list, titanium, though a very constant ingredient of soils in
the form of titanic dioxid, is not known as performing any important
function in soils, and is not, so far as known at present, ever taken
up by plants. Aluminum, in the form of its compounds with oxygen
and silicon, is a very prominent and physically very important soil
ingredient, but does not, apparently, perform any direct function in
plant nutrition, and is absent from their ash, except in the case of
some of the lower plants (horsetails and ferns).

Iodin appears to be normally present in all seaweeds, and occurs in
traces in some land plants. Fluorin is a normal ingredient of animal
bones, and its presence in plant ashes is often easily shown. The
remaining fourteen, however, are always present in plants; carbon,
hydrogen, oxygen and nitrogen forming the volatile or combustible part,
while the rest occur in the ashes.

It is true that other elements, or rather their compounds, are
sometimes found in plants, being taken up by them from solutions
existing in the soil. Thus the alkalies caesium and rubidium, also
barium, strontium, zinc, copper, boron and some others, may be absorbed
when present in soluble form. But they are neither necessary nor
beneficial to plant economy, and when in considerable amounts are
harmful. Thus fifteen elements, ommiting iodin and titanium, alone
require discussion.

_The Volatile Part of Plants_, as already stated, consists of carbon,
hydrogen, oxygen and nitrogen. Of these, carbon is obtained by the
plant exclusively from the carbonic (dioxid) gas of the air; hydrogen
and oxygen, from the soil in the form of water; nitrogen, directly
from the soil but indirectly also from the air, through the agency of
certain bacteria. The _ash ingredients_ of course are all derived from
the soil through the roots, and must all be present in the latter in
an available form, to a sufficient extent to supply the demands of
vegetation.

_The Agencies of Soil Formation._—With respect to their mode of
formation, soils may be defined as the residual product of the physical
disintegration and chemical decomposition of rocks; with, ordinarily,
a small proportion of the remnants of organic life. The agencies
producing these changes are those classed under the general term
“atmospheric” or “meteorological;” they include therefore the action
of _temperature_—heat and cold—that of _water_, and that of _air and
its ingredients_. In popular parlance, it includes the processes of
_weathering_; nearly the same processes are involved in the “fallowing”
of soils.




PART I.

THE ORIGIN AND FORMATION OF SOILS.




CHAPTER I.

THE PHYSICAL PROCESSES OF SOIL FORMATION.


Since the physical and mechanical effects of the agencies mentioned
above usually precede, in time, the chemical changes, which are
materially facilitated by the previous pulverization of the rocks, the
former should be first considered.

_Effects of heat and cold on rocks._—Most rocks are aggregates of
several simple minerals; a few only (limestone, quartzite and a few
others) expand or contract alike in all their parts. Of the minerals
composing the compound rocks, scarcely any expand to exactly the same
extent under the influence of the sun’s heat, especially when their
colors differ; nor, in the great majority of cases, does one and the
same mineral expand alike in all three directions. It follows that at
each change of temperature there is a tendency to the formation of
minute fissures between adjacent crystals or masses of different simple
minerals; and especially in the case of large crystals of certain
kinds, this action alone will gradually result in the disruption of the
rock surface, so that individual crystals may be detached with little
difficulty. In any case, the cracks so formed are gradually widened
by a frequent repetition of the changes of temperature, coupled with
access of air, water, dust, and the rootlets of plants; all of which
brings about a gradually increasing rate of surface crumbling. This is
especially conspicuous at the higher elevations of mountains, where the
temperature changes are very great and abrupt; and also in the clear
atmosphere of deserts, where owing to the extent and suddenness of
temperature-changes between day and night, caused by the free radiation
of heat into the clear sky, even homogeneous pebbles are known to be
almost explosively disrupted in the mornings and evenings of clear
days.

Such effects may often be strikingly observed on small surfaces of
compound crystalline rocks, such as granite, exposed on glaciers, where
the daily changes of temperature are often extreme, viz., from below
the freezing point to as much as 130 degrees Fahr. (54.4 degrees C). In
such cases one may sometimes scoop off the disintegrated rock by the
handful, while yet the mineral surfaces are almost perfectly fresh.

On a larger scale, the disruption and scaling off of huge slabs of
granite, and rocks of similar structure, may be observed in southern
California on the southwestern side of rock exposures, where slabs
from a few inches to ten and more feet in length and eight or ten
inches thick, have slid off, perhaps still leaning against the parent
rock, which has been rounded off by a succession of such events into
the domelike form so characteristic of granite mountains. Merrill[4]
reports similar exfoliations to occur especially on the peninsula of
California, on Stone Mountain in Georgia, and elsewhere.

[4] See Rocks, Rock-weathering, and Soils, page 246; also paper on
Domes and Dome Structure, by G. K. Gilbert, in Bulletins of the Geol.
Society Am., Vol. 15, pp. 29-36.

A striking exemplification of the effects of frequent and rapid changes
of temperature on rocks, and of humid and dry climates as well, is seen
in the case of the great monoliths of Egypt, one of which now stands in
the Central Park, New York. In the quarries of Syene in Upper Egypt,
where most of these monoliths were obtained, the rough blocks that
were in progress of quarrying when the work was abandoned, quite two
thousand years ago, still show an almost perfectly fresh surface; and
the same is true of the finished obelisks in Lower Egypt, where both
the changes of temperature and the rainfall are somewhat greater. It
is a matter of public note that one of “Cleopatra’s Needles” which was
set up in Central Park nearly thirty years ago, but originally erected
at Heliopolis on the Nile, is in great danger of destruction from the
influence of a totally different climate, in which both the temperature
changes and the rainfall are much more frequent and severe than in
Egypt. The large crystals of feldspar and quartz which compose the
(syenite) rock material have had fine fissures formed between them by
often-repeated expansion and contraction; which when filled with water
and subsequently changed to ice, the latter’s expansion in freezing
(see below) has still farther enlarged them and caused a scaling-off,
which threatens to obliterate the hieroglyphic inscriptions. Thus
temperature-changes and a rain followed by freezing may in a few days
produce a greater effect than a thousand years of Egyptian climate.

_Cleavage of rocks._—Many kinds of rocks have definite directions
of ready cleavage. The most common and obvious cases of this kind
are schists, slates and shales, cleaving readily into plates or
irregular flat or lens-shaped fragments. Such structure greatly favors
disintegration, especially when the layers are on edge at steep
angles. But there are other apparently structureless, massive rocks,
particularly basalts and other eruptive rocks related to them, as well
as many sandstones and claystones, that have a strong tendency to
cleave into more or less definite forms when struck; such as columns
or prisms, square, six-sided or diamond-shaped blocks, etc. Similar
forms are naturally produced in them under the influence of changes
of temperature; by the formation of minute cracks at first, then
enlargement of these by the several agencies already mentioned.

_Effects of freezing water._—The irresistible force exerted by the
expansion of water in freezing, amounting to about 9 per cent of its
bulk, is a powerful factor in widening and deepening fissures and
cracks of rocks; not uncommonly, whole masses of rock are rent into
fragments by this agency, which is one of the most common causes of
“rock falls” on the brink of precipices. By the freezing process cracks
and crevices are enlarged, and the surfaces exposed to weathering are
still farther increased; and the rock fragments or soil particles are
loosened and rendered more liable to be removed from the original site,
whether by gravity, wind or water.

_Glaciers._—Ice in the form of the glaciers that descend from mountain
chains (see figure 1), and of the moving ice sheets that have covered
large portions of North America and Europe in past ages and now cover
Greenland and the South Polar continent, exerts a most potent action
in abrading and grinding even the hardest rocks; not so much by the
direct friction of the moving ice itself, as by the cutting, scoring,
grinding and crushing action which the stones imbedded in the ice,
or carried and shoved by it, exert upon the rocky channels in which
the ice stream moves, as well as upon each other. The product of this
grinding process is largely very fine (hence “glacier flour”), so that
it remains suspended in the water of the glacier-streams until their
velocity is permanently checked when reaching a plain or lake. This
suspended stone-flour imparts to the glacier streams their distinctive
character of “white rivers,” as contradistinguished from the clear,
dark “green rivers” that have their origin outside of glaciated areas.
This difference can be readily observed in traveling along any of
the glacier-bearing mountain chains of the world, and is frequently
expressed in the names of the streams.

[Illustration: FIG. 1.—Zermatt Glacier (Agassiz).]

The physical analysis of mud from the foot of Muir glacier,[5] Alaska,
at its sea front, made by Professor Loughridge, shows the prevalent
fineness of the materials brought down by the glacier waters.

[5] Collected by Dr. W. E. Ritter of the University of California.

    PHYSICAL COMPOSITION OF GLACIER MUD.

    ============+====================+===============
     MATERIAL.  |     DIAMETER.      |   PER CENT.
    ------------+--------------------+---------------
    Clay        |        ?           | 16.57 } 70.31
    Fine silt   | .0023 -- .016 mm.  | 53.74 }
    Fine silt   | .016  to .025 mm.  |  4.38
    Medium silt | .025  to .036 mm.  |  7.06
    Coarse silt | .036  to .047 mm.  |  5.91
    Coarse silt | .047  to .072 mm.  |  3.76
    Fine sand   | .072  to .12  mm.  |  1.14
    Medium sand | .12   to .16  mm.  |  1.56
                |                    | -----
        Total   |                    | 94.12
    ------------+--------------------+---------------

It will be noted that over 70 per cent of this mud consists of
extremely fine, wholly impalpable materials; but little of which is
true clay.

The fineness of the glacier flour renders it peculiarly suitable for
the rapid conversion into soil, and such soils are usually excellent
and remarkably durable. The great and lasting fertility of the soils
of southern Sweden is traced directly to this mode of origin, and
doubtless the great American ice sheet of glacier times is similarly
concerned in the high quality of the soil of our “north central”
states, from the Ohio to the Great Lakes and the Missouri.

The accumulations of rocks and debris of all sizes in the “moraines”
or detrital deposits of glaciers and ice-sheets form another class of
glacier-made lands which cover extensive and important agricultural
areas (drift areas), both in the old and new worlds. Such lands are
undulating or slightly hilly, and the soil usually contains imbedded
in it stones of a great variety of kinds and sizes, partly angular,
partly rounded and polished by friction. Of course the frequent and
violent changes of temperature occurring on the surface of a glacier,
aid materially in reducing the rocks carried by it to the condition in
which we find the material of the moraines; which commonly form lateral
or cross ridges in valleys formerly occupied by glaciers.

_Action of flowing water._—The action of flowing water is doubtless at
this time the most potent mechanical agency of soil formation. From the
sculpturing of the original simple forms in which geological agencies
left the earth’s surface into the complex ones of modern mountain
chains, to the formation of valleys, plains, and basins out of the
materials so carried away, its effects are prodigious. The torrents
and streams in carrying silt, sand, gravel and bowlders, according to
velocity and volume, do not merely displace these materials; the rock
fragments of all sizes not only score and abrade the bed of the rill
or stream, but by their mutual attrition produce more or less of fine
powder similar to that formed by glacier action; usually more mixed in
its ingredients than the former, because derived from a wider range of
drainage surface. In the glacier stream itself, it is easy to trace the
gradual transition from the sharp stone fragments lying in the water as
it issues from the terminal ice cave at the lower end of the glacier,
to the rounded shingle found a few miles below.

[Illustration: FIG. 2.—Erosion of Hawaiian Hills, near Honolulu. (Phot.
by H. C. Myers.)]

On slopes where water flows only during rain or the melting of
snow, the same erosive effects may be seen as between the heads of
ravines and their outlets. (See figure 2,) It is there too that
the surprisingly rapid cutting-out of channels by the aid of water
charged with rock fragments or gravel, can readily be observed, and
the enormous power of water erosion convincingly shown. In the United
States the stupendous gorges of the Columbia and Colorado rivers,
the former cut to a depth of over 2000 feet into hard basalt rock,
the latter to over 5,000 feet, partly into softer materials, partly
into granite, are perhaps the most striking examples of this power;
the manifestations of which can, however, be as convincingly seen in
thousands of minor rivers and streams.

All the materials so carried off from the higher slopes are finally
deposited on a lower level; whether only a short distance away on a
lower slope (colluvial soils), or farther away in the flood plain of
streams, rivers, or lakes (alluvial soils). Other things being equal,
the finest materials are of course, carried farthest, and often into
the sea; in which, however, they cannot long remain suspended, but are
quickly thrown down, forming river bars, flood plains, and deltas. The
fineness of the material of delta soils, like that of those made from
glacier flour, insures them the same advantage, viz. great fertility
and durability.

It is calculated that the Mississippi River carries into the Gulf of
Mexico annually some 7469 millions of cubic feet of earthy deposits,
which would fill one square mile of surface to the height of 268 feet,
or would cover that number of square miles to the depth of one foot.

[Illustration: FIG. 3.—Cliffs and caves on sea-beach at La Jolla,
Calif. showing effects of Wave action.]

_Wave-Action._—The powerful effects of the beating of waves upon abrupt
shores of seas or lakes are in evidence all over the world, and these
effects are so characteristic that they can be recognized even where
no sea or lake exists at present. Gravel and sand are carried in the
surf and serve as grinding materials, wearing even the hardest rocks
into grooves, rills, channels and caves, defining sharply the varying
degrees of hardness or tough resistance in different parts of rocky
cliffs; frequently undermining them and causing extensive rock falls.
The latter then serve for a time to break the violence of the waves’
onset, and may even cause permanent shore deposits to be formed under
their lee.

Such deposits are very generally formed on gently sloping beaches,
and as the water gradually recedes, sometimes by elevation of the
ground, beach lines or beach-terraces are left, which indicate the
successive levels of the lake or sea. Such old beach lines or terraces
and level-surfaced “buttes” in the Great Basin country, and “bench
lands” elsewhere, show in their structure the characteristic lines of
wave-deposition.

_Effects of Winds._—The action of winds in transporting soil particles
(dust and sand) is familiar; and the accumulations that may be formed
under the influence of regular, continuous winds are sufficiently
obvious on lee shores having sandy beaches, inland of which the
formation of sand dunes at times assumes a threatening magnitude. Where
winds are irregular, frequently reversing their direction, of course
the local effects will be less obvious, and the transportation of
material actually occurring will often not be noticed. Yet there can be
no doubt of the importance of wind action in soil formation, and there
are cases in which no other agency can explain the facts observed over
widely extended areas. This is especially true with regard to the soil
masses of the high plains or plateaus of the dry continental interiors,
where not only the regularity of the prevailing winds, but also the
structure (or absence of structure) and pulverulent character of the
soil itself, renders this the only rational mode of accounting for its
presence where we find it.

    The effects that may be exerted by regular winds are well
    illustrated in the plains and deserts of Africa as well as
    those of central Asia. Here we find a distinct subdivision
    of the desert (rainless) areas into the _stony_, from
    which the wind has swept all but the bedrock and gravel
    and where scarcely any natural growth, and certainly no
    cultivation is possible in the almost total absence of
    soil. The next subdivision is the _sandy_ desert,
    to leeward of the stony area, where the winds are less
    violent and regular, and where, therefore, the sand has
    been dropped and is waited back and forth by “sand
    storms,” the surface being covered with moving sand dunes.
    Still farther to leeward we find the region in which the
    finer portions of the desert surface has been deposited;
    here we have “_dust_ storms” so long as the land is
    not irrigated: but the application of water renders the
    soil abundantly fruitful. Such is the case of the Oases
    and fertile border-lands of the Sahara and Libyan deserts.

In the cultivated portions of the Mojave and Colorado deserts in
California, plowing of the land during a dry time is not uncommonly
followed by a bodily removal of the loosened soil to neighboring
fields, sometimes leaving a gravel surface behind. Such “blown-out
lands” exist naturally at numerous points in the Colorado desert.

Sven Hedin (Central Asia and Tibet, Vol. II.) shows that from the
effects of the violent storms that prevail in the Gobi or Takla Makan
desert, Lop-nor lake, the sink of the Tarim river, has in the course
of time shifted its bed as much as fifty miles in consequence of the
excavation of the southern part of the desert by the wind; while the
sand so blown out, together with the deposits from the rivers, now
tends to fill up the present (southern) lake, which is gradually
returning northward toward its original site, now a desert, but around
which formerly a dense population existed.

The great plains of North America, the pampas of South America, the
plateaus of Mongolia and especially the fertile loess region of
northwestern China, are also cases in point. The dense dust storms of
these regions are familiar and unpleasant phenomena, which are often
observed even by vessels at sea off the east coast of South America,
where the dust-laden “pamperos” at times compel them to proceed with
the same precautions as in a fog; and the same is true of the northeast
winds blowing off the Sahara desert on the west coast of Africa.

The effects of windstorms carrying sand in the _erosion of rocks_ are
very obvious and striking in many parts of the world; nowhere probably
as much so as on the great plains of western North America, where the
geological composition of the “bad lands” is frequently impressed
upon the rock surfaces very prominently. The strikingly grotesque
forms are frequently brought out in this way, especially in the case
of “mushroom” rocks, where a hard stratum has remained as a covering
while softer layers underneath have been worn away. The illustration
annexed shows such a case on the plains of Wyoming as figured in the
Report of the U. S. Geological Survey, on the Central Great Plains,
by N. H. Darton. Striking examples of the same effects are seen on
the shores of Lake Michigan in the Grand Traverse region, where the
rocky cliffs are visibly worn away and carved under the influence of
the regular “sand-blasts” of northwest winds. On a smaller scale the
effects of these sand-blasts may be noted in the cobble-deserts, where
we frequently find the cobbles worn away on the windward side in a very
characteristic manner; the lee side remaining rounded and smooth, while
the structure of the rock is strongly outlined on the windward side.

[Illustration: FIG. 4.—“Mushroom rocks,” produced by Wind action,
Wyoming. (Darton.)]


CLASSIFICATION OF SOILS.

_The physical Constituents_ of soils are thus, in the most general
terms, first, _rock powder_ (“sand”) more or less changed by
weathering; second, _clay_, as one of the chief results of the
weathering process of silicate minerals; and thirdly, _humus_, the
dark-colored remnant of vegetable decay. According to the obvious
predominance of one or the other of these primary ingredients, soils
are popularly, in the most general sense, classed as “heavy” and
“light”; the former term corresponding as a rule to those in which
clay forms a prominent ingredient, while sandy and humous or “mold”
soils usually fall under the latter designation, because of their easy
tillage. For practical purposes these subdivisions are both convenient
and important, and they form the ordinary basis of land classification.
Beyond these, the degree of fineness of the rock debris, and their
physical and chemical constitution, determine distinctions such as
gravelly, sandy, silty, loamy, calcareous, siliceous, magnesian,
ferruginous, and others of less general application, though locally
often of considerable importance.

For the purposes of discussion and definition, however, another basis
of classification is needed, which essentially concerns both the origin
and the adaptations of lands.

[Illustration: FIG. 5.—Diagram illustrating the genetic relation of
different soil classes to each other.]

1. _Sedentary Soils._—When soils have been formed without removal
from the site of the original rock, by simple weathering, they are
designated as _sedentary_, or _residual_ soils, or “soils in place.”
In the case of these, the original rock underlies the soil or subsoil
at a greater or less depth, according to the intensity and duration
of the weathering process, and is usually more or less softened and
decomposed at the surface where it meets the soil layer. The latter of
course bears some of the distinctive characters of the parent rock, and
its composition and adaptations may, in a measure, be directly inferred
from that origin. Such soils usually contain, especially in their lower
portions, some angular fragments of the parent rock. In some cases
sedentary soils may have been partially derived from rocks that have
been removed from above the present country rock by erosion, and in
that case fragments of such vanished rock may also be present.

Sedentary soils are most commonly found on rock plateaus and on slopes
or plains underlaid by rock strata of but slight inclination, where the
velocity of the “runoff” rainfall is not sufficient to dislodge the
rock debris. Extended areas of such soils exist in the granitic areas
of the southern Alleghenies, in the “black prairies” of the Cotton
States, and on the “basaltic” plateaus of the Pacific Northwest.

2. _Colluvial Soils._—When the soil mass formed by weathering has been
removed from the original site to such a degree as to cause it to
intermingle with the materials of other rocks or layers, as is usually
the case on hillsides, and in undulating uplands generally, as the
result of rolling or sliding down, washing of rains, sweeping of wind,
etc., the mixed soil, which will usually be found to contain angular
fragments of various rocks, and is destitute of any definite structure,
is designated as a _colluvial_[6] one. Colluvial soil masses are
frequently subject to disturbance from landslides, which are usually
the result of water penetrating underneath, between the soil mass and
the underlying rock, or sometimes simply of complete saturation of the
former with water. Aside from such catastrophic action, they commonly
have a slow downward movement in mass (creep), which ordinarily becomes
perceptible only in the course of years; most quickly where there are
heavy frosts in winter, which act both by direct expansion, and by the
state of extreme looseness in which the soil mass is left on thawing.
Colluvial soils form a large portion of rolling and hilly uplands, and
are of very varying degrees of productiveness.

[6] The term “overplaced,” used for such soils in late memoirs of
the U. S. Geological Survey, is at least superfluous, in view of the
perfectly understood term already in general use, and does not seem to
commend itself for adoption by any special or superior fitness; nor
does the suggestion of Shaler (The Origin and Nature of Soils, 12th
Rep. U. S. Geol. Survey) to include the colluvial soils within the
alluvial class, commend itself either from a theoretical or practical
point of view, since but few useful generalizations can apply to both
classes.

3. _Alluvial Soils._—When soils are the result of deposition by
streams, the material having been gathered along the course of the
stream from various sources and carried to a distance before being
deposited, the soil is designated as _alluvial_. These are the soils
of the valleys, flood-plains, and sea- and lake-borders, past and
present. Being of mixed origin, their general character may vary
from one extreme to the other, both as regards physical and chemical
composition. Since, moreover, they represent the finer portions of
the soils of the regions drained by the water-courses, alluvial
soils are as a rule of a fine texture; and as representing the most
advanced decomposition products of the parent rocks, they are usually
preëminently fertile. This is proverbially true of the flood-plains of
rivers, and still more of their deltas—the bodies of lands formed near
their outlets into seas or lakes.

_Character of these soil-classes._—_Sedentary_ soils are as a matter of
course, other things being equal, dependent entirely on the parent rock
for their specific character; and taking into consideration the various
rocks (usually one or few) from which they may have been derived,
nearly the same is true of _colluvial_ soils, except that a portion of
the clay and finest pulverulent matters may in their case be carried
down on the lower slopes and into the valleys and streams, by the
hillside rills.

    According to the calculation of Merrill (_Rocks,
    Rock-weathering, and Soils_, p. 188) granite when
    transformed into soil _without loss_ would increase
    in weight by 88%; more than doubling its bulk. More
    usually, the leaching process _diminishes_ their
    volume as compared with the parent rock.

_Alluvial_ soils are also of course to a certain extent dependent upon
the character of the rocks and surface deposits occurring within the
drainage area of the depositing stream. As a rule their composition
is much more generalized; but their character as to the relative
proportions of sand and clay is essentially dependent upon the velocity
of the water current. Thus in the upper portions of valleys, where the
slope is relatively steep and the velocity therefore high, a large
proportion of cobbles and gravel is often present in the deposits,
sometimes to the extent of rendering cultivation impracticable, or at
least unprofitable. As the slope and velocity decrease, first coarse
and then fine sand will be the prominent component of the deposited
soil; while still lower down, in the region of slack water, the
finest sand or silt, together with clay, will predominate. According
to Hopkins,[7] flowing water will, at a velocity of three inches per
second, _carry_ in suspension only fine clay (and silt); at eight
inches it will carry sand as large as linseed. At one and one-third
inches, it will _move_ pebbles one inch in diameter; and at a velocity
of two inches per second, pebbles of egg size are moved along the
stream bed. Since the velocity of streams subject to freshets will
vary greatly from time to time, deposits of very different grain
will in such cases be found alternating with one another in the soil
stratum of the flood plain. In fact, this alternation and the more or
less stratified structure resulting therefrom, is the distinguishing
mark of alluvial soils as such. It is true that this peculiarity is
also sometimes found in the case of lands now lying far above the
flood-plains of present rivers; but this is due to the elevation of
the land or the depression of the river channels at a former period,
prior to which such lands (commonly known as river terraces, benches
or second bottoms) were formed. The same is true of lake terraces
(“mesas”), which cover enormous areas in some parts of the world,
more particularly in western North America. It must nevertheless be
remembered that such alluvial terrace or bench soils differ in some
respects from the modern alluvials, on account of their long exposure
to atmospheric action alone; one result of which is that they are
usually much poorer in humus, and therefore of lighter tints, than
the more modern soils of alluvial origin. Other differences will be
adverted to hereafter.

[7] Geikie, “Text-book of Geology,” 3d ed.

As a matter of course the above distinctions, especially between
colluvial and alluvial soils, cannot be rigorously maintained in all
cases. There are transitions from one class to the other, so that it
is sometimes optional with the observer to which of the two classes a
particular soil may be considered as belonging. On the lower slopes of
the hills bordering alluvial valleys the colluvial slope-soil may often
be found alternating with the alluvial deposits, or bodily washed away
to be redeposited as alluvium at a greater or less distance.

One characteristic of the flood-plain lands of all the larger rivers,
and more or less of all streams subject to periodic overflows, is
that the land immediately adjoining the banks is both higher and more
sandy than are the lands farther back from the stream. The cause of
this phenomenon is that as lateral overflow diminishes the velocity of
that flow, its coarser portions are deposited near the river banks,
while the finer particles are carried farther away, until finally only
the finest—clay-substance—reach the lagoons or lakes filled with the
overflow or backwater, and are there in the course of time deposited
as heavy clay “swamp” soils. The same occurs where rivers empty into
lakes or the sea; and these slack-water or delta lands are, as a rule,
the most productive on the river’s course. The continued productiveness
of alluvial soils is moreover in many cases assured by the deposition,
during overflows, of fresh soil-material brought down from the head
waters of the streams. The Nile, and the Colorado river of the West,
illustrate this point.

_Lowering of the land-surface by soil formation._—It is evident that
the soil-forming agencies must in the course of time materially affect
both the surface conformation and the absolute level of the land. The
sharp pinnacles and crests of rock are abraded into the rounded forms
now characterizing our uplands and lower ranges of hills and mountains;
and it is estimated that, _e. g._, the general level of the drainage
basin of the Mississippi river is lowered about one foot in 7,000
years, the material being carried into the lowlands and the sea.




CHAPTER II.

THE CHEMICAL PROCESSES OF SOIL FORMATION.


_Chemical Disintegration, or Decomposition._

It may be said that in general, the physical agencies of disintegration
are most intensely active in the dry or arid regions of the globe,
while chemical processes of decomposition are most active in humid
climates.

The chemical decomposition of rocks is primarily due to the action
of the atmosphere, the average composition of which may be stated as
follows:

    ================+==============================+==================
                    |      VOLUME PER CENT.        | WEIGHT PER CENT.
    ----------------+------------------------------+------------------
    Nitrogen        |            78.00             |   75.55
    Oxygen          |            21.00             |   23.22
    Carbonic dioxid |              .03-.04         |     .045-.060
    Ammonia         |      1 to 4 millionths       |
    Water vapor     | Variable; 48 to 83 grams per |
                    |  cubic meter, when saturated |
                    |  between O° and 50°C.        |
    ----------------+------------------------------+------------------

In addition to the above, air contains minute amounts of the very
indifferent and therefore practically negligible elements, argon,
krypton, neon, xenon and helium, the aggregate amount of which in air
is somewhat less than one per cent, of which the greater part is argon.
So far as known these elements take no part whatever in vegetable or
animal life, and possess no known chemical action or affinity.

The primary active agents in effecting chemical changes in rocks by
which soils are formed, are water, carbonic acid,[8] and oxygen; all
therefore ingredients of the atmosphere. Hence the chemical changes so
brought about are in the most general sense comprehended within the
term weathering, as applied to rocks; while the corresponding but more
complex action within the soil itself is usually termed fallowing.

[8] Owing to the universal presence of water (H₂O) in air as well as in
soils, it is usual and convenient to speak of carbonic dioxid (CO₂) gas
when so occurring as carbonic acid (H₂CO₃), of which it produces the
effects (CO₂ + H₂O = H₂CO₃).

_Effects of Water._—Since but few substances, particularly among
those forming rocks, are totally insoluble even in pure water,[9] and
some (such as gypsum) may be considered easily soluble in the same,
the rain water must exert solvent action wherever it penetrates.
In nature, however, strictly pure water does not occur, it being
difficult to obtain it even artificially. Among the “impurities” almost
always contained in natural water, there are several that materially
increase its solvent power. Foremost among these, both because almost
universally present and on account of its great ultimate efficacy,
is _Carbonic dioxid_, in contact with water forming _carbonic acid_,
the acidulous ingredient of all effervescent waters, the gas which
is produced in nature by innumerable processes, such as decay,
putrefaction, fermentation, the slow or rapid combustion of vegetable
and animal substances, such as wood, charcoal and all other fuels; by
the respiration of animals; in the burning of limestone, etc. It is
therefore of necessity contained in air, on an average to the extent
of about 1-3000 of its bulk in the general atmosphere, but locally
in considerably higher proportions because of proximity to sources
of formation, and of its greater density as compared with air (1½ as
against 1). It may thus accumulate in inhabited buildings, in cellars,
wells, mines, caves; and it is contained in considerable proportion in
the air of the soil. Moreover, being easily soluble in water (to the
extent of an equal volume at the ordinary temperature and barometric
pressure) it is contained in all natural water, whether of rains,
rivers, springs or wells, and largely of course in that percolating the
soil. Such waters may therefore be considered as being acid solvents;
and as such, they exercise a far more energetic and far-reaching effect
than would pure water.

[9] See Chapter 18.

_Carbonated water a universal solvent._—While limestones are the rocks
most obviously acted upon by carbonated water, few if any resist it
altogether. Even quartz rocks of the ordinary kinds are attacked by
it; only the purest white crystalline quartzite may be considered as
sensibly proof against it. Granite and the rocks related to it are
rather quickly acted upon, because of the presence of the feldspar
minerals containing potash, soda and lime as bases[10] together with
alumina.

[10] The increase of solvent power on feldspar when carbonated instead
of distilled water is used, was well exemplified in an experiment
made by Headden (Bull. 65, Color. Exp’t Sta., p. 29), who allowed
pure distilled and carbonated water respectively to act on fresh but
finely pulverized feldspar, with frequent shaking, for five days. The
distilled water dissolved .0081 gram, the carbonated water, .0723 gram
of solids, or nearly nine times as much as the distilled water. Both
residues gave strong reactions for potash with platinic chlorid.

The results of this action are highly important; one being the
formation of clay, so essential as a physical ingredient of soils; the
other the setting-free of potash, one of the most essential nutrients
of plants. Hornblende and the related minerals are similarly acted
upon so far as they contain the same substances. In all cases, of
course, the silica (silicic acid) set free by the carbonic acid remains
partially or wholly in the resulting soils, as such. Lime also at first
mostly remains behind in the form of the carbonate; but potash and
especially soda compounds, being mostly readily soluble in water, are
largely carried away by the latter.

The effect of carbonated water upon silicate minerals is greatly
increased by the presence of ammonia (ammonic carbonate), which always
exists in atmospheric water to a greater or less extent. This effect
may readily be noted on the windows of stables, or other places where
animal offal decays, by the dimming of the glass surfaces; also in
glass bottles containing solution of ammonic carbonate.

_Action of Oxygen._—The effects of atmospheric _oxygen_ on rocks
are of course confined to those containing substances capable of
farther oxidation. Chief among these are ferrous (iron monoxid) and
ferroso-ferric oxid the latter imparting bottle-green, bluish and black
tints to so many minerals and rocks that these colors may usually
be taken as indicating its presence. By taking up more oxygen the
ferrous and ferroso-ferric oxids are converted into ferric oxid or its
hydrate (rust), the tints mentioned passing thereby into brick-red or
rust color, according as the former or the latter (or sometimes their
intermixtures) is formed. In either case there is an increase in bulk;
and this when taking place in the cracks or crevices of minerals or
rocks, tends, like the freezing of water, to widen the cracks and thus
to increase the surface exposed to attack. Since ferrous compounds,
when soluble in water, are injurious to plant growth, this oxidation
is of no little importance, and in soils must be carefully maintained
against a possible reversal.

It is hardly necessary to insist that the action of all these
chemical agents continues in the soils themselves, and that owing to
the fineness of the material, resulting in an enormously increased
surface exposed to attack, such action acquires increased intensity.
This is the more true as in soils bearing vegetation there are always
superadded the effects of the humus-acids resulting from the decay
of vegetable matter, as well as of the acid secretions of the living
plants.


_Action of Plants and their Remnants in Soil Formation._

(_a_) _Mechanical action._—The direct action of plants in forcing
their roots into the crevices of rocks and minerals and thus both
widening them by wedging, and by exposing new surfaces to weathering,
has already been alluded to. That the mechanical force exerted by root
growth is very great, may readily be judged from their effects in
forcing apart, even to rupture, the walls of rock crevices; but actual
measurement has shown the force with which the root, _e. g._, of the
garden pea penetrates, to be equal to from seven to ten atmospheres,
say from 200 to over 300 pounds per square inch. Such a force, exerted
under the protection of the corky layer protecting the root tips, often
produces surprising effects.

(_b_) _Chemical action._—Vegetation takes a most important part, from
a chemical point of view, both in the first formation of soils and
in their subsequent relations to vegetable life. The lower forms of
vegetation are usually the first to take possession of rock surfaces;
foremost among these are the lichens. In humid climates we find
these crust-like plants incrusting more or less all exposed rock
surfaces, sometimes with a solid mantle that can be peeled off in wet
weather, showing the corroded rock-surface, and the beginnings of soil
clustering amid the root-fibrils beneath. A microscopic examination of
the substance of these lichens often shows as a prominent ingredient,
crystals of oxalate of lime, the lime having of course been derived
from the rock, while the oxalic acid has been formed by the plant and
used in the corrosion of the rock minerals. When it is remembered that
this acid is comparable in strength to hydrochloric and nitric acids,
the energy of the attack of the lichens is explained. Its progress can
often be traced, even beyond the visible root fibers, by a change in
the color of the rock; _e. g._, from rust-color to brick red.

When by the action of the lichens a certain depth of loosened rock
or half-formed soil has been produced, the next step is usually the
advent of various mosses, which gradually shade out the crust-like
lichens, while the erect kinds persist for some time. Eventually
the mosses, after having increased still farther the soil layer on
the rock surface, are themselves partially or wholly displaced by
the hardier species of ferns; and with these the higher flowering
plants, such as the stonecrops and saxifrages (the latter deriving
their name from their “rock-breaking” effect), the heather, and many
other or shallow-rooted plants, gradually take possession. The roots
of all plants secrete carbonic acid; and many of them, much stronger
vegetable acids, such as oxalic and citric. In the crevices of rocks
we commonly find the roots forming a dense network over the surfaces,
the marks of which show plainly the solvent effect produced on the rock
by the root secretions. This is most readily observable on a polished
marble surface, or on feldspathic rocks. Of course the progress of
soil-formation is very much more rapid when, as in the case of powdered
lava (volcanic ash) and rock debris resulting from the effects of frost
etc., the surface is very much increased. In tropical climates, where
both vegetative and chemical action is most intense, it takes some of
the higher plants only a few years after a volcanic eruption to take
possession of portions of the “ash” surfaces; thus helping to form a
soil on which after a few more years agricultural plants such as the
vine and olive yield paying returns.

To this direct action of the higher plants is always added, to a
greater or less extent, that of innumerable bacteria, as well as molds;
whose vegetative and secretory action materially assists that of the
roots, and the weathering process in general.

_Humification._—While the mechanical action of the roots and the
chemical effect of the acids of their root secretions are very
efficient in promoting the transformation of mere rock powder into
soil material proper, the efficacy does not end with the life of the
plant. In the natural process of decay to which the roots are subject
after death, and which also affects the leaves, twigs and trunks
falling on the surface, the vegetable matter suffers a transformation
which must be considered more in detail hereafter, and results in
the formation of the complex mixture of dark-tinted substances known
as vegetable mold or _humus_; the remnant of vegetation that imparts
to surface soils their distinctive dark tint. Its functions in soils
are both numerous, and important to vegetable growth; as regards soil
formation, it assists disintegration of the rock minerals both by the
formation of certain fixed, soluble acids capable of acting on them
with considerable energy, and by the slow but continuous evolution of
carbonic acid under the influence of atmospheric oxygen, which has been
alluded to above.

_Causes influencing chemical action and decomposition._—The chemical
processes causing rock decomposition are of course continued in the
soil, and there also are materially influenced by climatic and seasonal
conditions, which bring about great differences in the kind and
intensity of chemical action.

Within the ordinary limits of solar temperatures it may be said that,
other things being equal, the higher the temperature the more intense
will be chemical action in soil formation. Since, however, water is
a potent factor in the majority of these processes, the presence or
absence of moisture at the same time with heat will cause material
differences in the kind and intensity of chemical action. In view of
the importance of carbonic acid as a chemical agent, the presence or
absence of vegetable matter or humus, from which by oxidation or decay
carbonic and humus-acids are formed, will likewise be of material
influence.

The presumption that climatic and seasonal conditions must greatly
influence both the kind and rapidity of the soil-forming processes, is
fully borne out by observation and practice. Especially is the amount
and distribution of rainfall of great importance in this respect, and
should therefore be first considered.


INFLUENCE OF RAINFALL ON SOIL FORMATION; LEACHING OF THE LAND.

In the general consideration of the soil-forming processes, it has been
stated that soils formed by the disintegration of rocks “in place,” _i.
e._, without removal from the original locality, are also designated as
“residual”; meaning thereby that only a portion of the original rock
remains to form the soil mass, while another portion has been removed.
To a slight extent this removal occurs by the partial washing-away of
the finest clay and silt particles; but the most important action from
the agricultural point of view is the removal by leaching with the
carbonated water of the atmosphere and soil, of certain easily-soluble
compounds formed in the process of chemical decomposition of rocks and
resultant soils. The nature of these compounds is exemplified in the
subjoined table giving the composition of some waters flowing from
drains in unmanured fields, laid at depths of from two to three feet;
and for comparison with these, the composition of the water of some of
the world’s large rivers, showing what these largest drains carry into
the ocean.

The analyses have in all cases, where necessary, been recalculated to
parts per million, and to oxids, from the published data.

The letter “c” indicates that the preceding figure has in the absence
of a direct determination been stoichiometrically calculated from the
data given, in order to complete the comparison.

               COMPOSITION OF DRAINAGE WATERS FROM UNMANURED GROUND.
                              (PARTS PER MILLION.)

    ======================+=============+=============+==============+
                          | ROTHAMSTED. |  PROSKAU.   |   MOCKERN.   |
                          | (VOELKER.)  | (KROCKER.)  | (Ö. WOLFF.)  |
                          +-------------+-------------+------+-------+
                          |             |             | Rye  |Meadow.|
                          |             |             |Field.|       |
    ----------------------+------+------+------+------+------+-------+
    Potash, K₂O           |  1.7 |  5.4 |  2.0 |  2.0 |  8.5 |   3.4 |
    Soda, Na₂O            |  6.0 | 11.7 | 15.1 | 13.7 | 23.3 |   8.2 |
    Lime, CaO             | 98.1 |124.3 |133.0 |118.1 |122.6 |  22.5 |
    Magnesia, MgO         |  5.1 |  6.4 | 33.3 | 22.4 | 14.9 |   6.7 |
    Iron Oxid, Fe₂O₃      |  5.7 |  4.4 |  6.6 |  6.6 |}     |       |
    Alumina, Al₂O₃        |      |      |      |      |} 8.0 |   6.0 |
    Silica, SiO₂          | 10.9 | 15.4 |  7.0 |  6.0 |  7.0 |   4.0 |
    Carbonic Acid, CO₂    | 48.1 | 44.4 | 75.8 | 82.6 |      | 121.3 |
    Phos’ Acid, P₂O₅      |   .63|  9.1 |Trace.|Trace.|Trace.|  19.0 |
    Sulfuric Acid, SO₃    | 24.7 | 66.3 |122.7 | 67.3 |      |       |
    Chlorin, Cl           | 10.7 | 11.1 |  4.8 |  4.2 | 14.0 | Trace.|
    Nitrogenas, N₂O₅      |  3.90|  5.10|      |      |      |       |
    Nitrogenas, NH₃       |   .12|   .13|      |      |      |       |
    ----------------------+------+------+------+------+------+-------+
    Total Mineral Matter  |215.9 |295.5 |400.3 |322.9 |198.3 | 191.1 |
    Less O: Cl            |  2.35|  2.4 |  1.1 |   .9 |  3.1 |       |
    Corrected Total       |213.3 |293.1 |399.2 |322.0 |195.2 | 191.1 |
    Organic Matter        | 22.9 | 19.3 | 25.0 | 16.0 | 26.0 |  26.0 |
    ----------------------+------+------+------+------+------+-------+
    Total Solids          |235.2 |312.4 |424.2 |338.0 |221.2 | 217.1 |
    ----------------------+------+------+------+------+------+-------+
                          |  Farnham.   |   Munich.   |
                          |   (Way.)    |  (Zöller.)  |
                          +------+------+-------------+---------
                          |Wheat | Hop  |  Lysemeter  | Average.
                          |Field.|Field.|  Drainage.  |
    ----------------------+------+------+------+------+--------
    Potash, K₂O           |Trace.|Trace.|   6.5|  2.4 |   3.2
    Soda, Na₂O            | 14.3 | 45.7 |   7.1|  5.6 |  15.1
    Lime, CaO             | 69.3 |185.0 | 145.8| 57.6 | 107.6
    Magnesia, MgO         |  9.7 | 35.1 |  20.5|  8.9 |  16.3
    Iron Oxid, Fe₂O₃      | }    |}     |    .1|  6.3 |
    Alumina, Al₂O₃        | }5.9 |} 7.1 |      |      |
    Silica, SiO₂          |  1.35| 12.1 |  10.4| 11.3 |
    Carbonic Acid, CO₂    |      |      |      |      |
    Phos’ Acid, P₂O₅      |Trace.|  1.7 |   2.2|Trace.|   0.5
    Sulfuric Acid, SO₃    | 23.5 |135.8 |  17.5| 27.1 |  60.8
    Chlorin, Cl           | 10.0 | 37.4 |  57.5|  9.5 |  17.7
    Nitrogenas, N₂O₅      |102.4 |163.5 |      |      |
    Nitrogenas, NH₃       |   .25|   .03|      |      |
    ----------------------+------+------+------+------+---------
    Total Mineral Matter  |248.8 |623.5 | 267.6|128.7 |
    Less O: Cl            |  2.2 |  8.2 |  12.7|  2.14|
    Corrected Total       |246.6 |615.3 | 254.9|126.6 | 285.7
    Organic Matter        |100.0 |105.7 |  20.5| 12.6 |
    ----------------------+------+------+------+------+---------
    Total Solids          |346.6 |721.0 | 275.4|139.2 | 352.6
    ----------------------+------+------+------+------+---------

                 COMPOSITION OF RIVER WATERS. PARTS PER MILLION.
      ====================+==========+==========+=============+===========
                          | Yukon,   |  Dwina,  |St. Lawrence,| Missouri,
                          | Alaska.  |  above   | Pointe des  | Montana.
                          |          |Archangel.|  Cascades.  |
     ---------------------+----------+----------+-------------+-----------
     Potash, K₂O          |  Trace   |   12.58  |      1.40   |    1.90
     Soda, Na₂O           |   8.10   |   23.38  |      6.90   |   30.10
     Lithia, Li₂O         |          |          |             |
     Lime, CaO            |  30.40   |   37.30  |     45.30   |   58.00
     Magnesia, MgO        |   7.30   |   36.25  |      9.70   |   18.10
     Manganese, Mn₃O₄     |          |          |             |
     Ferric Oxid, Fe₂O₃   |          |    1.63  |             |}   3.10
     Alumina, Al₂O₃       |   1.80   |          |             |}
     Silica, SiO₂         |   7.60   |    3.05  |     32.60   |   18.90
     Carbonic Acid, CO₂   |  33.00   |   54.01  |     68.40   |   65.20c
     Phosphoric Acid, P₂O₅|          |     .40  |   Trace     |     .22
     Nitric Acid, N₂O₅    |          |          |             |
     Sulfuric Acid, SO₃   |   8.50   |   29.62  |     47.70   |   21.90
     Chlorin, Cl          |    .40   |   33.09  |      2.40   |   18.00c
     Ammonia, NH₃         |          |          |             |
                          |  -----   |  ------  |    ------   |  ------
     Total Mineral Matter |  97.10   |  231.31  |    214.40   |  225.42
     Less O : Cl          |    .10   |    7.33  |       .55   |    4.10
                          |  -----   |  ------  |    ------   |  ------
     Corrected Totals     |  97.00   |  223.98  |    213.85   |  221.32
     Organic Matter       |          |          |             |
                          |  -----   |  ------  |    ------   |  ------
     Total Solids         |  97.00   |          |    213.85   |  221.22
     ---------------------+----------+----------+-------------+-----------
                          |  F. W.   |     C.   |    T. S.    |
                          | Clarke,  | Schmidt, |    Hunt,    | Traphagen,
                          |Jour. Am. | Jahresb. |  Geol. of   |Bull. Mont.
                          |Chem. Soc.|d. Chemie,|   Canada,   | Expt. Sta.
                          |Feb. 1905,|   1873.  |    1863.    |  No. 190.
                          |  p. 112. |          |             |
     ---------------------+----------+----------+-------------+-----------
     ====================+============================================
                         |         Mississippi near Carrollton, La.
                         |
                         |        Average of one year.    |
                         |                                |
                         +------+-----------+-------------+-----------
                         |      |  Min.     |    Max.     |  May 1905.
                         |      |           |             |
                         |      |           |             |
    ---------------------+------+-----------+-------------+-----------
    Potash, K₂O          |      |           |             |      2.80
    Soda, Na₂O           |19.80c|           |             |     13.50
    Lithia, Li₂O         |      |           |             |
    Lime, CaO            |      | 49.75 Dec.|  33.81 Feb. |     41.20
                         |      |           |             |
    Magnesia, MgO        | 40.80|16.39 March|  10.32 Feb. |     11.30
    Manganese, Mn₃O₄     | 12.40|           |             |       .16
    Ferric Oxid, Fe₂O₃   |} 2.10|           |             |       .11
    Alumina, Al₂O₃       |}     |           |             |       .17
    Silica, SiO₂         |  8.70|11.45 March|  5.94 April |      7.40
    Carbonic Acid, CO₂   | 45.10|           |             |     33.16c
    Phosphoric Acid, P₂O₅|      |           |             |       .33
    Nitric Acid, N₂O₅    |   .13|           |             |       .23
    Sulfuric Acid, SO₃   | 16.10| 24.72 July|   8.18 Jan. |     23.90
    Chlorin, Cl          |  9.60|1 4.50 June|   6.90 Dec. |     16.10
    Ammonia, NH₃         |      |           |             |       .16
                         |------|---------- | ----------  |    ------
    Total Mineral Matter |154.73|           |             |    150.52
    Less O : Cl          |  2.10|           |             |      3.63
                         |------|---------- | ----------  |    ------
    Corrected Totals     |152.63| 180.0 July| 110.0 Dec.  |    146.89
    Organic Matter       |      |           |             |
                         |------|---------- | ----------  |    ------
    Total Solids         |152.63|           |             |    146.89
    ---------------------+------+-----------+-------------+-----------
                         |           Porter,              |    Stone,
                         |        Rep. New Orleans        |     U. S.
                         |       Sewerage and Water       | Reclamation
                         |            Board.              |  Service.
                         |                                |
                         |                                |
    ---------------------+--------------------------------+-----------
     ====================+===========+================+========+==========
                         |Rio Grande,|                |Average |Average
                         |Ft. Craig, |Nile near Cairo.|of the 7|19 Great
                         |   N. M.   |                | rivers.|Rivers of
                         |           |                |        |the World.
                         +-----------+-------+--------+--------+----------
                         |           | High. |  Low.  |        |
                         |           | Aug.  | May 13,|        |
                         |           | 1874. |  1875. |        |
    ---------------------+-----------+-------+--------+--------+----------
    Potash, K₂O          |     .80   | 15.01 |   4.04 |    4.80|     2.40
    Soda, Na₂O           |   43.40   |  5.87 |  13.01 |   18.20|     7.10
    Lithia, Li₂O         |           |       |        |        |      .20
    Lime, CaO            |   22.80   | 44.22 |  51.78 |   43.50|    43.20
                         |           |       |        |        |
    Magnesia, MgO        |    2.10   | 10.33 |  10.29 |   13.10|    14.70
    Manganese, Mn₃O₄     |           |       |        |        |     1.20
    Ferric Oxid, Fe₂O₃   |           |       |   1.80 |    2.80|
    Alumina, Al₂O₃       |           |       |   1.80 |    3.10|
    Silica, SiO₂         |           | 11.29 |   6.71 |   10.80|    16.40
    Carbonic Acid, CO₂   |   10.25   | 42.81 |  40.91 |   38.10|    46.00
    Phosphoric Acid, P₂O₅|           |       |        |     .24|      .30
    Nitric Acid, N₂O₅    |           |       |        |     .18|     3.80
    Sulfuric Acid, SO₃   |   47.00   | 18.37 |  29.31 |   26.90|     8.00
    Chlorin, Cl          |   36.00   |  6.28 |  17.37 |   15.50|     3.70
    Ammonia, NH₃         |           |   .043|    .014|        |      .07
                         |  ------   |-------| -------|  ------|   ------
    Total Mineral Matter |  162.35   |154.223| 173.434|  173.12|   152.97
    Less O : Cl          |    8.05   |  1.40 |    4.13|    3.50|      .72
                         |  ------   |-------| -------|  ------|   ------
    Corrected Totals     |  154.30   |152.823| 169.304|  169.62|   152.25
    Organic Matter       |           |       |        |        |    16.4
                         |  ------   |-------| -------|  ------|   ------
    Total Solids         |  154.30   |164.683| 200.594|  169.62|   168.65
    ---------------------+-----------+-------+--------+--------+----------
                         | O. Loew,  |                |        | John
                         |  U. S.    | Letheby, Jour. |        |Murray,
                         |  Geogr.   |  of the Khediv.|        |Scottish
                         |  Survey   |   Agr. Society.|        | Geogr.
                         |  W. of    |                |        |  Mag.,
                         |  100th    |                |        | Vol. 3,
                         |  Merid.   |                |        |  1887.
                         |  Vol. 3.  |                |        |
    ---------------------+-----------+----------------+--------+----------

It will be noted that in all the drain waters, lime is the ingredient
most abundantly leached out, and as reference to the acids shows,
mainly in the form of carbonate, also in that of sulfate. Magnesia is
next in amount among the bases; next in amount is soda, largely in
the form of sodium chlorid or common salt. Potash is present only in
small but rather uniform amounts. Of the acids the carbonic is the most
abundant, sulfuric next; chlorin and silicic acid come next, in about
equal amounts. Nitric acid passes off in small, but still relatively
considerable amounts.

Comparison of the drain waters with the river waters, while showing a
general qualitative agreement, also shows a marked diminution of total
solids (from 285.7 to 188.7; hence “soft river water”), and especially
of lime (from 107.6 to 43.2), together with the carbonic acid with
which it is mostly combined; indicating a deposition of lime carbonate
in the river deposits or alluvial lands. There is, on the other hand,
little if any general difference in the magnesia content of the two
classes of waters; nearly the same is true of soda, so that these two
bases really show a considerable relative increase when the diminished
total is considered. Potash remains about the same all through, viz.
two parts or a little more; phosphoric acid shows a fraction of one
millionth; nitric acid varies greatly but is usually higher in the
drain waters, sometimes showing a heavy depletion of the land by the
leaching-out of this important plant food.

It has been computed by John Murray, as quoted by Russell,[11] that the
volume of water flowing into the sea in one year, including all the
land areas of the earth, is about 6524 cubic miles. From the average
composition of river waters as given above, it would follow that nearly
five billions (4,975,117,588) of tons of mineral matter are annually
carried away _in solution_ from the land into the sea. The amount of
_sediment_ carried at the same time is many times greater; in the case
of the Mississippi river, it is more than five times the amount of the
matter carried in solution.

[11] Rivers of North America, p. 80.

Comparison of the river waters among themselves shows less of any
consistent relation to climatic conditions than might have been
anticipated. The waters of the arctic streams Yukon and Dwina show
wider differences than any two other waters in the list, unless it
be the St. Lawrence, another northern stream. The Missouri and Rio
Grande show by their high content of soda, chlorin and sulfuric acid
their origin in arid climates, where alkali lands prevail. The water
of the Nile is here represented by two analyses,[12] one showing the
season when the water is “red” and of high fertilizing quality because
of the sediment it brings down from the mountains of Abyssinia; the
other the “green” and relatively clear water which comes from the
great lakes and through the “sudd ” or grassy swamp region near the
junction of the Gazelle river with the Nile. Of the analyses given of
the Mississippi river water, the first represents the average of a full
year’s observations made weekly under the auspices of the New Orleans
Commission on Sewerage and Drainage, by J. L. Porter. The fourth is
an analysis made of water taken at the same point in May, 1905; the
analysis having been made in full by Mr. Stone, of the Reclamation
Service of the U. S. Geol. Survey, the direct determination of potash
and soda being in this case included. As will be seen, and might be
expected, the average of the Mississippi water corresponds quite nearly
to that of nineteen of the world’s great rivers as given by Murray. The
very great variation in the content of sulfates is evidently due to the
occasional heavy influx of the gypseous waters of the Washita and Red
rivers when in flood; while the minimum content (in January) agrees
almost precisely with the general average. Murray’s table would hardly
be changed if these analyses of Mississippi water were incorporated
therein, owing doubtless to the large and varied drainage area of the
great river.

[12] The correctness of Letheby’s analyses has been disputed, partly
because of their disagreement with former analyses in the very high
amount of lime, partly because of the high potash-content in the
Low-Nile water. The lime content is, however, confirmed by the partial
analyses made by Mathey in 1887, which gives an average of 44.1 for the
year, while the older analyses, made in Europe, of _transported_ water
gave only half as much. Letheby working on the spot was doubtless more
nearly right in this respect. His figure for potash in the “Low-Nile”
water agrees with former determinations, but that in the “High-Nile” is
approached only by that in the Dwina water. It may be suspected that
the soda is too low and potash too high in this analysis.

_Sea Water._—The nature of the substances permanently leached out is
also seen by considering the composition of sea water, since the ocean
is the final reservoir for all the leachings of the land. It might be
objected that the ocean may have received its salts from other sources;
but this objection is overborne by the fact that substantially the
same salts are found in landlocked lakes, in which, as they have no
outflow, the leachings of the adjacent regions are perforce, as a rule,
the only possible source of the salts. It is true that the nature of
the salts differs somewhat in different lakes, as might be expected;
but a general statement of that nature will, after all, be the same as
that made in regard to sea-water. The following table of the average
composition of sea-water, according to Regnault, illustrates these
facts.

            MEAN COMPOSITION OF SEA-WATER.

    Sodium Chlorid (common salt)          2.700
    Potassium chlorid                      .070
    Calcium sulfate (gypsum)               .140
    Magnesium sulfate (Epsom salt)         .230
    Magnesium chlorid (bittern)            .360
    Magnesium bromid                       .002
    Calcium carbonate (limestone)          .003
    Water (and loss in analysis)         96.495
                                        -------
                                        100.000

The average saline contents of sea-water would thus be 3.505 per cent
In twenty-one determinations of the saline contents of the Atlantic
Ocean, the percentage ranged from 3.506 to 3.710 per cent Of this
mineral residue, common salt constitutes from about 75 to over 80 per
cent.

    We see that most prominent among the ingredients mentioned
    here is common salt (sodium chlorid), which forms nearly
    four-fifths of the total solid contents. Next in quantity
    are the compounds of magnesium, viz. Epsom salt and bittern,
    with a very small amount of the bromin compound. Next come
    the compounds of calcium (lime), of which gypsum is the
    more abundant, while the carbonate, so abundant on the land
    surface in the various forms of limestone, is present in
    minute amounts only, yet enough to supply the substance
    needed for the shells of shellfish, corals, etc. Least in
    amount of the metallic elements mentioned is potassium.
    Calculating the total amounts of chlorin, we find that it
    exceeds in weight any one other element present in the salts
    of sea-water, being two-sevenths of the whole solids.

    Substantially the same result, with variations due to
    local causes, as exemplified in the varying composition of
    river and drain waters, is obtained when we consider the
    saline ingredients of lakes having no outlet, and in which
    therefore, the leachings of the tributary land area have
    accumulated for ages. The Great Salt Lake of Utah, the
    landlocked lakes of the Nevada basin, of California, Oregon,
    and of the deserts of Asia, Africa, and Australia, all tell
    the same tale, which may be summarized in the statement that
    the chlorids of sodium and magnesium, and the sulfates of
    sodium, magnesium and calcium constitute the bulk of the
    leachings of the land; while of other substances potassium
    alone is present in relatively considerable amount.

    While the above analysis shows the ingredients of sea-water
    so far as they can at present be directly determined
    by chemical analysis, yet the presence of many others
    is demonstrable, directly or indirectly, from various
    sources. One is, the mother-waters from the making of
    sea-salt, in which such substances accumulate so as to
    become ascertainable by chemical means, and even become
    industrially available in the cases of potash and bromin.
    Another is the ash of seaweeds, which is indisputably
    derived from the sea-water, and contains, among other
    substances not directly demonstrable in the original
    water, notable quantities of iodin (of which this ash is a
    commercial source), iron, manganese, and phosphoric acid.
    Again, the copper sheathing of vessels, as it is gradually
    corroded, becomes more or less rich in silver, manifestly
    thrown down from the sea-water, and the silver so obtained
    is associated with minute amounts of gold. Copper, lithium,
    and fluorin likewise have been found in sea water; and it
    is probable that close search would detect very many of the
    other chemical elements as ordinary ingredients in minute
    amounts. This is what must be expected from the fact that
    few mineral substances known to us are entirely insoluble in
    pure water, and still fewer in water charged with carbonic
    acid. The latter is always present in sea-water and holds
    the lime carbonate in solution; on evaporation or boiling,
    this substance is the first to be precipitated; and thin
    sheets of limestone from this source are commonly found at
    the base of rock-salt beds, which, themselves, are evidently
    the result of the evaporation of segregated bodies of
    sea-water in past geological ages.

Summing up the facts concerning the water of the sea and of landlocked
lakes, with reference to the ingredients of soils needful for the
nutrition of plants, it appears that the rock ingredients leached out
in the largest amounts (lime alone excepted) are those of which the
smallest quantities only are required by most plants; while of those
specially needful for plant nutrition, only potash is removed in
practically appreciable amounts by the stream drainage.

_Result of insufficient Rainfall; Alkali Soils._—When the rainfall
is either in total quantity, or in consequence of its distribution
in time, insufficient to effect this leaching, the substances that
otherwise would have passed into the drainage and the sea are wholly
or partially retained in the soil; and when the rainfall deficiency
exceeds a certain point, the salts thus retained may become apparent
on the surface in the form of saline efflorescences, or as it is
usually termed in North America, “alkali.”[13] Their continued presence
modifies in various ways the process of soil formation and the nature
of the soils as compared with those of regions of abundant rainfall
(“humid climates”); one of the most prominent and important results
being that, besides the easily soluble salts mentioned above, the
_carbonate of lime_ formed in the process of decomposition is also
retained, and imparts to the soils of regions of deficient rainfall
(“arid climates”) the almost invariable character of _calcareous_
lands. There is thus in the United States a marked and practically very
important contrast between the soils of the arid region west of the
Rocky Mountains and those of the “humid” region between the immediate
valley of the Mississippi and the Atlantic coast. These differences and
their practical bearings can be best discussed after first considering
more in detail the chemical decomposition of the several soil-forming
minerals.

[13] In some cases the soluble salts originate in rocks impregnated
with salts from marine lagoons or landlocked lakes, or directly from
their evaporation residues. But this is the exception rather than the
rule.




CHAPTER III.

THE MAJOR SOIL-FORMING MINERALS.


Since the several stratified rocks, such as sandstones, shales,
claystones, clays, limestones, etc., are themselves but the outcome of
the same disintegrating and decomposing influences upon the crystalline
rocks by which soils are now formed, we must study the action of these
influences upon the minerals composing the latter rocks in order to
gain a comprehensive understanding of the subject. While the number
of different minerals known to science is very large, such study need
not go beyond a small number of the chiefly important, rock-forming
species which are so generally distributed as to require consideration
in this connection. These minerals are the following: Quartz and its
varieties; the several feldspars; hornblende and augite; the micas;
talc and serpentine. Calcite, gypsum and dolomite, though not contained
in the older rocks, must be considered because of their forming large
rock deposits by themselves; and zeolites require mention because,
though rarely forming a large proportion of rocks, they are of special
importance as soil ingredients.

_Quartz_ and the minerals allied to it consist essentially of dioxid of
silicon, usually without (quartz proper) but partly also with water in
combination (opal and its varieties). _Silicon_ is next to oxygen the
most abundant element found on the earth’s surface. It occurs largely
in the various forms of quartz, alone, or as one ingredient of compound
rock-masses; the rest, in combination (as silica) with various metallic
oxids, forms the important group of silicate minerals, constituting the
bulk of most rocks.

Quartz occurs frequently in crystals (rock crystal; six-sided prisms
terminated by six-sided pyramids), clear or variously colored; but more
abundantly as quartz rock or quartzite, readily known by its hardness,
so as to strike fire with steel, and by its glass-like, irregular
fracture. Besides the crystalline quartz rock we find close-grained and
at least partly non-crystalline varieties, such as hornstone and flint.
Sandstones most commonly consist of grains of quartz cemented by some
other mineral, or by silica itself; in the latter case the siliceous
sandstone frequently passes insensibly into true quartzite. The loose
sand so well known to common life is prevalently composed of quartz
grains, whose hardness and resistance to weathering enables them to
survive longest the soil-forming agencies.

Quartz and its allied rocks—jasper, hornstone, siliceous schist,
etc., are all, as already stated, acted on with difficulty by the
“weathering” agencies. Crystalline quartz rock may be considered as
practically refractory against all but the mechanical agencies, and
hence remains in the form of sand and gravel, more or less rounded
by attrition, as a prominent component of most soils; sometimes to
the extent of over 92 per cent, even in soils highly esteemed in
cultivation, especially in the arid region. Such soils are mostly
the result of the disintegration of sandstones, the cement of which
has been dissolved out in the course of weathering; or they may be
derived directly from geological deposits of more or less loose and
unconsolidated sand. Among crystalline rocks, granites, gneiss and
mica-schists are those most usually concerned in the formation of
sandy soils; since in common parlance, quartz is understood to be the
substance of the sand unless otherwise stated. The exceptions are
especially important in the regions of deficient rainfall.

But while crystalline quartz is practically insoluble in all natural
solvents, the same is not true of the jaspers and hornstones. These
consist of a mixture of crystalline and amorphous (non-crystalline)
silica, which is more readily soluble than the crystalline, and is
attacked by many natural waters, especially by those containing even
very small amounts of the carbonates of potash or soda. We thus
often find that hornstone and jasper pebbles buried in the soil,
while still hard internally, have externally been converted into a
friable, almost chalky substance, consisting of crystalline quartz
from which the cementing amorphous silex has been removed by the
soil water. In the course of time such pebbles may be completely
destroyed by this process, so as to be light and chalky throughout, and
readily crushed in tillage. The change is the more striking when, as
frequently happens, the hornstone pebble is traversed by small veins of
crystalline quartz, which remain as a skeleton.

_Solubility of Silica in Water._—It is easily shown experimentally that
the compound of silica with water (hydrate) is under certain conditions
readily soluble not only in pure water, but also in such as contains
carbonic acid. It thus occurs in nearly all spring and well waters;
some hot springs deposit large masses of it (sinter); and geological
evidence clearly demonstrates that quartz veins have as rule been
formed from water-solutions of silica.

That silica in its soluble form circulates freely in the soil water, is
abundantly evident from the large amounts of it which are secreted on
the outside of the stems of grasses, horsetail rushes and other plants,
imparting a gritty roughness to their outer surface. In the case of
the giant bamboo grass of Asia, the silica accumulated on the outside
of the joints forms a hard sheath of considerable thickness, known to
commerce as tabashir.

That among the first products of rock decomposition we often find
small amounts of the silicates of the alkalies (potash and soda) has
already been mentioned. It cannot be doubted that the same continues to
be formed in soil containing the proper minerals; and there they also
take part in the formation of the easily decomposable hydrous silicates
designated as _zeolites_, which are largely instrumental in retaining
the “reserve” of mineral plant-food in soils.


SILICATE MINERALS.

Silica occurs in nature combined with the oxids of most metals, forming
silicates; but most abundantly with the earths (lime, magnesia,
alumina) and alkalies (potash and soda). These compounds are the most
important in soil formation; and among them the following are the chief:

The _Feldspars_, which may be defined as compounds of silicates of
potash, soda or lime (either or all) with silicate of alumina. They
are prominent ingredients of most crystalline rocks; potash feldspar
(orthoclase) with quartz and mica forms granite and gneiss; feldspars
containing soda and lime (either or both) form part of many other
crystalline rocks, such as basalt, diabase, diorite, gabbro and most
lavas. The feldspars are decomposed by weathering rather readily, and
are important in being the chief source of clays as well as of potash
in soils. When acted upon by carbonated water, the bases potash, soda,
and lime or carbonates, the silica being mostly displaced; while the
silicate of alumina takes up water and forms _kaolinite_, the essential
basis of clays, and one of the most important constituents of soils;
imparting to them the necessary firmness and cohesion, together with
other important physical properties, discussed more in detail hereafter.

While thus on the one hand feldspars are the source of clay, on the
other they supply one of the most essential ingredients of plant food,
viz. potash; which is first dissolved by the water in the forms of
carbonate and silicate, but in most cases soon becomes fixed in the
soil by forming more complex (zeolitic) combinations. The soda not
being retained by the soil as strongly as is potash is washed through
into the country drainage; while if lime is present, it mostly remains
in the form of the carbonate.

Orthoclase feldspar contains nearly 17% of potash; Leucite, a related
mineral occurring in some lavas, contains 21.5%. The other feldspars
contain only a few per cent, sometimes none.

Other silicate minerals, so far as they contain the same bases, are
acted upon similarly to the feldspars.

    In the decomposition of the feldspars by carbonated water,
    the compounds of potash and soda so formed are soluble in
    water, those of lime and magnesia are insoluble or nearly
    so. Hence pure clays can be formed only in the decomposition
    of the potash-and soda-feldspars (orthoclase, albite) while
    in the case of lime feldspar (labradorite) and the mixed
    feldspars (plagioclase, anorthite) calcareous clays (marls)
    are the result. Lime feldspar resists decomposition more
    tenaciously than do those containing large proportions of
    the strong bases potash and soda; potash feldspar especially
    is attacked most readily, and is the main source of the
    formation of the valuable deposits of porcelain earth or
    _kaolin_, which is essentially a mixture of kaolinite
    with fine silex and more or less of undecomposed feldspar,
    and is of a chalky texture.

_Formation of Clays._—When instead of remaining in place, this kaolin
is washed away and triturated in the transportation by water, it is
partially changed from its original chalky condition to that plastic
and adhesive form which is the characteristic ingredient of all clays.
The remarkable properties of this substance and the part it plays
in the physical constitution of soils, will be discussed in another
chapter. Its lightness and extreme fineness of grain (if grain it can
be called) cause it to be carried farther on by the streams than any
other portion of the products of rock decomposition save those actually
in solution; it can therefore be deposited only in water that is almost
or quite still (as in swamps) so long as the latter is fresh. So soon
however as brackish or salt water is encountered, clay promptly gathers
into floccules (“flocculates”), and thus enveloping the finest-grained
silts that may have been carried along with it, it quickly settles
down, forming the “mud banks” and heavy clay soils that are so
characteristic of the lower deltas of rivers, as well as of swamps
formed by the backwater or overflow of the same.

When instead of potash feldspar alone, the lime- or soda-lime feldspars
are also concerned in the decomposition process, the resulting clay
soils will be more or less calcareous, while the soda, as stated above,
is for the greater part leached out permanently.

_Hornblende_ (Amphibole) and _Pyroxene_ (Augite). These are two very
widely diffused minerals, differing but little in composition though
somewhat differently crystallized, mostly in short columnar forms. The
typical and most abundant varieties of these minerals appear black to
the eye, though in thin sections they are bottle-green; they form the
black ingredient of most rocks.

    The color is due to ferroso-ferric (magnetic) oxid of iron;
    the mineral as a whole may be considered as a silicate of
    lime, magnesia, alumina and iron, varying greatly in their
    absolute proportions; alumina and iron being sometimes
    almost absent. When iron is lacking the mineral may be
    almost white (tremolite, asbestos), and its weathering is
    then much retarded, since the oxygen of the air cannot take
    part in the process of disintegration.

The black variety of hornblende is not only the most abundant as
a rock-ingredient, but it also the one most easily decomposed and
therefore most commonly concerned in soil formation. The black
hornblende owes its easy decomposition under the atmospheric influences
to two properties; one, its easy cleavage, whereby cracks are readily
formed and extended by the agencies already mentioned (pp. 1-3). The
other is its large content of ferrous silicate (silicate of iron
protoxide), whereby it is liable to attack from atmospheric oxygen;
the latter forms ferric hydrate (iron rust) out of the protoxide, thus
causing an increase of bulk which tends to split the masses of the
mineral in several directions, while the silex is set free. At the same
time the carbonic acid of the air converts the silicate of lime and
magnesia, which forms the rest of the mineral, into carbonates; and the
alumina present forms kaolinite, as in the case of the feldspars. There
is thus formed from this mineral, when alone, a strongly rust-colored,
more or less calcareous and magnesian clay, constituting the material
for rather light-textured “red” soils. In most cases however the
hornblende is associated in the rock itself with the several feldspars,
(mostly lime- and soda-lime feldspars) as well as with more or less
quartz. The rust-colored soils are therefore most commonly the joint
result of the weathering of these several minerals. This is well
exemplified in the case of the “red” soils formed from the so-called
granites and slates of the western slope of the Sierra Nevada of
California.

_Pyroxene_ or _Augite_ so nearly resembles hornblende in its chemical
composition and crystalline form, that what is said of the latter may
be considered as applying to augite also. Owing however to the absence
of any prominent tendency to cleavage, the smooth crystals of this
mineral are attacked much less readily than is hornblende, so that
we often find them as “black gravel” in the soils formed from rocks
containing it. Such soils are particularly abundant and important in
the region covered by the great sheet of eruptive rocks (basalts,
so-called) in the Pacific Northwest, and on the plateau of South
Central India (the Deccan), and result likewise from the decomposition
of the black lavas of volcanoes; thus in the Hawaiian islands, and in
the Andes of Peru and Chile.

Both hornblende and augite being either free from, or deficient in
potash, of coarse the soils formed from them are apt to lack an
adequate supply of this substance for plant use. This is markedly true
of hornblende schist or amphibolite rocks.

_Mica_, commonly known as isinglass, is so conspicuous wherever it
occurs that it is more readily recognized than any other mineral.
It occurs in glittering scales in soils and sands, and in rocks it
sometimes forms sheets of sufficient size to supply the small panes for
the doors of stoves, lamp chimneys, etc., which being flexible are not
liable to break, but only gradually scale into very thin films, into
which it can also be split by hand. When white, (muscovite, phlogopite)
its scales are sometimes mistaken for silver by mine prospectors; when
yellow, for gold; but their extreme lightness should soon remove these
delusions. The composition of mica is not widely different from that of
the two preceding minerals; like these it sometimes contains much iron,
and is then dark bottle-green (biotite); this variety in weathering
becomes bright yellow, and soon disintegrates.

This mineral is so abundant an ingredient of many rocks and soils, that
one naturally looks for it to play some definite or important part in
soil formation. By its ready cleavage it favors the disintegration of
rocks; but it seems that owing to the extremely slow weathering of its
smooth, shining cleavage surfaces, it exerts no notable effect upon
the chemical composition of the soil, although, owing to its peculiar
character of fine scales, it sometimes adds not immaterially to the
facility of tillage in otherwise somewhat intractable soils. So far as
is known at present, its presence or absence does not constitute, in
itself, any definite cause or indication of the quality of any soil.
It may nevertheless be said that the rock in which it usually occurs
most abundantly—mica-schist, a mixture of mica and quartz—is known to
form, as a rule, lands of poor quality. On the other hand, the soils
derived from granites and gneisses, even when rich in mica, are usually
excellent, on account of their content of feldspars, and frequently of
other associated minerals.

_Hydromica_ differs from the preceding mainly in containing a larger
proportion of combined water; but it hardly decomposes more readily,
and the rocks in which it mainly occurs (hydromica schists) are
refractory to weathering, and in any case do not yield soils of any
fertility, the mineral being associated simply with quartz.

_Chlorite_, essentially a silicate of alumina and iron, somewhat
resembles mica but is deep green or black, in small scales. It forms
part of certain rocks (chlorite schists), which greatly resemble
the hornblende schists, but are usually inferior to the latter as
soil-formers, containing but little of any direct value to plant life.

_Talc and Serpentine_, Hydrous silicates of magnesia, are extensive
rock-materials in some regions, and as such require mention as
soil-formers also. Serpentine usually forms blackish-green rock-masses,
that although soft disintegrate very slowly in the absence of definite
structure, and are attacked with some energy only when charged—as is
frequently the case—with ferrous oxide. The conversion of this into
ferric hydrate, so common in nature, here also serves as the point of
attack on a rock otherwise very stable; causing it to crumble, even
though slowly.

_Talc_ (the true “soapstone”) being usually free from iron, would be
even more slow than serpentine to yield to weathering, but that its
extreme softness and ready cleavage greatly facilitate its abrasion.
Thus talc schist, which is usually a mixture of talc with more or less
quartz, undergoes mechanical disintegration quite readily.

But the soils formed from either serpentine or talcose rocks are
almost always very poor in plant food, and sometimes totally sterile.
Magnesia, though an indispensable ingredient of plant food, is rarely
deficient in soils and unlike lime does not influence in any sensible
degree the process of soil formation. Magnesian rocks as a whole are
practically found to be not specially desirable soil-formers, even in
the form of magnesian limestones. They do not even, as a rule, contain
as many useful accessory minerals as are commonly found in limestones.
Moreover, an excess of magnesia over lime is injurious to most crops,
as is shown later (chapt. 18).

_The Zeolites._—Zeolites may be defined as hydro-silicates containing
as bases chiefly lime and alumina, commonly together with more or
less of potash and soda, more rarely magnesia and baryta. The water
is easily expelled by heating, but is present in the basic form, not
merely as water of crystallization. All zeolites are readily decomposed
by chlorhydric and other stronger acids.

    The zeolites proper are not original rock ingredients,
    but are formed in the course of rock decomposition by
    atmospheric agencies, heated water, and other processes not
    fully understood. They are therefore usually found in the
    cavities and crevices of rocks that have been subject to the
    influence of atmospheric or thermal waters, most frequently
    in eruptive rocks, particularly in the vesicular cavities
    characterizing what is known as amygdaloids. They are also
    found in the crevices of sandstones and shales percolated
    by water, as well as in nodules of infiltration (geodes),
    in which they are frequently associated with quartz. Those
    found in the cavities of rocks are usually well crystallized
    wherever room is afforded, and are readily recognized by
    their crystalline form; they are mostly colorless, sometimes
    yellow or reddish.

_Exchange of bases in Zeolites._—Although zeolites rarely form a large
proportion of rock-masses and therefore do not enter directly into the
soil minerals to any great extent, their interest in connection with
soil-formation is very great, because of the continuation, within the
soil, of the same processes that bring about their formation in rocks.
Under the conditions existing in soils they will naturally rarely
form crystals, but will appear in the pulverulent or gelatinous form,
leaving the zeolitic nature of the material to be inferred from its
chemical behavior. Among these characters the ready decomposability
by acids has already been mentioned; another of special importance in
the economy of soils is the fact that when a pulverized zeolite is
subjected to the action of a solution containing either of the stronger
bases usually present (potash, soda or lime), such base or bases
will be partially or wholly taken up by the zeolitic powder, while
corresponding amounts of the bases originally present will pass into
solution.

    Thus when a hydrosilicate of soda and alumina is digested
    with a solution of potassic chlorid or sulphate, the soda
    may be partially or wholly replaced by potash, while the
    corresponding sodium salt passes into solution. In the case
    of zeolites containing lime or magnesia or both, the action
    of potassic or sodic chlorid will be to partially replace
    the lime, while calcic and magnesic chlorids pass into
    solution, resulting in the partial or complete replacement
    of the lime by one or the other, or by both bases. It is
    important to note that, other things being equal, potash
    is usually absorbed in greater amounts and is held more
    tenaciously than soda. The process may frequently be
    partially or wholly reversed again, by subsequent treatment
    with large amounts of solutions of the displaced base or
    bases. Thus while a solution of potassic chlorid may be made
    to expel almost completely the sodium present in analcite,
    subsequent treatment with sodic chlorid solution will again
    almost completely displace the potash before taken up.
    The same happens when the natural mineral potash leucite,
    (see p. 32) of frequent occurrence in certain lavas, is
    pulverized and treated with a sodic solution; resulting
    finally in the production of a mass corresponding to natural
    analcite, the sodium mineral corresponding to leucite.

In other words, in any zeolitic powder the alkaline or alkaline earth
bases present may be partially or wholly displaced by digestion with an
excess of solution of any of these, varying according to the amount of
solution employed, and the length of time and temperature of action.

This characteristic behavior of zeolites is exactly reproduced in
soils. Few soils permit any saline solution to pass through them
unchanged; solutions of alkaline chlorids filtered through soils almost
invariably cause the passing through of calcium and magnesium chlorids,
while a part of the alkaline base is retained; and as a matter of fact,
we find that this absorbing power of soils for alkaline bases is more
or less directly proportional to the amount of matter which may be
dissolved or decomposed with elimination of silica, by means of acids.

    This absorption of bases from solutions by chemical
    fixation will be farther discussed later on; but it should
    be mentioned here that both naturally and artificially,
    rock-masses are very commonly cemented, wholly or in part,
    by zeolitic material. Hydraulic concretes may be considered
    as sandstones or conglomerates whose grains are cemented by
    a zeolitic cement consisting of silica, lime and alumina,
    with usually some potash or soda, and of course containing
    the basic water; hence unaffected by the farther action of
    the latter substance after the time of setting has expired,
    which varies somewhat according to the nature of the
    material used. That similar cements should occur in natural
    sandstones is to be expected; thus we find not unfrequently
    that certain sandstones are materially softened, and their
    resistance destroyed, by treatment with even moderately
    dilute acid, while silica and the usual zeolite bases pass
    into solution. It is not often, however, that zeolitic
    material alone cements the sandstone; it is most frequently
    associated with siliceous, calcareous and sometimes even
    with ferruginous cementing material.[14]

[14] A zeolitic mass, at first gelatinous and then becoming
granular-crystalline is frequently observed oozing from the lower
surface of newly made concrete reservoir dams: just as we find similar
oozes consolidated into natrolite crusts in the crevices of natural
sandstones.


CALCITE AND LIMESTONES.

_Calcite_ or calcareous spar is one of the minerals most commonly known
in the crystallized form, and is readily recognized by its perfect
cleavage in three directions, producing cleavage forms with smooth,
rhomb-shaped faces (rhombohedrons); these are sometimes colorless and
perfectly transparent, and laid on printed paper show the letters
double. But it may be whitish-opaque, and of various colors, which
may also be imparted to the limestones formed from it. It is readily
distinguished from quartz, which it sometimes resembles, by its
cleavage, its inferior hardness, being easily scratched with a knife;
and by its effervescence with acids, the latter being the crucial test
when other marks are unavailable, as when it forms soft granular masses
or “marls.” In all cases it can be recognized by its crystalline form
under the microscope, even when the substance containing it has been
pulverized in a mortar. The great importance of this compound—calcic
carbonate—from the agricultural point of view renders it desirable that
it, as well as limestones as such, should be recognized, when seen, by
every farmer.

    In mass the pure mineral constitutes white marble; colored
    or variegated marbles are more or less impure from the
    presence of other minerals. Some compact limestones also
    are nearly pure; and as supplying only a single ingredient
    of plant food these would not be much better soil-formers
    than quartz or serpentine. But it is quite otherwise with
    _common_ limestones; the mass of which, it is true,
    is formed of calc-spar, but owing to its origin, is in the
    great majority of cases so far commingled with other matters
    of various character, that limestones are popularly reputed
    to form the very best soils. “A limestone country is a rich
    country” is a popular axiom to which there are, on the
    whole, but few exceptions.

_Origin._—Actual observation of what is happening at the present time,
as well as the examination of the rock as anciently formed, prove
conclusively that with insignificant exceptions, all limestones have
been formed from the framework and shells, and to some extent from the
bones, of marine and fresh-water organisms, ranging in size from the
extinct giants of the lizard relationship to those recognizable only by
the microscope. Owing to the solubility of lime carbonate in carbonated
water, the organic forms have often (in crystalline limestones) been
almost completely obliterated in some portions, but in others are so
preserved as to prove undeniably the similarity of origin of the whole,
and that they have been formed in relatively shallow water, as they are
to-day.

_Impure Limestones as Soil-formers._—From what has been said regarding
the composition of sea-water, it will readily be inferred that a pure
deposit of any one kind cannot easily be formed in it; moreover,
the matter held in mechanical suspension everywhere near the coasts
must very commonly be included within the calcareous deposits formed
off-shore. Hence few limestones dissolve in acids without leaving a
residue of sand, clay and various other substances, usually even some
organic matter not fully decomposed; sometimes less than half of the
mass is really lime carbonate. It is obvious that when the solvent
action of carbonated water is exerted upon such impure limestones,
a loose residue of earthy matters will remain behind. It is by this
process that a considerable proportion of the richest soils in the
world have been formed, which have given rise to the popular maxim
above quoted. They are emphatically “residual” soils; sometimes, it is
true, somewhat removed, by washing-away, from their point of origin,
but in many cases forming a compact soil-layer on top of the unchanged
rock, into which there exists every shade of transition. Striking
examples of such residual soils in place are seen in the black prairies
of the southwestern United States; they are mostly rather “stiff”
(clayey), and hence has arisen a local popular error, to the effect
that clay or “heavy” soils are always calcareous. On the other hand,
the blue-grass region of Kentucky, and most of the lands of the arid
regions are prominent examples of “light” calcareous soils.

    _Caves, Sinkholes, Stalactites._—Perhaps the most
    striking exemplification of the solvent power of carbonated
    water is seen in the formation of limestone caves. As a
    matter of fact, the vast majority of all existing caves
    is found in limestone formations; and such formations,
    as will be more fully discussed hereafter, nearly always
    bear a luxuriant vegetation. The water filtering through
    the vegetable mold, in which carbonic acid is constantly
    being formed, becomes charged with it, and on reaching
    the underlying rock, dissolves to a corresponding extent
    the lime carbonate of which this rock wholly or chiefly
    consists. When penetrating crevices it soon enlarges these,
    to an extent proportioned to the length of time and the
    strength of the solvent; and thus gradually subterranean
    passages or caves are formed, which at first are almost
    always the bed of a stream, the mechanical action of which
    accelerates the process of enlargement, until after some
    time the water is perhaps drained off through some crevice
    to a lower level, where the same process is repeated.

    Sometimes the ceiling gives way, forming the funnel-shaped
    “sinkholes” or “lime-sinks” so familiar in some of the
    Mississippi Valley States. Sometimes the lime solution
    on reaching the ceiling of the cave, instead of dropping
    down, evaporates there and eventually forms icicle-like
    “_stalactites_” out of the dissolved substance; while
    when dropping on the floor and thus growing upwards, the
    corresponding formation is called “_stalagmite_.” These
    caves, subterranean rivers, sinkholes, natural bridges and
    tunnels, etc., mostly owe their origin to this solvent
    action of carbonated water on limestone formations.[15]

[15] T. M. Reade (in his treatise on Chemical Denudation in Relation
to Geological Time) calculates that 143.5 tons of lime carbonate are
annually removed by solution from each square mile of land in England
and Wales, and that the average amount thus removed annually from each
square mile of the earth’s surface is about fifty tons.

The same occurs on a small scale, when calcareous land is underdrained;
the lime carbonate dissolved from the soil is partially deposited in
the drain pipes, which it frequently obstructs. Similarly, an impure,
porous deposit of calcareous tufa is frequently formed on the surface,
at the foot or in rills of calcareous hills. When “hard” water, being
usually such as contains lime carbonate dissolved in carbonic acid, is
boiled, or long exposed to the air, carbonic gas escapes and the lime
salt is deposited partly on the walls of the kettle, partly forming a
pellicle on the surface of the water.

_Dolomite_, or bitter spar, greatly resembles calcite in its aspect
and properties, although containing nearly half its weight (47.6%)
of magnesic, together with calcic carbonate. It is, however, nearly
always whitish-opaque; its crystalline and cleavage surfaces are
usually somewhat curved; and its effervescence with acids is much
less lively than in the case of calcite. Like the latter it often
forms pure granular rock deposits, frequently used instead of marble
and limestone, and under that designation. The dolomite rocks,
however, are much more subject to weathering than the non-magnesian
limestones, and it is a curious fact that in contradistinction to the
limestone regions proper, those having strongly magnesian limestones or
dolomites as their country rock are frequently remarkably sterile. In
some portions of Europe dolomite areas are sandy deserts, whose sand
consists of weathered dolomite, so pure as to offer no adequate supply
of mineral food to plants. In the United States, magnesian limestones
underlie the “barrens” of several States and thus seem to justify their
European reputation of being poor soil-formers. The exact cause of
this difference is not fully understood, for at first sight it is not
clear why the presence of the magnesian carbonate should interfere with
the well-known beneficial effects of the lime compound. O. Loew and
May[16] and others have, however, shown that a certain excess of lime
over magnesia in the soil is necessary to prevent the injurious effects
exerted by magnesic compounds on plant nutrition, in the absence of an
adequate supply of lime. This point will be discussed more in detail
farther on.

[16] Bull. No. 1, U. S. Dept. Agr. Veg. Path. and Physiol. Investig.

_Selenite_ or _Gypsum_, sulfate of lime with about 14 per cent of
water, though not as abundant in nature as the carbonate or limestone,
is a very widely disseminated mineral and often occurs in large
masses over considerable areas. These are undoubtedly in most cases
the result of evaporation of sea water (see p. 26), more rarely of
the transformation of limestone. In mass it frequently resembles the
latter, but is readily distinguished by its softness; it does not grit
between the teeth, is readily cut with a knife and does not effervesce
with acids. Very commonly it occurs in crystals, which are easily split
into thin plates. The crystals are very frequently found imbedded in
gray or bluish, tough clays, in rosettes, or flat sheets which mostly
show characteristic incurrent angles (caused by twinning), and are
hence known as “swallowtail” crystals. Such sheets of selenite are
popularly called “isinglass,” which name however is equally applied to
the mineral mica (see p. 35).

Gypsum is only exceptionally an abundant ingredient of soils; yet
such soils prevail quite extensively on the upper Rio Grande, in New
Mexico and adjacent portions of Chihuahua, Coahuila, and on the Staked
Plains of Texas. Here whole ranges of hills are sometimes composed of
gypseous sand, bear a scanty, peculiar vegetation, and are ill adapted
to agricultural use. It may be said in general that few naturally
gypseous soils are very productive. This is largely because of the very
heavy clays which commonly accompany it, as the compound itself is not
only not hostile to plant life but is in extended use as a valuable
fertilizer (“land plaster”) for special purposes. From causes not fully
understood as yet, it particularly promotes the growth of leguminous
plants, notably the clovers; and as stated in chapter 9, it also
specially favors nitrification in soils. In the arid region it renders
important service in the neutralization of “black alkali” or carbonate
of soda in alkali soils. Being soluble in 400 parts of water, it easily
penetrates downward in most soils, and in doing so effects changes in
the zeolitic portions, setting free potash from silicates and thus
indirectly supplying plants with this essential ingredient in a soluble
form. About 200 pounds per acre is an ordinary dose.

For agricultural use the rock gypsum is ground in mills so as to be
easily distributed, and dissolved by the soil water. Frequently,
however, it occurs in the soft granular form (gypseous marl) requiring
only light crushing; thus in the hills bordering the Great Valley of
California, and in parts of New Mexico and Texas.

_Iron Minerals._—In connection with calcite and dolomite, the several
minerals constituting the common iron ores require mention. One of
these is:

_Iron Spar_ or siderite; carbonate of iron, corresponds in composition
to calcite and dolomite and crystallizes in the same form. It sometimes
occurs in large masses and is an important iron ore, brownish-white
in color, and when compact resists the attack of atmospheric oxygen
remarkably well. Like the carbonates of lime and magnesia, it is
soluble in carbonated water, and its deposits are undoubtedly formed
from such solutions. The latter are copiously formed wherever
fermenting or decaying organic matter is in contact with iron-bearing
materials, such as rust-colored sands or clays; and if the solution so
formed can percolate without coming in contact with air, iron spar is
formed. But whenever the solution comes in contact with air, it absorbs
oxygen and the ferrous carbonate is converted into ferric hydrate or
rust, mineralogically known as:

_Limonite_ or brown iron ore. This ore is frequently found deposited
on the upper surface of clay layers traversing sandy strata, the clay
having arrested the carbonate solution and thus given time to the air
to effect the change. Sometimes such deposits form great masses in
rock-caves, fissure-veins, or crevices; and like siderite, it is an
important iron ore, though frequently quite impure, as in the case
of _bog ore_, which is formed in ill-drained subsoils. It is also
sometimes found as the residue from the weathering of rocks rich in
hornblende or pyroxene, and in this, as well as in other cases, is
pulverulent, constituting _yellow ochre_. It makes a rust-colored
streak on biscuit porcelain or unglazed queensware. _It is the coloring
material of all yellow or “red” soils and clays_, as well as of brown
sandstones, which are cemented by it.

As is well known, such clays and sandstones become dark red by heating
or “burning,” as in the case of common brick clays; the brown or yellow
ferric hydrate losing its water and becoming red ferric oxid. The
latter sometimes occurs in nature in the impure, pulverulent condition,
constituting “red ochre”; but more commonly and abundantly it is found
in the form of

_Hematite_ or red iron ore, which is sometimes formed in nature by
limonite losing its water, but more commonly in different ways. It
is but rarely found in soils and is of no special interest in that
connection.

A fourth form of iron ore, quite common in the soils of some regions, is

_Magnetite_ or magnetic iron ore, also known as lodestone. This
mineral, the oxygen-compound of iron corresponding to “blacksmith’s
scale,” also occurs in large masses and is an important and usually
a very pure iron ore. It occurs very commonly disseminated through
certain rocks, and in their weathering it remains unattacked and thus
passes unchanged into the soils and sands, constituting the “black
sand” so well known to gold miners and almost universally present in
the alluvial soils of the Pacific coast. These black grains are of
course attracted by the magnet and can thus be easily recognized and
extracted. In soils they are simply inert, like quartz sand.

But while the ore is of little interest to the farmer, it is quite
otherwise with the compound of this oxid with water, the ferroso-ferric
hydrate; intermediate in composition between the white ferrous and the
brown ferric hydrates. As mentioned above, the black silicate minerals,
such as hornblende and pyroxene, are bottle-green when seen in thin
sections. Nearly the same color, with modifications running toward
blue and bright green, is often seen in natural clays and rocks, and
is almost always caused by the ferroso-ferric hydrate. Such materials
always become red or reddish when heated by the formation of red ferric
oxid; while when exposed to damp air, they assume the rust color of
ferric hydrate.

_Reduction of ferric hydrate in ill-drained soils._—When such oxidized,
rust-colored clays or soils are exposed to the action of fermenting
organic matter, the first effect observed is the change of color
from rusty to bluish or greenish, by the reduction of the ferric to
ferroso-ferric hydrate. Afterward, if the action is continued, the
solution of ferrous carbonate (see above) may be formed, and the
greenish or bluish color may disappear.

The importance of this reaction to farming practice lies in the fact
that the blue or green tint, wherever it occurs, indicates a lack
of aeration, usually by the stagnation of water, in consequence of
imperfect drainage. Such a condition, always injurious to plants,
becomes doubly so when it is associated with the formation of a
metallic solution, such as ferrous carbonate, and promptly results in
the languishing or death of plants in consequence of the poisoning of
their roots. In the presence of sulfates such as gypsum, the formation
of iron pyrite (ferric bisulfid) and sulfuretted hydrogen, is likely to
take place. Moreover, under the same conditions the phosphoric acid of
the soil may be concentrated into ferrous or ferric phosphate, which
pass into deposits of bog ore in the subsoil.




CHAPTER IV.

THE VARIOUS ROCKS AS SOIL-FORMERS.


_Rock-weathering in arid and humid Climates._—From what has been
said in the preceding chapters of the physical and chemical agencies
concerned in rock-weathering, it is obvious that climatic differences
may materially influence the character of the soils formed from one
and the same kind of rock. Since kaolinization is also a process of
hydration, the presence of water must greatly influence its intensity,
and especially the subsequent formation of colloidal clay; so that
rocks forming clay soils in the region of summer rains may in the arid
regions form merely pulverulent soil materials. Many striking examples
of these differences may be observed, _e. g._, in comparing the outcome
of the weathering of granitic rocks in the southern Alleghenies with
that of the same rocks in the Rocky Mountains and westward, especially
in California and Arizona. The sharpness of the ridges of the Sierra
Madre, and the roughness of the hard granitic surfaces, contrasts
sharply with the rounded ranges formed by the “rotten” granites of the
Atlantic slope, where sound, unaltered rock can sometimes not be found
at a less depth than forty feet; while at the foot of the Sierra Madre
ridges, thick beds of sharp, fresh granitic sand, too open and pervious
to serve as soils, cover the upper slopes and the “washes” of the
streams, causing the latter to sink out of sight. A general discussion
of the kinds of soils formed from the various rocks must, therefore,
take these differences into due consideration.


GENERAL CLASSIFICATION OF ROCKS.

Rocks may be broadly classified into three categories, viz:

1. _Sedimentary_ rocks, formed by deposition in water and hence more or
less distinctly stratified.

2. _Metamorphic_ rocks, formed from rocks originally sedimentary, by
subterranean heat in presence of water. Usually crystalline, that is,
composed of more or less distinct (large or minute) crystals of one or
several of the minerals mentioned above.

3. _Eruptive_ rocks, ejected in the molten state from volcanoes or
fissures; crystalline or not, according to slow or rapid cooling.

_Sedimentary Rocks._—Sedimentary rocks are forming to-day by deposition
from either sea or fresh water, precisely as they were in the most
remote geological times; the oldest clearly sedimentary rocks being
sometimes undistinguishable in their nature and composition from the
very latest immediately preceding our present time. They may for the
purposes of the present work be simply classified as follows:

1. _Limestones_, formed in comparatively shallow seas, or fresh water
basins, from the calcareous shells or skeletons of various organisms.

2. _Sandstones_, and conglomerates (sometimes called pudding-stones)
formed from the debris of pre-existing rocks disintegrated by the
agencies described above, (chap. 1-2), re-cemented by means of
solutions of one or several substances, such as silex, carbonate of
lime, ferric hydrate and others. Loose sands and gravels are the
initial stages of such rock formation as well as the results of their
disintegration.

3. _Clays_, _Claystones and Clay shales_, consisting of clay substance
with more or less sand, and soft or hard according to the nature of the
waters or solutions that may have acted upon them, with or without the
aid of heat. These rocks can only be formed in comparatively quiet or
“back” waters, since clay would not ordinarily be deposited in moving
water.

_Metamorphic Rocks._—The effects of subterranean heat or metamorphism
upon the sedimentary rocks may be roughly stated as follows:

_Limestones_ are transformed into marbles of various degrees of purity,
according to the nature of the original rocks.

_Sandstones_ when cemented by silex are transformed into quartzite, of
greater or less purity according to the nature of the “sand” entering
into its composition. When cemented by materials other than quartz,
these also will be segregated in the form of various minerals in the
body of the rock.

The _clay rocks_ form the most varied products under the influence of
(aqueo-igneous) metamorphism; granites, gneiss, syenite and hornblendic
schist are among the most common. The great variations in the
composition of clayey materials account for the correspondingly great
variations in the nature of the resultant metamorphic rocks.

_Igneous or Eruptive Rocks._—These are usually divided into two groups;
the one characterized by a large proportion of free quartz (silicic
acid), and hence designated as _acidic_, and usually of a light tint;
the other the _basic_, containing little or no free quartz, and
commonly of a dark tint caused by the presence of a large amount of
iron (contained in pyroxene, more rarely in hornblende).

    Of the latter class are the dark “basaltic” rocks
    constituting the mass of the enormous eruptive sheet
    of the Pacific Northwest, covering the greater part of
    Washington, Oregon and northeastern California. The lavas
    of the Hawaiian islands are of the same class and even more
    basic; while the eruptives of Nevada, middle and southern
    California, and eastward to the Rocky Mountains, are mostly
    of the light-colored, acidic type. The same is largely true
    of the rocks of the Andes of Central and South America, the
    gray “Andesites,” also represented in the Caucasus.

As one and the same eruptive material may, according to the greater or
less rapidity of cooling, appear as a glassy mass (obsidian, pumice,
volcanic ash, tuff, etc.,) or as a crystalline rock resembling coarse
granite in structure, it is not easy to identify them in all their
various forms. This can frequently be done only by ascertaining their
component minerals by the microscope, or by chemical analysis. The
same is sometimes true of metamorphic rocks; and as in the latter, the
several feldspars and quartz, with pyroxene instead of hornblende,
constitute the predominant soil-forming minerals. More rarely, garnet,
chrysolite, leucite and other silicates require consideration.


_Generalities regarding the Soils derived from various Rocks._

It is hardly necessary to insist that as in the case of the rocks
composed of single minerals, already referred to above, the predominant
mineral or minerals of compound rocks determine the facility of
weathering, as well as the quality of the soil resulting therefrom.
Since rocks are named essentially in accordance with the _kinds_ of
minerals that constitute their regular mass, the _proportion_ in which
the several constituents stand to each other may vary greatly. Thus a
_granite_ may consist, over considerable areas, mainly of a mixture of
potash feldspar and quartz; in others, mainly of quartz and mica with
little feldspar. Very frequently, hornblende replaces mica partially
or wholly. The latter will weather much more slowly than feldspar
or hornblende, and will produce an inferior soil when decomposed.
Allowing for such variations, a fairly approximate general estimate
of the quality and peculiarities of soils from crystalline rocks may
nevertheless be made. To some extent such estimates must make allowance
not only for the chief ingredients, but also for those which are
called “accessory” or characteristic, and which while not present in
large amount, may nevertheless exert a considerable influence upon the
quality of the soil.

_Soils from granitic and crystalline rocks._—In the case of the
(potash-feldspar) granite soils it is generally admissible to expect
that they will be fairly supplied with phosphoric acid, because in
the great majority of cases, minute crystals of apatite (phosphate of
lime) are more or less abundantly scattered through it. From the potash
feldspar present, granite soils may always be relied on for a good
supply of potash for plant use; on the other hand, unless hornblende be
present, they are pretty certain to be deficient in lime, since neither
lime, feldspar nor calcite are probable accessory ingredients of this
rock.

Granite is exceedingly apt to weather by mechanical disintegration
far in advance of its chemical decomposition. It is therefore common
to find in sedentary soils overlying granite, a gradual increase of
grains of its component crystalline minerals as we descend in the
subsoil; until finally the latter grades off into rock almost unchanged
save in lacking coherence. This is seen strikingly in the southern
Appalachians, as well as in the Sierra Nevada and Sierra Madre of
California; at Cintra in Portugal, at Heidelberg in Germany, and
elsewhere.

But of the rocks that resemble granite and are popularly so called, a
good many are not “true to name” and therefore form soils differing
materially from the type just mentioned.

    Thus the so-called granite areas of the Sierra Nevada of
    California are largely occupied by a rock containing,
    besides quartz, chiefly soda-lime feldspar and some
    hornblende, and scarcely any mica. It is more properly
    a diorite (grano-diorite); the soils formed from it are
    rather poor in potash, not strongly calcareous, and quite
    poor in phosphoric acid. On account of the small proportion
    of hornblende (unusual in diorites), these soils are
    light-colored (not “red”), and bear a growth of small pine
    instead of the usual oak growth of the lower Sierra slopes.

What is said of granite soils is also generally true of those formed
from _Gneiss_, which is composed of the same minerals as granite,
but has a slaty cleavage and on that account when upturned on edge,
weathers rather more rapidly than most granites. Owing to the frequent
occurrence of lenticular masses of quartz in gneiss, its soils are
more commonly of a siliceous nature than are those of the true granite
regions, and not as “strong” as the latter. This is the more true
since gneiss often passes gradually into _mica schist_, which, being a
mixture of quartz and mica only, not only weathers very slowly but also
supplies but little of any importance to plants, to the soils formed
from it. Such soils would mostly be absolutely barren but for the
frequent occurrence in the rock, of accessory minerals that yield some
substance to the soil. Yet it remains true that inasmuch as gneiss and
mica-schists are among the rocks in which mineral veins most commonly
occur, the proverbial barrenness of mining districts is very frequently
traceable to these rocks. The same may be said of some of the related
rocks, such as gabbro, minette and others.

Normal _diorite_ consists of hornblende and soda-feldspar, with more or
less quartz.

The soils derived from certain diorites of the Sierra Nevada of
California have just been referred to. But these granite-like diorites
are on the whole exceptional; it should be added that the (diabasic)
“greenstones” of the Eastern United States and of the Old World, which
are usually much finer-grained, do not form the mass of fine, angular
debris constituting the subsoil in the Sierra Nevada, but weather into
rounded masses and fine-grained soils possessing, on the whole, a fair
fertility, though liable to contain an excessive proportion of silex in
various forms.

Of the _eruptive rocks_ as a class it is often said that they form
very productive soils; yet, as these rocks differ widely from each
other in composition, this statement must be taken with a great deal of
allowance. Very many of them decompose with extreme slowness on account
of their glassy nature; this is particularly true of obsidian, pumice
stone, and the “volcanic ash” derived from its pulverization, and which
is found unchanged, in sharp scales, among the decayed minerals of
other rocks in complex soils. Other volcanic ash, however, being formed
by the pulverization of crystalline or of basic lavas, weathers rather
readily, as already stated; so that certain plants take possession in
the course of a few years. The general classification into basic and
acidic rocks, given above, is of importance in connection with soil
formation from eruptive masses; for the basic rocks are much more
easily attacked by the atmospheric agencies than the acidic class.

    A broad distinction must, however, be made between the
    basic rocks of the basaltic class, which contain black
    pyroxene as a prominent ingredient, and those which, like
    many trachytes, are rich in feldspathic minerals. The
    latter are naturally rich in alkalies (potash and soda)
    which they impart to the corresponding light-colored soils;
    while the black basaltic rocks and lavas weather into “red”
    soils, sometimes containing extraordinary amounts of iron
    (ferric hydrate) and (from the lime-feldspars they contain)
    a fair supply of lime, but oftentimes very little potash.
    Experience seems to prove that the red basalt soils are
    mostly rather rich in phosphoric acid; this is especially
    true of the country covered by the great eruptive sheet of
    the Pacific Northwest, in the rocks of which the microscope
    readily detects the presence of numerous needles of
    apatite (lime phosphate). The same is true of the highly
    iron-bearing soils from the black basaltic lavas of the
    Hawaiian islands, even though they have been leached of
    all but traces of lime and potash. All these soils are
    physically “light” and easily workable, since the rocks
    in question contain but little alumina from which to form
    clay; they are sometimes extremely rich in iron, even to the
    extent of being capable of serving as iron ores.

The soils derived from _trachytes_ and trachytic lavas are generally
light-colored and light in texture; the latter from the presence of
a large proportion of volcanic glass, together with undecomposed
crystalline minerals. These are usually rich in potash, but poor in
lime and phosphates. The high quality of the wines of the lower Rhine
has been ascribed to these soils, which however vary greatly within
the areal limits of the production of the high-grade wines, not only
from gray trachytes to dark colored, highly augitic basalt, but also to
acidic quartz porphyries or rhyolites, and clay-slates.

The rhyolites on the whole yield the poorest soils among the eruptive
rocks; they are slow to weather at best, and the soils produced are
poor and unsubstantial, largely from the predominance of quartz and
undecomposable, glassy material; of which the phonolites are the
extreme type, resisting the influence of the atmospheric agencies just
as would so much artificial glass. Soils consisting largely of volcanic
glass may be found covering considerable areas in the Sierra Nevada of
California. Such “volcanic ash” soils are usually very unthrifty, and
bear a growth of small pines.

_Soils from sedimentary rocks._—_Limestones_, when pure and hard,
are very slow to disintegrate, and are also very slowly attacked by
carbonated water (see chap. 3, page 41). Soft impure and vesicular
limestones are, however, very rapidly attacked, especially when
underlying a surface clothed with the luxuriant vegetation that usually
flourishes on soils rich in lime. The popular adage that “a limestone
country is a rich country,” is of almost universal application and
stamps lime, from the purely practical standpoint, as one of the most
important soil ingredients.

_Residual Limestone Soils._—Striking examples of the formation of
large, fertile soil areas by the leaching out of limestones are found
in the States of Alabama, Mississippi, Louisiana and Texas, where the
fertile black prairies have been largely thus formed. The “blue-grass”
country of Central Kentucky is another case in point.

The following table shows a representative example of the relative
composition of the (cretaceous) “Rotten Limestone” of Mississippi, and
the “residual” soil-stratum derived from it. The average thickness of
the layer of residual clay above the limestone is about eight feet,
but ranges from seven to ten; the upper layers of the limestone are
somewhat softened, but the rock is always fresh at twelve feet, from
which depth the sample analyzed was taken, in a cistern adjoining
the field from which the soil and subsoil were procured. The black
soil varies in depth from 8 to 15 inches; then there is a change to
a brownish subsoil, reaching down to about two feet, and in drying
cleaving into prismatic fragments. The black soil has here in the
highest degree the peculiarity of crumbling in drying from its
water-soaked condition, so that it may be plowed when wet without
injury, although in the roads it works up into the toughest kind
of mud. The prairie is sparsely timbered with compact, fair-sized
black-jack oak, accompanied originally by red cedar.

The limestone derives its popular name of “rotten” from its being
usually soft enough to be cut with a knife or hatchet, and is therefore
somewhat used for building, and for burning lime.

    COMPOSITION OF LIMESTONE, AND RESIDUAL SOIL AND SUBSOIL,
          FROM BLACK PRAIRIE, MONROE CO., MISSISSIPPI.

    ================================+============+=========+========
                                    |  “ROTTEN   | SUBSOIL |  SOIL
                                    |LIMESTONE.” |(YELLOW).|(BLACK).
    --------------------------------+------------+---------+--------
             FINE EARTH.            |Depth 12 ft.| 2-3 ft. |15 ins.
    Chemical analysis of fine earth.|            |         |
    --------------------------------+------------+---------+--------
    Insoluble matter                |    10.90   |  71.54  |   78.29
    Soluble silica                  |            |         |
    Potash (K₂O)                   |      .25   |    .54  |     .33
    Soda (Na₂O)                     |      .32   |    .23  |     .08
    Lime (CaO)                      |    45.79   |   1.08  |    1.37
    Magnesia (MgO)                  |      .88   |    .77  |     .36
    Br. Ox., of Manganese (Mn₃O₄)   |            |    .05  |     .14
    Peroxide of Iron (FeO)          |     1.42   |   5.42  | }
    Alumina (Al₂O₃)                 |     1.96   |  13.15  | } 14.22
    Phosphoric Acid (P₂O₅)          |            |    .05  |     .10
    Sulfuric Acid (SO₃)             |            |    .04  |     .03
    Carbonic Acid (CO₂)             |    35.73   |         |
    Water and Organic matter        |     2.84   |   6.99  |    5.75
                                    |   ------   |  -----  |  ------
          Total                     |   100.09   |  99.86  |  100.67
                                    |            |         |
    Humus                           |            |         |    1.93
      “   Ash                       |            |         |    4.38
    Hygroscopic moisture            |            |  10.35  |   12.82
      absorbed at °C                |            |   19°   |   19°
    --------------------------------+------------+---------+--------

It appears from the above table that in the change from the original
limestone to the soil mass as found at three feet depth, 81.5% of the
lime carbonate has been eliminated by leaching, leaving behind somewhat
less than one fifth of the original mass. Taking the average depth of
the soil mass at 8 feet, this thickness of material has required about
45 feet of the rotten limestone. Considering that notwithstanding the
tenacity of the clay soil, some of it must in the course of time have
been washed away, we may safely assume that the original rock surface
was from 50 to 60 feet higher than at present.

_Sandstone Soils._—The indefiniteness of the nature of “sandstones” as
such renders generalizations in regard to the soils formed from them
rather difficult, save as to their physical qualities, which in the
nature of the case are always “light.” In the Old World and in the
humid region generally, sandstone and sandy soils are usually spoken
of as being poor, because there the sand almost always consists of
quartz grains only, and hence the fine portions alone can be looked to
for plant nutrition. Consequently, the more sand is seen in a soil,
the poorer it is usually presumed to be. But this presumption would be
wholly erroneous in the arid regions. (See chapt. 6, p. 86).

Clearly, the nature of the soils produced by the weathering of
sandstones depends upon two points: first, the nature of the cement
binding the sand grains, and second the character of the latter
themselves.

_Varieties of Sandstones._—As has been stated above, the cements may
be roughly classified into five kinds, and their intermixtures, to
wit: _quartzose_ or siliceous, _calcareous_, _ferruginous_, _aluminous
or clayey_, and _zeolitic_. As regards the first, it is obvious
that siliceous sandstones will disintegrate with great difficulty,
since neither the cement nor the grains are susceptible of material
change by weathering. Such sandstones frequently pass insensibly
into quartz rock, and the light, unsubstantial soils they produce
are of the poorest, containing often mere traces of the plant-food
ingredients. This of course, is true, not only of the soils formed by
the actual weathering of sandstones, but equally of those consisting of
quartz-sand deposited by water or drifted by winds.

    Of this character are the pine-forest soils of the coast
    region of the Gulf of Mexico, particularly the “Sand
    hammocks” of the immediate Gulf border, from Mississippi
    Sound to Charlotte Harbor, Florida; the sandy lands of the
    Grand Traverse region of Michigan, and many other minor
    areas in the United States, usually characterized by a pine
    growth, often more or less stunted, according to the nature
    of the sand grains.

_Calcareous sandstones_ usually form a very much better class of soils,
partly for the intrinsic reason given above as regards limestones as
soil-formers. The calcareous cement is very rarely pure calcite; in
most cases it is very impure, as, most commonly, is also the “sand”
itself. This is explained from the fact that such rocks (mostly soft
and often quite unconsolidated) are, like limestones themselves, the
result of deposition in shallow seas or lakes, receiving deposits from
the land drainage, and enriched by the animal and vegetable life of
such waters. Not uncommonly they contain, disseminated through them,
grains of the mineral glauconite (a hydrous silicate of iron and
potash), which readily supplies available potash; while the remnants
of animals and plants furnish more or less of available phosphates.
Thus the general presumption regarding calcareous sandstones is that
the derived soils are of good quality, frequently of the very best. The
same, however, does not appear to be true of sandstones cemented by
dolomite; the soils derived from magnesian sandstones are in many cases
noted for their unproductiveness. (See chapt. 3, p. 42).

_Ferruginous Sandstones_ manifestly derive no important soil
ingredients from their cement when the latter is measurably pure ferric
hydrate; and when in addition the sand itself is purely siliceous, the
soils resulting from the disintegration of the rocks are very poor.

    Such are, e.g., the soils derived from the ferruginous
    sandstones of the Lafayette formation in a part of northern
    Mississippi and adjacent portions of Tennessee and Alabama,
    characterized by small scrubby oak or dwarfed pine. On the
    whole, however, such purely ferruginous quartz sandstones
    are exceptional, and should not detract from the favorable
    inferences usually to be drawn from the iron-rust tint of
    soils (see chapter 15).

Sandstones with purely _zeolitic_ cement are on the whole not of
frequent occurrence, the zeolites forming, more commonly, the hard
portion of a clay-sandstone cement, which disintegrates by their
weathering-out.

    In regions where the tufaceous rocks of eruptives prevail,
    we not uncommonly find the “volcanic ash” solidly cemented
    by a zeolitic mass, which is then usually apparent in
    cavities or crevices in the form of crusts or crystals. Such
    tuffs are commonly rich in alkalies and lime, but mostly
    poor in phosphates, and in disintegration form soils of a
    corresponding nature. They are largely represented in the
    valleys off Puget Sound, as well as in portions of central
    Montana, and northward.

_Clay-Sandstones_ (argillaceous sandstones) when soft, as is mostly
the case, form as a rule desirable loam soils, of a generalized
composition, difficult to predict. It is here that the composition of
the sand grains themselves most frequently comes into play in modifying
the soil quality. From clay-sandstones to claystones of various degrees
of sandiness there is, of course, every grade of transition, the soils
ranging correspondingly in the scale of lightness or clayeyness.
As a general rule, the potash contents of such soils are sensibly
proportioned to the clayey ingredient, at least in the humid regions.

_Claystones_ (i. e., clays hardened by some one or more of the cements
mentioned in connection with sandstones), will in the nature of the
case, when disintegrated from the condition in which they lie in the
geological formations, make correspondingly clayey, heavy soils, which
as experience shows are usually rich in the ingredients of plant food,
but frequently too heavy and intractable in tillage to be readily
utilized.

    There are, of course, exceptions; such as soils formed
    from pipe-clays, in which little if any mineral plant-food
    remains, and which are best used for other purposes than
    agriculture, unless under special conditions it may be worth
    while to reclaim them by fertilization.

_Natural Clays._—Clays occur in nature in a great variety of
modifications that have received designations known in common life.
Such are porcelain clay, pipe-clay, fire-clay, potters’ clay,
brick-clay, and many others of more or less local use only. As these
materials practically concern the farmer in very many cases, they may
properly find a brief discussion here.

The variety-names enumerated above in the order of the actual contents
of the materials in true clay substance (“colloidal clay”), are partly
based upon that fact, partly upon the degree of plasticity attained
by that substance, and essentially upon the nature and amount of
foreign admixtures associated with it. Thus, porcelain clay is chalky
kaolinite, sometimes associated with enough of pure white plastic
clay to render it workable in the potter’s lathe, but more commonly
requiring to be molded in porous molds; it is very refractory to heat.
Pipe-clay is also white, but more plastic and usually less refactory.
Fire-clay is a refractory pipe-clay commingled with some coarse
infusible material, such as quartz sand (or the same clay burnt and
crushed), in order to prevent excessive contraction and change of shape
in drying and burning. Potters’ clay is a much less pure, and from that
cause more fusible clay, which when burnt forms at a moderate heat
a semi-fused, more or less hard mass, such as crockery and pottery
ware. Brick-clay is a still more impure clay, or loam, containing
considerable sand and usually iron oxid, and largely falls already
within the limits of tillable soils or subsoils, rendered fusible by
the presence of relatively considerable amounts of iron, magnesia and
lime.

    _Iron_ colors natural clays either red, yellow, green
    or blue; the latter two colors turning to yellow or red on
    exposure to the air, and to red on burning. Black color is
    usually due to carbon, such clays often turning white on
    heating.

    Clays containing much lime are usually of a gray or whitish
    tint, and like the soft crumbly limestones are often called
    marls, and are used as such for land improvement. But it
    should be understood that the colors of clays, mostly
    derived from some iron compound, have little to do with
    their uses in the arts, except that no deeply colored clay
    (black excepted) is refractory in the fire.


_“Colloidal” Clay._[17]

In connection with soils, clay may be defined, in the most general
terms, as being the substance which imparts plasticity and adhesiveness
to soils when wetted and kneaded, and which, when heated to redness,
loses this property completely and permanently, becoming hard and
coherent in proportion to the degree of heat to which it is exposed.

[17] This term was first employed by Th. Schloesing, in communications
to the French Academy of Sciences, and reported in the Comptes Rendus
of that body; first in 1870. Unaware of Schloesing’s work, the writer
began a full investigation of the subject of mechanical soil analysis
in 1871, and published the results in 1873 (Am. Jour. Sci., Oct. 1873).
Up to that time the limited resources of the library of the University
of Mississippi had not given him an opportunity to see Schloesing’s
publication. The two independent investigations, though conducted on
somewhat different lines, gave of course practically the same results,
and complement each other.

In common life, however, the name is applied to the whole of any
naturally occurring earth which on wetting and kneading assumes a
reasonable degree of plasticity and adhesiveness. When the latter
property becomes nearly or quite insensible, the earth is designated as
a “loam,” more or less “clayey” according to the amount of the pure,
plastic and adhesive material associated with the mineral powders and
sand that form the bulk of most soils.

Chemically, the pure clay substance[18] probably consists (as has been
stated above) of silica and alumina in the proportion of nearly 46 to
40, the rest (14%) being water of hydration, which is lost on burning
the clayey material. But while it is true that such is the composition
of the plastic substance of clays, plasticity and adhesiveness are by
no means invariable properties of this compound. In its purest state,
as kaolinite, it is readily mistaken for chalk, (and is sometimes
used as such), being powdery to the touch and entirely devoid of
plasticity[19] when wetted and kneaded. The microscope shows this
chalky kaolinite to consist of minute, mostly rounded, originally
six-sided, thin plates, which when pure resemble to the touch powdered
talc (soapstone) or even black-lead, rather than any clay known to
common life. But being exceedingly soft, the kaolinite substance
is easily ground or triturated into an extremely fine powder; and
Johnson and Blake[20] succeeded in producing sensible plasticity and
adhesiveness by long-continued trituration of kaolinite with water in a
mortar. A similar process, but continued much longer by the mechanical
agencies concerned in soil-formation (see chapt. I), is unquestionably
the chief factor concerned in the formation of natural plastic clays;
but whether this is the _only_ process by which the powdery kaolinite
may be transformed into plastic clay, is a question not definitely
settled. It is at least possible that repeated freezing and thawing, as
well as the action of hot water, may take a part in the transformation,
beyond that by which they destroy the crumbly (flocculated) structure
of soils and clays, and render them plastic; as is done in the maturing
of clays by potters.

[18] There is still some discussion as to the chemical identity of
colloidal clay with Kaolinite; but the objections are not convincing.

[19] It has of late been attempted to extend the meaning of this
word to the behavior of all powders when wetted with water. But the
adhesive plasticity of clay stands almost alone, in that (aside
from contraction) it preserves in drying the form into which it may
have been molded while wet, even when struck, whereas other powdery
substances similarly treated at once collapse back into the original
powder. The exclusive use of clay in modeling offers the typical
example of plasticity as generally understood. The addition of any
powdery substance, however fine, diminishes the plasticity of clay.

[20] American Journal of Science, 2d Ser., Vol. 43, p. 357.

_Causes of Plasticity._—In any case the property of plasticity and
adhesiveness is restricted to the particles so fine that they fail to
settle, in the course of 24 hours, through a column of pure water eight
inches (200 m) high, while some are so extremely minute that they will
not settle for many months, and even for several years.[21] Such turbid
“clay water” may sometimes be found existing in nature, in moist,
secluded places, for weeks after the subsidence of the overflows of
rivers whose water is exceptionally free from dissolved mineral matter.

[21] Williams (Forsch. Agr. Phys. Vol. 18, p. 225 ff.) claims that
the diameter of the minutest clay particles is one-thousandth of
a millimeter, their form being that of scales showing continual
(Brownian) motion in water. He maintains that the plasticity of clay
is due to this minute size, and this view has gained wide acceptance
in late works on the subject. But this assumption cannot be maintained
in the face of the fact that nothing like the adhesive plasticity of
clay can be attained even by the finest powders of other substances,
least of all by those having the closest mineralogical resemblance to
kaolinite, such as graphite and talc. Above all, the most persistent
trituration with water utterly falls to restore plasticity to clay once
baked so as to expel its water of hydration, although the fineness
of the particles is thereby not only not diminished, but actually
increased, by contraction in heating. No powders however fine can
replace the functions of clay in soils, _viz._ the maintenance of
floccules, and tilth dependent thereupon; and they distinctly impair
the plasticity of clay. The fine “slickens” of quartz mills merely
render soils containing them more close and impervious, and more
difficult to flocculate. Even gelatinous masses like hydrated ferric
and aluminic oxids fail to replace clay in its adhesive functions.

    _Separation of Colloidal Clay._—This property of
    the plastic clay substance, of diffusing in pure water,
    furnishes the means of separating from it the coarser, sandy
    and silty portions of soils and natural clays, and observing
    its characteristic properties, so far as the almost
    unavoidable admixture of some other substances, presently to
    be considered, permits.

    In natural soils the clay particles usually incrust the
    powdery ingredients, cementing them together; or themselves
    form complex aggregates (floccules) of large numbers
    of individual particles. These may be loosened from
    their adhesion or cohesion either by prolonged, gentle
    kneading of the wet clay, or by more or less prolonged
    digestion (soaking) in hot water, or more expeditiously,
    by lively boiling with water. The boiling should not,
    however, be prolonged beyond the time actually required
    for disintegration, since (as Osborne[22] has shown)
    long-protracted boiling tends to render the clay permanently
    less diffusible.

    From the turbid clay-water the diffused clay may be obtained
    either by evaporating the water (which as the bulk is very
    large, is usually inconvenient), or, more conveniently,
    by throwing it down from its suspension by the action of
    certain substances which possess the property of curdling
    (coagulating) the clay substance into flocculent masses
    that settle quickly. Of all known substances, lime, in the
    form of lime-water, acts most energetically in producing
    this change; but other solutions of lime, as well as most
    salts and mineral acids, produce the same effects when
    used in sufficient quantity. Common salt is among the most
    convenient, because it can most readily be leached out of
    the clay precipitate thus thrown down. This when white,
    resembles boiled starch, but being usually colored by iron
    might be easily mistaken for the mixed precipitate of ferric
    hydrate and alumina so commonly obtained by chemists in
    soil analysis. When separated from the water and dried,
    the jelly-like substance (“colloidal clay”) shrinks as
    extravagantly as would so much boiled starch, into hard,
    shiny crusts or flakes, which when struck in mass are
    sometimes even resonant, and bear more resemblance to glue
    than to the clay of everyday life. Like glue, too, but much
    more quickly and tenaciously, the dried colloidal clay
    adheres to the tongue, so as to render the separation
    painful; when wetted it quickly bulges with great energy,
    and in a short time resumes its former jelly-like condition.
    When moistened with less water it assumes a highly plastic
    and adhesive condition, so that it is difficult to handle
    and almost as sure to soil the operator’s hands as so much
    pitch.

[22] Rep. Conn. Agr. Expt. Stn., 1886, 1887.

_Effects of Alkali Carbonates upon Clay._—The carbonate of potash and
soda, when in very dilute solution (.01 to .05%) exert upon diffused
clay an effect the reverse of the acids and neutral salts. They destroy
the flocculent aggregates formed by precipitation with these, or
naturally existing in the soil, and tend to puddle the clay so as to
render it impervious to water. It is thus that in the alkali lands of
the arid regions we often find the soil or subsoil consolidated into
a very refractory “hardpan,” difficult to break even with a sledge
hammer and impossible to reduce to tilth until the alkali carbonate is
destroyed by means of a lime salt, such as gypsum. (See chapt. 23).
Ammonia water also helps to cause the diffusion of clay in water, but
its effect of course disappears upon drying. It is probable that this
property of sodic carbonate can be utilized in rendering earth dams
firmer and more secure against the penetration of water.




CHAPTER V.

THE MINOR MINERALINGREDIENTS OF SOILS; MINERAL FERTILIZERS; MINERALS
INJURIOUS TO AGRICULTURE.


(A.) MINERALS USED AS FERTILIZERS.

Of minerals important in soil-formation, not usually present in large
amounts in rocks, but extensively used in fertilization, the following
require mention:

_Apatite_; phosphate of lime containing more or less of the chlorids
and fluorids of the same metal; the mineral from which the phosphoric
acid of the soil is mostly derived. In the crystallized condition when
perfectly pure it is colorless; but it is mostly of a greenish tint
(hence “asparagus stone”). The pure crystalline mineral rarely occurs
in large masses (as in Canada); but small to minute crystals are found
widely disseminated in many rocks (granites, “basalts” of the Pacific
Northwest), thus passing into the soils formed from these rocks. These
crystals are readily recognized, being regular six-sided prisms with a
flat or obtusely pyramidal termination (distinction from quartz), and
do not effervesce with acids (distinction from calcite). By far the
largest deposits of this mineral occur in connection with carbonate of
lime, in the rock materials known as _phosphorites_. Lime phosphate
being, like the carbonate, soluble in carbonated water, the two
naturally frequently pass into solution, and are subsequently deposited
together. Most limestones contain a small proportion of lime phosphate,
being, as already stated, formed from the shells and the framework of
animal organisms usually containing also phosphates. But the content of
phosphates in limestones is not readily apparent to the eye, and the
richest deposits, save such as contain animal bones, have long passed
unsuspected as to their being anything else but limestone. Systematic
search has now revealed the presence of phosphate rock in numerous
localities, chiefly where limestone formations occur. In the United
States, in South Carolina, Florida, Alabama, Tennessee, Kentucky,
Nevada; in South America, on Curaçoa island, Venezuela; in the Antilles
on Sombrero, St. Martins and Navassa islands. In Africa, in Algiers and
Tunisia; in Europe, in Spain (Estremadura, one of the first deposits
known), France, Belgium and the adjacent parts of Germany; in Bohemia
and Galicia in Austria; and very extendedly in European Russia. Many
islands of Oceanica supply phosphorites derived from the decomposition
of bird guano by the coral limestone.

    Unfortunately the percentage of phosphate in a large
    proportion of these materials is not sufficiently high to
    make their conversion into water-soluble superphosphate
    economically possible at the present time; since all the
    calcic carbonate present must also be converted into
    comparatively worthless sulphate (gypsum) by the use of
    sulfuric acid; and as yet no practicable method for avoiding
    this difficulty has been found.

    “_Thomas Slag._”—Probably the nearest approach to such
    a method is indicated by the fact that a compound containing
    four instead of three molecules of lime to one of P₂O₅, such
    as is contained in the “Thomas slag” of the basic process of
    steel manufacture, is nearly or in some cases (“sour” soils)
    _quite_ as effective for the nutrition of plants as the
    water-soluble superphosphate. This discovery has rendered
    available for agricultural use the phosphoric acid contained
    in the enormous deposits of limonite iron ore known as bog
    ore, which contains a large proportion of ferric phosphate
    and from that cause has until lately been excluded from the
    manufacture of wrought iron and steel. It is reasonable
    to hope that by some analogous process the low-grade
    phosphorites, such as those of Nevada and the plains of
    Russia, will also in the course of time become available
    for agricultural use. Extremely fine grinding and washing
    (producing “floats”) has been resorted to for the purpose of
    rendering the raw phosphorites effective in fertilization.
    But while this is successful on some soils, on others the
    “floats” remain almost inert; so that this method has found
    only limited acceptance.

_Animal bones_, which consist of from 24 to 30% of animal substance and
70 to 76% of “bone earth,” (or when fossil are free from the former),
are largely used for the manufacture of superphosphate. The bone-earth
consists in the main of tri-calcic phosphate with from one to two per
cent of calcium fluorid (much as in natural apatite), a small amount of
magnesic phosphate, and about 4 to 6% of calcic carbonate. Bone meal
can therefore supply to plants both phosphoric acid and nitrogen, and
the presence of the latter has been largely the cause of a material
overestimate of its efficacy as a fertilizer in the past. Wagner’s and
Maerker’s experiments have shown that at least in sandy soils poor in
humus, it cannot be considered an adequate source of phosphoric acid
for annual crops, and that in these soils its immediate effects are
almost wholly due to its nitrogen-content. The slow availability of
the phosphoric acid renders it unprofitable as a source of the latter,
outside of the heavier lands with abundance of humus; in “sour” lands
(notably on meadows) bone meal produces its best results. In soils
naturally calcareous, or in such as have received heavy dressings of
lime either as carbonate or in the caustic condition, the manurial
effects of bone meal are seriously diminished. Nagaoka (Bull. Coll.
Agr. Tokyo, Vol. 6, No. 3) shows that the crop of rice fertilized
with bone meal was reduced to less than half when limed, and that the
phosphoric acid taken up by the crop was reduced to one-sixth. In any
case it is most important that bone meal should be as finely ground as
possible, as in the case of the phosphorites; and this can best be done
when it has first been freed from fats by boiling with water, and then
steamed under pressure. It can then also be most readily converted into
superphosphate.

The phosphate minerals and the fertilizers manufactured therefrom are
of primary importance to agriculture. The phosphoric-acid content of
soils is mostly very small, and only a fraction of it is usually in
an immediately available form. Hence for permanent productiveness,
and especially for intensive farming or gardening, a cheap supply of
phosphate fertilizers is of first importance in all soils and climates.

Other phosphate minerals occur frequently, but as a rule only in small
amounts, in connection with the ores of most metals. The only ones
of these of interest to agriculture are _Vivianite_ and _Dufrenite_,
the phosphates respectively of the protoxid and peroxid of iron. The
former occurs in mineral deposits as small blue crystals, or more
frequently as blue earthy masses or streaks, in the substrata of rich
alluvial ground (Louisiana, California). Dufrenite sometimes results
directly from the oxidation of the protoxid mineral, which then turns
greenish and finally brown. Unfortunately these minerals, rich as
they are in phosphoric acid, cannot readily be utilized as sources of
phosphate fertilizers, because of the difficulty of getting rid of the
iron. Their occurrence usually suggests the presence of abundance of
phosphoric acid in the soil. But that which is actually combined with
the iron oxids is practically unavailable to plants; especially so in
the case of the peroxid compound, the formation of which is a common
source of loss of phosphoric acid when soils rich in iron are submerged
for any length of time; a point which is discussed below (chapt. 13).

Among the iron phosphate minerals, may also be mentioned “bog ore,”
which results from the reductive maceration of swamped ferruginous
soils, and accumulates in the subsoils and in the bottom of swamps or
moors, forming “moorbedpan”; a dark brown, rather soft mass, which is
sometimes used as an iron ore, especially since the invention of the
“basic process” of iron smelting, one of the products of which is the
phosphate or Thomas slag. (See above).

_Nitrate of Soda or Chile saltpeter._—This mineral being (like all
nitrates) easily soluble in water, can only occur in regions nearly
or quite destitute of rainfall. Such is the case in the Plateau of
Tarapacà in Northern Chile, where it occurs in large quantities; it
is likewise found, but to much smaller extent, in Nevada, southern
California, Egypt and India. By far its most extended occurrence is
that in Chile, where, together with common salt, it fills cavities and
crevices in a gravelly clay that forms the surface of a plateau from
three to six thousand feet above the sea. It is never pure, but always
mingled with a large proportion (up to 50% and over) of common salt;
also some Glauber’s salt (sulfate of soda) and some sodic perchlorate
and iodid; hence it forms an important commercial source of iodine.

    The mixed mineral mass, called “Caliche,” when taken out
    of the ground is dissolved in water; and the solution
    boiled down, during which process the common salt is first
    deposited and is raked out of the pans; the nitrate is
    afterward farther purified by crystallization. As brought
    into commerce for agricultural purposes it constitutes a
    moist gray saline mass, somewhat resembling common salt,
    of which substance it usually contains a few per cent;
    occasionally also a small amount of sodic perchlorate (which
    acts injuriously on vegetation). Aside from its use as a
    fertilizer, Chile saltpeter serves for the manufacture
    of nitric acid; and either directly, or after previous
    transformation into potassic nitrate, for that of gunpowder.

    The Chilean locality is the only one from which the
    commercial article is derived; the deposits elsewhere are
    too limited in extent to compete commercially with the South
    American product. Caliche ranging as high as 80% of nitrate
    of soda has been sent to the writer from the Colorado
    Desert in Southern California, but the exact locality of
    occurrence has not been divulged. Extended areas of clay
    hills impregnated with nitrates exist in the Death Valley
    region of California, but in the absence or extreme scarcity
    of water in that region, it is doubtful whether these
    impregnations can be made practically available. Another
    locality is that near White Plains, Nevada, where Caliche
    averaging about 50% purity is found in cavities and crevices
    of a reddish volcanic rock. The rainfall in this region is
    so slight that the greater part of the dust or sand blown
    about by the wind consists of Glauber’s salt. Here also,
    as in Chile, the niter deposits appear to be restricted to
    within a short distance from the surface, and the total
    amount thus far observed appears to be insufficient to
    encourage large-scale exploitation.

    _Origin of Nitrate Deposits._—The probable origin
    of these niter deposits has given rise to a great deal of
    discussion, and a wide difference of opinion exists as
    to the source from which the nitrogen may reasonably be
    supposed to have been derived. According to the present
    state of our knowledge, it must be presumed that its
    sources have been organic, and that the niter has been
    produced by the activity of the same bacteria which now
    produce nitrates in our soils, rendering the nitrogen of
    humus available to plants. But it is by no means clear what
    that organic material could have been; for at the present
    time the plateau of Tarapacà is almost wholly destitute of
    vegetation, if not of animal life. The latest and apparently
    most reasonable suggestion is that of _Kuntze_, who
    calls attention to the fact that the vicuñas and llamas
    which are at home in this portion of the Andes, and are
    known to have roamed over that region in countless herds,
    have the curious habit of always depositing their manure in
    one and the same place whenever at liberty. Each herd
    of these animals has its definite dunging place at some
    convenient point. That such herds have existed in the region
    from time immemorial is obvious from historical as well as
    collateral evidence; and as their manure accumulated, its
    nitrification would progress rapidly under the prevailing
    arid conditions. The common salt would naturally be derived
    from the urine and excrements, and the alkaline salts
    which exist throughout this region as the products of soil
    decomposition, would be quite sufficient to account for
    the alkaline bases in the caliche. On the other hand, the
    presence of iodine points to seaweeds as the organic source.

_Intensity of Nitrification in Arid Climates._—Of the efficacy of
nitrification under arid conditions abundant evidence may be found
within the State of California. In the alkali lands of southern
California the nitrates of soda, lime and magnesia are almost
universally present; they form at times as much as one-fifth and even
more of the entire mass of alkali salts, and in one case the total
amount in the soil has been found to reach two tons per acre, with
an average of twelve hundred pounds over ten acres. In the plains of
the San Joaquin Valley, spots strongly impregnated with niter are
found, especially under the shadows of isolated oak trees, where the
cattle have been in the habit of congregating for a long time; a case
quite analogous to that supposed by _Kuntze_ to exist in the Chilean
locality. Of course it is only in arid climates that the accumulation
of nitrates can usually occur; for in the region of summer rains the
nitrates formed during the warm season will inevitably be washed into
the subdrainage, unless restrained by absorption by the roots of
vegetation. The heavy losses occasionally occurring from this cause in
the course of a rainy winter on summer-fallowed land have been amply
demonstrated by many investigations.

POTASH MINERALS.—By far the most abundant occurrence of potash in the
earth’s crust is that in silicates and notably in orthoclase or potash
feldspar, which contributes so largely to soil-formation. But in the
absence of any economically successful artificial method for producing
potash compounds from feldspars on a commercial scale, almost the
entire supply of potash salts was, until a comparatively late period,
derived from plant ashes, viz., the “potashes” of commerce. At the
same time, almost the entire demand for alkalies for industrial uses
bore upon the same product, until the invention, toward the end of the
last century, of _LeBlanc’s_ process for the manufacture of soda from
common salt; for until that time, soda in the various forms in which
it was imported from the Orient or prepared from seaweed ashes, was a
comparatively costly product. LeBlanc’s invention was most timely in
that it very quickly diminished materially the production of potashes
which, in view of the increased demand for alkalies for industrial
uses, seriously threatened the depletion of agricultural lands, and of
woodlands as well, of one of its most essential ingredients. Yet as
there are many industrial uses in which soda cannot replace potash,
the manufacture of potashes continued to a greater or less extent, as
no other available source except the ashes of land plants, was then
known. The production of potassic chlorid from the mother-waters of sea
salt in the spontaneous evaporation of sea water for the manufacture
of common salt, was on too small a scale to influence materially the
manufacture of potashes.

_Discovery of Stassfurt Salts._—The depletion of potash had become so
serious a matter in the agricultural lands of Europe, that for a time
much research was bestowed, and prizes offered for an economical method
of producing potash salts from feldspar, on a commercial scale. But
the problem had not been satisfactory solved when, in the year 1860,
attention was called to the fact that the saline deposits overlying
certain large rock-salt beds that had been developed by borings near
Stassfurt in Prussia, contained so large a proportion of potash
salts, as to render their purification and conversion into fairly
pure sulphate and chlorid technically feasible. The impulse having
been given, the potash industry developed rapidly in that region as
well as in the adjacent portions of Saxony, where the same formation
underlies; the production of “Stassfurt Salts” rapidly assumed a
greater development than that of the rock-salt which had originally
prompted the enterprise, and numerous additional boreholes demonstrated
an unexpectedly wide extension of the same beds. At the present time,
in consequence of such development, the manufacture of potashes from
plant ash has almost ceased, outside of Canada and Hungary; and the
production of potash salts in the Stassfurt region now supplies the
demand of the entire world, both for industrial and agricultural
purposes.

The cheapening of potash as a fertilizer has rendered possible the
profitable cultivation of large areas of land which were naturally
too poor in that substance for ordinary cultures; and has likewise
rendered possible the restoration to general culture of lands that
had ceased to produce adequately, on account of the depletion caused
by long-continued cropping. It has likewise served to intensify
agricultural production wherever desired; and between this supply and
that of phosphoric acid from the phosphorites (see above), and the
discovery of the nitrogen-absorbing power of leguminous plants, which
can be used for green-manuring, farmers have been enabled to dispense,
in many regions, with the production and use of stable-manure, which
until then had been considered an indispensable adjunct to agriculture
everywhere. Even within the last fifty years it was proclaimed by high
authority in Germany that stable-manure constituted, as it were, the
farmer’s raw material, from which he manufactured the various products
of the field through the intervention of the plant-producing power of
the soil.

    _Origin of the Potash Deposits._—The manner in which
    this accumulation of potash salts has been formed deserves
    explanation. It is abundantly evident that nearly all
    deposits of rock-salt thus far known have been formed by the
    evaporation of sea-water at times when bays or arms of the
    sea were cut off from open communication with the ocean.
    The composition of sea-water has already been given and
    discussed (chap. 2, p. 26); and by the slow evaporation of
    sea-water on a small scale we can quite successfully imitate
    the phenomena observed in natural rock-salt deposits. When
    sea-water is heated a slight deposit of lime carbonate
    (usually containing a little ferric oxid and silica) is soon
    formed; and a corresponding thin deposit of ferruginous
    limestone is commonly found at the base of rock-salt-bearing
    deposits. Next above this we almost invariably find a
    deposit of gypsum, sometimes of great thickness; in the
    artificial evaporation of sea-water the same thing occurs
    so soon as the brine has reached a certain degree of
    concentration. It constitutes the major portion of the
    “panstone” of salt-boilers. Next above follows a deposit of
    rock-salt, at base somewhat mixed with gypsum; its thickness
    varies greatly according to circumstances. Above it lie the
    potash salts.

    In the manufacture of sea-salt by evaporation in shore
    lagoons or “saltpans,” the solution remaining after the salt
    has been deposited (known as “mother-waters,” or “bittern”),
    of course remains on the surface of the salt unless allowed
    to drain off, as is done in the process of manufacture. When
    not drained off, the water gradually evaporates, and there
    remains a saline crust of a composition exactly resembling
    that of the upper layers at Stassfurt, containing a large
    proportion of potash salts.

    If it be asked why the Stassfurt salts are not found
    overlying every rock-salt deposit in the world, the answer
    is that in a great many cases the concentrated mother-waters
    have had an opportunity to flow off from the surface of the
    rock-salt by the action of tides, the inflow of fresh water
    from the land or from other causes. Their presence therefore
    depends upon the fulfilment of accidental conditions not
    nearly always realized in the natural evaporation of
    sea-water, but which happened to occur on a very large scale
    in that portion of the North-European continent.

    _Nature of the Salts._—The potash is present in
    the Stassfurt salts in the form of complex sulfates and
    chlorids containing, besides, sodium, calcium and magnesium
    in various proportions and modes of combination. The most
    abundant of the potassic chlorid minerals is carnallite, a
    hydrous chlorid of potassium and magnesium. The chlorids
    characterize chiefly the upper portions of the deposit, the
    sulfates the lower.

_Kainit._—Of the products derived from the Stassfurt salt industry
for agricultural use, the two requiring special consideration are
“kainit,” a natural mixture of the several chlorid minerals in varying
proportions; and “high-grade sulfate.” Being a natural product,
“kainit” is the cheapest source of potash available to the farmer;
but on account of its variability in composition it must be sold
and purchased on guaranteed assay. On account of its large content
of chlorin it is not desirable in the production of certain crops,
especially in the arid region, where alkali soils, and even those not
visibly alkaline, often contain already large amounts of chlorin.
Moreover, kainit usually contains a considerable proportion of
common salt. For the arid region therefore the sulfate is generally
preferable, although it is somewhat higher in price for the same
amount of potash. The potash content of commercial kainit (calculated
as K₂O) ranges from 16 to 35%, while the sulphate frequently ranges
from 80 up to 95% of the pure sulfate; thus costing materially less in
freight charges than the lower-grade kainit. Its potash content ranges
from 43 to over 50% of K₂O.

_Potash Salts in Alkali Soils._—The sulfates and chlorids of potassium,
however, occur not only in connection with rock-salt deposits, but are
also found in the alkali soils of the arid region. They are, in fact,
never absent where such salts occur at all, and their percentage in the
total of salts ranges all the way from about 4 to as much as 20% of
potash sulphate. In numerous cases it has been found that the content
of this salt to the depth of four feet amounts to from 1200 to 1500
pounds per acre. In such lands, of course, additional fertilization
with potash salts is totally uncalled for, the more as such soils
invariably contain, besides the water-soluble potash, an unusually
large percentage of the same in the form of easily decomposable
silicates, or zeolites.

_Farmyard or Stable Manure._—In connection with the subject of mineral
fertilizers, it will be proper to discuss briefly the uses and special
merits of stable manure, composts, etc. Up to within the last century,
these were practically the only fertilizers known and used, and the
exclusive use of this manure might have continued indefinitely but for
the discovery that as time progressed, stable manure and with it grain
crops, for the production of which it was necessary, became less and
less in amount, so as to threaten bread famines. The cause of this
diminution was, of course, the incompleteness of the return of the
soil-ingredients taken off by the crops, when these were exported to
feed the cities or foreign countries. Thus the attention of chemists,
and notably that of Liebig, was attracted to the solution of the
problem of keeping up production even with an insufficient supply of
stable manure; and the discovery of the use of mineral fertilizers was
the result of their activity.

The chemical composition of stable manure does not, alone, suffice to
explain its remarkable efficacy and the difficulty of replacing it
by any other material. The composition of manure of course differs
not only with different animals but also with the different feeds
consumed by them; but the average composition of farmyard manure is
approximately given thus by Wolff and others:

                ANALYSES OF VARIOUS FARMYARD MANURES.

    ==========================================================
                    |   1.  |   2.  |   3.  |    4.    |   5.
                    |       |       |       |          |
    Water           | 71.00 | 75.00 | 79.00 |  79.95   | 72.33
    Dry Matter      | 29.00 | 25.00 | 21.00 |  20.05   | 27.67
    Ash ingredients |  4.40 |  5.80 |  6.50 |          |  5.87
    Potash          |  0.52 |  0.63 |  0.50 |  0.84[23]|  0.69
    Lime            |  0.57 |  0.70 |  0.88 |          |  0.85
    Magnesia        |  0.14 |  0.18 |  0.18 |          |  0.14
    Phosphoric acid |  0.21 |  0.26 |  0.30 |   0.40   |  0.30
    Ammonia         |       |       |       |          |  0.02
    Total Nitrogen  |  0.45 |  0.50 |  0.58 |   0.78   |  0.46
    ----------------+-------+-------+-------+----------+-------

    1. Average composition of fresh farm manure (Wolff).

    2. Average composition of moderately rotted farm manure (Wolff).

    3. Average composition of very thoroughly rotted farm manure
       (Wolff).

    4. Mixed cow and horse manure from a bed two feet thick, accumulated
       during the winter in a large covered yard, and packed solid by
       the tramping of cattle (The analysis by F. E. Furry).

    5. “Box Manure,” consisting of mixed manure of bullocks, horses, and
       pigs (Way, Royal Agric. Soc. Journ., 1850, II., 769).

[23] And soda.

It is thus seen that the percentage of the important plant-foods in
stable manure are minute when compared with those commonly found in
“commercial” fertilizers. Nor are they so much more available for plant
absorption than the latter; a very large proportion is not utilized
at all the first year, and unless the amount applied is very large it
hardly carries the supply needed for the usual crops.

It is now well understood that its efficacy is largely due to the
important physical effects it produces in the soil. It helps directly
to render heavy clay soils more loose and readily tillable. If well
“rotted” or cured it also serves to render sandy, leachy soils more
retentive of moisture; and the humus formed in its progressive decay
imparts to all soils the highly important qualities discussed later on
(chapt. 8). More than this, the later researches have shown that stable
manure acts perhaps most immediately upon the bacterial activity in the
soil, greatly increasing it not only directly by the vast numbers of
these organisms it brings with it, but also in supplying appropriate
food for those normally existing in the soil (see chapt. 9). In
so doing it serves indirectly to render the soil ingredients more
available, and to impart to the soil the loose condition required in a
good seed-bed—a “tilth” which cannot be brought about by the operations
of tillage alone.

The only possible substitute for the use of stable manure is found
in green-manuring with leguminous crops conjointly with the use of
commercial or mineral fertilizers. Unless this is done the use of the
latter, alone, ultimately leads to a depletion of humus substances,
which renders the acquisition of proper tilth by the seed-bed
impossible, and causes a compacting of the surface soil which no
tillage can remedy.

_Proper method of using stable manure in humid and arid climates._—In
the humid region it is a common practice to spread the stable manure
on the surface of the fields and leave it there without any special
operation to put it into the soil; trusting to the rains, earthworms
and subsequent tillage for its being brought into adequate contact
with the roots; it is rarely plowed in. In the arid region this
mode of using it is impracticable; it would remain on the surface
indefinitely without advancing in its decay because of the dryness, and
unless plowed in very deep the ordinary, strawy manure would ruin the
seed-bed by rendering it too pervious to the dry air, thus preventing
germination. Much of this valuable material has therefore been, and to
some extent is still being burnt, thus causing a severe depletion of
the land, both of humus and of mineral plant-food. The best way to deal
with stable manure in the arid regions is to thoroughly rot or cure it
before putting it on the land, and then plowing it in. To do this of
course it must be put in piles and wetted regularly; a procedure which
at the high prices of labor is thought to be too expensive, but which
in the end would be found eminently profitable, unless green-manuring
is regularly done. The very small proportion of humus generally present
in arid soils renders this precaution indispensable, if production and
proper tilth is to be maintained. The saving of stable manure and of
all composting material, even if less needful as a means of supplying
plant-food in the rich soils of the arid regions, is fully as essential
in order to maintain the humus supply.


(B.) MINERALS UNESSENTIAL OR INJURIOUS TO SOILS.

The minerals heretofore mentioned contribute to soil formation either
one or several ingredients, important to plant growth either by
their mechanical or chemical action. It remains to consider some not
intrinsically desirable, but frequently present in certain soils,
which should be known to the farmer in order that he may be enabled to
counteract or remove their injurious effects. Leaving aside such as are
of only casual or rare occurrence, the following may be mentioned as
among those which not unfrequently affect soils desirable for culture
to such extent as to make them unavailable for general farming purposes:

_Iron Pyrite_; sulphid of iron containing two molecules of sulphur
to one of iron; a mineral exceedingly common in deposits of metallic
ores, and whose deceptive gold-like color has caused it to be mistaken
for gold so often as to cause it to be designated as “fool’s gold”
among miners. While it frequently does contain some gold and is often
associated with valuable ores, it is practically valueless when
occurring outside of mineral veins, in rock-masses; and more especially
in sedimentary rocks, such as sandstones, limestones, shales and clays.

When present in soils it sometimes becomes a source of trouble to the
farmer, because in contact with air it is soon transformed into ferrous
sulfate or copperas, which, like the carbonate referred to above, is
injurious to plants. Sometimes indeed iron pyrite is actually _formed_
in badly-drained soils alongside of the carbonate of iron, when much
sulfate (such as gypsum) is present; and then its injurious effects
subside more slowly than do those of the carbonate (see above, p. 46).

    _Recognition of Iron pyrite._—The mineral is easily
    recognized by its golden or brass-yellow tint; the latter
    color being the one most commonly shown in the “sulphur
    balls” occurring in marls or soft limestones. A very easy
    test is to pulverize it and then heat it on a shovel over
    a fire, when it will soon itself take fire, burning with a
    blue sulphur flame, and upon more complete roasting, leaving
    behind a red powder, viz., “Venetian red” or red ochre;
    that is, ferric oxid. In clays it commonly occurs in large,
    well-defined cubes, which do not readily form copperas but
    rather become covered with a crust of limonite or brown iron
    ore.

When a subsoil is found to contain pyrite, or when “sulfur balls”
have been accidentally introduced with dressings of marl, the remedy
is thorough and persistent aeration of the material. In the case of
marls nothing more need be done; but in that of ill-drained subsoils
it is best to add lime in moderate dressings, to accelerate the
transformation into ferric hydrate or iron rust, and gypsum; whereby
the copperas becomes not only innocuous but adds two beneficial
ingredients to the soil. The same policy will render available manure
or other materials which have been disinfected by means of solution of
copperas.

_Halite_ (rock-salt), or common salt, has already been mentioned as
to its occurrence in connection with the Stassfurt potash salts (see
above, page 71); but as rock-salt it rarely exerts any injurious
influence upon lands. It is, however, a common ingredient of seashore
lands, and is also present to a certain extent in the alkali lands
of the arid countries. While it is true that occasionally small
quantities of common salt are used as an ingredient in fertilization,
its usefulness in that direction is exceedingly subordinate; and it
is far more generally to be considered as an injurious ingredient of
all cultivatable soils whenever present to a larger extent than a few
hundredths of one per cent It is usually considered that one-fourth
of one per cent of common salt renders lands unfit for most culture
plants. Only a few, such as asparagus, the beet, the saltbushes and
some others, succeed when it is present in this or in larger amounts.
In the case of sea water it is usually accompanied by a still more
injurious ingredient, magnesic chlorid or bittern; which is detrimental
to plant growth in much smaller quantities than the common salt itself.

    _Recognition of Common Salt._—The presence of common
    salt may, as a rule, be detected by the taste, well-known
    to every one; when this taste is very intense or somewhat
    bitterish, it indicates the presence of bittern. The
    presence of salt, however, is easily verified without the
    use of chemical reagents, by slowly evaporating some of
    the clear water leached from the soil in a clean silver
    spoon. If the last few drops are allowed to evaporate
    spontaneously, it will be easy to distinguish, even with
    the unaided eye, the square, cubical crystals, sometimes
    combined into cross-shape, which are characteristic of
    common salt. It is always an unwelcome addition to the land,
    and as its action cannot be neutralized in any way, it can
    be gotten rid of only by leaching-out. This process is
    usually accomplished in seashore lands by the action of
    rain, or by the overflow of fresh-water streams, after the
    tide has been excluded by means of drains provided with
    check-valves to prevent the inflow of tidewater; or else by
    underdrainage, and flooding when possible.

_Mirabilite_, (Glauber’s salt) or sulfate of soda, exists not
unfrequently in the soils of the arid region and sometimes encrusts
extended areas of lowlands during the dry season. When present in the
soil it will commonly be seen blooming out on the surface after a rain,
in light, feathery, needle-shaped crystals, sometimes to such an extent
that it can be collected by the handful. Subsequently, when wafted
by the wind, it is reduced to a fine white dust, which constitutes a
goodly proportion and sometimes the entire mass of the “alkali dust”
that is so annoying on the plains of Nevada, and in the desert regions
generally, during the hot summer. Near White Plains, Nevada, it forms
a thick layer of “_white sand_,” in which the foot sinks deeply, and
which is carried about by the wind with great ease.

Glauber’s salt is never a desirable soil-ingredient. It is largely
produced as a by-product in several industries, but cannot be utilized
for agricultural purposes to any extent. It is, however, much less
injurious to plant growth than common salt; according to experience
in California it may be considered about three times less so. It
constitutes the major portion of what is commonly known as “white
alkali,” which is well known to be much less injurious to crops than
the “black” kind, which contains carbonate of soda.

_Trona and Urao_ are natural forms of carbonate of soda or salsoda.
Like Glauber’s salt, it commonly occurs as a surface efflorescence or
crust in dry or desert regions; either from the evaporation of standing
water, as in the case of the soda lakes of Nevada, Hungary and Egypt,
or as an efflorescence on the surface of the soil, as in the western
United States, Mexico (“urao”), North Africa (“trona”), and at many
points in the Old Continent. In the United States it is commonly known
as “black alkali,” because of the black spots formed on the surface by
evaporation; practically the same name (“kara”) is given it in Arabia
and Asia Minor, whence impure soda has long been imported into Europe;
while in north India it forms part of the “reh” salts that incrust
large areas (usar lands) in the Indo-Gangetic plain.

                                                                 The
    natural mineral always contains an excess of carbonic acid over
    the “normal” salt, nearly in the proportion of four parts of
    carbonic dioxid to three of soda; it is sometimes designated as
    sesqui-carbonate. In hot sunshine it may lose most of this excess
    for a time; while within the soil itself it may, in presence of
    abundant carbonic acid, become temporarily converted wholly into
    hydrocarbonate or “bicarbonate,” which is less corrosive than the
    monocarbonate or common salsoda.

_Injury caused in soils._—Like common and Glauber’s salt, carbonate
of soda is always an unwelcome soil ingredient; more so, in fact,
than either of the other two, since less than a tenth of one per
cent is sufficient to render certain soils wholly untillable, by the
deflocculation or puddling of the clay; at the same time rendering it
impervious to water. It is by far the most injurious ingredient that
ordinarily occurs in otherwise good, arable soils; for in addition to
the physical effect just mentioned, it dissolves the humus-substance
of the soil, forming an inky-black solution which, especially when
evaporating on the surface and forming black spots, has given rise to
the popular name of “black alkali.” As will be more fully explained
hereafter, wherever such is the case, the first step necessary toward
reclamation is the transformation of the carbonate of soda, at least in
part, into the relatively innocuous sulfate, by means of gypsum in the
presence of water; while carbonate of lime remains in the soil.

In its direct action on the plants themselves, soda is also most
injurious; as when accumulated to any extent near the surface by
evaporation it will corrode the root-crown or stem, and sometimes
completely girdle the same, destroying the bark. Farther details on
this subject are given in chapter 22.

_Epsomite_, or Epsom salt, or sulfate of magnesia, is another one
of the water-soluble minerals frequently found efflorescent on the
surface of the ground; more commonly in saline seashore lands than
in the alkali region proper, although it is rather common in the
northeastern portion of the arid region of the United States. Whether
on the soil surface or in the crevices of rocks, its needle-shaped,
feathery crystals greatly resemble those of Glauber’s salt, but are
readily distinguished by the more intensely bitter taste. Epsom salt
is frequently the last remnant of sea-salts left in the soil after
reclamation. Though probably somewhat more injurious to plant growth
than Glauber’s salt, the mineral Kieserite, one of the Stassfurt
salts and consisting essentially of Epsom salt, is sometimes used as
an application to calcareous lands instead of gypsum, and with good
results. Yet gypsum is usually the safer, and equally effective.

_Borax_ (bi-borate of soda) occurs much more rarely than the salts just
described; most frequently in certain portions of California, forming
part of the “alkali” in the soil. It is injurious to plant growth, but
is as readily dealt with as is the carbonate of soda, by dressings of
gypsum, whereby inert borate of lime is produced.

It is hardly necessary to say that saline waters containing any of
the above salts in notable amounts must be used for irrigation very
cautiously. The measures to be observed in this respect will be
discussed later.




PART SECOND.

PHYSICS OF SOILS.




CHAPTER VI.

PHYSICAL COMPOSITION OF THE SOILS.


As has already been stated (chapt. 1, p. 10), the general physical
constituents of soils are _rock powder or sand_ and _silt_, more or
less decomposed according to the nature of the original rocks; _clay_,
the product of the decomposition of feldspars and some other silicates;
_humus_, the complex product of the decomposition of vegetable and
animal matters on and in the soil mass; as well as vegetable matter
not yet humified. Each of these several constituents must now be
considered more in detail. Since clay is the substance whose functions
and quantitative proportions influence most strikingly the agricultural
qualities of land, it should be first discussed.


_Clay as a Soil Ingredient._

The plasticity and adhesiveness of clay, together with the extreme
fineness of its ultimate particles (said to reach the 1-25000 of an
inch), explains its great importance as a physical soil ingredient.
It serves to hold together and impart stability to the flocculent
aggregates of soil particles that compose a well-tilled soil; for
without clay the sand would collapse into close-packed single grains
so soon as dried, and loose tilth would be impossible. Sand drifts
illustrate this condition.

On the other hand, the fineness of the particles serves to render clay
very retentive of moisture as well as of gases and of solids dissolved
in water, imparting these important properties to soils containing it;
while coarse sandy soils are oftentimes so deficient in them as to
render them unadapted to any useful culture, despite the presence of an
adequate supply of plant-food.

When to these essential physical properties of clay, there is added
the fact that usually the clay-substance as it exists in soils
contains the most finely pulverized and most highly decomposed
portions of the other soil-minerals, and therefore the main part of
the available mineral plant-food, it is easy to understand why soils
containing a good supply of clay should be called and considered
“strong” land by the farmers of all countries. “Poor” clay soils are
exceptional; but sometimes the clay content reaches such a figure that
the difficulties of tillage render them too uncertain of production for
profitable occupation.

_Amount of Colloidal Clay in Soils._—Any and all of the kinds of clay
mentioned (p. 57) as occurring naturally may, of course, enter into and
form part of soils. But as the amount of true, plastic clay substance
contained in them is very indefinite, it becomes necessary, in order
to classify soils in respect to their tillableness, to ascertain more
definitely the amount of pure, or nearly pure, colloidal clay substance
contained in the several classes of soils ordinarily recognized and
mentioned in farming practice. That this determination can at best be
only approximate, is obvious from the fact mentioned above (chapt. 4,
p. 59), that pure kaolinite itself is not plastic, and only becomes
so by the indefinite comminution and hydration it experiences in the
processes of soil-formation. As the progress of this process is also
indefinite, the same soil containing particles ranging from the finest
to the chalky scales of pure kaolinite, the drawing of a line must be
more or less arbitrary and empirical.

    From numerous experiments and comparisons made, the writer
    has been led to place the limits of “plastic clay” at and
    below such grain sizes as will remain suspended (afloat) in
    a water column eight inches high, during 24 hours. To go
    beyond this point in the examination of soils for practical
    purposes, would render such examinations so laborious and
    hence so rare, that this kind of work would be practically
    excluded from ordinary practice. According to this view
    the following percentages of such “clay” correspond
    approximately to the designations placed opposite:

        Very sandy soils           .5 to  3% clay
        Ordinary sandy lands      3.0 to 10%  “
        Sandy loams              10.0 to 15%  “
        Clay loams               15.0 to 25%  “
        Clay soils               25.0 to 35%  “
        Heavy clay soils         35.0 to 45% and over.

    It must be distinctly understood, however, that these
    figures make no claim to accuracy or invariability. For,
    the tilling qualities of a soil containing one and the
    same amount of such “clay” may be very materially modified
    according to the kind and amount of each of the several
    grain-sizes of rock powder or sand they contain.

_Influence of fine powders on plasticity and adhesiveness._—An
admixture of a large amount of fine powders diminishes materially the
adhesiveness of a clay soil, even though it may render it even more
“heavy” in tillage; while the admixture of coarse sand, even in very
considerable proportions, does not greatly influence the adhesiveness
of the clay. The latter alone cannot therefore serve as a proper guide
or basis for the classification of soils in respect to tillage; we
must also take into consideration the nature and amount of the several
granular sediments mixed with it.

Moreover, the nature and especially the adhesiveness of the clay
substance as obtained by analysis may vary considerably in the presence
of a very large amount of the finest grain-sizes; among which ferric
hydrate or iron rust is especially apt to accumulate predominantly
in the clay, considerably increasing its apparent weight and greatly
diminishing its adhesiveness.[24] In strongly ferruginous soils,
therefore, it becomes necessary to take into special consideration the
amount of the ferric hydrate or rust which accumulates in the clay
substance. The presence of large amounts of humus or vegetable mold
also influences materially the adhesiveness and physical properties
of the clay obtained by the method described, although most of it
remains with the finer powdery sediments or grain-sizes. There are,
besides, other colloidal or at least amorphous substances present in
all soils, such as silicic, aluminic and zeolitic hydrates, which are
all non-plastic, and yet sufficiently fine to form part of the “clay”
obtained as above specified.

[24] This fact emphasizes the impossibility of explaining the
plasticity and adhesiveness of clay simply as a function of fineness of
grain.

Despite these imperfections, (which however can in a measure be
taken into consideration in judging of a soil’s tilling qualities by
its clay content), the figures given in the above table approximate
much more nearly to a tangible basis for such estimate, than the
utterly indefinite mixtures which under the older methods of analysis
have been, and still are to some extent, used as a basis for soil
classification by writers on agriculture.


_Rock Powder; Sand, Silt and Dust._

The powdery (sandy and silty) constituents of soils usually constitute
the greater part of their mass; and the proportions present of the
several grades of fineness exert a most decisive influence upon their
cultural qualities, and very commonly upon their agricultural value
also. It is needless to add that the kind of mineral of which they
consist or from which they were formed, is also of great importance in
determining the quality of soils from the standpoint of the chemist,
with respect to their content of mineral plant-food.


WEATHERING IN HUMID AND ARID REGIONS.

_Sands of the Humid Regions._—As has already been stated, “sand” is
usually understood to be, in the main, quartz more or less finely
pulverized, generally intermingled with a few grains of other minerals.
With this understanding, since quartz is practically inert with respect
to plant nutrition, it follows that soils consisting mainly of this
substance contain but little plant-food; hence the common expression
“poor, sandy land,” the outcome of the experience had in Europe and
in the Eastern United States, and which until recently has been held
to be of general application. The “sands of the desert” have, both in
ordinary life and in poetry, always stood as the symbol of sterility.

    Thus the sandy lands (“sand hammocks”) of Florida, the
    (long-leaf) pine lands of the Gulf States, the “pine
    barrens” of New Jersey and of Michigan, are noted both
    for their sandy soils and their sterility after brief
    cultivation; necessitating fertilization within a few
    years from the time of occupation. In Europe, the “Heide”
    (heather) soils of northeastern Germany are of the same
    cultural character.

_Sands of the Arid Regions._—The experience of arid countries however,
has long ago shown that some very sandy lands—_e. g._, such as form
the oases of the north African deserts—may be extremely productive
when irrigated, and also of considerable durability. Actual experience
and close investigation given this subject in the arid regions of the
United States has fully demonstrated that lands appearing to the
casual observer to be hopelessly sterile sandy deserts, very commonly
prove to be even more productive than the more clayey lands of the same
regions. Examination of the sand shows, in these cases, that instead
of mere grains of quartz, the minerals of the parent rock, partially
decomposed, themselves constitute a large proportion of the sandy mass.
But in the regions of deficient rainfall, as has already been stated,
(p. 47) the formation of clay (kaolinization) is exceedingly slow;
hence the decomposition of the rock powder results in the production
of predominantly pulverulent instead of clayey soils. But the mineral
plant-food is not on that account less available, provided other
physical conditions necessary for the success of plant growth are
fulfilled. Among these moisture stands foremost; hence the relative
proportions of the several grain-sizes are of vital importance, since
upon this depends to a great extent the proper supply and distribution
of moisture, without which no amount of plant-food will avail.
Moreover, the finest and most highly decomposed powder is the portion
from which the roots draw their chief food-supplies.

    The point last mentioned is well shown in the results
    obtained by Dr. R. H. Loughridge, from the analysis of each
    of the several grain-sizes into which he had resolved a very
    generalized soil of the State of Mississippi, representing a
    very large land area in that State as well as in Tennessee
    and Louisiana. The details of this investigation are given
    farther on; but summarily it may be stated that he found
    practically the whole of the acid-soluble mineral plant-food
    accumulated within the portion of the soil the fineness of
    whose grains was below .025 millimeters (one-thousandth of
    an inch); ingredients so fine as to be wholly impalpable
    between the fingers. Moreover, two-thirds of the total
    amount was found in the portion described above as “clay.”
    It is thus readily understood why clay soils are in the
    regions of summer rains commonly designated as “strong”
    lands.

    The corresponding later investigations of Rudzinski (Ann.
    Agr. Inst. Moscow, Vol. 9, No. 2, pp. 172-234; Exp. Sta.
    Record, Dec. 1904, p. 245) and of Mazurenko (Jour. Exp.
    Landw. 1904, pp. 73-75; Exp’t Stn. Record, Dec. 1904, p.
    344) fully corroborate Loughridge’s conclusions, for typical
    soils of European Russia.

In the arid or irrigation regions, however, the case is different, for
the reason that much of the decomposed rock-substance remains adherent
to the surface of the larger grains, and plastic clay is formed to a
much less extent. Much available plant-food may therefore, in arid
lands, be present even in rather coarsely sandy soils almost devoid
of clay; such as in humid climates would be likely to be found wholly
barren. (See chapt. 19).


PHYSICAL ANALYSIS OF SOILS.

_Use of Sieves._—Down to a certain point the separation of the soil
into its several grain-sizes may be accomplished by means of sieves.
We may thus separate coarse gravel from fine gravel and from sand;
and the latter may itself be separated into several sizes by the same
means. This presupposes, of course, that the soil has been previously
prepared for the purpose by crushing the lumps consisting of aggregates
of finer particles, that in the operation of tillage would again be
resolved into their fine constituents, or be penetrated by roots. But
this preparation of the soil for sifting must not be carried beyond
the point mentioned, for a grain consisting of particles somewhat
firmly cemented together will under ordinary conditions play in the
soil precisely the same part as a solid sand-grain, and must not
therefore be broken up, if the soil is to be examined in its natural
condition. The pressure of the fingers or of a rubber pestle is as
far as trituration should go. The disintegration of these compound
particles by means of acids, as prescribed and practiced by the French
soil chemists, may wholly change the physical nature of the soil by
the breaking-up of mechanical aggregations which in the usual course
of tillage would remain intact. This is especially true of strongly
calcareous soils, and particularly those containing calcareous sand.

    The sieves used for this purpose should not be ordinary wire
    sieves, but should have bottoms of sheet brass perforated
    by _round_ holes of the various diameters desired, of
    fractions of inches, or preferably of millimeters. For the
    finer grain sizes, silk bolting cloth is used by the U. S.
    Bureau of Soils.

    In the sifting process it will be found that so soon as the
    finer grain-sizes of the sand are approached, the sieve
    fails to act satisfactorily; the more so, the more clay was
    originally contained in the material. The fine particles
    flock together, forming little pellets, which refuse to be
    separated by the sieve. This difficulty can, of course, be
    partly overcome by previously separating the clay from the
    sand by means of water, as detailed above; but even then
    it will be found that so soon as the grain-sizes fall much
    below ¹/₅₀ of an inch (½ millimeter) the same difficulty is
    experienced, so long as the sand is dry. By playing a small
    stream of water upon the sieve, however, all the particles
    beyond the ¹/₅₀₀ of an inch may be successfully separated
    from the coarser portion; and for many practical purposes
    the separation need be carried no farther.

    _Use of Water for Separating Finest Grain-Sizes._—The
    scientific investigator, however, must of necessity proceed
    to separate the finer grain-sizes from each other, since,
    as will presently be shown, they influence the tilling
    qualities of the soil to a much greater degree than do
    the coarser particles. Such farther separation can be
    accomplished only by the aid of water.

_Subsidence Method._—When a small amount of soil is stirred up in
water, and is afterward allowed to stand for some time, the different
grain-sizes will settle consecutively in accordance with their
sizes (or weights); the smallest ones settling latest, and the clay
only remaining suspended, as stated above. So long, however, as any
considerable amount remains suspended in the water, the latter is not
only denser but especially more viscid than if the clay were absent.
In order therefore to obtain correct results by any method involving
the use of water, it is necessary to remove the clay before proceeding
to the separation of the granular sediments. This, as has been already
stated, is approximately accomplished by allowing the soil, when
diffused in water after proper disintegration, to settle for 24 hours
from a column of water 200 mm. high, whereby all grain-sizes, of
and above .01 mm. diameter are removed from the turbid liquid. This
sedimentation is then repeated until after 24 hours the water becomes
clear. The clay is then determined in the “clay water” by evaporation
or precipitation; the granular sediments may then be successfully
separated by sedimentation.

The U. S. Bureau of Soils uses for the separation of clay, instead of
subsidence for 24 hours, the more expeditious process of centrifuging
the turbid soil water in appropriate glass cylinders, by the aid of
an electric motor; and thus in a relatively short time obtains “clay”
in which the upper limit of size is one-half of that mentioned above,
viz., .005 mm. But for the costliness of the appliances required,
including the entire time of an operator, this method of separating the
clay would undoubtedly be preferable to the elimination by subsidence;
the more as a more minute grain-size for the clay group is thus secured.

[Illustration: FIG. 6.—Schöne’s Elutriator.]

The separation of the clay having been accomplished, the various sizes
of silt and sand may be separated by again suspending them in water;
and interrupting the settling process at stated times, the grain-sizes
corresponding to definite velocities in settling may be segregated and
weighed. When this process of settling and decanting is carefully and
repeatedly carried out, very good results are obtained.

_Hydraulic Elutriation._—The sedimentation (or “beaker”) method,
long practiced in the arts is, however, quite tedious, requiring the
constant close attention of a skilled observer. The desired results
may, in the writer’s judgment, be more conveniently obtained by the
hydraulic method, whenever no very large volume of work of this kind is
required to be done at once.

When instead of allowing the soil to settle in quiet water, the latter
is used as an ascending current of regularly graded velocities, it
is clear that the soil particles will be carried off by this current
in exact conformity with their several sizes (or strictly speaking,
volume-weights); and when maintained in such a current for a sufficient
length of time, the entire quantity of the sediment corresponding to
the prevailing velocity will be carried away. It is of course easy
to ascertain to what grain-sizes certain velocities of the upward
current (regulated by a stopcock with arm moving on a graduated scale)
correspond, and to regulate accordingly the intervals between the
different velocities to greater or less detail, as may be desired. A
number of instruments have been devised for this purpose.

_Schöne’s Elutriator_ is the one commonly used in Europe; in it the
upward current ascends in a conical glass tube, (see figure 6) entering
through a narrow, curved inlet tube, in which the soil sample is kept
agitated by the current itself. The objection to this plan is twofold:
first, the narrow, curved inlet-tube is readily clogged by the soil
mass at the lower velocities, which are thereby changed, so that,
unless a very small amount of soil only is employed, the whole mass is
not kept properly stirred; second, the circulating currents brought
about by the conical shape of the tube cause the sediment-particles to
coalesce into complex, larger ones (floccules), which will then settle
down and fail to pass over at the current-velocity corresponding to
their individual component parts.

[Illustration: FIG. 7.—The Churn Elutriator (Hilgard’s) for the
physical analysis of soils.]

_Churn Elutriator with Cylindrical Tube._—The errors just alluded
to are obviated by an arrangement devised by the writer, in which
a rapidly revolving stirrer, placed at the base of a cylindrical
tube in which the washing process is conducted and which eliminates
counter-currents, continually disintegrates these compound particles,
and thus enables the entire quantity of the sediment corresponding to
the prevailing current-velocity to pass off with a comparatively slight
expenditure of time on the part of the operator (see figure 7). A wire
screen interposed between the churn and cylindrical glass tube prevents
communication of the whirling motion to the column. As the apparatus
works automatically, the analyst has only to observe from time to time
whether or not the turbidity near the top of the tube has disappeared;
and as the sediment accumulates at the bottom of the tall receiver
bottle,[25] no harm is done if the attendant should neglect to change
the velocity in time, except that water will run to waste.

[25] The figure given of this elutriator in Bulletin No. 24, on
physical soil analysis, published by the U. S. Bureau of Soils, shows
as the receiver a bottle entirely too low to insure the complete
retention of the sediments by settling. The receiving bottle should not
be less than twelve inches high and five inches wide.

The conical relay glass below the churn serves to retain the coarser
grades of sediments which are not concerned in the velocities employed
in the elutriator tube, and thus prevents injurious attrition. But
these sediments can at any time be stirred up by the incoming current
and brought into the washing tube if desired. In the same manner the
passing-off of the finer sediments can be materially accelerated by
running off rapidly about two-thirds of the turbid column of water
every twenty minutes.

It should be fully understood that prior to attempting such separation,
the “colloidal clay” must first be removed by the subsidence or
centrifugal method, since otherwise much larger grain-sizes may be
carried off at a given velocity.

_Yoder’s Centrifugal Elutriator._—A very ingenious instrument which
combines the elutriation and sedimentation processes into one, has
been devised by P. A. Yoder, of the Utah Expt. Station. The elutriator
bottle is placed in a centrifuge driven by an electric motor; it is
closed by a glass stopper carrying a delivery tube to a short distance
above the bottom of the elutriator bottle, as well as an outflow tube
ending at the base of the stopper; the latter also carries a funnel
coinciding with the center of rotation. Into this funnel flows
gradually the muddy water containing the soil in suspension; and the
rate of its flow, together with the velocity of rotation, determines
the size of the sediment-granules that will be deposited in the
slack-water below the mouth of the delivery tube. The muddy soil-water
is kept agitated in a funnel-shaped reservoir by air-bubbles from a
constant-pressure chamber.

While the principle of this instrument is good, it is quite complicated
and the results obtainable from it in practice have not as yet been
made public. The inventor claims that an analysis may by its means be
completed in less than three hours.

In all hydraulic elutriators a provision for constant pressure in
the reservoir supplying the current of water is needed; although in
Schöne’s and some other instruments a gradually decreasing pressure in
a plain reservoir is employed. A large glass bottle or carboy fitted
with the proper tubes so as to constitute a Mariotte’s bottle (in which
the air enters near the bottom of the vessel), is a very convenient
arrangement.

_Number of Sediments._—The number of grain-sizes or sediments into
which the soil mass is to be segregated is of course entirely within
the option of the operator. Experience has shown that it is unnecessary
to discriminate very closely between the several sizes of the coarser
portion of the sand, such as those lying between one-fourth and
one-half of a millimeter. But below this point, and especially between
one-tenth of a millimeter and the clay, a proper discrimination becomes
very important. The series first devised by the writer in 1872 is based
upon a consecutive doubling of the velocities of the current from
a quarter of a millimeter per second to thirty-two millimeters per
second; the sediment of sixty-four millimeter-velocity corresponding to
a diameter of one-half of a millimeter, will remain in the elutriator.
Above this, as before remarked, the sieve (especially when aided by a
jet of water) effects a satisfactory segregation.

The table below shows the elements of these series both as regards
current-velocities and maximum quartz-grain diameters carried off by
each. In a great many cases, however, it is altogether unnecessary to
go into such detail, and a subdivision into six or seven divisions
is quite sufficient. Such a subdivision, based upon the doubling of
grain-sizes instead of current-velocities, has been adopted by Prof.
Milton Whitney, of the U. S. Department of Agriculture, and others.

    TABLE OF DIAMETERS AND HYDRAULIC VALUES OF SEDIMENTS.

    ===============================================================
                             |Velocity per second,|Maximum diameter
    Designation of materials.|    or hydraulic    |   of quartz
                             |       value.       |    grains.
    -------------------------+--------------------+----------------
                             |       _Mm._        |     _Mm._
                             |                    |
    Grit                     |       (?)          |     1-3
                           { |       (?)          |    .5-1
                           { |     32-64          |        .50
    Sand                   { |     16-32          |        .30
                           { |      8-16          |        .16
                           { |      4- 8          |        .12
                           { |      2- 4          |        .072
                           { |    1.0- 2          |        .047
    Silt                   { |     .5- 1          |        .036
                           { |     .25-0.5        |        .025
                           { |         0.25       |        .016
                           { |       < 0.25       |        .010
                             |                    |
    Clay                     |       < 0.0023     |
    -------------------------+--------------------+----------------

_Results of such analyses._—A tabular presentation of the results of
analyses made in accordance with the above plan will give a good idea
of the differences between the various grades of soils recognized
in farm practice, to any one accustomed to the study of figures.
But a much more satisfactory showing is made by placing the several
grain-sizes segregated, into small vials or tubes of identical diameter
and placing them in parallel series alongside of each other.[26] The
curves formed by the surfaces of the several sediment-columns in each
series show to the eye very strikingly the relations of the several
grades of soils to each other, and suggest at once that while gentle
slopes or gently undulating curves belong to soils of intermediate,
loamy character, steep grades and zigzags show soils of extreme types.
This is exemplified in the subjoined Figures:

[26] Convenient stands for this purpose, used by the writer since 1872,
may be cut from L-shaped moldings of wood, such as can be readily
ordered from any planing mill. The vials can be cemented, wired or
tied.

[Illustration: FIG. 8.—Illustration of Results of Hydraulic
Elutriation, showing extremes of soil texture, and intermediate loam.]

[Illustration: FIG. 9.—Illustration of Results of Hydraulic
Elutriation, showing Alluvial Silts and Pine-Woods Soil.]

_Physical composition corresponding to popular designations of Soil
quality._—The subjoined table illustrates the physical composition
of a number of soils from the State of Mississippi, selected for
their representative character, in order to deduce therefrom
approximate definitions of physical character corresponding to popular
designations. This table, published in 1873 in accordance with results
obtained during the two preceding years, does not require any material
modification on account of subsequent investigations. It lacks,
however, a characteristic representative of the predominant soils of
the arid region, viz., the silty soils so prevalent in dry climates,
only approximately represented by No. 165 of the table; hence two such,
from California, exemplifying respectively the valley deposits of the
Sacramento and Colorado rivers, have been added to the list.

It must not, however, be understood that these typical soils
necessarily represent correctly the physical constitution of all
soils falling under the same popular designation; for we are far from
being able as yet to predict accurately in every case the tilling
qualities of a soil material from its physical composition. To do this
it would be necessary not only to know with some degree of precision
the several physical coefficients of each of the several grain-sizes,
and perhaps of many more intermediate ones; but we would also have to
construct a formula according to which each could be given its proper
weight when present in varying proportions, and of varying shapes,
surface condition, and material. For this our present knowledge is
wholly inadequate, if indeed the problem is not beyond the limits of
mathematical computation. We must for the present at least be satisfied
with the empirical approximations afforded us by the constantly
increasing number of such analyses, correlated with farming experience.

        Since the finest grain-sizes above those classed as “clay”
        do not tend to “lighten” soils, but even to render them more
        intractable (“putty soils”), while coarser ones gradually
        change the dense clay-texture into the “loamy,” it is clear
        that in between there must be a neutral point, some grain
        sizes which by themselves do not influence soil texture
        either way. Discussion of numerous physical analyses, and
        some direct experiments, have led the writer to conclude
        that this theoretically neutral grain-size lies at or
        near the diameter of .025 mm., or .5 mm. hydraulic value.
        In correlating the results of analysis with the tilling
        qualities of the soil as to “heaviness and lightness,”
        therefore, that grain-size may usually be left out of
        consideration.

                  PHYSICAL ANALYSES OF SOILS AND SUBSOILS.

        (A) = 238 White Pipe Clay. Tishomingo Co.
        (Z) = Hygroscopic Moisture (+7 = to +21°C)
        ===========+==============+================+=======
                   |   Diameter   |    Velocity    |
                   |(Millimeters) |   (Hydr. V.)   |
        Designation|              |                |
            of     +--------------+----------------+-------
        Materials. |              |                |
                   |              |  Millimeters   |  (A)
                   |              |    per sec.    |
        -----------+--------------+----------------+-------
        Grit       |    1-3       |                |
                   |              |                |
                 { |   .5-1       |                |
                 { |       .50    |    64          |}
        Sand     { |       .30    |    32          |} 0.06
                 { |       .16    |    16          |}
                 { |       .12    |     8          |  0.08
                   |              |                |
                 { |       .072   |     4          |  0.02
                 { |       .047   |     2          |  0.04
        Silt     { |       .036   |     1          |  0.08
                 { |       .025   |     0.5        |  0.08
                 { |       .016   |     0.25       |  2.00
                 { |       .010   |    <0.25       | 21.15
                   |              |                |
        Clay       |      ?       |    <0.0023     | 74.65
                                                   +-------
                                                   | 98.16
                                                   |
        (Z)                                        |  9.09
        Ferric Oxid                                |  0.13
        -------------------------------------------+-------

        (B) = 248 Tallahoma Subsoil. Jasper Co.
        (C) = 165 Flatwoods Soil. Chickasaw Co.
        (D) = 206 Pine Hill Soil. Smith Co.
        (E) = 209 Pine Hill Subsoil. Smith Co.
        (F) = 397 Oxford Subsoil. Lafayette Co.
        (G) = 219 Table Lands Subsoil. Benton Co.
        (H) = 173 Prairie Subsoil. Monroe Co.
        (I) = 230 H’y Flatwoods Soil. Pontotoc Co.
        (J) = 246 Red Hills Subsoil. Attala Co.
        (K) = 196 Hog-wallow Subsoil. Jasper Co.
        (Z) = Hygroscopic Moisture (+7 = to +21°C)
        ==========+===============================================================
                  |                  MISSISSIPPI UPLANDS.
                  +------------------+------------------+-------------------------
       Designation|       Sandy.     |      Loam.       |           Clay.
            of    +-----+------+-----+-----+-----+------+-----+------+-----+------
        Materials.|     |      |     |     |     |      |     |      |     |
                  | (B) |  (C) | (D) | (E) | (F) |  (G) | (H) | (I)  | (J) |  (K)
                  |     |      |     |     |     |      |     |      |     |
        ----------+-----+------+-----+-----+-----+------+-----+------+-----+------
        Grit      | 6.94| 2.90 | 0.36| 0.36|     |}     |}    | 0.33 |}    |  0.83
                  |     |      |     |     |     |} 0.23|}2.10|      |}1.97|
                 {|17.65| 6.96 | 2.98| 0.83|     |}     |}    | 0.35 |}    |  1.19
                 {|18.81| 2.81 | 6.62| 6.21|}    |  1.47|}    |      | 0.72|  1.96
        Sand     {|10.16| 4.41 | 7.75| 3.38|}0.79|  2.33|}0.62|      | 2.32|  1.64
                 {| 2.66| 3.13 | 3.01| 3.85|}    |  1.17|}    |      | 2.09|  0.88
                 {| 1.66| 2.02 | 1.59| 1.49| 0.18|  0.78| 0.20| 0.23 | 0.70|  0.26
                  |     |      |     |     |     |      |     |      |     |
                 {| 1.02| 2.23 | 1.19| 0.64| 0.78|  0.76| 1.26| 0.18 | 1.29|  0.19
                 {| 0.88| 5.06 | 3.56| 2.63| 3.56|  9.79| 2.92| 1.61 | 1.80|  2.49
        Silt     {| 1.96| 9.67 | 6.50| 5.40|13.12|  7.26| 7.36| 2.66 | 3.60|  3.67
                 {| 7.89|14.18 |13.97| 7.77|16.64| 13.14| 8.81| 9.13 | 2.73|  5.39
                 {| 8.40|22.03 |14.20|16.65|27.28| 15.07| 7.85|26.64 | 3.30| 10.31
                 {|15.53|15.62 |29.36|37.75|18.87| 26.50|35.22|32.35 |25.33| 24.18
                  |     |      |     |     |     |      |     |      |     |
        Clay      | 8.63| 7.86 | 4.58|10.70|17.23| 19.19|33.16|25.48 |40.25| 47.03
                  +-----+------+-----+-----+-----+------+-----+------+-----+------
                  |99.28|98.68 |95.67|97.77|98.35| 97.65|99.50|97.87 |96.11|100.00
                  |     |      |     |     |     |      |     |      |     |
        (Z)       | 1.80| 3.36 | 2.48| 7.69| 8.79|  7.24|11.35| 9.33 |18.60| 14.48
       Ferric Oxid| 1.10|(1.45)| 1.25| 4.45| 2.53|  5.11| 5.42|(5.90)|10.50|  4.00
        ----------+-----+------+-----+-----+-----+------+-----+------+-----+------

        (L) = 390 Buckshot Soil. Issaquena Co.
        (M) = 237 Loess. Claiborne Co.
        (N) = 365 Tallahatchie All. Soil. Panola Co.
        (O) = 377 Frontland Subsoil. Sunflower Co.
        (P) = 395 Dogw. Ridge Soil. Coahoma Co
        (Q) =     Southwest Pass. Plaquemine Par.
        (R) =     Southwest Mud-lump. Plaquemine Par. La.
        (Z) = Hygroscopic Moisture (+7 = to +21°C)
        ==========+=======================================================
                  |           MISSISSIPPI RIVER BOTTOM.
                  +-------+-------+---------------------------------------
       Designation| Swamp.| River.|       River Deposit.        Delta
            of    +-------+-------+-------+-------+-------+-------+-------
        Materials.|       |       |       |       |       |       |
                  |  (L)  |  (M)  |  (N)  |  (O)  |  (P)  | (Q)   |  (R)
                  |       |       |       |       |       |       |
        ----------+-------+-------+-------+-------+-------+-------+-------
        Grit      |  0.09 |}      |}      |       |       |       |
                  |       |} 0.24 |} 0.09 |       |       |       |
                 {|  0.05 |}      |}      |       |       |       |
                 {|       |  0.37 |  0.04 |} 0.32 |  0.15 |} 0.18 |} 0.10
        Sand     {|  0.36 |  0.61 |  0.05 |}      |       |}      |}
                 {|       |  0.93 |  0.21 |  2.79 |       |  0.47 |  5.02
                 {|  0.31 |  1.65 |  1.30 |  2.41 |  3.75 |  7.03 |  3.68
                  |       |       |       |       |       |       |
                 {|  0.27 |  1.95 |  2.68 | 16.90 | 21.46 | 12.38 |  5.34
                 {|  1.56 | 14.25 |  9.38 | 19.97 | 21.83 | 13.27 | 10.09
        Silt     {|  2.23 | 16.20 |  9.88 | 13.90 | 14.01 | 15.87 |  5.58
                 {|  3.68 | 20.08 | 20.37 |  4.27 |  9.93 |  8.25 |  9.54
                 {|  8.97 |  5.59 | 19.79 |  1.89 |  9.58 |  7.26 |  8.01
                 {| 38.19 | 33.38 | 25.30 | 30.08 |  8.65 | 19.67 | 34.46
                  |       |       |       |       |       |       |
        Clay      | 44.30 |  2.51 |  9.64 |  5.51 | 10.35 | 12.20 | 18.18
                  +-------+-------+-------+-------+-------+-------+------
                  |100.01 | 97.74 | 98.73 | 98.04 | 99.72 | 96.58 |100.00
                  |       |       |       |       |       |       |
        (Z)       | 14.31 |  4.18 |  6.12 |  5.68 |  3.95 |       |
       Ferric Oxid|(5.82) |  3.27 |  2.58 |  2.31 |  2.69 |       |
        ----------+-------+-------+-------+-------+-------+-------+-------

        (S) =  10 Sacramento. Sacramento Co.
        (T) = 506 Gila. San Diego Co.
        (Z) = Hygroscopic Moisture (+7 = to +21°C)
        ===========+==================
                   | CALIFORNIA.
                   +------------------
       Designation |   River Deposit.
            of     +---------+--------
        Materials. |         |
                   |   (S)   |   (T)
                   |         |
        -----------+---------+--------
        Grit       |         |
                   |         |
                 { |         |
                 { |         |     .13
        Sand     { |         |     .15
                 { |         |     .11
                 { |     .32 |     .75
                   |         |
                 { |    3.16 |    2.51
                 { |   10.27 |    8.32
        Silt     { |   13.67 |   12.64
                 { |   13.11 |   11.28
                 { | } 43.61 | } 31.79
                 { | }       | }
                   |         |
        Clay       |   12.06 |   23.97
                   +---------+--------
                   |   96.20 |   91.71
                   |         |
        (Z)        |    9.18 |    9.26
       Ferric Oxid |         |
        -----------+---------+--------

_Number of soil grains per gram._—It is of some interest to consider
the number of grains of different sizes that may be contained in, _e.
g._, a gram of soil. If for this purpose we assume all the soil grains
to be spherical, we shall obtain the minimum figures, for most other
shapes will pack more closely. King (Physics of Agriculture, p. 117)
calculates such figures for different grain-sizes, assuming the density
to be that of quartz (2.65), with the result that while with a diameter
of one millimeter (1-25 inch) the number of grains would be 720, and
with one-tenth of a mm. 720,000; if made of the finest particles only,
viz., one thousandth of a mm., the number would be 720,000 billions.
Probably few of the clayey soils we ordinarily deal with are of this
order; it is doubtless approached in certain fine plastic clays.

_Surface afforded by various grain-sizes._—The amount of _surface_
afforded by a similar amount of soil must naturally be considered in
this connection, since upon it depends not only the amount of moisture
which the soil may hold in the form of superficial films, but also the
extent of surface upon which the weathering agencies as well as the
root hairs of plants may act. Quoting again from King’s work, we find
on the same premises given above for the _number_ of grains, that their
_surface_ would in the case of grains of one mm. diameter be eleven
square feet per pound (about half a pint) of material; while in the
case of the finest grade we should have 110,538 square feet, or more
than two and a half acres.

From actual experiments made with the flow of air through various
soils, King calculates that while in ordinary loam soils the total
surface is about an acre per cubic foot, in fine clay soils it rises to
as much as four acres. If we imagine this large surface to be covered
with even a very thin film of water, it is readily seen how large an
amount may be present in a cubic foot of moist soil.

    E. A. Mitscherlich (Bodenkunds für Land-und-Forstwirthe;
    Berlin, 1905) attributes to the surface offered by the
    soil particles supreme importance in determining the
    productiveness of soils. According to him the internal
    soil-surface determines directly the ease with which roots
    can penetrate the soil; and he proposes the determination
    of this factor by means of the heat produced in wetting
    the soil (“Benetzungswärme”), measured in a calorimeter,
    as a substitute for all methods of physical soil analysis,
    which are vitiated by the varying shapes and densities
    of the particles; while his method gives directly the
    actual surface. To the consumption of energy required
    by difficult penetration he attributes most of the
    differences in production, and hence refers to the internal
    soil-surface as governing nearly all the other physical
    factors. The introduction of many arbitrary assumptions,
    and the failure to show that the admitted inaccuracy of
    the ordinary mechanical soil analyses are of any practical
    importance, greatly detract from the cogency of the rigorous
    mathematical discussion carried through his work by
    Mitscherlich.

_Influence of the several grain-sizes on soil texture._—Undoubtedly
the most potent of all the sediments appearing in the above table in
influencing soil texture, is the “clay.” That the materials included
under this empirical designation may vary considerably in different
soils, has already been sufficiently insisted on; and it is doubtful
that in the present imperfect state of our knowledge of the functions
of the several physical grain-sizes, we would be much wiser were we to
go to the extreme advocated by Williams (Forsch. Agr. Phys., vol. 18,
p. 225, ff.), of determining with precision the actual amount of such
extremely fine clay particles as cease altogether to obey the law of
gravity when once suspended in water. It is at least doubtful that the
essential property of adhesive plasticity belongs only to these, for
this property doubtless increases gradually as the size diminishes,
although unquestionably not a mere function of the latter, since it
belongs only to the hydrated silicate of alumina.

    _Ferric Hydrate._—Probably the body which most
    commonly modifies materially the adhesive and contractile
    properties of the clay substance, is ferric hydrate;
    the more as on account of its high density it tends to
    exaggerate materially, in many cases, the apparent content
    of true clay, and the estimate of the soil’s plasticity
    based upon it. A good example in point is the case of soil
    No. 246 (Miss.) of the above table. This is a heavy clay
    soil, yet not excessively adhesive; scarcely as much so as
    No. 230 (Miss.), the heavy gray “flatwoods” soil, and not
    nearly as “sticky” when wet as No. 173 (Miss.), the prairie
    subsoil, although containing apparently 15% more clay than
    the former, and 7% more than the latter. But No. 246 is a
    highly ferruginous clay, in which the ferric hydrate is in
    a very finely divided condition, and materially influences
    the physical qualities of the clay substance. Were it all
    accumulated in the “clay,” it would diminish the percentage
    of true clay by 11.75%, reducing the clay-percentage to
    28.5% which accords more nearly with the soil’s only
    moderate adhesiveness, and not excessively heavy tillage.

But it must be remembered that the iron oxid shown in the analysis is
not nearly always in this finely diffused condition. Frequently it
incrusts the sand grains; quite commonly it forms small concretions of
limonite, which themselves act as sand grains; and again, it may be
present in the form of “black sand” or magnetic oxid, as is commonly
the case in California and on the Pacific slope generally. To take
this point properly into account, therefore, it would be necessary to
determine the amount of ferric hydrate actually present in the “clay”
as separated by subsidence of the granular constituents.

_Other substances._—This circumstance as well as the inevitable
presence of other modifying substances, clearly shows the desirability
of being enabled to examine the physical properties of this “clay”
directly, by collecting its entire amount as obtained in analysis,
instead of merely determining it by weighing fractional portions. When
this is done the analysis is much more valuable as indicating the true
tilling qualities of the land. The increase of bulk suffered by this
substance after wetting, is a very fair index of its content of true
clay, and is preferable to the chemical analysis proposed by some
investigators. For it is quite impossible to distinguish the silica and
alumina derived from the kaolinitic substance proper, from that which
is due to the decomposition of zeolites.

It is possible, however, to determine the possible _maximum_ of the
kaolinite ingredient by taking into consideration the quantitative
ratio according to which silica and alumina combine to form it, viz.,
approximately 46% of the former to 40 of the latter, the rest being
water. By using this calculation we can often demonstrate clearly the
presence in the “clay” of considerable amounts (up to 33%) of _aluminic
hydrate_; since no zeolitic mass can contain as much alumina as does
kaolinite. Whether the aluminic hydrate be in the form of gibbsite,
bauxite, diaspore,[27] or in the gelatinous state, the nature of the
soils containing it proves that it is totally destitute of plasticity
and adhesiveness; and this consideration will often serve to explain
the fact that soils showing in their chemical analysis high percentages
of alumina, nevertheless show quite low degrees of plasticity,
adhesiveness and water absorption. What part it may take in modifying
the physical properties of the soil we can thus far only conjecture.

[27] Bauxite is not only the most abundant of the three hydrates of
alumina known to occur naturally, but also stands nearly midway between
the two others in its water content, viz., a little over 25%; that of
diaspore being nearly 15%, gibbsite about 35%.

_Influence of the granular sediments upon the tilling qualities of
Soils._—Considering the granular sediments by themselves, in the
absence of clay, it may be stated in a general way that while in a
moist condition they flocculate sufficiently to produce a fair tilth,
they will nevertheless on drying collapse into a close arrangement
resulting from the single-grain structure. The form of the grains
being angular instead of rounded, they are apt to form a very closely
packed mass far from suitable to vegetable growth; as will be seen by
an example taken from one of the culture stations of the University of
California, from a piece of land which on the surface would be called
a very sandy loam, but after we descend increases in its content of
fine grains until at a depth varying from eighteen inches to three
feet we find what appears to be a hardpan, which is equally impervious
to roots and water and causes the water to stagnate to such an extent
that after heavy rains the land becomes so boggy as to render plowing
almost impossible without endangering the team. A close examination of
this hardpan shows that, unlike others, it is devoid of any cement, and
when taken out can be readily crushed between the fingers, and softens
in water, but does not become plastic. Its imperviousness is therefore
due solely to the close packing of the sand grains, for it contains
practically no plastic clay, and under the microscope the grains
are seen to be angular-wedge-shaped and composed of the remnants of
granite. The physical analysis shows the following result:

    MECHANICAL ANALYSIS OF HARDPAN.

    ==================================
    Designation.|Diameter.|Percentage.
    ------------+---------+-----------
              { | .50 mm. |   10.93
    Sand      { | .30  “  |   21.23
              { | .12  “  |    7.27
                |         |
              { | .072 “  |    9.63
              { | .047 “  |   12.00
    Silt      { | .036 “  |    7.19
              { | .025 “  |    1.25
              { | .016 “  |   14.20
                |         |
    “Clay”      |    ?    |    8.64
    ------------+---------+-----------

It is doubtful whether this condition of things can be remedied by the
usual measure of breaking up the hardpan either by hand or by means
of giant-powder blasting. Experience seems to show that the effect
is only temporary, and that in the course of time, by the action of
the percolating waters, the particles settle back into their original
impervious condition. It is just possible, however, that if once
penetrated by roots, the intervention of these would permanently
destroy the close structure, so as to make this a fair subsoil for the
growth of trees and other plants. The writer is not aware that this
kind of purely physical hardpan without cement has ever been observed
elsewhere.

This physical condition is doubtless responsible for two other
phenomena, viz., the “putty soils,” and also certain difficulties
experienced in irrigation.

“_Putty Soils_” is the name popularly given in the Cotton States, and
probably elsewhere, to soils usually occurring in low ground and also
known as “cray-fishy.” They consist of very uniform, powdery sediment,
with little or no coarse sand and still less of clay to render them
coherent. When wet these soils behave precisely as would glazier’s
putty, adhering to the surface of even the best-polished plowshare, so
that no furrow-slice can be turned and the plow is soon dragged out
of the ground. At a very closely limited condition of moisture such
lands may plow fairly well; but when this limit is passed in the least
(as sometimes happens in the course of a single day), it turns up only
hard clods, which in a few hours of sunshine become so hard that no
instrument of tillage short of a sledgehammer will make any impression
upon them. The physical analysis of these usually gray soils shows that
they contain only a trifling amount of clay; perhaps 1 or 2%, playing
the part of linseed oil in making putty out of whiting. Even the
addition of lime does not help such soils much, because there is little
or no clay to flocculate. They are, as a matter of fact, among the most
refractory lands the farmer has to deal with. A soil showing similar
behavior, though not quite as extreme as in the case of the Gulf or
Cotton States’ soils in question, occurs at the culture substation at
Paso Robles, California, and is probably closely correlated to the
physical hardpan referred to above. The physical analysis of this soil
yielded the following result:

       MECHANICAL ANALYSIS OF SOIL.

    ==================================
    Designation.|Diameter.|Percentage.
    ------------+---------+-----------
              { | .50 mm. |   14.24
              { | .30  “  |   15.17
    Sand      { | .16  “  |    8.88
              { | .12  “  |    5.60
                |         |
              { | .072 “  |    6.75
              { | .047 “  |    8.35
    Silt      { | .036 “  |    8.55
              { | .025 “  |    6.03
              { | .016 “  |   17.77
                |         |
    “Clay”      |    ?    |    7.50
    ------------+---------+-----------

It would seem the best and almost only remedy to be applied to
such soils as these is the introduction of vegetable matter or
green-manuring, by which their texture is loosened: for the hauling of
mere clay upon the land would hardly accomplish the purpose intended,
within the limits of farm economy.

_Dust Soils_, which during the dry season are even in their natural
condition so loose as to rise in clouds and render travel very
uncomfortable, are not uncommon in arid countries, _e. g._, in
Washington and adjacent parts of Oregon, on the uplands bordering the
Columbia, Yakima and Snake rivers. The physical analyses of three of
such soils, given in the table below, will convey some idea of their
peculiarities in this respect.

               PHYSICAL ANALYSIS OF DUST SOILS.

    =========================================================
              |Hydr. Value.|Diameter. |No 17.|No. 37.|No. 79.
    ----------+------------+----------+------+-------+-------
    Clay      |<.0023. mm. |<.10--?   |   .93|  3.59 |  1.27
            { |<.25 mm.    | .010     | 30.93| 13.06 | 32.29
    Silt    { | .25 to   .5| .016     |  3.20|  5.82 | 12.75
            { | .5  to  2.0| .025-.047|  7.18| 27.37 | 37.51
            { |2.0  to  8.0| .047-.120| 21.88| 43.78 | 10.92
    Sand      |8.0  to 64.0| .12- .50 | 32.39| 49.57 |  3.97
              |            |          +------+-------+-------
        Total |            |          | 96.57| 98.18 | 98.72
    ----------+------------+----------+------+-------+-------

_Slow penetration of Water._—Soils of this class are wetted with
extreme slowness by irrigation water; so that when first taken under
cultivation it sometimes takes twenty-four hours to soak the land for
twelve inches in each direction. Irrigation furrows must be placed
very close together and in large numbers, in order to ensure the
wetting of the soil so that the crop shall not suffer from lack of
moisture at a distance of two or not more than three feet. Where the
irrigation furrows are drawn farther apart a fine stand of grain may
be seen within eighteen inches of the same, while farther away the
crops may be dying from lack of moisture. This difficulty is by no
means infrequent in the arid region, and is difficult to overcome
except by frequent and thorough tillage, which gradually increases the
rapidity of water-penetration; as has been shown in the soils of the
alluvial prairies of the Yakima country in the State of Washington.
It is necessary, however, to take care that they shall always contain
an adequate amount of humus or vegetable matter, in order to prevent
re-consolidation by the burning-out of the humus during the warm,
rainless season.

There is an unmistakable resemblance between these dust soils of the
Northwest and the “putty” soils mentioned above; both showing a very
low percentage of clay with a relatively large amount of the finest
sediments, with a sudden downward break of the curve before the coarser
grain-sizes are reached. It would seem as though the absence of these
intermediate grains favors the close packing of the fine sediments
in the interstices of the coarse ones, thus bringing about the
imperviousness, which is the chief obstacle to their cultivation.

_Effects of coarse Sand._—Coarse sand intermingled with heavy clay
soils has but little effect in improving the tilling qualities, unless
carried to such excess as renders it financiallyimpracticable. In
actual practice it is frequently possible to improve such soils by
properly distributing upon them the washings of the adjacent hills,
which will always carry sands of many grades; and when it is intended
to improve garden land by hauling sand it is important to choose the
latter so as to complement the deficient grain-sizes of the soil.
The sand of wind drifts or dunes is generally well adapted to such
improvement, being, as Udden[28] has shown, of a fairly definite
composition of sufficiently wide range of grain-sizes for the purpose.

The effects of humus in modifying soil texture are discussed farther on.

[28] The Mechanical Composition of Wind Deposits, Bull. No. 1,
Augustana Library Publications; 1898.




CHAPTER VII.

THE DENSITY AND VOLUME-WEIGHT OF SOILS.


Aside from the humus-substances the specific gravity of the common
soil constituents, taken individually, do not vary widely; kaolinite
being the lightest (2.60), feldspar next (2.62); then quartz (2.65),
calcite (2.72). Mica and hornblende range (according to their iron
contents) from 2.72 to over 3.0. The average specific gravity of soils
of ordinary humus content only will thus range between 2.55 and 2.75;
sandy soils approaching very closely to that of quartz alone.

_Volume-Weight._—The specific gravity of the soil is, however, of
little practical consequence compared with the “_volume-weight_,” _i.
e._, the weight of the natural soil as compared with an equal bulk of
water. A cubic foot of water weighs 62½ pounds; a similar volume of
soil usually weighs more, but in the case of peaty lands may actually
(when dry) weigh less. The extreme range is from 110 pounds for
calcareous, and somewhat less for siliceous sand, to as little as 30 to
50 pounds in the case of peaty and swamp soils. It may be conveniently
remembered that while average arable loams range from 80 to about 95
pounds per cubic foot, “heavy” clay soils range from 75 pounds down
to 69, observed by the writer in the case of certain alluvial soils,
poor in humus,[29] of the Sacramento river, California. Manured garden
soils, and the mold surface soil of deciduous forests, generally
contain so much humus as to depress their weight considerably, varying
according to their state of tilth from 66 to 70 pounds per cubic foot.

[29] This remarkable soil seems to have been derived from the finest
“slickens” of the hydraulic gold mines.

_Weight per acre-foot._—As for practical purposes and calculations it
is often desirable to know approximately the weight in pounds of an
acre (43,560 square feet) one foot deep, it is convenient to remember
that in the case of sandy land, this weight (per “acre-foot”) may be
assumed at four millions of pounds; for loams, at 3½ millions; for
clay lands, 3¼ millions; for humus or garden land and woods earth,
about 3 millions of pounds; for reedy swamp and peaty lands, 2 to 2½
millions.

    The loose tilth and humus-content of the surface soil will
    in general cause it to weigh less, bulk for bulk, than the
    underlying subsoil, even when the latter is more clayey;
    moreover, the continuous pressure from above will tend to
    consolidate the subsoil and substrata. Warington (Phys.
    Properties of Soils, pp. 46, 47) gives interesting data on
    this point from the Rothamstead fields, as follows:

    Old pasture, first nine inches   71.3 pounds per cub. ft.
    Same,        fourth  “    “     102.3   “     “   “    “
    Arable land, first   “    “      89.4   “     “   “    “
    Same,        fourth  “    “     101.4   “     “   “    “

    The influence of humus and unhumified organic matter, as
    well as of tillage, in diminishing the volume-weight of
    soils is here strikingly shown.

_Air-space in Natural Soils._—The difference between the specific
gravity as usually determined, and the volume-weight of soils, is of
course caused by the large amount of air contained in them when dry,
but which in wetting them is partially or wholly replaced by water.

[Illustration: FIG. 10.—Various possible arrangements of soil
particles.]

Theoretically, assuming all soil grains to be globular, and packed as
closely as possible (in oblique order), the space not filled by them
would be the same for all sizes, whether that of marbles, or so minute
as to be hardly felt between the fingers; and would be 25.95 per cent
of the soil volume.[30] If the same globular particles were packed
as loosely as possible, _i. e._, in square instead of oblique order
(see figures 10 and 11), the vacant space would be 47.64 per cent If
however we imagine each sphere to be itself composed of a number of
smaller ones, the empty space will obviously be greatly increased, to
an extent proportionate to the diminution of solid mass thus brought
about. The pore-space might in that case, with the oblique arrangement
of the globules as shown in Fig. 10, be as high as 74.05 per cent But
since the soil particles may be of all shapes and sizes within the same
soil, and usually fit much more closely than would globular grains,
the empty space rarely approaches (only in certain alluvial soils and
in loose mulches) to the figure last named. In sandy soils it may fall
as low as 20%, and in coarse gravelly soils even as low as 10%. Most
cultivated soils range between 35 and 50% of empty space.

[30] King, Physics of Agriculture, p. 116, ff.

_Effects of Tillage._—That these figures can be only approximations is
obvious from the consideration that one and the same soil will vary
materially in its volume-weight according to its temporary condition
of greater or less compactness. After land has been beaten by winter
rains, its volume-weight will be found to have materially increased
from the well-tilled condition brought about by thorough cultivation.
This difference is strikingly seen when, in plowing, the height of
the ground on the land side is compared with that of the turned
furrow-slice in well conditioned loamy land. This loose condition is
called _tilth_, and it results from the formation of relatively large,
complex crumbs[31] or floccules, between which there are large air
spaces that were wholly absent in the untilled land; the floccules
themselves being also more loosely aggregated than was the case before
tillage.

[31] The word crumbs, which is generally understood as meaning a
relatively large, loose aggregate, seems preferable to the word
kernels, suggested for the same by King (Physics of the Soil, p.
110). Kernels are understood to be bodies rather more solid than the
surrounding mass, and do not convey the idea of loose aggregates. The
word “Krümelstructur” (crumb-structure), adopted by Wollny for this
phenomenon, has both fitness and priority in its favor.

[Illustration: FIG. 11.—Land before and after plowing. The compactness
of the soil is indicated by the density of dotting. Before plowing
there is a compact surface crust (s), below which the soil becomes less
and less compact as we go deeper. After plowing we find the soil (fs,
furrow-slice) converted into a loose mass of crumbs (floccules), with
increase of bulk. Compacted plowsole at pl.]

[Illustration: FIG. 12.—A soil-crumb, magnified to show the particles
of which it is composed. The particles are held together by the
water-menisci, just as are the hairs of a brush when wetted. The white
spaces between the particles represent air.]

_Crumb or Flocculated structure._—Figure 11 illustrates the difference
between the unplowed land, consolidated especially on the surface by
winter rains, and in its upper portion consisting largely of single
grains; while the plowed land, toward which the furrow-slices have
been turned, is greatly increased in height and volume and consists
almost wholly of variously-shaped and-sized aggregates or floccules,
loosely piled upon one another and separated by large interspaces. The
increase in volume from consolidated clay to crumb-structure is given
by Wollny (Forsch., vol. 20, p. 13, 1897) at 41.9%, to powder as 33%.
On moistening dry clay increased 36.9%, quartz powder 8.01%. When land
is plowed in the proper moisture-condition the crumbs of floccules are
held together by the surface tension of the capillary films (menisci)
of water at the points of contact. In the case of sands, the crumbs
will collapse into single grains whenever the water-films evaporate,
unless some cementing substance was dissolved or suspended in the
water. (See figure 12). Lime carbonate is one of the substances most
commonly found permanently cementing the floccules; hence the ready
tillage of most calcareous soils, and especially the loose texture of
the “loess” of the western United States, and of Europe and Asia. In
these deposits we find sandy and silt aggregates or concretions ranging
from ten or more inches in length (loess puppets) to microscopic size,
held together by lime carbonate, but collapsing into silt and sand when
the material is treated with acid so as to dissolve the cement. The
rough surfaces of these aggregates, gripping into each other, explain
the stability of the steep loess cliffs in the United States, as well
as in northeastern China, as observed by Von Richthofen and Pumpelly.

_Clay_ is most frequently the substance which imparts at least
temporary stability to the crumbs and crumb-structure; this is one of
its most important functions in soils, as it serves to _maintain_ tilth
once imparted by cultivation, even after the land dries out. Beating
rains, and cultivation while too wet, will in this case of course
destroy the crumbs and the loose tilth.

Other substances which greatly aid the maintenance of tilth are the
several humates (of lime, magnesia, iron), which when fresh are
colloidal (jelly-like) like clay itself, but unlike the latter, when
once dried do not resume their plastic form by wetting (Schloesing).
The crumbs thus formed are therefore quite permanent and contribute to
the looseness of soils rich in humus. One part of lime humate is said
by Schloesing to be equal in cementing power to eleven parts of clay.

Silica, silicates and ferric hydrate are sometimes found cementing soil
crumbs, wholly or in part.

The importance of the ready penetration of air, water and roots thus
rendered possible is obvious; and the question arises how it happens
that wild plants are able to do without tillage.

_How Nature Tills._—When we examine the undisturbed soil of woods or
prairie in the humid region, we will as a rule find the natural surface
soil in a very good condition of tilth; the obvious cause being the
presence in it of an abundant network of surface roots and rootlets of
grasses and herbs, which in connection with the fallen foliage prevent
the beating and compacting of the soil surface; which can be seen to
happen before the observer’s eyes whenever a heavy rain falls on a bare
land surface, however well tilled.

_Crusting of Soils._—In some soils, especially of the Gulf States, the
beating of rain followed by warm sunshine so effectually compacts the
surface that in the case of taprooted plants like cotton, it becomes
necessary to cultivate after each rain, so as to break the crust that
would otherwise not only prevent the proper circulation of air, but
would also serve to waste the moisture of the land. The same land in
the wild condition suffered no such change, being protected by the
native vegetation, and by fallen leaves. (See chapt. 8).

_Soils of the arid region._—In the regions of deficient rainfall the
conditions are modified in several respects. Grass sward rarely exists,
nearly all grasses assuming the habit of growing in tufts or bunches
some distance (a foot or two) apart; hence the name of “bunch grass”
commonly used, which however means not any one definite kind of grass,
but serves to distinguish the grasses of the uplands from those of the
moist lowlands, where true sward may be found. Between these bunches
of grass the soil is fully exposed, and being free from roots and
leaf-covering is compacted, _unless its nature is such that the usually
gentle rains do not produce a serious crusting of the surface_.

That such is actually the predominant nature of the soils formed under
arid influences has already been stated; and thus the hard-baked
soil-surface so often seen in the Eastern United States in unplowed
bare land, or during the prevalence of a drought, is rarely seen in
the arid region. The clay lands that do exist are usually sufficiently
calcareous to possess the property of “slaking” into crumbs whenever
wetted after drying. But where this is not the case, the stony hardness
brought about by the long dry and warm season is long in being removed
by the winter rains.

_Changes of soil-volume on wetting and drying._—The behavior of
colloidal clay in the above respects has already been described above
(see chapt. 4, page 59). It is obvious that whenever soils contain a
large proportion of such clay, their behavior on wetting and drying
will approximate to those of the pure clay. This is exemplified in the
heavy clay, or so-called “prairie soils” of the United States, which
when thoroughly wetted in spring will, during a dry summer, form wide,
gaping cracks. These in the long summers of the arid region may extend
to the depth of several feet, with a width of as much as three and
more inches at the surface of the ground. This, of course, contributes
greatly to the drying-out of the soil to the same depth, and results
as well in the mechanical tearing of the root-system of growing
plants; sometimes causing the total destruction of vegetation. In some
clay soils it happens that after a rain or irrigation, the shrinkage
occurring upon the advent of warm sunshine will cause the surface crust
to so contract around the stem, _e. g._, of grain, as to constrict and
_injure the bark_, causing serious injury to the crop. In soils of
this character very _thorough tillage in preparing for a crop_, and
the maintenance of _a loose surface during its growth, are of course
extremely essential_.

In the arid region it will frequently happen that such soils when not
tilled to a sufficient depth, will during the later part of the summer
so shrink and crack beneath the shallow-tilled surface layer that the
latter will bodily fall into the cracks, exposing the roots to all the
deleterious influences of mechanical lesion and drying-out. It is thus
obvious that the cultivation of such soils should not be undertaken at
all by those not naturally able and willing to bestow upon them, to
the fullest extent, the deep and thorough tillage which is absolutely
essential in the utilization of their usually high productive power.

    _Extent of Shrinkage._—The extent of this shrinkage
    in drying, and subsequent expansion in wetting, have been
    measured by the writer by the use of the sieve cylinder
    described below (chapt. 11, p. 209), as serving for the
    determination of the water capacity of soils. When a soil of
    the kind above referred to is placed in the sieve cylinder
    in the tilled (flocculated) condition, then allowed to
    absorb its maximum of water and then dried at 100 degrees
    C., the contraction in drying can be very strikingly seen,
    and its amount measured by filling up the empty space with
    mercury; then measuring the latter after expelling the
    surplus by means of a ground glass plate laid on top. The
    contraction of several heavy clay soils, thus measured,
    has been found by the writer to range from 28 to as much
    as 40 per cent of the original bulk.[32] The soil thus
    contracted, when again wetted, does not return altogether to
    its original bulk, but remains in a more or less compacted
    condition, like that of a soil which has been rained upon.

[32] Wollny (Forsch. Vol. 20, p. 13 ff., 1897) records similarly high
shrinkages in his experiments.

The expansion and contraction of a heavy clay soil on wetting and
drying are well illustrated in the figure below, in which the soils are
shown in the shallow cylinder which serves for the determination of
water-holding power (see chapt. 11, p. 209). The middle figure shows
in profile the expansion of a dry, pulverized “black adobe,” struck
level, when allowed to absorb its maximum of water; it rises above
the rim of the sieve-box to nearly the half height of the latter. The
outside figure to the right shows the same soil after drying; that to
the left, a red clay soil similarly treated. It is easily seen that
these variations in volume may bring about very marked results in the
fields; the surface of which, apart from the cracks usually formed,
may be several inches lower in the dry season than during wet weather.

[Illustration: RED CLAY SOIL. BLACK “ADOBE” CLAY SOIL.

FIG. 13.—Expansion on Wetting and Contraction on Drying of heavy clay
soils.]

_Contraction on Wetting._—In the case of alkali soils containing much
carbonate of soda, a very notable _contraction_ occurs in _wetting_
the loose, dry soil. The cause is here obviously the collapse of the
crumbs, formed in dry tillage or crushing, into single grains, closely
packed. The same result is observed in the naturally depressed “alkali
spots” (see chapt. 22).

“_Hog-wallows._”—In the _field_ the wetting of cracked clay soils
produces some very curious effects. The effect of the first light rains
usually is to crumble off the edges or angles near the surface, the
materials thus loosened falling into the lower portion of the cracks.
This is repeated at each successive shower followed by sunshine,
the crevices thus becoming partly filled with surface soil. When,
subsequently, the heavier and more continuous rains wet the land fully,
also causing the consolidated mass in the crevices to expand, the
latter cannot close on account of the surplus material having fallen
into them; the result being that the intermediate portions of the
soil are compelled to bulge upward, sometimes for six or more inches,
creating a very uneven, humpy surface, well-known in the southwestern
United States as “hog-wallows.”[33]

[33] A totally different kind of “hog-wallows,” occurring in California
and the arid region generally, have been described in a previous
chapter under the head of Aeolian soils (See chapt. 1, p. 9).

Such a surface is always therefore an indication of an _extremely heavy
soil, difficult to cultivate_; yet embracing some of the most highly
and permanently productive lands known in the United States, and in
India, where the “regur” lands of the Deccan are of this character;
they have been cultivated without fertilization for thousands of years.
The subjoined physical analyses of lands of such extreme character as
to be almost uncultivatable will serve to exemplify their physical
composition.

                 PHYSICAL ANALYSES OF HEAVIEST CLAY SOILS.

    ===================================================================
                                             |No 242 Miss.|No. 643 Cal.
                                             +------------+------------
                                             |Hog-wallows |Black Adobe.
                                             |   soil.    |Contra Costa
                                             | Jasper Co. |    Co.
                                             |Mississippi.|California.
    -----------------------------------------+------------+------------
    Weight of gravel over 1.2 mm. diameter } |      .83   |
      “         “    between 1.2 and 1 mm  } |            |
      “         “    between 1 and 0.6 mm    |     1.19   |
    Fine earth                               |    97.98   |   100.00
                                             +------------+------------
                                             |   100.00   |   100.00
              FINE EARTH.                    |            |
                                             |            |
          Hydr. Value.       |   Diameter.   |            |
    -------------------------+---------------+            |
    Clay     <.0023 mm       |       ?       |    48.00   |    45.96
                             |               |            |
            { <0.25 mm       |     .010      | }  35.18   |    37.64
            {  0.25 mm       |     .016      | }          |
    Silt    {  0.5 mm        |     .025      |     5.50   |     2.74
            {  1.0 mm        |     .036      |     3.74   |     3.31
            {  2.0 mm        |     .047      |     2.54   |     2.95
            {  4.0 mm        |     .072      |      .20   |     2.39
                             |               |            |
            {  8.0 mm        |     .120      |      .27   |     1.68
            { 16.0 mm        |     .160      |      .90   |      .79
    Sand    { 32.0 mm        |     .30       |     1.67   |     2.36
            { 64.0 mm        |     .50       |     2.00   |
                             |               +------------+------------
                             |               |   100.00   |   100.00
    -------------------------+---------------+------------+------------

It will be noted that in both these extremely heavy soils the sum of
the clay and finest sediments is a little over 83%.

It should be stated that both these soils after being thoroughly
wetted become so adhesive that it is almost impossible to travel over
the tracts occupied by them, and that they are practically almost
untillable, being too adhesive when wet; yet if allowed to dry to a
certain extent (varying within very narrow limits) they turn up by
the plow in large clods, which after a few hours of sunshine become
of stony hardness and will resist all efforts at pulverization or the
production of tilth.[34]

[34] In driving a light carriage over the land represented by No. 643
above, after a light rain, the wheels gathered up so much soil within
a hundred yards as to render it necessary to stop and chop it off the
tires by means of a hatchet. This is a common experience in the black
prairie lands of Texas.

_Calcareous Clay Soils crumble on drying._—The heavy clay soils of some
of the calcareous prairies of the Southwest, instead of contracting
into a stony mass on drying, on the contrary resolve into a mass of
crumbs, thus producing excellent tilth. This occurs even though the
land may have been plowed when wet, and of course is a great advantage.
The most striking exemplification of this peculiarity occurs in the
heavy but profusely fertile “buckshot” clay lands of the Yazoo bottom,
in Mississippi, where it is usual to plant corn and sweet potatoes in
the semi-fluid mud left after an overflow, after turning a shallow
furrow, then covering by turning another. To the onlooker it seems
impossible that such plantings could be successful; but within a short
time the muddy surface becomes a bed of crumbs (“buckshot”), forming a
seed-bed not readily excelled by any made by artificial means. Hence,
largely, the almost invariable success of crops in the Yazoo region.

_Port Hudson Bluff._—The same clay produces a most unpleasant result
at the foot of the Port Hudson bluff, where it crops out some feet
above low water. When after a freshet the water level falls below this
stratum, on drying the clay disintegrates into crumbs just as does the
Yazoo buckshot soil; with the result that at the next rise, the loose
mass subsides into the river as a flood of mud. Thus the foot of the
bluff is being constantly undermined, and the falling of the bluff
scarp has obliged the town above to recede many hundreds of feet from
its original historic site.

The exact proportions of lime carbonate necessary to produce this
phenomenon, and its necessary relations to clay substance and other
physical soil ingredients, yet remain to be investigated.[35]

[35] Schübler (Grundsätzed. Agrikulturchemie, 1838) ascribes the
crumbling of calcareous clay soils to the difference in the contraction
of calcareous sand and the clay substance. But it is doubtless more
directly connected with the flocculation of the latter by lime.

_Loamy and Sandy Soils._—It is largely the _absence_ of these extreme
changes of volume that renders the cultivation of loamy or even sandy
lands so much more easy, and the success of crops so much more safe,
than is the case in clay soils. Whenever the content of colloidal clay
diminishes below 15%, the shrinkage in drying from the wet condition
becomes so slight as to cause no inconvenience; while in sandy soils
properly speaking, no perceptible change in volume occurs.

Peaty soils, however, and all those containing a relatively large
amount of humus, are also liable to visible shrinkage when passing from
the wet to the dry condition. But on account of their looseness and
porosity such shrinkage does not usually result in the formation of
cracks or rupture of the roots, as is the case in heavy clay lands. The
entire mass of the soil then shrinks downwards, but rarely forms cracks
on the surface. Hence the introduction of humus into “heavy” soils is
among the best means of improving their tilling qualities.

_Formation of Surface Crusts._—Some soils, especially those of a
clay-loam character, are very liable to the formation of hard surface
crusts from the beating of rains, and from surface irrigation; owing,
doubtless, to the ready deflocculation of their clay substance. It is
not easy to define the precise physical composition conducive to this
crust formation; but the subjoined physical analyses show examples
of soils in which this tendency is very prominent and is frequently
annoying, in that when they occur in the regions of frequent summer
rains, it becomes necessary after each one to till the surface in hoed
crops (_e. g._, in cotton-fields) in order to prevent the injurious
effects of such consolidation of the surface. It may, of course, be
prevented by mulching, or on the large scale by green-manuring, to such
extent as to prevent contraction.

    The subjoined physical analyses of two soils from the
    Brown-Loam region of Northern Mississippi (see chap. 24),
    shows the composition of lands excellent in every respect
    other than the tendency to crust after each rain:

            PHYSICAL ANALYSES OF CRUST-FORMING SOILS.

    ===========================================================
                      |Diameter.|Hydr. Value.|No. 219.|No. 197.
    ------------------+---------+------------+--------+--------
    Coarse materials  | 1-3 mm. |            | }      |
                      |         |            | }  .23 |
                    { |.5-1  “  |            | }      |
                    { |  .50    |   64 mm.   |   1.47 |
    Sand            { |  .30    |   32  “    |   2.33 |    .79
                    { |  .16    |   16  “    |   1.17 |
                    { |  .12    |    8  “    |    .78 |    .18
                      |         |            |        |
                    { |  .072   |    4  “    |    .76 |    .78
                    { |  .047   |    2  “    |   9.79 |   3.56
    Silt            { |  .036   |    1  “    |   7.20 |  13.12
                    { |  .025   |     .50    |  13.11 |  16.64
                    { |  .016   |     .25    |  15.07 |  27.28
                    { |  .010   |    <.25    |  26.36 |  18.87
                      |         |            |        |
    Clay              |    ?    |    <.0023  |  19.10 |  17.23
    ------------------+---------+------------+--------+--------

    These soils agree in having a sufficient amount of clay (17
    to 19%) to characterize them as clayey loams, associated
    with a very large proportion of the grain-sizes of less than
    .025 mm., or .5 mm. hydraulic value. A higher proportion of
    clay, even though associated with a similarly high or even
    larger proportion of these fine sediments, seems to prevent
    crusting, probably because the swelling of the clayey
    ingredient on wetting and its extravagant contraction in
    drying breaks up the continuity of the surface. The heaviest
    clay soils, such as those shown on a preceding page, neither
    crust nor crumble on drying after wetting, but contract into
    lumps of stony hardness, _as a whole_.

The burning-out of the humus from well-tilled surface soils during the
extended heat and dryness of rainless summers, brings about such a
contraction or packing of the surface soil of orchards in California
as to greatly reduce their productiveness, and to render necessary
diligent green-manuring as the only practical remedy. In many cases,
liming of the surface also serves well to prevent this injurious
effect, which to some extent of course follows surface irrigation as
well as rains.

In most soils, repeated alternate wetting and drying _in place_
produces a loose, flocculated texture, so long as no deflocculation is
brought about by mechanical causes, such as beating rains or running
water.

_Effects of Frost on the Soil._—The expansion suffered by water in
freezing necessarily tends to separate the soil particles previously
held together by the surface tension of the capillary water, or
otherwise flocculated or cemented. Freezing of the soil is therefore of
material assistance in _disintegrating_ cloddy, ill-conditioned soils,
leaving them in loose, crumbly condition after the ice has melted and
the surplus water drained off; so as to materially facilitate tillage
and root penetration. When, however, soils thus circumstanced are
tilled or trodden while too wet, they quickly become puddled, being
practically reduced to single-grain structure. (See this chapt. p.
110). Hence the injury caused by allowing cattle to range in winter on
cultivated land subject to freezing and thawing, which it sometimes
takes years to correct.

A disagreeable effect often produced by the freezing and thawing of
wet lands is the “heaving-cut” of grain, resulting from the upward
expansion of the surface soil in freezing, that may readily rupture the
roots; while on thawing, the soil surrounding the upheaved stool is apt
to settle down, especially in case of a rain, leaving the stool and
roots exposed either to drying or freezing, as the case may be. Hence
the desire of grain farmers in northern climates, for a sufficient
covering of snow to protect the fall-sown grain, rather than an “open
winter,” during which the grain is exposed to alternate freezes and
thaws, or extreme cold.

In certain soils, notably in those liable to crusting (p. 117), instead
of heaving the soil, the water in freezing emerges bodily from small
cracks, in foliated or wire-like forms (“ice-flowers”) resembling those
of native silver, and formed substantially in the same way, by a kind
of “wire-drawing” process, aided by crystallization.

    Small ice-crystals formed on the surface of small crevices
    filled with water cause others to be formed at their lower
    ends, and the expansion occurring in freezing, forces the
    ice upward; the process repeating itself under favorable
    conditions, until the stalks or sheets of ribbed ice grow to
    a height of several inches. This phenomenon is especially
    frequent in the middle cotton States—Arkansas, Tennessee,
    northern Mississippi, etc., where frequent changes from
    rainstorms or thaws to cold northwest winds occur in winter.




CHAPTER VIII.

SOIL AND SUBSOIL.


CAUSES AND PROCESS OF DIFFERENTIATION. HUMUS.

_Soil and Subsoil Ill-defined._—While the general mass of rock
debris formed by the action of the agencies heretofore discussed as
soil-material, may under proper conditions become soil capable of
supporting useful plant growth, universal experience has long ago
recognized and established the distinction between soil and subsoil:
by which are ordinarily meant, respectively, the portion of the
soil-material usually subjected to tillage, and what lies beneath.
There can be no question about the practical importance of this
distinction; but the definition of the two terms, as commonly given in
some works of agriculture, is both incomplete and, in its application
to many cases, partly misleading.

The differentiation of soil and subsoil is due partly to the action of
organic matter and micro-organisms, partly to physico-chemical causes,
now to be discussed in detail.


THE ORGANIC AND ORGANIZED CONSTITUENTS OF SOILS.

_Humus in the Surface soil._—The most obvious mark of distinction
between soil and subsoil is, usually, the darker tint of the former,
due to the presence of humus or vegetable mold, which becomes most
apparent by darkening of the tint when the soil is moistened. Thus
soils having a gray tint when dry, may become almost black when wetted.
When no such deepening of color occurs in wetting, the absence or great
deficiency of humus may safely be inferred. The only other substance
whose presence may invalidate the conclusions based upon the darkening
of the soil tint, is ferric hydrate (iron rust), which itself possesses
the property of darkening on wetting, and may effectually cover either
the presence or the absence of humus.

Since the formation of the humus depends upon the decomposition of
organic matter (mostly of the cellulose group) derived partly from
the roots, partly from the leaves and stems of plants growing and
dying on the soil, its accumulation near the surface is natural. But
since the depth to which roots penetrate varies greatly not only with
different plants, but very essentially in conformity with the greater
or less penetrability of the soil and subsoil, the depth to which the
dark humus tint may reach vertically varies correspondingly, from two
or three inches to several feet. In the case of soils that have been
formed by the gradual filling-up of swamps or marshes, the humus-tint
may reach to several yards depth.

_Surface Soil, and Subsoil._—It is thus apparent that the term “surface
soil,” while commonly confined by the farmer to the portion turned by
the plow or usually reached in cultivation by any implements, may or
may not belong, functionally, to layers of greatly varying thickness.
Similarly the term _subsoil_ may or may not refer, in individual
cases, to parts of the soil mass materially different from the surface
soil. Yet this distinction is of no mean practical importance, because
the efficacy of one of the most common measures of soil improvement,
viz., subsoil plowing or “_subsoiling_,” depends materially upon the
differences between soil and subsoil in each particular case. Most of
the diversity of opinion regarding the merits of this operation is
simply the result of a corresponding diversity in the natural facts and
cultural practice of each case.

_Causes of the Differentiation of Soil and Subsoil._—One of the
prominent points of difference between surface soils and subsoils has
already been mentioned in the usual predominance of root-mass in the
upper layers; to which is added a part at least of the substance of
fallen leaves and stems of its vegetation. How much of this vegetable
mass ultimately becomes converted into humus, as well as the nature of
the product formed, depends upon a great variety of circumstances; some
of which have already been mentioned in connection with the general
discussion of humification (chapt. 2, p. 20). Briefly stated, the main
controlling conditions are: the amount of water or moisture present,
the access of air (oxygen), a proper temperature, and the presence of
the several organisms which in the course of time take part in the
process of soil-formation.

_Ulmin Substances; Sour Humus_ (Germ. Rohhumus).—In the presence of
so much moisture or liquid water as will materially impede the access
of air, and with the concurrence of reasonably low temperatures, the
organisms that at first take the chief role in the transformation of
the vegetable tissues into humus-like substances are bacteria. But the
antiseptic nature of the compounds thus formed[36] soon puts an end
to their activity, and thereafter the process seems to be a purely
chemical one, and very slow. In peat bogs, the transition from the
fresh, dead stems and roots to brown peat is easily followed downward,
white cellulose fibers remaining apparently unchanged to some depth;
so that such fiber has been used for tissues and paper. The solid
decomposition-products are brown substances, partly soluble in water
and imparting to it a brown or coffee color (frequently seen in the
drains of marshes) and an acid reaction; the latter due to ulmic (as
well as apocrenic) acid, readily soluble in caustic and carbonated
alkalies, and forming insoluble salts with the earths and metals; while
another portion, ulmin, is insoluble in the same, but gradually becomes
soluble by oxidation.

[36] The antiseptic properties of sour humus are well exemplified in
the perfect state of preservation in which the remains of animals,
wood implements, etc., are found in bogs into which they have sunk in
prehistoric times.

The gaseous products formed under these conditions are carbonic dioxid
and “marsh gas” (methan, CH₄), the former predominating in the early
stages; while later, the carburetted hydrogen predominates, rendering
the gas readily inflammable.

_Sour Soils._—The “sour” soils thus produced in nature in presence of
excess of water bear only “sour” growth, such as sedges and rushes, of
little agricultural value; they usually require reclamation processes
before becoming adapted to ordinary crops. In old forests of northern
climates a peaty and more or less acid layer is sometimes formed on the
surface, above the black woods-earth, and retards somewhat the full
production of such land when taken into cultivation.[37]

[37] See Müller, Natürliche Humusformen.

Marshes and swamps, both fresh and salt, as above stated usually
show coffee-colored waters, which are also characteristic of the
streams that drain them, until by intermixture with waters containing
lime salts, the ulmic substances are neutralized and precipitated.
Such neutralization, preferably by means of lime, is the first step
towards the reclamation of lands bearing “sour” vegetation. The acid
reaction characterizing the ulmic substances is also characteristic
of many woodlands, notably in the United States of the soils of
the “Long-leaf-pine” region of the Cotton States, both upland and
lowland, as well as of many deciduous forests in northern climates.
Hence liming, whether artificial or natural, effects a most notable
improvement, together with a marked change of vegetation, in these
lands.

    It has been long known that after long-continued
    cultivation, soils originally of neutral or slightly basic
    reaction become acid: and the liming of such lands is an
    ancient practice in Europe. The matter, however, received
    but scant attention until Wheeler and Hartwell, of the
    Rhode Island Experiment Station, demonstrated the almost
    universal acid condition of the older lands of that State,
    and the excellent effects produced by neutralization with
    lime, or even with the alkali carbonates.[38] The current
    neutralization of the humus-acids is unquestionably one of
    the cardinal advantages of calcareous lands; for such as
    contain only small amounts of lime carbonate will of course
    become acid more quickly under cultivation.

[38] Reports of the Rhode Island Exp’t Station, 1895, and ff.

_Humin Substances._—In the presence of only a moderate amount of
moisture, therefore under the influence of a more or less rapid
circulation of air, and in the presence of earthy carbonates
(especially that of lime) to prevent the formation of acids, or to
neutralize them as formed, the normal process of humification occurs;
mainly under the influence of fungous instead of bacterial growths. The
various molds take a prominent part in the conversion of the vegetable
substance into black, neutral, insoluble humus compounds. Such fungous
vegetation is always accompanied by the evolution of carbonic gas,
and the resulting fungous tissues are markedly richer in nitrogen and
carbon than the substance of the higher plants from which they were
derived (see chapt. 9). Comparative analyses show that in the normal
process of humification of vegetable substances, oxygen and hydrogen
are eliminated in the form of water and carbonic dioxid, while at
the same time there is an increase in the percentage of carbon, and
generally also of nitrogen; the latter more particularly in the case of
vegetable matter not very rich in that element. When once humification
is complete, oxidation, especially under arid conditions, bears mainly
upon the carbon and hydrogen, so that the nitrogen content may rise to
very high figures; while another portion is ultimately wholly oxidized,
with the formation of nitrates, under the influence of the nitrifying
bacteria, this being the process chiefly efficient in the nutrition of
vegetation with nitrogen.

    As a matter of course, the several organic compounds
    contained in plants may continue to exist in soils for
    some time, varying according to conditions of temperature
    and moisture. Thus dextrin, glucose, and even lecithin and
    nuclein have been reported to be found. The activity of
    the numerous fungous and bacterial ferments under favoring
    conditions will, of course, limit the continued existence of
    such compounds somewhat narrowly, so that they can hardly be
    considered as active soil ingredients save in so far as they
    favor the development of the bacterial flora.

_Porosity of Humus._—One of the essential features of natural humus is
its great porosity, whereby it not only becomes highly absorbent of
water and gases, but is also gradually oxidized, probably under the
influence of bacteria. For this oxidation, as measured by the evolution
of carbonic gas, progresses most rapidly under the same conditions
as to moisture, temperature and access of air, that are known to be
most favorable to fungous and bacterial growth. Hence the formation
of carbonic dioxid in the soil is assumed to be the measure of the
intensity of such activity.

_Physical and Chemical Nature of the Humus Substances._—The humus
substances are gelatinous when moist, but are neither markedly adhesive
or plastic. Like the other colloidal substances of the soil, they serve
to retain both gases and vapors, including moisture, liquid water,
and its dissolved solids. In the natural, porous condition they are
powerfully absorbent of gases, including especially aqueous vapor.
Dry humus swells up visibly when wetted, the volume-weight increasing
to the extent of two to eight times; so that humus stands foremost in
this respect among the soil constituents. The _density_ of natural
humus is about 1.4, being the lightest of the soil constituents. Hence
soils rich in humus are “light” not only in the farmer’s sense of being
easily tilled when not too wet, but also of light weight for equal
volumes when compared with clayey and sandy soils. Some data bearing
upon these points are given in the table[39] below, for the substances
moderately and uniformly packed:

            VOLUME-WEIGHTS OF

    Humus.[40]    Clay.    Quartz Sand.

     .3349      1.0108       1.4485

When saturated with water, the same substances gave the following
figures:

                    Air-dry.   Saturated     Increase.
                               with water.       %

    Humus[41]          .3565      1.1024        209.2
    Clay             1.0395      1.6268         55.9
    Quartz sand      1.4508      1.8270         25.9

[39] Wollny, Zersetzung der Organischen Stoffe, pp. 242, 243.

[40] Peat pulverized and extracted with alcohol and ether to remove
resinous substances.

[41] Peat pulverized and extracted with alcohol and ether to remove
resinous substances.

These data show strikingly the effects produced by the several physical
soil constituents upon some of its physical properties.

_Chemical Nature._—While humus artificially produced by the action of
caustic alkalies upon sugar or cellulose is free from nitrogen, all
naturally occurring humus contains the latter.

    It is not, however, present in the form of ammonia, as it
    cannot be set free by treatment in the cold with lime or
    alkalies. When, however, natural humus is _boiled_
    with these substances, ammonia is slowly given off, but the
    process continues indefinitely and it seems to be impossible
    to expel all the nitrogen in this manner. This behavior
    being characteristic of amido-compounds, it is presumable,
    in view of the slightly acid nature of the humus substances,
    that natural humus is largely of an amidic constitution.
    Artificial humic acid, formed by the action of caustic
    alkalies upon sugar, gums or cellulose, combines with
    ammonia as with other bases, and at first the ammonia
    can be readily expelled from this as from other ammonia
    salts. But after the lapse of some time it seems that the
    amidic condition is assumed, so that caustic lye acts but
    very slowly and cannot expel the whole of the nitrogen
    present. This is very important in connection with the
    practice of fertilization, as any ammonia taken up by or
    generated in the soil is thus in the course of time rendered
    comparatively inert, and unavailable to vegetation until
    nitrified.

_Progressive Changes._—The natural neutral humin and ulmin, as found,
_e. g._, in the lower portions of peat beds, are in the course of time
by oxidation converted into ulmic and humic acids, capable of combining
with bases; by still farther oxidation they form apocrenic and crenic
acids, readily soluble in water and in part forming soluble salts with
lime, magnesia and other bases. These acids act strongly upon the more
readily decomposable silicates of the soil, and in the course of time
may dissolve out, and aid in the removal by leaching, of most of the
plant-food ingredients as well as the ferric hydrate of a soil. Thus
red or rust-colored soils may be rendered almost white by continued
“swamping” with stagnant water, and be greatly impoverished; and it is
doubtless largely through this agency that the underclays of coal beds
and the lower portions of peat beds, as well as peat and coal ashes,
are almost wholly destitute of mineral plant food.

_The Phases of Humification._—The progressive changes involved in
the process of humification of vegetable matter are illustrated in
the table below,[42] together with the farther changes by which such
matter may ultimately be transformed into the several varieties of
coal, and finally into anthracite, which already represents nearly pure
carbon, but in nature has sometimes been still farther transformed into
graphite (black-lead) and diamond.

[42] Data recalculated, omitting ash.

       PROGRESS OF HUMIFICATION, AND FORMATION OF COAL.
        (MOISTURE AND ASH OMITTED FROM CALCULATIONS.)
    ========+==========+======================+============+
            |          |      Oak Wood.       |            |
            |          +------+--------+------+   Humin    |
            |Cellulose.|Fresh.|Decayed.|      |    and     |
            |          |      +--------+------+   Humic    |
            |          |      | Light  | Dark |   Acid.    |
            |          |      | Brown. |Brown.|            |
    --------+----------+------+--------+------+------------|
    Carbon  |  44.44   | 50.60|  53.60 | 56.20|49.4 to 59.7|
    Hydrogen|   6.17   |  6.00|   5.20 |  4.90| 2.5 “   4.5|
    Oxygen  |  49.38   |      |        |      |35.8 “  47.3|
            |          | 43.40|  41.20 | 38.90|            |
    Nitrogen|          |      |        |      |  .3 “  18.7|
    --------+----------+------+--------+------+------------+

    ========+======================+================================
            |       Peat.[43]      |            Coals.
            |--------+-------------+--------+-----------+-----------
            | Brown  |   Black.    | Lignite|  Scotch   |  Penn’a
            |Surface.+------+------+  Brown |  Splint   |Anthracite.
            |(Ulmin.)|40 in.|80 in.|  Coal. |Bituminous.|
            |        |      |      |(Bovey).|           |
    --------+--------+------+------+--------+-----------+-----------
    Carbon  |  57.80 |62.00 |64.10 |  69.50 |   84.20   |   94.80
    Hydrogen|   5.40 | 5.20 | 5.00 |   5.90 |    5.80   |    2.60
    Oxygen  |  36.00 |30.70 |26.80 |  24.00 |    8.80   | }
            |        |      |      |        |           | }  2.60
    Nitrogen|   .80  | 2.10 | 4.10 |    .60 |    1.20   | }
    +-------+--------+------+------+--------+-----------+-----------

[43] Detmer, Landw. Versuchst., Vol. 14, 1871.

The steady increase of carbon and nitrogen, together with a
corresponding decrease of oxygen, are well illustrated in the analyses,
especially in the strictly comparable series of peat samples from
various depths. In this case there is also a steady decrease of
hydrogen, and an increase of ash from 2.72% in the surface layer, to
9.16 at 80 inches depth. This increase is due in the main, of course,
to the progressive volatilization of the organic matter in the forms of
carbonic dioxid and marsh gas (methan, CH₄).

In considering this table it should not be forgotten that while normal
humus stands very close to peat, and the latter when compressed in
certain stages would be undistinguishable from lignite or brown coal;
yet both peat and lignite are known to be formed under conditions
permitting much less access of air or oxygen than occurs in the
formation of normal black soil-humus. Hence even black peat cannot at
once stand in place of soil-humus when removed from its watery bed, but
requires considerable time and aeration (oxidation), and in most cases
neutralization with lime or marl, before it can serve the purposes of
humus in the soil.

    Lignite and the progressively more carbonaceous coals
    are and have been formed under the conjoined action of
    submergence and pressure, sometimes also aided by heat; and
    thus they cannot perform the function of soil-humus, any
    more than the fire-clays or shales underlying them can resume
    their original soil-functions without prolonged weathering.

    _Amounts of Humus and Coal Formed from Vegetable
    Matter._—Only very general and indefinite estimates
    can be given of the amount of humus or coal formed from a
    given quantity of vegetable matter, since these must vary
    according to the conditions under which the transformation
    occurs. The greater or less access of air and of moisture,
    the temperature and pressure under which the process occurs,
    will modify very materially the quantitative as well as
    the qualitative result. In the hot arid regions the fallen
    leaves may wholly disappear by oxidation on the surface of
    the ground, while under humid conditions they are mostly
    incorporated with the surface soil. If we assume that in
    the humification of plant debris (estimating their average
    nitrogen content at 1%), no nitrogen is lost, it would seem
    that in the humid region one part of normal soil-humus might
    be formed from 5 to 6 parts of (dry) plant debris; while
    in the extreme regime of the arid regions, from 18 to 20
    parts of the same would be required. But as most probably
    some nitrogen also is lost in the process of humification, a
    considerably larger proportion of original substance may be
    actually required.

[Illustration: FIG. 14.—Section of lignitized log showing contraction
into solid lignite on drying.]

    As to coal, it is usually assumed that it requires about
    8 parts of vegetable matter for one of bituminous coal.
    Much higher estimates are made by some, and an observation
    made by the writer at the Port Hudson bluff, Mississippi,
    in 1869, would seem to justify such estimates. The
    above figure, from a sketch made at the time, shows the
    proportions to which a pine log about eight inches in
    diameter had shrunk in drying into a small sheet of
    lignitized wood; the original trunk, projecting from a bed
    of sand some forty feet below the surface, being so porous
    and spongy that when wet it flattened somewhat by its own
    weight; it was connected with the little sheet of lignite by
    a spirally twisted, tapering stipe.

    Here evidently the proportion of lignite formed was a very
    minute one, doubtless because of the long leaching to which
    the trunk had been subjected. It thus seems impossible, as
    in the case of humus, to assign any definite proportion as
    between woody matter and coal formed from it.

_Normal humification_ takes place only under the influence of moderate
temperature. When the temperature is too low, bacterial and fungous
growth are repressed or arrested; when too high, the fungous vegetation
assumes a different phase, the result of which is the almost total
oxidation of the organic matter, sometimes so accelerated as to
initiate rapid combustion “fire-fanging” of dung; leaving in any case
but a trifling organic residue of very high ash contents.[44]

[44] A striking illustration of this is afforded by Naegeli’s
experiment of enclosing several loaves of bread in a loosely closed
tin-box. After eighteen months there remained only seventeen per cent
of air-dry mouldy matter, totally destitute of starch.

_Eremacausis._—In the absence of a sufficient degree of moisture to
co-operate with the other agencies of humification, the final result
in the soil is practically the same as in the “fire-fanging” of dung.
The organic matter is almost wholly destroyed by direct oxidation
(eremacausis) with or without the aid of minute organisms; leaving
essentially only the ash behind to be reincorporated with the soil.
This is to a very great extent the predominant process in the arid
regions of the Globe; most of the soils formed in these climates being,
therefore, very poor in humus-substances, and deriving it almost
entirely from the decay of roots only.

The extent to which the humus of a soil may be derived from the
vegetable debris falling or growing upon the surface, varies greatly
with the climatic conditions as well with the nature of the soil. In
the forests of humid climates with loamy soils, not only does the
autumnal leaf-fall, as well as decaying twigs and trunks, become
obviously incorporated with the surface soil as decay progresses on the
lower surface, but active animal agencies (see below) carry the organic
remnants bodily down. But where heavy clay soils prevail, these animal
agencies are much restricted by the compactness of the material; only a
light surface-layer of mold would be formed, and the humus of the lower
soil layers must of necessity be derived from the decay of the roots
only. This origin is claimed by Kosticheff[45] for the high content
of black humus in the tchernozem or black earth of Russia. Following
Hellriegel in determining the weight of roots contained in successive
equal layers of soil from the surface downwards, Kosticheff gives for
each six inches the following data as found in the tchernozem, taking
as 100 the root-content of the surface layer:

    ===========+======+======+======+======+======+======
      Number.  |  1   |  1   |  2   |  2   |  3   |  3
      Depth.   |Roots.|Humus.|Roots.|Humus.|Roots.|Humus.
    -----------+------+------+------+------+------+------
     6 inches. |100.  | 5.42 |100.  | 8.11 |100.  | 9.64
    12   “     | 89.1 | 4.83 | 63.9 | 5.19 | 80.3 | 7.77
    18   “     | 66.9 | 3.62 | 48.3 | 3.92 | 70.0 | 6.71
    24   “     | 47.3 | 2.56 | 35.0 | 2.84 | 58.4 | 5.61
    30   “     | 47.3 | 2.59 | 26.0 | 2.11 | 38.2 | 3.57
    36   “     | 34.6 | 1.88 | 18.1 | 1.47 | 33.0 | 3.18
    42   “     | 23.9 | 1.29 |  6.3 |  .51 | 16.2 | 1.56
    48   “     | 14.4 |  .78 |      |  .70 |      |
    54   “     |  6.7 |  .36 |      |      |      |
    -----------+------+------+------+------+------+------

[45] Abstract in Ann. de la Science Agronomique, Tome 2, 1887.

    It will be seen that there is a very close correspondence of
    the humus content with the root development in the several
    layers, and it seems as if though but little of the humus
    could be derived from the surface growth, which is that of
    the grasses of the steppe.

    The climate of the black-earth country of Russia is, though
    not properly arid, yet one of rather deficient and uncertain
    rainfall. But as a consequence of extremely arid conditions,
    and in sandy lands, it may even happen that the immediate
    surface soil contains _less_ humus than what, in the
    farmers’ habitual parlance, would be called the subsoil;
    because of the penetration of slow combustion for some
    distance into the porous soils. It will then be lower down
    that, in the presence of a favorable degree of moisture and
    lower temperature, the conditions of normal humification are
    fulfilled.

It is not always, then, that the commonly recognized distinction
between surface soil and subsoil based upon humus content can be
maintained. But the observation of everything bearing upon this point
is of the utmost importance in determining both the agricultural value
and the mode of treatment of the land.

_Losses of Humus from Cultivation and Fallowing._—The fact that humus
accumulates in woodlands and meadows, where no cultivation is given,
would naturally lead to the converse conclusion, viz., that cultivation
causes loss of humus and of its constituents. That this is actually
the case is recognized and widely acted upon in practice, and there is
no question that the general acceptance of stable manure as the most
widely useful fertilizer, despite its usually low content of plant-food
ingredients, is based upon the fact that it supplies vegetable matter,
in a condition highly favorable to its conversion into humus. The most
direct and cogent proof of the depletion of the soil of both humus and
nitrogen by continuous cultivation of cereal grains has been given
by Snyder,[46] who determined the loss both of humus and of nitrogen
suffered by a Minnesota soil during eight years’ continuous cultivation
of wheat. The total loss of nitrogen was 1700 pounds per acre, while
only 350 pounds were utilized by the crop; about 1400 pounds being
dissipated as gas or leached out as nitrates. A conservative estimate
of the loss of humus suffered during the same period was about a ton
per acre annually, and this loss seriously decreased not only the
nitrogen-content, but rendered the soil more compact and less retentive
of moisture. But by rotation of the wheat with clover in alternate
years, very nearly an equilibrium of both humus and nitrogen-content
was obtained. In addition, the amount of available mineral plant-food
was decreased by continuous grain culture. Ladd has made similar
observations in North Dakota, with similar results.

[46] Bull. No. 70 Minn. Exp’t Station, 1905.

That excessive aeration results in serious losses of humus as well as
of nitrogen, is very obvious in the arid region, where it is the habit
to maintain on the surface of orchards and vineyards during the dry,
hot summers, a thick mulch of well-tilled soil, thus preventing loss of
water by evaporation. In the course of years this surface soil becomes
so badly depleted of humus that good tilth becomes impossible, the soil
becoming light-colored and compacted; while the loss of nitrogen is
indicated by the small size of the orchard fruits. Similar losses are
of course sustained in the practice of bare summer-fallow, which at one
time was almost universal in portions of the arid region. The complete
extirpation of weed growth thus brought about, at first considered an
unmixed benefit, has ultimately had to be made up for by the practice
of green-manuring; since in the arid region the use of stable manure
encounters many difficulties.

    _Estimation of Humus in Soils._ It has been usual to
    determine the amount of humus in soils by means of (dry or
    wet) combustion, calculating the humus from the carbonic
    dioxid so formed, while measuring the nitrogen gas directly.
    But in this process the entire organic matter of the soil,
    humified and unhumified, is indiscriminately included;
    and it is wholly uncertain to what extent the latter will
    ultimately become humus, from the nitrification of which
    plants are presumed to chiefly derive their nitrogen.[47] In
    order to obtain definite results, the actual, functional
    humus must be extracted from the soil mass by some solvent
    which discriminates between the humified and unhumified
    organic matter. This cannot be done by direct extraction
    with caustic soda or potash, which inevitably dissolve
    unhumified matters and tend to expel ammonia from the humus;
    besides themselves acting as humifiers (see this chapter, p.
    125.)

    _Grandeau Method: Matière Noire._—The only method
    now known which accomplishes this separation, practically
    excluding the unhumified while fully dissolving the humified
    matter—is that of _Grandeau_: the extraction of the
    soil, first with dilute acid, in order to set the humic
    substances free from their combinations with lime and
    magnesia; and their subsequent extraction with moderately
    dilute solutions of ammonia (or other alkali hydrates). Upon
    the evaporation of the ammonia solution the humus is left
    behind in the form of a black lustrous substance (“matière
    noire” of Grandeau) much resembling the crust of soot formed
    in flues from wood fires. As it contains a variable amount
    of ash, it must be burnt and the ash subtracted from the
    first weight.

[47] The humus determinations thus made, which include nearly all
those made by German chemists, give the humus-content from 40 to 50%
too high. The French determinations are mostly made by the method of
Grandeau.

_Amounts of Humus in Soils._—While in peat, marsh and muck lands the
humus-content may rise above twenty per cent, in ordinary cultivated
lands it rarely exceeds about five per cent, and very commonly falls
below three per cent, even in the humid regions. In properly arid soils
we find a very much lower average, rarely exceeding one per cent,
and frequently falling to .30 and even less. This scarcity of humus
manifests itself plainly in the prevalently light gray tint of the arid
soils.

Meadows and woodlands generally show the highest humus-content in their
surface soils, gradually increasing while in that condition; while when
taken into cultivation the humus-content gradually decreases, owing
to the free aeration and consequent “burning-out” caused by tillage.
Hence the humus must be from time to time replaced by the use of stable
manure, or green-manure crops, to prevent injurious changes in the
tilling qualities of the land. Not only humus as such, but according to
Schloesing also the insoluble colloid humates, produce in the soil a
loosening effect or tilth (Germ. Bodengare), which apparently cannot be
brought about by any other substance.[48]

[48] The decrease of humus from wheat culture in the soils of
Minnesota and North Dakota has been studied by H. Snyder and E. F.
Ladd, respectively. In the prairie lands of the latter State the total
organic matter in the first six inches of soil ranges from 15 to as
much as 26%, and the humus alone from 4 to 7.8%.

_Humates and Ulmates._—That the insoluble humates of lime, magnesia,
iron, manganese and alumina are present in most soils is conclusively
shown by the composition of the solution obtained by the extraction
of soils with weak acid, as above mentioned in connection with the
quantitative determination of humus according to Grandeau; since
these bases are almost always extracted by the weak acid. When the
brown solution of alkali humate obtained in this process is carefully
neutralized with sulfuric or hydrochloric acid, or is mixed with
solutions of the above bases, flocculent, insoluble precipitates are
formed, while the solution is discolored. Similar precipitates may be
obtained with other metallic solutions, notably with that of copper,
which precipitates the humus-acids most completely. Doubtless these
compounds contribute greatly to the conservation of the humus-content
of soils, protecting it to a certain extent from oxidation, and also
preventing excessive acidity. The brown tint of certain subsoils in
the northern humid regions have been shown by Tollens and others to
be due not to ferric hydrate, as had been supposed, but to calcic,
magnesic and aluminic humates. _None of the mineral bases or acids
present can be detected in the humic solution by the usual reagents._

_Mineral Ingredients in Humus._—That the mineral plant-food ingredients
present in the humus extracted by the Grandeau process, and which
remain as ash when the matière noire is burned, are capable of
nourishing plant growth, was directly shown by Grandeau, Snyder and
others. The former was inclined to consider that those substances were
mainly thus taken up by plants, under natural conditions. This theory,
however, has not been sustained by subsequent investigations; the
mineral plant-food thus extracted is not a measure of the immediate
productiveness of the soils, as demonstrated by Snyder, and the
residual soils are not sterile. It is also still doubtful to what
extent the mineral bases and acids are _naturally_ combined with the
humus-substances, it being contended by some that they are brought
into organic combination by the acid and ammonia extraction. The
investigations of Snyder and Ladd, above referred to, prove however to
some extent at least that the humus-substances are naturally combined
with them, and that probably they are largely made available to plants
through the direct and indirect action of the humus compounds. This
subject is farther considered in chapter 19.

The nature and amounts of these mineral substances are well exemplified
in the subjoined full analysis by Snyder, of the ash of the humus and
humates extracted from a compound sample of prairie soils of Minnesota,
which had been thrown down from the ammonia solution by simply
neutralizing the liquid:[49]

    ASH OF HUMUS FROM MINNESOTA PRAIRIE SOILS.

    Insoluble matter[50]          61.97
    Potash (K₂O)                  7.50
    Soda (Na₂O)                   8.13
    Lime (CaO)                    0.09
    Magnesia (MgO)                0.36
    Peroxid of Iron (Fe₂O₃)       3.12
    Alumina (Al₂O₃)               3.48
    Phosphoric acid (P₂O₅)       12.37
    Sulfuric acid (SO₃)            .98
    Carbonic acid (CO₂)           1.64

[49] Precipitation with an excess of acid does not greatly change the
results.

[50] In California soils this is mostly silica soluble in carbonate of
soda.

The large amounts of the soluble alkalies potash and soda thrown
down with the humic matters are very striking, as is the very large
proportion of phosphoric acid. Lime and magnesia had, of course, been
mainly eliminated by the preliminary acid treatment.

_Functions of the Unhumified Organic Matter._—The unhumified plant
debris in the soil are not to be regarded as useless, even aside
from their potential conversion into active humus. Not only do these
remnants of vegetation lighten the soil, rendering it more pervious
to air and water, but in their progressive decay they give off
carbonic gas, which is active in soil-decomposition; and they serve as
nourishment to the soil bacteria upon which its thriftiness so greatly
depends. See below, chapter 9.

_The Nitrogen-Content of Humus._—Since soil-humus is doubtless the
chief depository of soil-nitrogen, and the main source from which,
through the process of nitrification, the nitrogen-supply to plants
is usually derived, its content of that element is a matter of
great interest. It has been customary to estimate approximately the
nitrogen-content of soils by the proportion of humus-substance present;
and as the light tints of the soils of the arid region indicate a small
humus-content, a scarcity of nitrogen seemed to be also indicated
for these lands. As this in a number of cases did not seem to accord
with actual experience, an investigation of the subject was made at
the California experiment station,[51] with the results shown in the
subjoined table. In considering these results it must be kept in mind
that while arid conditions can rarely be fulfilled in the humid region,
humid conditions are quite frequently locally represented in the arid,
in lowlands and on high mountains; while moderately moist benchlands
represent the semi-arid regime.

[51] _Hilgard and Jaffa._ On the Nitrogen-content of Soil-humus in the
Humid and Arid regions. Rep. Cal. Exp’t Station for 1892-4; Agric.
Science, April, 1894; Wollny’s Forsch. Geb. Agr. Phys., 1894.

                 HUMUS PERCENTAGE AND NITROGEN CONTENT IN
                   SOILS OF THE ARID AND HUMID REGIONS.
      =====+==============================================+=====+========+========
           |                                              |Humus|Nitrogen|Nitrogen
    Station| Soils arranged in order of nitrogen          | in  |   in   | in
     Number|        percentages in humus.                 |soil,| Humus, |soil,
           |                                              | per |  per   | per
           |                                              |cent.| cent.  |cent.
      -----+----------------------------------------------+-----+--------+--------
           |   SOILS OF THE ARID REGION (California).     |     |        |
           |                                              |     |        |
      2061 |Dark clay loam, Arroyo Grande Valley,         | 3.06|  22.00 |  .670
           |    San Luis County                           |     |        |
      2291 |Red soil, Orland, Glenn Co.                   |  .71|  21.10 |  .150
      1904 |Sediment Soil, Porterville, Tulare Co.        |  .90|  19.50 |  .180
      1901 |Sandy soil near Ceres, Stanislaus Co.         |  .64|  18.75 |  .120
       704 |Sandy soil of plains, near Fresno, Fresno Co. |  .60|  18.66 |  .112
         6 |Black adobe soil, Stockton, San Joaquin Co.   | 1.05|  18.66 |  .196
      1679 |Black adobe soil, Berkeley, Alameda Co.       | 1.20|  18.58 |  .203
      2324 |Clay soil of desert, Imperial, San Diego Co.  |  .38|  18.40 |  .070
      1167 |Black clay loam soil, near Tulare, Tulare Co. | 1.66|  18.19 |  .302
      1536 |Brown loam soil, Windsor Tract, Riverside,    |  .20|  18.00 |  .036
           |    Riverside County                          |     |        |
      1126 |Sandy loam soil, Paso Robles,                 |  .55|  17.27 |  .095
           |    San Luis Obispo Co.                       |     |        |
      2301 |Red hill soil, Upper Lake, Lake Co.           |  .81|  16.90 |  .137
      1607 |Plateau soil of desert, Lancaster,            |  .25|  16.80 |  .042
           |     Los Angeles Co.                          |     |        |
      1159 |Sandy plains soil, Tulare, Tulare Co.         |  .37|  16.75 |  .062
      1900 |Sandy soil, near Modesto, Stanislaus Co.      |  .84|  16.65 |  .140
      1113 |Clay loam soil (slate), Jackson, Amador Co.   |  .54|  16.60 |  .090
      1149 |Adobe clay soil, near Paso Robles,            |  .47|  16.18 |  .074
           |  San Luis Obispo County                      |     |        |
      1538 |Mesa soil, Chino, San Bernardino Co.          |  .65|  16.08 |  .105
      1147 |Sandy loam soil, Paso Robles,                 |  .66|  16.06 |  .106
           |    San Luis Obispo Co.                       |     |        |
      2403 |Valley Soil, Wheatland, Yuba Co.              | 1.50|  16.00 |  .240
      1281 |Red Mesa soil, Pomona, San Bernardino Co.     |  .58|  15.50 |  .090
      1117 |Sandy granitic soil, near Jackson, Amador Co. |  .80|  15.27 |  .123
      1406 |Red loam soil, Arlington Heights, Riverside,  |  .30|  15.00 |  .045
           |  Riverside County                            |     |        |
      1172 |Red clay loam soil, east of Tulare, Tulare Co.|  .72|  14.75 |  .106
      1958 |Sandy Mesa soil, Nipomo, San Luis Obispo Co.  |  .85|  14.45 |  .122
      1423 |Chocolate-red soil, Carisa plain,             |  .39|  14.36 |  .056
           |   San Luis Obispo County                     |     |        |
      1291 |Sandy hill land, near Jackson, Amador Co.     |  .76|  14.34 |  .109
       585 |Wire-grass loam soil, Visalia, Tulare Co.     | 1.00|  14.10 |  .146
       863 |Red ridge loam soil, Grass Valley, Nevada Co. | 2.89|  13.91 |  .402
      1907 |Dark loam soil, near Chino, San Bernardino Co.|  .92|  13.26 |  .121
      1115 |Sandy granitic soil, near Jackson, Amador Co. |  .85|  13.20 |  .112
       332 |Plateau desert soil, Mojave, Los Angeles Co.  |  .28|  12.50 |  .035
      2126 |Gravelly soil, East Highlands,                |  .62|  11.75 |  .070
           |    San Bernardino Co.                        |     |        |
      1910 |Ojai Valley soil, Nordhoff, Ventura Co.       | 1.64|  11.21 |  .183
      2187 |Sandy loam soil, Soledad, Monterey Co.        |  .97|  11.10 |  .110
      1759 |Sandy soil, Perris Valley, Riverside Co.      |  .53|  11.04 |  .059
       774 |Bench slope soil, Ontario, San Bernardino Co. | 1.29|  10.85 |  .140
      1984 |Red soil, East Highlands    “      “       “  |  .58|  10.50 |  .060
      2325 |Silt soil of desert, Imperial, San Diego Co.  |  .65|  10.70 |  .070
      1906 |Light sandy soil, Pomona, San Bernardino Co.  |  .95|   9.80 |  .093
      2430 |Hillside adobe, Berkeley, Alameda Co.         | 1.85|   8.70 |  .160
           |                                              +-----+--------+--------
           |          Average of arid uplands             |  .91|  15.23 |  .135
           |                                                    |     |        |
           |  SUB-IRRIGATED ARID SOILS (California).      |         |          |
           |                                              |     |        |
       586 |Sandy plains soil, Tulare, Tulare Co.         | 1.14|  10.79 |  .123
      1466 |Loam soil, Miramonte, Kern Co.                |  .60|  10.66 |  .064
      1284 |Moist land loam soil, Chino,                  | 1.99|  10.20 |  .203
           |    San Bernardino Co.                        |     |        |
      1148 |Swale soil, near Paso Robles,                 | 1.16|   9.65 |  .112
           |    San Luis Obispo Co.                       |     |        |
      1714 |Bench soil, Santa Clara River, Piru,          |  .78|   9.56 |  .074
           |    Ventura Co.                               |     |        |
        77 |Alluvial soil, Tulare Lake bed, Tulare Co.    |  .47|   9.37 |  .045
      1880 |Creek bench soil, Niles, Alameda Co.          | 1.19|   8.90 |  .109
      1903 |Sediment soil, Porterville, Tulare Co.        | 1.12|   8.50 |  .140
       168 |Alluvial soil, Santa Clara river, Santa Paula,|  .84|   7.99 |  .067
           |      Ventura Co.                             |     |        |
      1760 |Green-sage land, Perris Valley, Riverside Co. |  .91|   7.70 |  .070
       506 |Alluvial soil, Colorado River, Yuma,          |  .75|   7.47 |  .050
           |   San Diego Co.                              |     |        |
      1636 |Red soil, Manton, Tehama Co.                  | 2.00|   6.86 |  .137
      1758 |Alkali soil, Perris Valley, Riverside Co.     |  .60|   6.83 |  .071
      1963 |Sandy loam soil, Willows, Glenn Co.           |  .36|   6.05 |  .022
      2080 |Sandy soil, Santa Maria Valley,               | 1.64|   5.36 |  .090
           |    Santa Barbara Co.                         |     |        |
           |                                              |     |        |
           |       Average of sub-irrigated arid soils    | 1.06|   8.38 |  .099
           |                                              |     |        |
           |   HUMID SOILS FROM ARID AND HUMID REGIONS    |     |        |
           |                   (California).              |     |        |
           |                                              |     |        |
       207 |Eel River Alluvial soil, Ferndale,            | 1.25|   6.96 |  .085
           |    Humboldt Co.                              |     |        |
      2319 |Alluvial soil, Hupa Valley, Humboldt Co.      | 7.83|   6.70 |  .514
       213 |Marsh soil, Novato, Meadows, Marin Co.        | 1.54|   6.36 |  .089
      1704 |Valley soil, Hollister, San Benito Co.        |  .94|   5.21 |  .049
      2295 |Tule soil, Upper Lake, Lake Co.               | 1.70|   4.50 |  .077
       110 |Alluvial soil, Putah Creek, Dixon, Solano Co. | 1.71|   4.25 |  .072
        37 |Redwood Valley soil, Pescadero, San Mateo Co. | 2.28|   3.07 |  .070
           |                                              |     |        |
           |       Average for California                 | 2.45|   5.29 |  .135
           |                                              |     |        |
           |                  OTHER STATES.            |         |        |
           |                                              |     |        |
        26 |Bog soil, Michigan[52]                         |33.02|   6.08 | 2.012
           |Back-land clay loam, Houma, Louisiana         | 5.07|   4.20 |  .218
           |Duff soil, Oregon                             |13.84|   3.49 |  .483
           |Sandy prairie soil, Harris Co., Texas         | 2.13|   3.66 |  .184
           |                                              |     |        |
           |       Average for other States               | 7.01|   3.78 |  .295
           |                                              |     |        |
        23 |Red soil, Oahu Island, Hawaii (maximum)       | 1.57|   5.07 |  .078
        27 |Guava soil, Hawaii Island (minimum)           | 9.95|   1.71 |  .170
           |Average of 5 soils, Oahu Island               | 3.01|   6.07 |  .237
           |Average of 2 soils, Maui Island               | 9.07|   2.13 |  .286
           |Average of 4 soils, Hawaii Island             | 6.17|   2.54 |  .146
           |                                              |     |        |
           |       Average for Hawaiian Islands           | 5.26|   3.69 |  .169
           |                                              |     |        |
           |       Total for Humid soils, average         | 4.58|   4.23 |  .166
    -------+----------------------------------------------+-----+--------+-------

[52] Introduced only for comparison of the nitrogen percentage in Humus
and not included in the average.

It thus appears that on the average the humus of the arid soils
contains about three and a half times as much nitrogen as that of the
humid; that in the extreme cases, the difference goes as high as over
six to one (see Nos. 37 and 704); and that in the latter cases, the
nitrogen-percentage in the arid humus considerably exceeds that of the
albuminoid group, the flesh-forming substances.

It thus becomes intelligible that in the arid region a humus-percentage
which under humid conditions would justly be considered entirely
inadequate for the success of normal crops, may nevertheless suffice
even for the more exacting ones. This is more clearly seen on
inspection of the figures in the third column, which represent the
product resulting from the multiplication of the humus-percentage of
the soil into the nitrogen-percentage of its humus; as appears in
comparing the respective averages, or Nos. 1167 and 110 and others. An
additional consideration is the probable greater ease with which the
nitrifying bacteria can act upon a material so rich in nitrogen.

We must not, then, be misled by the smallness of many humus-percentages
in the arid region, into an assumption of a deficiency in the supply of
soil-nitrogen.

    _Decrease of Nitrogen-Content in Humus with
    Depth._—Since the oxidation of the carbon and hydrogen
    in the humus-substance, and the consequent increase of its
    relative nitrogen-content, are manifestly dependent upon the
    presence of air and heat, it is reasonably to be expected
    that the nitrogen-percentage of the humus should decrease
    with the depth of the soil. That this is really the case
    is plainly shown in the subjoined table, which gives the
    humus-percentages and the nitrogen-content of the humus from
    the surface foot down to twelve feet, in a soil on the bench
    of the Russian River, Cal., which is sub-irrigated, and
    liable to more or less rainfall during the summer. It will
    be seen that not only does the absolute humus-percentage
    decrease quite regularly down to seven feet, at which point
    there evidently was at one time a strong root development,
    causing a notable increase of the humus-content; from which
    again there is a regular decrease down to the twelfth
    foot. It will be noted that the nitrogen-percentage in the
    humus, while not decreasing with the same regularity as the
    humus-content itself, yet exhibits a general recession from
    5.30 to 1.15 in the ninth foot, to which direct oxidation
    doubtless never penetrates.

         HUMUS AND NITROGEN-CONTENT OF RUSSIAN RIVER SOIL.
    ==============+==============+=================+================
    Depth in feet.|  Per cent    |    Per cent     |  Per cent
                  |    Humus     |    Nitrogen     | Humus-Nitrogen.
                  |   in soil.   |    in Humus.    |  in soil.
    --------------+--------------+-----------------+----------------
           1      |     1.21     |      5.30       |   .064
           2      |     1.16     |      4.32       |   .054
           3      |     1.14     |      3.87       |   .044
           4      |     1.17     |      3.76       |   .044
           5      |      .74     |      2.16       |   .016
           6      |      .60     |      2.66       |   .016
           7      |      .47     |      2.54       |   .012
           8      |      .78     |      1.54       |   .012
           9      |      .54     |      2.24       |   .012
          10      |      .52     |      1.15       |   .006
          11      |      .53     |      1.51       |   .008
          12      |      .44     |      1.81       |   .008
    --------------+--------------+-----------------+----------------

_Influence of the Original Materials on the composition of Humus._—The
great variability of the composition of humus formed from different
substances is well shown in the subjoined table, representing the
results of experiments made by Snyder,[53] who caused various
substances to humify by mixing the pulverized material intimately with
a soil poor in humus, and allowing the process to continue for a year.
At the end of that time the humus formed was extracted by the method of
Grandeau, outlined above, and analyzed, with the following results.

    ========+======+======+=======+======+========+=======+==========
            |Sugar.| Oat  | Green |Wheat |Sawdust.| Meat  |  Cow
            |      |Straw.|Clover.|Flour.|        |Scraps.|Manure.[54]
    --------+------+------+-------+------+--------+-------+----------
    Carbon  | 57.84| 54.30|  54.22| 51.02|  49.28 |  48.77|   41.93
    Hydrogen|  3.04|  2.48|   3.40|  3.82|   3.33 |   4.30|    6.26
    Nitrogen|  0.08|  2.50|   8.24|  5.02|   0.32 |  10.96|    6.16
    Oxygen  | 39.04| 40.72|  34.14| 40.14|  47.07 |  35.97|   45.63
    --------+------+------+-------+------+--------+-------+----------
            |100.00|100.00| 100.00|100.00| 100.00 | 100.00|  100.00
    --------+------+------+-------+------+--------+-------+----------

[53] Bull. No. 53, Minn. Exp’t Station, p. 12, Chem. of Soils and
Fertilizers, p. 94.

[54] The figures for cow manure are so far out of range with any others
thus far observed, that it seems reasonable to suppose that they are
influenced by unchanged substances present in the excreta.

While it may be questioned whether the process of humification had in
these materials really reached the point of sensible completion in
all cases (notably in those of sawdust and cow manure), the great
variability of the products from different materials is very striking.
When the nitrogen-content is deducted the percentage composition of
the products agrees more nearly. Considering that the nitrogen is
probably present in the amid form, it is natural that hydrogen should
in a measure vary with it, as in the case of the clover, flour and
meat humus. Nitrogen being the most variable ingredient of humus, it
seems probable that the variation of the proportion of the humus-amids
present is the most potent factor in the variability of the composition
of natural soil-humus.

Arranging these results in the order of their nitrogen-content as in
the table below, we see that the latter approximately corresponds to
the original protein-content of the humified substances.

    Humus from meat scraps    10.96 % Nitrogen.
      “    “   green clover    8.24
      “    “   cow manure      6.16
      “    “   wheat flour     5.05
      “    “   oat straw       2.50
      “    “   sawdust          .32

While the above data prove the correlation between the first products
of humification and the original substance, it must be remembered that
subsequently, under proper conditions, the nitrogen-percentage in humus
may, in the course of time, increase very greatly, even to a proportion
considerably above that contained in flesh itself. When we consider
that ordinarily, the latter, and the albuminoid substances generally,
decompose in contact with air with an abundant evolution of ammonia
compounds, sometimes leaving only a little fat (adipocere) behind, it
is surprising that the decomposition _within_ the soil should have
exactly the opposite result, viz., an _accumulation_ of the nitrogen.
The causes of this marked difference are not yet well understood, but
it is probably due to the differences in the kinds of bacteria that are
active in the two cases.

Snyder has also shown that the richer the organic matter humified is
in nitrogen, the more energetically it acts in rendering available
the mineral matters of the soil for plant nutrition. Correspondingly,
Ladd[55] has shown that with the increase of humus in the soil, there
is also a corresponding increase in the amounts of mineral plant-food
extracted from the soil by a four per cent solution of ammonia, such as
is employed in the Grandeau method of humus-determination.

[55] Bull., S. Dakota Station, Nos. 24-32, 35, 47.




CHAPTER IX.

SOIL AND SUBSOIL (_Continued_).


ORGANISMS INFLUENCING SOIL CONDITIONS; BACTERIA, ETC.

MICRO-ORGANISMS OF THE SOIL.

Intimately correlated with the humus-substances of the soil, as well
as with its temporary contents of the carbohydrates (cellulose, gums
and sugars) from which humus is formed, is the multitudinous flora of
micro-organisms always present and exercising important functions in
connection with the growth of the higher plants. Extended researches
by Adametz, Schloesing and Müntz, Miquel, Koch, Fraenkel, Winogradsky,
Frank and many others, have thrown light upon the immense numbers and
great variety of minute organisms, especially of the bacterial group,
present in soils, and upon their distribution and activities in the
same. It has been shown that their numbers are greatest near (although
usually not at) the surface, decreasing rapidly downward and generally
disappearing wholly at depths between seven and eight feet; the latter
depth varying of course according to the nature and porosity of the
soil, and both depth and numbers being greatest in summer.

_Numbers of Bacteria in Soils._—Adametz found in one gram of soil,
38,000 bacteria at the surface, 460,000 at ten inches depth; in a
loam soil at the surface 500,000, at ten inches 464,000 in each gram
of earth. Of mould and similar fungous germs there were only 40 to 50
in the same, 6 species being true molds, while four were ferments,
including the yeasts of wine and beer. Fraenkel found in virgin land
from near Potsdam, a sudden, marked decrease at depths of from three
to five feet; while in earth from inhabited places within the city of
Berlin, considerable numbers were still present at eight and even ten
feet, in some cases.

In the researches lately made by Hohl at the bacteriological station
at Liebefeld, near Bern, it was found that in cultivated soils the
number of bacteria greatly exceeds the figures given by Fraenkel.
He found a gram of moist soil to contain from three to fifteen
millions of bacteria. In the cultivated soil of Liebefeld he found
5,750,000, in meadow land 9,400,000, in a manure pile 44,500,000 per
cubic centimeter. These figures seem high for so small a quantity
of material, but taking the average size of a bacterium, a cubic
centimeter might readily contain six hundred millions. (Grandeau, Ann.
Sci. Agronomique, vol. 1, p. 461, 1905).

    Mayo and Kinsley (Rep. Kansas Exp’t Station for 1902-3) have
    made elaborate investigations of the numbers and kinds of
    bacteria found in various soils in Kansas, in connection
    with different crops. It is noteworthy that in most cases
    their figures exceed considerably those given by European
    observers, as they often reach high into the millions, in
    one case to over fifty millions, per cubic centimeter.[56]

[56] The mode of statement in the paper is not always quite clear as
to the manner in which the averages given were calculated. It must be
remembered that these data refer to cubic centimeters of soil, or about
twice the amount (1 gram) used by European observers.

    Five fields with different soils were investigated; the
    land being described as follows: “Field No. 1 is a black
    loam containing considerable humus; field No. 2 is similar
    to field 1 but contains more humus; field No. 3 is a thin
    soil with clay gumbo subsoil; fields Nos. 4 and 5 are black
    loams, but not as rich in humus as either No. 1 or No. 2.”

    The average bacterial contents of the several fields are
    given as follows:

    Field No. 1   33,931,747 per cubic centimeter.
      “   No. 2   53,596,060  “   “        “
      “   No. 3       78,534  “   “        “
      “   No. 4    8,643,006  “   “        “
      “   No. 5    3,192,131  “   “        “

    “The crop records of these fields for the past ten years
    indicate that the crop yield has been (more or less?)
    directly proportional to the bacterial content of the soil
    of each field; field 2 has produced the largest yield, field
    3 the least.”

    Unfortunately no chemical analyses of any of these soils are
    communicated; but at the request of the writer samples of
    the soils of the first three fields were sent from the
    Kansas station for humus determinations (courteously made by
    Dr. H. C. Myers), which gave the following results:

    Field No. 1   2.19% of Humus.
      “   No. 2   3.07% “    “
      “   No. 3   1.85% “    “

    While these humus-percentages are not directly proportional
    to the bacterial content, a favoring effect of high
    humus-content is clearly shown. The bacterial and the
    humus-content of these soils are sensibly, even if not
    directly, correlated; which might reasonably be expected,
    since the organic matter and the humus are the bacterial
    food.

    The investigation also showed wide differences in the
    bacterial content of the same soil when different crops
    were growing on it. Thus in samples taken on Aug. 15, there
    were found in the first twelve inches of a black loam soil
    bearing timothy and clover, 1,380,000, in the same with
    alfalfa and clover, 21,091,000, with maize from one to over
    two millions. In soils from the western part of Kansas,
    the bacterial content of the same crops was much less (as
    doubtless is the humus-content), and it is noteworthy that
    the prairie buffalo grass shows throughout a relatively high
    bacterial content in the first foot of the soil, ranging
    next to alfalfa. The root bacteria living on the legumes
    will naturally increase the bacterial content of the soils
    on which they grow, more than plants which, like maize, do
    not directly utilize bacterial action.

    _Multiplication of the Bacteria._—Marshall Ward and
    Duclaux have made some special observations in regard to
    the rapidity with which certain bacteria multiply. Duclaux
    summarizes the final conclusion thus: taking as a basis
    the time of 35 minutes for the subdivision into two, which
    has been frequently observed by Ward, there would be four
    millions of bacteria produced in twelve hours. The first
    filaments had plenty of room in a drop culture of one cubic
    millimeter; but at the end their total volume amounted to
    the tenth part of the total volume of the drop. At the above
    rate, making 48 generations in 24 hours, 281,500 billions
    of organisms would be produced. (Grandeau, Ann. Sci. Agron.
    Vol. 1, 1905, p. 456).

_Aerobic and Anaerobic Bacteria._—As may readily be inferred, the
cultural and other surface conditions exert a potent influence both
upon the kinds and abundance of the bacteria and molds; since the
life-functions of some are dependent upon the presence of free oxygen
(“aerobic”), while others flourish best, or only, in the absence of
air (“anaerobic”), or are able to avail themselves of the presence
of _combined_ oxygen, by reduction of oxids present. Their number is
found, in general, to be greatest in cultivated lands, and bacteria
are there by far predominant over the moulds. On the other hand, the
moulds gain precedence in woodlands and meadows, at least so far as air
can gain access; while in the deeper layers of the same, as well as in
peaty lands, bacterial life is always scanty. This holds particularly
in respect to the nitrifying organisms, and others whose life-functions
are dependent upon abundant access of oxygen (aerobic).

_Food Material Required._—All bacteria, like the fungi, are dependent
for their development upon the presence of adequate amounts of some
organic food-material, best apparently in water-soluble form. In the
soil it seems to be chiefly compounds of the carbohydrate group,
especially various gums derived from the decaying plant substance, or
from stable manure; in artificial cultures, glucose is mostly found
to be a highly available food. When the decaying substance reaches
the state of humus, the latter seems to be available as food only to
comparatively few bacteria. The very abundant development of bacterial
life seems to be among the most important effects produced by stable
manure upon the surface soil, in establishing good tilth (“Bodengare”
in German).

_Functions of the Bacteria._—While there is still much uncertainty as
to the exact functions performed by most of these bacteria in respect
to soil-formation and plant growth, there are several kinds whose
activity has been proved to be of the utmost importance in one or both
directions; it having been shown that when the soil is sterilized
either by heat or antiseptic agents, certain essential processes are
completely suppressed until the soil is re-infected and the conditions
of bacterial life restored.

Probably the chief in importance are those connected with the processes
of _nitrification_ and _denitrification_, bearing as they do upon the
supply to plants of the most costly of the three substances furnished
by fertilizers. These organisms have been first extensively studied by
Winogradsky, while the conditions of their activity have been largely
developed by R. Warington.

[Illustration: FIG. 15.—Nitrosomonas. (Winogradsky).]

[Illustration: FIG. 16.—Nitrobacterium. (Winogradsky).]

_Nitrifying Bacteria._—The conversion of ammonia into nitrates is
accomplished under proper conditions by two organisms, or groups of
organisms; the first stage being the formation of nitrites by the
round, often flagellate cells of _nitrosomonas_ (or nitrosococcus).
The second, the oxidation of the nitrites into nitrates by very minute
rod-shaped bacilli, named _nitrobacteria_. The conditions under which
these bacteria can act are quite definite in that, aside from a supply
of the nitrifiable substance, a fairly high temperature (24° C. or 75°
F.) and a moderate degree of moisture, there must be a free access of
oxygen (air); and there must be present a base (or its carbonate) with
which the acids formed by oxidation can immediately unite. In an acid
medium (“sour” soils) nitrification promptly ceases; as it also does
whenever the amount of base present has been fully neutralized. The
bases most favorable to nitrification are lime and magnesia in the form
of carbonates, an excess of which does no harm; while in the case of
the carbonates of potash and soda, the amount must be strictly limited.

    _Conditions of Activity._—Dumont and Crochetelle found
    that up to .25 per cent, potassic carbonate acted favorably
    on the process; which was, however, completely stopped by
    as much as .8 per ct. Warington has shown that ammonic
    carbonate similarly prevents nitrification when exceeding
    about .37 per ct. Ammonia salts in general appear to be
    antagonistic to the transformation of nitrites into nitrates.

Aside from the carbonates, some neutral salts favor nitrification very
markedly; while others tend to depress it. Deherain found that .5 per
cent of common salt suffices to prevent nitrification altogether, while
smaller amounts retard it proportionally. According to Dumont and
Crochetelle, potassium chlorid acts favorably up to .3 per cent, but at
.8 per cent suppresses nitrification. Earthy and alkaline sulfates, on
the contrary, seem to act favorably throughout, at least up to .5 per
cent. This is especially true of gypsum, which, according to Pichard,
accelerates the process more than any other substance known. Taking
the effect of gypsum as the maximum, he found that, other things being
equal, the amounts of nitrates formed were as shown in the table below,
the effect of gypsum being taken as 100:

    Gypsum               100
    Sodic Sulfate         47.9
    Potassic Sulfate      35.8
    Calcic Carbonate      13.3
    Magnesic Carbonate    12.5

    The above estimates are markedly confirmed by the
    observations of the writer in the alkali soils of
    California. In these, nitrates exist most abundantly when
    the salts contained in the soil are mainly sulfates; while
    wherever common salt or sodic carbonate are present in
    considerable amounts, the amounts of nitrate found are
    notably less. In saline seashore lands nitrates are usually
    present in traces only. Wollny has moreover shown that
    the nitrates themselves exert a repressive influence on
    nitrification.

_Effects of Aeration and Reduction._—While the fostering effect of
sulfates upon nitrification is very energetic in well aerated soils,
they become injurious whenever by a reductive process in ill-drained
lands, the sulfates are reduced to sulfids. Under such conditions
the process will in any case be much impaired. On the other hand,
the favoring effect of abundant aeration was strikingly shown in
the experiment made by Deherain, in which a cubic meter of soil was
left unmoved for several months, while a similar mass was thoroughly
agitated once a week during the same time. The proportion of nitrates
formed in the latter case was as 70 to 1 formed in the quiescent soil
mass. It follows that the intensity of nitrification is essentially
dependent upon the porosity of the soil; and that it is thus greatly
favored in the pervious soil-strata of the arid regions. It also
follows that thorough and frequent tillage and fallowing greatly favor
nitrification; thus explaining one of the beneficial results of these
operations. At the same time, it is true that we may thus in a short
time seriously diminish the reserve stock of nitrogen contained in the
soil in the form of humus-amids; and since nitrates are exceedingly
liable to be lost from the soil in several ways, such excessive
nitrification is to be avoided.

_Unhumified Organic Matter does not Nitrify._—There can be little doubt
that the formation of ammonia from the amido-compounds in humus is
also the work of bacteria; but this, really the initial phase of the
nitrogen-nutrition of plants, has not yet been fully elucidated. That,
however, it is essentially only the ready-formed humus and not the
unhumified debris of the soil which participate in nitrification was
shown by the experiments of the writer, see chapter 19.

[Illustration: FIG. 17.—Bacillus denitrificans I. (Burri.)]

_Denitrifying Bacteria._—Among the sources of _loss of nitrates_ in
the soil is the action of denitrifying bacteria; some of which cause
merely the reduction of nitrates to nitrites and progressively to
ammonia, while others cause gaseous nitrogen to be given off from
nitrites and nitrates, resulting in their complete loss to the soil.
While there are probably several kinds of the latter class, the most
rapidly effective is an organism contained abundantly in fresh horse
dung, and also on the surface of old straw. This can readily be shown
by subjecting a very dilute solution (1-3 per cent.) of Chile saltpeter
to the action of fresh horse dung in a close flask, when nitrogen and
carbonic dioxid gases are evolved, and in a few days the nitrate has
totally disappeared. In the course of time this power of horse-manure
disappears; so that “rotted manure” is practically free from it and
under proper conditions serves nitrification so effectively, that in
the past it has served extensively for the production of saltpeter in
the “niter-plantations” for the industrial purposes; the material
of which was loose earth, marl and manure, kept moist and frequently
forked over for better aeration. Saltpeter is similarly produced in
stables, corroding the mortar of brick foundations. Nevertheless, it
is necessary to avoid the use, either together or at short intervals
apart, of Chile saltpeter and fresh manure; the manure if used first
should be allowed to remain at least two months in the soil before
saltpeter is applied.

    The reduction of nitrates to nitrites and ammonia is brought
    about by quite a number of bacteria, mostly anaerobic,
    and such as consume combined oxygen in their development.
    Thus the butyric ferment, which in the absence of readily
    reducible compounds evolves free hydrogen, will in presence
    of nitrates reduce the latter to nitrites, or form ammonia
    by addition of hydrogen to nitrogen just set free by
    reduction. Such reductive processes of course occur chiefly
    in soils rich in organic matter, or ill-aerated. The ammonia
    so formed, while at first simply combining with any humus
    acids present, may in the course of time be itself reduced
    to the amidic condition, being thereby rendered relatively
    inert, until again brought into action by ammonia-forming
    bacteria.

_Ammonia-forming Bacteria._—A large number of different bacteria appear
to be concerned in the formation of ammonia from compounds of the
albuminoid group, (and probably from humus). Among these is one of the
most common in soils (_Bacillus mycoides_, root bacillus), which while
forming ammonia carbonate in solutions of albumen, is also capable of
reducing nitrates to nitrites and ammonia in presence of a nutritive
solution of sugar.

The “hay bacillus” (_B. subtilis_), so abundantly developed in hay
infusions, and one of the most abundant in cultivated soils, has
together with B. ellenbachensis, B. megatherium, B. mycoides, and
others, by some been credited with important action in favoring
vegetation; so that a fairly pure culture of B. ellenbachensis has
been brought out commercially in Germany under the name of “Alinit.”
Rigorous culture experiments made by Stutzer and others have,
however, failed to show any general benefit from the use of alinit in
infecting either land or seeds. But there is no doubt of the _Effects
of Bacterial Life on Physical Soil Conditions._—It is apparent that
all conditions favoring the life of aerobic (air-needing) bacteria
tend also to produce the loose, porous state (tilth) of the surface
soil so conducive to the welfare of culture plants, designated by
German agriculturists as “Bodengare.” Whether or not this condition is
directly due to bacterial processes, as is thought by Stutzer (Landw.
Presse, 1904, No. 11) it is assuredly a highly important point to
be gained, and is essentially connected with the presence of humus
in adequate amounts, which is also a favoring condition of abundant
bacterial life. It seems that the preference given to the shallow
putting-in, or even surface application of stable manure, existing in
Europe, is largely based upon the marked effect upon the looseness of
the surface soil, generally credited to the physical effect of the
manure substance itself, but apparently largely due to the intensity of
bacterial action thus brought about.

[Illustration: FIG. 18.—Bacillus subtilis. (Wollny, after Brefeld.)]

[Illustration: FIG. 19.—Bacteria producing ammoniacal fermentation:
_A_, _C. mycoides_: _B_, _B. stutzeri_. (From Conn, Agr. Bacteriology.)]

[Illustration: FIG. 20.—Bacillus megaterium. (From Migula.)]

ROOT-BACTERIA OR RHIZOBIA OF LEGUMES.—Among the most important
bacteria, agriculturally, is that which enables plants of the
leguminous order—(peas, beans, vetches, clovers, lupins, etc.),—to
obtain their supply of nitrogen from the air independently of those
contained in the soil. The source of nitrogen to plants was long a
disputed question; it was at first supposed (by de Saussure) that it
was obtained directly from the soil by the absorption of humus; but
this was disproved, and Liebig then contended that it was derived
directly from the atmosphere through the ammonia in rain water. This
was then shown to be wholly inadequate; and Boussingault proved
conclusively that plants do not take up nitrogen gas from the air.
This was subsequently denied by Ville; but investigation at the
Rothamstead agricultural station by Lawes and Gilbert definitely
confirmed Boussingault’s results. At the same time they also proved
very definitely that while grass and root crops deplete the soil
of nitrogen, clover and other leguminous crops leave in the soil
more nitrogen than was previously present, even when the entire,
itself highly nitrogenous, leguminous crop is removed from the land.
The improvement of lands for wheat production by rotation with
clover had long ago become a practical maxim; but the cause was not
understood until, in 1888, Hellriegel and Wilfarth announced that the
variously-shaped excrescences or tubercles which had long been observed
as frequently deforming the roots of legumes, are caused by the attacks
of bacilli capable of absorbing the free nitrogen of the air and thus
enabling the host-plant to acquire its needed supply by absorbing
the richly nitrogenous matter thus accumulated in the excrescences.
The minute rod-shaped organism was named _Bacillus radicicola_ by
Beyerinck; _Rhizobium leguminosarum_, by A. Frank, who has published an
extensive treatise on the subject.[57]

[Illustration: FIG. 21.—Microscopic section of cell tissue, from a
nodule of Square-pod pea, showing cells filled with Rhizobia.[58]]

[57] Uber die Pilzsymbcose der Leguminosen, Berlin, 1890.

[58] Original figure from drawing by O. Butler, Asst. in Agr. Dep’t
Univ. of California.

Microscopic examination of the nodules shows their tissues to contain
partly motile, free bacteria, partly others (bacteroids), which have
assumed a quiescent condition, and are of much greater dimensions
than those of the motile form. These relatively thick, and sometimes
forked, forms, differing somewhat in each of the group adaptations
mentioned below, constitute the bulk of the cell-contents of the
nodules, and ultimately serve for the nutrition of the host-plant
with nitrogen. When the growth of the excrescence is completed, the
swollen, quiescent bacteroids gradually collapse and become depleted
of their nitrogenous substance; and finally the apparently empty husk
remains or drops off, carrying with it the minute cocci which in the
soil become active bacteria again. The nodules are thus found mainly
on the actively-growing roots, and at the time when vegetation and
assimilation are most active in the plant. In autumn, or when the
plants are in fruit, the roots may be wholly destitute of nodules.

[Illustration: FIG. 25.—Square-pod pea.—Tetragonolobus purpureus. FIG.
26.—White Lupin.—Lupinus albus.]

The adhesion of the nodules to the roots is mostly very loose, and
their falling-off when the seedlings are carelessly transplanted,
doubtless accounts for much of the difficulty generally found in
transplanting legumes when once established.

The figures annexed show the various forms assumed by the nodules
in different plants, and with them also the corresponding forms of
the bacteroids of each. The latter, here shown magnified about 1000
times, are taken from the inaugural dissertation of D. Brock on this
subject, published at Leipzig in 1891. It appears that the forms of the
bacteroids are quite as much varied as are those of the nodules they
form.

[Illustration: FIG. 22.—Common Vetch.—Vicia sativa.

FIG. 23.—Bur clover.—Medicago denticulata.

FIG. 24.—Garden pea.—Pisum sativum.]

_Varieties of Forms._—While these bacilli seem to be normally present
in most soils, it seems to be necessary that they should adapt
themselves for this symbiosis[59] with each of several groups of the
legumes in order to exert their most beneficial effects. In many
soils there appears to exist a “neutral form”, which requires about
a season’s time or more to adapt itself specially to the several
leguminous groups so that a great advantage is gained by infecting
either the seeds or the soil with the forms already adapted, when no
similar plant has lately occupied the same ground. Thus the bacillus of
the clover root is of little or no benefit to beans, peas or alfalfa,
and the root-bacilli of each of the latter are relatively ineffectual
when used to infect either of the other groups. The same is true of the
bacilli of lupins and of acacias, as applied to leguminous plants of
any other groups.[60]

[59] “Living together” beneficially; in contradistinction to
parasitism, which is injurious to the host plant.

[60] It is asserted by some observers that the root-bacilli producing
differently-shaped excrescences upon different legumes are distinct
species; but this view is not sustained by the experiments of Nobbe and
Hiltner, and seems intrinsically improbable.

_Mode of Infection._—The infection is especially effectual when applied
to the seeds before sowing; and for that purpose there may be used
either the turbid water made by stirring up in it some earth of a
properly infected field, or else water charged with a pure culture of
the appropriate kind, commercially known under the name of nitragin,
now manufactured for the purpose. Or else, the field to be sown may be
infected by spreading on it broadcast, _and promptly harrowing in_, a
wagon-load of earth per acre from a properly infected field. Such earth
must not be allowed to dry, or to be long exposed to light.

    Specially effective (“virulent”) and hardy forms of such
    bacteria have been produced under artificial culture by Dr.
    Geo. T. Moore of the U.S. Department of Agriculture. These
    cultures can be sent by mail on cotton imbued with them, for
    the infection of seeds.

It is very important that the bacillus should be present in the
_earliest_ stages of the growth of the seedlings; otherwise the latter
will undergo a longer or shorter period of starvation, unless the soil
contains, or is furnished with, a sufficiency of available nitrogen
to supply their immediate wants. When such a supply is very abundant,
the legume crop will sometimes develop no nodules at all; but the best
crops appear to be the result of a thorough infection, and abundant
formation of the excrescences.

_Cultural Results._—The marked results obtained in certain soils by
inoculation with the legume-root bacillus are exemplified in the
following table, showing results of experiments by J. F. Duggar, at the
Alabama Experiment station.[61]

       TABLE SHOWING INCREASE OF PRODUCTION BY SOIL INOCULATION.
    ==============================+=======+========+================
              PER ACRE.           | TOPS. | ROOTS. |   NITROGEN.
                                  | lbs.  |  lbs.  |  lbs. | Value.
    ------------------------------+-------+--------+-------+--------
    Hairy vetch, not inoculated   |   194 |   387  |   7   | $ 1.05
      “     “   inoculated        |  3045 |  1452  | 106   |  15.90
    Crimson clover not inoculated |   106 |   266  |   4.3 |    .65
      “        “   inoculated     |  4840 |  1452  | 143.7 |  21.25
    ------------------------------+-------+--------+-------+--------

[61] Bull. Ala. Exp’t Station, No. 96, 1898.

Such marked _increases_ from soil inoculation cannot of course be
expected in cases where the soil has previously borne leguminous crops
of similar nature and therefore already contains the root bacteria.
Hence Duggar found no increase of production when inoculating for
cowpea, land that had borne that crop two years before and already
contained the root bacteria. In the arid region, where the almost
universally calcareous soils usually bear a natural growth largely
composed of various leguminous plants, inoculation is likely to be less
commonly effective than in the humid region east of the Mississippi,
where leguminous plants are much less generally present in the native
flora.

The distinctive agricultural function of supplying nitrogen to the
soils on which they grow, renders inexcusable the persistence of some
writers and teachers in designating all forage plants as “grasses.”
Whatever excuse there may have been for this practice so long as the
nitrogen-gathering function of the legumes was unknown, disappears
with this discovery, and the misleading misnomer should be banished
from agricultural publications and lectures, at the very least.

_Other Nitrogen-Absorbing Bacteria._—An increase in the
nitrogen-content of some soils, aside from the action of leguminous
root-bacteria, has long been observed. As already stated, this increase
was at first ascribed to certain green algæ often seen to develop on
the soil surface; but it has now been shown that the nitrogen-gathering
function belongs to at least two bacteria, one of which (_Clostridium
pastorianum_) was discovered by Winogradski, the other (_Azotobacter
chroococcum_) by Beyerinck, and has since been farther investigated by
Koch, Kröber, Gerlach and Vogel, and last by Lipman and Hugo Fischer.
According to the latter it seems likely that Azotobacter chroococcum
lives in symbiosis with the green algæ, all of which, like the
Azotobacter itself, develop with special luxuriance on calcareous soils.

    Lipman (Rep. Agr. Exp’t Station, New Jersey, 1903 and 1904)
    describes as _Azotobacter vinelandii_ a form somewhat
    different from the A. chroococcum, the nitrogen-assimilating
    power of which he tested quite elaborately. He exposed to
    air pure cultures of _A. vinelandii_ in nutritive
    solution containing the proper mineral ingredients, and
    glucose 20 grams per liter. 100 cub. centimeters of this
    solution was exposed in flasks of respectively 250, 500
    and 1000 cc. content, therefore having greater surface in
    the larger flasks. After ten days, the amounts of nitrogen
    fixed were found to be respectively 1.67, 3.19 and 7.90
    milligrams. When mannite solution was employed instead of
    glucose, a similar fixation was observed; and it was also
    shown that the presence of combined nitrogen in the forms of
    nitrates or ammonium salts discouraged the fixation by the
    bacillus.

    It was thus clearly proved that _A. vinelandii_ at
    least does not need symbiosis with algæ to fix atmospheric
    nitrogen; but experiments with mixed cultures of the above
    bacillus and another (designated as No. 30 by Lipman) proved
    that when these two co-operate the absorption of atmospheric
    nitrogen is nearly doubled. As it is probable that this is
    the case also with other soil bacteria, the importance of
    this source of nitrogen to plants is obvious; provided of
    course that the proper nutritive ingredients are present
    in available form. Lipman shows that among the organic
    nutrients, besides the sugars, glycerine and the salts of
    propionic and lactic acids, and probably also others of the
    same groups, can serve as nourishment to the nitrogen-fixing
    bacteria.


DISTRIBUTION OF THE HUMUS WITHIN THE SURFACE SOIL.

The uniform distribution of the humus-contents of the surface soil, as
shown in sections of the same, is by no means easily accounted for.
The roots from which its substance is so largely derived are not so
universally distributed as to account for it; but least of all can the
rapid disappearance of the leaf-fall and other vegetable offal from
the surface be accounted for without some outside agencies. Of these,
the action of fungous vegetation, and of insects and earthworms, are
doubtless the chief ones.

_Fungi._—When we examine a decaying root, we find radiating from it
a zone of deeper tint, as though from a colored solution penetrating
outward. But since under normal conditions humus is insoluble, this
explanation cannot stand. Microscopic examination, however, reveals
that the outside limit of this zone is also the limit to which the
fungous fibrils concerned in the process extend; and as these fibrils
are much more finely distributed and much more numerous than the
roots of any plant, it is natural that the humus resulting from
their decomposition should be more evenly distributed than the roots
themselves.[62]

[62] Kosticheff, Formation and Properties of Humus; in abstract Jour.
Chem. Soc., 1891, p. 611.

Such fungous growth is not, however, confined to dead and decaying
roots only. A large number of trees and shrubs, among them pines and
firs, beeches, aspen and many others, also the heaths, and woody plants
associated with them, appear to depend largely for their healthy
development, notably in northern latitudes, upon the co-operation
(“symbiosis”) of fungous fibrils that “infest” their roots, enabling
them to assimilate, indirectly, the decaying organic (and inorganic)
matter which would otherwise be unavailable, and at the same time
converting that matter into their own substance. Fungous growths thus
mediate both the decomposition and rehabilitation of the vegetable
debris.

The vegetative fibrils (mycelia) of several kinds of molds are
constantly present in the soil, and while consuming the dead tissue
of the higher plants, spread their own substance throughout the soil
mass. The same is true of the subterranean or “root” mycelia of the
larger fungi, toadstools, mushrooms, which are commonly found about
dead stumps and other deposits of decaying vegetable and animal offal.
All these being dependent upon the presence of air for their life
functions, remain within such distance from the surface as will afford
adequate aeration; the depth reached depending upon the perviousness of
the soil and subsoil. In the humid region this will usually be within
a foot of the surface, but in the arid may reach to several feet.
Ultimately these organisms contribute their substance to the store of
humus in the land.

    On the surface of moist soils we frequently find a copious
    growth of green fibrils, which may be either those of algæ,
    such as Oscillaria, or the early stages (prothallia) of
    moss vegetation. This vegetation has been credited with
    absorption of nitrogen from the air, thus enriching the
    soil; but later researches have shown this effect to be due
    to symbiotic bacteria (see above p. 156).

_Animal Agencies._—Darwin first suggested that wherever the common
earthworm (_Lumbricus_) finds the conditions of existence, it exerts a
most important influence in the formation of the humous surface-soil
layer; and the limitation imposed upon these conditions by the
subsoil has doubtless a great deal to do with the sharp demarcation
we often find between it and the surface soil. Briefly stated, the
earthworm nourishes itself by swallowing, successively, portions of
the surrounding earth, digesting a part of its organic matter and then
ejecting the undigested earth in the form of “casts,” such as may
be seen by thousands on the surface of the ground during or after a
rain. Darwin (The Formation of Vegetable Mold, 1881), has calculated
from actual observation that in humid climates and in a ground fairly
stocked with these worms, the soil thus brought up may amount to from
one-tenth to two-tenths of an inch annually over the entire surface;
so that in half a century the entire surface foot might have been
thus worked over. Aside from the mechanical effect thus achieved in
loosening the soil, and the access of air and water permitted by their
burrows, the chemical effects resulting from their digestive process,
and the final return of their own substance to the soil mass; also
their habit of drawing after themselves into their burrows leafstalks,
blades of grass and other vegetable remains, renders their work of no
mean importance both from the physical and chemical point of view. The
uniformity, lack of structure and loose texture of the surface soil,
especially of forests, as compared with subsoil layers of corresponding
thickness, is doubtless largely due to the earthworms’ work. It has
frequently been observed that when an unusual overflow has drowned out
the earthworm population of a considerable area, the surface soil layer
remains compacted, and vegetation languishes, until new immigration
has restocked the soil with them. Again, the humus formed under their
influence is always neutral, never acid.

    Wollny (Forsch, Agr., 1890, p. 382), has shown by
    direct experimental cultures in boxes, with and without
    earthworms, surprising differences between the cultural
    results obtained, and this has been fully confirmed by the
    subsequent researches of Djemil (Ber. Physiol. Lab. Vers.
    Halle, 1898). In Wollny’s experiments, the ratio of higher
    production in the presence of the worms, varied all the way
    from 2.6 per cent in the case of oats, 93.9 in that of rye,
    135.9 in that of potatoes, 300 in that of the field pea, and
    140 in that of the vetch, to 733 per cent in the case of
    rape. Wollny attributes these favorable effects in the main
    to the increased looseness, and perviousness of the soil to
    air, and diminished water-holding power. Djemil’s results
    all point in the same direction; and he shows, moreover,
    that the allegation that the roots penetrate more deeply in
    the presence of the worms by following their burrows, is
    unfounded, the descending roots often passing close to and
    outside of these.

The work of earthworms is especially effective in loamy soils and in
the humid regions. In the arid region, and in sandy soils generally,
the life-conditions are unfavorable to the worm, and the perviousness
elsewhere brought about by its labors already exists naturally in most
cases. It is stated by E. T. Seton (Century Mag. for June, 1904) that
the earthworm is practically non-existent in the arid region between
the Rocky Mountains and the immediate Pacific coast, from Manitoba to
Texas. In the Pacific coast region, however, they are abundant, and do
their work effectually.

_Insects_ of various kinds are also instrumental in producing, not only
the uniform distribution of humus in the surface soil, but also the
looseness of texture which we see in forest soils especially. Ants,
wasps, many kinds of beetles, crickets, and particularly the larvæ
of these, and of other burrowing creatures, often form considerable
accumulations, due directly both to their mechanical activity, and to
their excrements.

The work of _ants_ is in some regions on so large a scale as to attract
the attention of the most casual observer. Especially is this the case
in portions of the arid region, from Texas to Montana, where at times
large areas are so thickly studded with hills from three to twelve feet
in diameter, and one to two feet high, that it is difficult to pass
without being attacked by the insects. The “mounds” studding a large
portion of the prairie country of Louisiana seem also to be due to the
work of ants, although not inhabited at present.

Larger burrowing animals also assist in the task of mixing uniformly
the surface soils, and aiding root-penetration, as well as, in many
cases, the conservation of moisture. Seton (loc. cit.) even claims that
the pocket gophers (Thomomys) in a great degree replace the activity of
the earthworms in the arid region, where they, together with the voles
(commonly known there as field mice), exist in great numbers. Of course
the work of these animals, as well as that of the prairie dogs, ground
squirrels, badgers, etc., is incompatible with cultivation. But the
effects of their burrows on the native vegetation, and the indications
they give of the nature of the subsoil, are eminently useful to the
land-seeker.

    Thus in the rolling sediment-lands of the Great Bend of
    the Columbia, the observer is surprised to see the “giant
    rye grass,” usually at home in the moist lowlands, growing
    preferably on the crests of the ridges bordering the
    horizon. Examination shows that this is due to the burrowing
    of badgers, whereby the roots of the grass are enabled to
    reach moisture at all times, even in that extremely arid
    region.




CHAPTER X.

SOIL AND SUBSOIL (_Continued_)

THEIR RELATIONS TO VEGETATION.


_Physical Effects of the Percolation of Surface Waters._—The muddy
water formed by the beating of rains on the soil surface will, in
penetrating the soil, carry with it the diffused colloidal clay to a
certain depth into the subsoil. We should therefore expect that as
a rule every subsoil will be more clayey than its surface soil; and
this is found to be almost universally the case in the humid region.
Subsoils are therefore almost always less precious and more retentive
of moisture, as well as of plant-food substances in solution, than
their surface soils, unless these are very rich in humus; and as the
finest particles are usually those richest in available plant-food,
it follows that subsoils will as a rule be found to contain larger
supplies of the latter than the surface soil. Common experience as well
as comparative analysis confirm both of these inferences so thoroughly,
that it becomes unnecessary to adduce examples in this place.

On the other hand, the reverse, upward movement of moisture caused
by surface evaporation tends constantly to bring any soluble salts
contained in the soil mass nearer to the surface, thus increasing the
stock of easily available plant-food in the surface soil. In extreme
cases, especially in the arid region, this accumulation of salts may
become excessive, and seriously injurious to plant growth. (See “Alkali
Soils”, chapters 21, 22.)

_Chemical effects of Water-Percolation._—The accumulation of plant-food
in the subsoil is not, however, due only to the mechanically-carried
particles, but also to the ingredients carried in solution from the
surface soil and redeposited in the more retentive subsoils. Especially
is this true of _lime carbonate_, which is dissolved by the carbonic
acid formed chiefly within the humic surface soil, and is often carried
off in amounts sufficient to obstruct drain tiles by its deposition
in contact with air (see chapt. 3). In the case of moderate rains,
however, it is carried no farther than the subsoil, and is there
redeposited, in consequence of the penetration of air, following the
water, and causing the carbonic gas to diffuse upward; thus leaving the
lime carbonate behind. In the majority of cases this results simply in
a gradual enriching of the subsoil in this substance; while the surface
soil may become so depleted as to require its artificial replacement by
liming or marling. The same general process occurs to a less extent, in
the case of magnesia.

_Calcareous Subsoils._—The fact that subsoils are more calcareous than
the corresponding surface soils is often of great practical importance,
in enabling the farmer to enrich his depleted surface soil in lime by
subsoil plowing. The accumulation of lime carbonate in the subsoil also
tends in a measure to offset the extreme heaviness sometimes resulting
from the accumulation of clay.

_Calcareous Subsoils and Hardpans._—When soils are very rich in lime,
and rains occur in limited showers rather than continuously, the
lime carbonate dissolved from the surface soil may accumulate in the
subsoil so as to either form calcareous “hardpan” by the cementing of
the subsoil mass; or it may accumulate and partly crystallize around
certain centers and thus form white concretions, known to farmers as
“white gravel.” The latter is the form usually assumed in the regions
of summer rains; while in the arid regions the deficient rainfall
causes this substance to accumulate, and calcareous hardpan to form, at
definite depths depending upon the maximum penetration of the annual
rainfall; sometimes in crystalline masses of veritable limestone
(“kankar” of India), or sometimes merely as crystalline incrustations
loosely cementing the subsoil.

_“Rawness” of Subsoils in Humid Climates._—From the greater compactness
of the subsoil which is almost universal in the humid regions, the
absence of humus and of the resulting formation of carbonic and humic
acids, it follows that its minerals are less subject to the weathering
process than are those of the surface soil. In the farmer’s parlance,
the subsoil is “raw” as compared with the surface soil; it is not so
suitable for plant-nutrition, and therefore must not be brought to the
surface to form the seed-bed, or be incorporated with the surface soil
to any considerable extent _at any one time_, if crop-nutrition is to
be normal. It is only in the course of time, by exposure to atmospheric
action as well as to that of the humus, and of plant roots, that it
becomes properly adapted to perform the functions of the surface soil.

_Soils and Subsoils in the Arid Region._—But however pronounced and
important are these distinctions and differences in the humid region,
they are found to be profoundly modified in the arid; where, as before
stated, the formation of colloidal clay is very much diminished,
so that most soils formed under arid conditions are of a sandy or
pulverulent type. There is then little or no clay to be washed down
into the subsoil, hence there is no compacting of the latter; the air
consequently circulates freely down to the depth of many feet.

Thus one of the most important distinctions between soil and subsoil
is to a great extent practically non-existent in the arid region, at
least within the depths to which tillage can be made to reach; so that
the limitations attached to subsoil-plowing in the countries of summer
rains do not apply to the characteristic soils of the arid regions.

Even the distinction in regard to humus is here largely obliterated
by the circumstance, already alluded to, that most of that substance
must, in the arid regions, be derived from the decay of roots, which
moreover reach to much greater depth in these soils. Hence even in the
uplands of the arid region it is common to find no change of tint from
the surface down to three feet, and even more. This, like the free
circulation of the air in consequence of porosity, tends to render the
distinction of soil and subsoil practically useless; since it disposes
of the objection to “subsoiling” based upon the inert condition of the
subsoil, which in humid climates so effectually interferes with the
welfare of crops unless subsoiling is restricted to a fraction of an
inch at a time.

These fundamental differences in the soils of the two regions are
illustrated schematically in the subjoined diagram, which shows on the
left the contrast between clay or clay loam soils, in which the depth
of the surface soil-sample to be taken is prescribed as nine inches by
the rules of the Association of Am. Official Chemists (in the writer’s
experience it is more nearly six inches as a rule). Alongside of the
Eastern soil thus characterized is placed a typical “adobe” soil from
the grounds of the California Experiment station, of which a sample
showing uniform blackness to three feet depth was exhibited at the
World’s Fair at Chicago in 1893. At the right is a profile of the noted
hop soil on the bench lands of the Russian river, Cal., in which the
humus-content was determined down to twelve feet, the humus-percentage
being .44% at that depth against 1.21% in the surface foot (see chapt.
8, p. 139). In this and similar soils the roots of hops reach down to
as much as fourteen feet without much lateral expansion; as shown in
plate No. 31 of this chapter. Similar conditions prevail in the sandy
uplands, as, _e. g._, in the wheat lands of Stanislaus county, Cal.,
mentioned above.

Taking the clay soils as a fair type for comparison, it would seem that
the farmer in the arid region owns from three to four farms, one above
another, as compared with the same acreage in the Eastern states.

_Subsoils and Deep-plowing in the Arid Region._—Up to the present time
this advantage is but little appreciated and acted upon by the farmers
of the arid region. They still instinctively cling to the practice
taught them by their fathers, and which is still promulgated as the
only correct practice, in most books on agriculture. There are of
course in the arid as well as in the humid region, cases in which deep
plowing is inadvisable; viz, that of marsh or swamp lands, as well as
sometimes in very sandy, porous soils, the cultural value of which
often depends essentially upon the presence of a somewhat consolidated,
and more retentive subsoil, which should not be broken up. But in most
soils not of extreme physical character, it is in the arid region not
only permissible, but eminently advisable to plow, for preparation, as
deeply as circumstances permit, in order to facilitate the penetration
of the roots beyond the reach of harm from the summer’s drought; while
for the same reason, subsequent cultivation should be to a moderate
depth only, for the better conservation of moisture, and the formation
of a protective surface mulch (see chapter 13).

[Illustration:

       |TYPE OF EASTERN SOILS|  TYPES OF CALIFORNIA SOILS.
    ---+---------------------+--------------------+-----------
    Ft.|       Upland.       | Up land clay-loam. | Beach land.
    ---+---------------------+--------------------+-----------

FIG. 27.—Soil Profiles illustrating differences in Soils of Humid and
Arid Region.]

It must not be forgotten that there are in the lowlands of the arid
region (river swamps or tules, sea-coast marshes, etc.,) soils in which
surface soil and subsoil are differentiated as fully as in the humid
countries; at least so long as they have not been fully drained for a
considerable length of time. In swamp areas that have been elevated
above the reach of overflow or shallow bottom-water by geological
agencies, even the heavy swamp clays are fully aerated down to great
depths, and roots penetrate accordingly.

_Examples of Plant-growth on Arid Subsoils._—The fact that in the arid
region the surface-soil conditions reach to so much greater depths
than in the East and in Europe, is so important for farming practice
in that region that experimental evidence of the same should not be
withheld. Of such, some cases well established as typical of California
experience are therefore cited.

    It is well known that in the Sierra Nevada of California
    the placer mines of the Foothills, worked in the early
    times, have long disappeared from sight, having been quickly
    covered by a growth of the bull pine (_P. ponderosa_).
    Much of this timber growth has for a number of years past
    been of sufficient size to be used for timbering in mines,
    and a second young forest is springing up on what was
    originally the red earth of the placer mines, which appears
    to the eye as hopelessly barren as the sands of the desert.
    In this same red sandy earth not unfrequently cellars and
    house foundations are dug, and the material removed, even to
    the depth of eight feet, is fearlessly put on the garden and
    there serves as a new soil, on which vegetables and small
    fruits grow, the first year, as well as ever. In preparing
    such land for irrigation by leveling or terracing no heed is
    taken of the surface soil as against the subsoil, even where
    the latter must be removed to the depth of several feet, so
    long as a sufficient depth of soil material remains above
    the bedrock.

    The same is generally true of the benchlands; the irrigator
    levels, slopes or terraces his land for irrigation with
    no thought of discrimination between soil and subsoil,
    and the cultural result as a rule justifies his apparent
    carelessness. It is only where from special causes a
    consolidated or hardpan subsoil is brought to the surface,
    that the land when leveled shows “spotted” crops. Such
    is the case in some of the “hog-wallow” areas of the San
    Joaquin valley of California, and in some cases where by
    long cultivation and plowing to the same depth, a compact
    soil-layer or plowsole has been formed, and the land is
    then leveled for the introduction of irrigation. In these
    cases a section of the soil mass will usually show a marked
    difference in color and texture. But, as a rule, in taking
    soil samples, no noticeable difference can be perceived
    between the first and the second, and oftentimes as far
    down as the third and fourth foot. The extraordinary
    root-penetration of trees, shrubs and taprooted herbs, whose
    fibrous feeding-roots are found deep in the subsoil and
    are sometimes wholly absent from the surface soil, fully
    corroborate the conclusion reached by the eye. The roots
    of grape vines have been found by the writer at the depth
    of twenty-two feet below the surface, in a gravelly clay
    loam varying but little the entire distance. In a similarly
    uniform and pervious material, the loess of Nebraska,
    Aughey[63] reports the roots of the native Shepherdia to have
    been found at the depth of fifty feet.

[63] See Merrill, Rocks and Rock weathering.

_Resistance to Drought._—These peculiarities of the soils of the arid
region explain without any resort to violent hypotheses, the fact
that many culture plants which in the regions of summer rains are
found to be dependent upon frequent and abundant rainfall, will in
California, and in the country west of the Rocky Mountains generally,
thrive and complete their growth and fruiting during periods of four
to six months of practically absolute cessation of rainfall; when east
of the Mississippi a similar cessation for as many _weeks_ will ruin
the crops, if not kill the plants. In continental Europe, in 1892,
a six weeks’ drought caused almost all the fruit crops to drop from
the trees, and many trees failed to revive the next season; while at
the very same time, the same deciduous fruits gave a bountiful crop
in California, during the prevalence of the usual five or six months’
drought. This was without irrigation, or any aid beyond careful and
thorough surface tillage following the cessation of rains in April
or May, so as to leave the soil to the depth of five or six inches
in a condition of looseness perfectly adapted to the prevention of
evaporation from the moist subsoil, and of the conduction of the
excessive heat of the summer sun. This surface mulch will contain
practically no feeding-roots, the paralysis or death of which by heat
and drought would influence sensibly the welfare of the growing plant.

[Illustration: FIG. 28.—Root of an Eastern (Wisconsin) Fruit Tree.
(Photograph by Prof F. H. King.)]

_Root-system in the Humid Region._—It is quite otherwise where a dense
subsoil not only obstructs mechanically the deep penetration of any
but the strongest roots, but at the same time is itself too inert
to provide sufficiently abundant nourishment apart from the surface
soil, which is there the portion containing, alongside of humus, the
bulk of the available plant-food, and in which alone the processes of
absorption and nutrition find the proper conditions; such as access
of air and the ready and minute penetration of even the most delicate
rootlets and root-hairs. The largest and most active portion of the
root-system being thus accumulated in the surface soil, it follows
that unless the latter is constantly kept in a fair condition of
moistness, the plant must suffer material injury very quickly; hence
the often fatal effects of even a few weeks’ drought. The same occurs
in the arid region when often-repeated shallow plowing has resulted in
the formation of a “plowsole” which prevents the deep penetration of
roots; when a hot “norther” will often in a short time not only dry the
plowed soil, but will heat it to such extent as to actually bake the
roots it harbors. Under the same weather-conditions an adjoining field,
properly plowed, may almost wholly escape injury.

[Illustration: FIG. 29.—Prune Tree on Peach Root, at Niles, Cal.]

_Comparison of root development in the arid and humid regions._—Figures
28, 29 given here show the differences as actually seen in the case of
fruit trees as grown in Wisconsin and California, respectively, both in
the absence of artificial water-supply.

_Adaptation of humid species to arid conditions._—Figures, in No. 30,
show the root systems respectively of the riverside grape (_Vitis
riparia_) as grown in the Mississippi Valley states, and the natural
development as found in the Rock grape of Missouri and also in the wild
grape vine of California. It will be noted at once that the latter
directs its cord-like roots almost vertically from the first, until
it reaches a depth varying from 12 to 18 inches, where it begins to
branch more freely, but still with a strong downward tendency in all.
The roots of the riverside grape, on the contrary, tend to spread
almost horizontally, branching freely at the depth of a few inches and
manifestly deriving its supply both of plant-food and moisture mainly
from the surface soil. It is curious to observe the behavior of this
vine when cuttings are planted in California vineyards as a resistant
grafting-stock. Its first roots are sent out horizontally, very much as
is its habit in the East, so long as the soil moisture is maintained
near the surface. But as the season advances, the more superficial
rootlets are first thrown out of action by the advancing dryness and
heat of the surface soil, and many finally die the first year.

[Illustration: FIG. 30.—Root Growth of Resistant Grape Vines.]

Not unfrequently the entire root system developed by the uppermost bud
perishes; but usually its main roots soon begin to recede from the
threatening drought and heat of the surface, curving, or branching
downward in the direction of the moisture supply, and without detriment
to their nutrition because of the practical identity of the surface
soil and subsoil. As the portions of the roots near the surface thicken
and mature, their corky rind soon prevents their being injured by the
arid conditions to which they are subjected; while the root-ends,
finding congenial conditions of nutriment and aeration in the moist
depths, develop without difficulty as they would in their humid home.
Practically the same process of adaptation takes place in every one of
the trees, shrubs, or perennials belonging to the humid climates, until
their root system has assumed nearly the habit of the corresponding
native vegetation.

The photograph of the roots of a hop plant, grown on bench lands of the
Sacramento river, shows the roots extending to 8 feet depth, but where
broken off the main root is still nearly two millimeters in thickness,
proving that it penetrated at least two feet beyond the depth shown in
figure 31.

In the case of native annuals, either the duration of their vegetation
is extremely short, ending with or shortly after the cessation of
rains; or else their tap roots descend so low, and the nutritive
rootlets are developed at such depth, as to be beyond reach of the
summer’s heat and drought. For while it is true that rootlets immersed
in air-dry soil may absorb plant-food, this absorption is very slow
and can only be auxiliary to the main root system which, instead of
terminating in the surface soil as in the humid region, will be found
to begin to branch off at depths of 15 and 18 inches, and may then in
sandy lands descend to from 4 to 7 feet even in the case of annual
fibrous-rooted plants like wheat and barley.[64] In the case of maize
the roots of a late-planted crop may sometimes be found descending
along the walls of the sun-cracks in heavy clay land poorly cultivated;
and it frequently matures a crop without the aid of a single shower
after planting. See figures 33, 34.

[64] Shaler (Origin and Nature of Soils; 12th Rept. U. S. Geol.
Survey, p. 311) says: “Annual plants cannot in their brief period of
growth push their roots more than six to twelve inches below their
root-crowns”—a generalization measurably true for the humid region
only. According to F. J. Alway, the roots of cereals penetrate to 5-7
feet in Saskatchewan, also.

[Illustration: FIG. 31.—Hop Root from Sacramento Bench-land.]

The annexed plate (No. 32) shows the main roots of two native perennial
weeds of California, the goosefoot (_Chenopodium californicum_) and the
figwort (_Scrophularia californica_), common on the lower slopes of the
coast ranges. The soil was a heavy clay loam or “black adobe” resulting
from the weathering of the clay shale bedrock, fragments of which are
so abundantly intermixed with the substrata that excavation of the
roots became very difficult. Yet the main root of the goosefoot went
down below the depth of eleven feet.

The main root of the figwort, also, was followed below the depth of ten
feet without reaching the extreme end. This proves clearly that the
great penetration of the goosefoot was not, as might be supposed, due
to its bulbous root. Yet such thickening of the root just below the
crown is a rather common feature in arid-region plants, and can here be
noted even in the figwort, within whose botanical relationship bulbous
roots are almost unknown.

Any one accustomed to the cornfields of the Middle West, where in the
after-cultivation of maize it is necessary to restrict very carefully
the depth of tillage to avoid bringing up a mat of white, fibrous
roots, will be at once impressed with the remarkable adaptability of
maize to different climatic conditions, as exhibited in such cases and
shown in figures 33, 34. In southern California, in the deep mesa or
bench soils, corn stalks so tall that a man standing on horseback can
barely reach the tassel, and with two or three large ears, are quite
commonly grown under similar rainfall-conditions.

_Importance of proper Substrata in the Arid Region._—The paramount need
of deep penetration of roots in the arid region renders the substrata
below the range of what is usually understood by subsoil in the humid
climates, of exceptional importance. A good farmer anywhere will
examine the subsoil to the depth of two feet before investing in land;
but more than this is necessary in the arid region, where the surface
soil is often almost thrown out of action during the greater part of
the growing season, while the needful moisture and nourishment must be
wholly drawn from the subsoil and substrata; an examination of which
should therefore precede every purchase of land, or planting of crops.

[Illustration: FIG. 32.—Deep-Rooting of Native California Goosefoot and
Figwort.]

[Illustration: FIG. 33.—Kentucky Maize, grown in region of Summer
Rains. (Photography by A. M. Peter.)]

[Illustration: FIG. 34.—California Maize, Grown Without Rain or
Irrigation.]

    Such examinations are most quickly made by means of a probe
    consisting of a pointed, square steel rod five or six feet
    long, provided at one end with a loop for the insertion of
    a cross-handle like that of a carpenter’s auger. The handle
    being grasped with both hands, the probe is forced into the
    soil with a slight reciprocating motion, by the weight of
    the operator; who soon learns how to interpret the varying
    kinds of resistance, and on withdrawing the probe carefully
    will generally be able to determine if bottom water has
    been reached. Should this easy method of examination not
    convey all the needful information, the post-hole auger may
    be resorted to; and it is desirable that extra (three-foot)
    rods or gaspipe joints be provided for the purpose of
    lengthening the probe or auger, when necessary, to nine
    or twelve feet. It will rarely be necessary to go to the
    trouble of digging a pit for such examinations; but even
    this is to be recommended rather than “buying a cat in a
    bag” in the guise of an unexplored subsoil.

_Faulty Substrata._—A number of examples of “faulty lands,” _i. e._,
such as are underlaid by faulty substrata, are given in the annexed
diagram Fig. 35; the examples being taken from California localities
because of their having been most thoroughly investigated. Similar
cases, as well as others not here illustrated, of course occur more or
less all over the world.

No. 1 shows a case which, though at first sight an aggravated one of a
rocky substratum, is in reality that of some of the best fruit lands
in the State. The limited surface-soil is very rich, and is directly
derived (as a “sedentary” soil) from the underlying bedrock slate.
But this it will be noted stands _on edge_, and the roots of trees
and vines wedge their way along the cleavage planes of the slate to
considerable depth, deriving from them both nourishment and moisture.
Under similar conditions the California laurel, usually found on the
banks of streams, grows on the summits of rocky ridges in the Coast
Ranges.

The case of No. 2 is quite otherwise. Here the shale lies horizontally,
and though much softer than the slate of the first column, obstinately
resists the penetration of roots; so that the land, though fairly
provided with plant-food, is almost wholly useless for cultivation. It
is naturally covered with low, stunted shrubs or chaparral; only here
and there, where a cleft has been caused by earthquakes or subsidence,
a large pine tree indicates that nourishment and moisture exists within
the refractory clay stratum, and suggests blasting as a means of
rendering the land fit for trees at least.

[Illustration: FAULTY LANDS. CALIFORNIA

FIG. 35.—Faulty Lands, California.]

No. 3 is a case similar to that of No. 2, only there is here a dense
unstratified mass of red clay, of good native fertility. It is here
that the expedient of blasting the tree holes with dynamite was first
successfully employed, in central California. For lack of this,
extensive tracts of similar land in southern California, planted to
orchards, have completely failed of useful results after three years of
culture.

No. 4 shows a typical case of calcareous hardpan obstructing the
penetration of roots, even though usually interrupted at intervals,
because of the formation occurring mostly in swales, along which
the sheets lie more or less continuously. Here also, blasting will
generally permit the successful growth of trees and vines, whose roots
frequently will, in time, wholly disintegrate the hardpan and thus
render the land fit for field cultures. The depth at which such hardpan
is formed usually depends upon the depth to which the annual rainfall
penetrates. (See below, page 183).

Nos. 4, 5, and 6 all illustrate cases of intrinsically fertile, very
deep soils, shallowed by obstructions which in the case of No. 4 are
hardpan sheets, while in No. 5 the intervention of bottom water limits
root penetration, hence restricts the use of the land to relatively
shallow-rooted crops, and the use of only a few feet of the profusely
fertile soil. Such is the case where bottom water has been allowed to
rise too high, through the use of leaky irrigation ditches.

No. 6 illustrates a case not uncommon in sedimentary lands, where
bottom water is quite within reach of most plants, but is prevented
from being utilized by the intervention of layers of coarse sand or
gravel, through which the water will not rise; and the roots, while
they would be able to penetrate, are not near enough to feel the
presence of water underneath and therefore spread on the surface of
the gravel, suffering from drought within easy reach of abundance of
water. The “going-back” of large portions of orange orchards in the
San Bernardino Valley of California has been thus brought about; and
unfortunately this state of things is almost beyond the possibility of
remedy.

[Illustration: FIG. 36.—Almond Tree on Hardpan. Paso Robles Substation,
Cal.]

_Injury from Impervious Substrata._—The injurious effects of a
difficultly penetrable subsoil have already been discussed and are
self-evident. When the substratum is a dense clay, the rise of moisture
from below being very slow, it can easily happen that the roots cannot
penetrate deep enough in time for the coming of the dry season, and
that thus the crop will suffer. The case will be still worse when
hardpan cemented by lime or silex limits root-penetration, as well as
proper drainage. In such cases the culture of field crops often becomes
impracticable, even with irrigation, as its frequent repetition,
besides being costly, can rarely be commanded. In the case of trees,
the limitation of root-penetration results in the spreading-out of the
roots on the surface of the impenetrable layer; as shown in figure 36,
which exhibits a root-development that would be quite normal in the
regions of summer rains, but is wholly abnormal in the arid region, and
results in the unprofitableness or death of the trees. It has often
been attempted in such cases to plant trees in large holes dug deep
into the subsoil and refilled with surface earth and manure. All such
attempts result in failure, if only because the excavation inevitably
fills with water, which will soak away but very slowly into the dense
substrata, and will thus injure or drown out the roots. Besides, the
latter will remain bunched in the loose earth, and will thus be unable
to draw either moisture or nourishment from the surrounding land. It
is absolutely necessary to remedy this by loosening the substrata, if
success is to be attained.

_Shattering of Dense Substrata by Dynamite._—The permanent loosening
of dense substrata is best accomplished by moderate charges (½ to
¾ pounds) of “No. 2” dynamite at a sufficient depth (3 to 5 feet).
The shattering effect of the explosive will be sensible to the depth
of eight feet or more, and will fissure the clay or hardpan to a
corresponding extent sidewise. If properly proportioned the charge
will hardly disturb the surface; but if this be desired, from 1½ to
2½ pounds of black powder placed above the dynamite will throw out
sufficient earth to plant the tree without farther digging. Where labor
is high-priced this proves the cheapest as well as the best way to
prepare such ground for tree planting; and it has often been found that
in the course of time, the loosening begun by the powder has extended
through the mass of the land so as to permit the roots to utilize it
fully, and even to permit, in after years, of the planting of field
crops where formerly they would not succeed.

_Leachy Substrata._—While we may thus overcome the disadvantages of a
dense subsoil or hardpan, there is another difficulty not uncommonly
met with in alluvial lands, which cannot be so readily remedied. It
is the occurrence, at from two to six feet depth, of coarse sand or
gravel, through which capillary moisture will not ascend, but through
which irrigation water will waste rapidly, leaving the overlying soil
dry. Then unless very frequent irrigation can be given, the crop will
suffer from drought, unless indeed the gravel itself is filled with
bottom water upon which the root-ends can draw.

This case is a common one in the larger valleys of the arid region, and
in time of unusual drought the sloughs originally existing, but since
filled up, will be clearly outlined by the dying crops, while outside
of the old channels there may be no suffering.

_“Going-back” of Orchards._ On such land as this, and on such as has a
shallow soil underlaid by an impervious subsoil, trees will often grow
finely for three to five years; then suddenly languish, or turn yellow
and die, as the demand of their larger growth exceeds what moisture or
plant-food the shallow soil and subsoil can supply. Enormous losses
have arisen from this cause in many portions of the arid region, but
more especially in California, owing to the implicit confidence reposed
even by old settlers, and still more by newcomers, in the excellence
of the lands, as illustrated by farms perhaps a short distance away,
but differently situated with respect to the country drainage and the
geological formations. All such disappointments could have been avoided
by an intelligent observation of the substrata, either by probing or
digging. Important as is such preliminary examination in the region of
summer rains, it is a _vitally_ needful precaution in the arid region,
where the margin between adequate and inadequate depth of soil and
moisture-supply is much smaller.

    When farmers note such distress in the orchard, the first
    idea usually is that fertilization is needed. This in the
    almost universally very rich lands of the arid region
    is rarely the case until after many years of exhaustive
    cultivation, and is scarcely ever of more than passing
    benefit in such cases. The first suggestion should always
    be an _examination of the substrata_, and especially
    of the deeper roots; in the diseased or thirsty condition
    of which the cause of the “die-back” or yellowing will
    commonly be found. Of course no amount of fertilization can
    permanently remedy such a state of things, arising from
    impervious substrata, coarse gravel, or shallow bottom water.

_Hardpan._—By “hardpan” is understood a dense and more or less hardened
layer in the subsoil, which obstructs the penetration of both roots
and water, thus materially limiting the range of the former both
for plant-food and moisture, and giving rise to the disadvantages
following such limitation, as described in the case of dense subsoils.
The hardpans proper differ from the latter, however, in being usually
of limited thickness only; the direct consequence of their mode of
formation, which is not direct deposition by water or other agencies,
but the infiltration of cementing solutions into a pre-existing
material originally quite similar to that of the surface soil. Such
solutions usually come from above, more rarely from below, and are of
very various composition. The solutions of lime carbonate in carbonated
water have already been referred to in this connection; as has also the
fact that corresponding solutions of silica, associated more or less
with other products of rock decomposition (see chapters 2 and 4) are
constantly circulating in soils. The surface soil being the portion
where rock-weathering and other soil-forming processes are most active,
these solutions are chiefly formed there; and according as their
descent into the substrata is unchecked, or is liable to be arrested
at some particular level, whether by pre-existing close-grained layers
or by the cessation of rains, the subsequent penetration of air, and
evaporation of the water alone by shallow-rooted plants, may cause the
accumulation of the dissolved matter at a certain level, year after
year. Finally there is formed a subsoil-mass more or less firmly
cemented by the dissolved matters, sometimes to the extent of stony
hardness (lime carbonate in the arid regions, kankar of India), more
usually soft enough to be penetrated by the pick or grubbing hoe, and
sometimes by the stronger roots of certain plants; but resisting both
the penetration and the assimilation of plant food by the more delicate
feeding roots.

_Nature of the Cements._—The nature of the cements that serve to
consolidate the hardpan mass is substantially the same as those already
mentioned in the discussion of sandstones (chapt. 4, p. 55); with
the addition of those formed, usually in connection with siliceous
solutions, by the acids of the humus group. The latter class of
hardpans is especially conspicuous in the case of swampy ground and
damp forests, where “moorbedpan” and reddish “ortstein” (the latter
particularly developed in the forests of northern Europe, where it has
been studied in detail by Müller and Tuxen[65], are characteristic.)
The latter gives for a characteristic sample of the reddish hardpan
underlying a beech forest in Denmark a content of from 2.20 to 4.40%
of ulmic compounds, and shows that the color is due to these and not,
as had been supposed, to ferric oxid, which is present only in minute
quantities.

[65] See “Studien über die natürlichen Humusformen,” by Dr. P. E.
Müller.

_Bog ore, Moorbedpan, Ortstein._—It is otherwise with moorbedpan, which
often consists of a mass of bog iron ore permeated more or less with
humous substances, which impart to it the dark brown tint so often seen
also in the “black gravel” spots of badly-drained land. On the whole,
however, ferric cements are much less frequently found in hardpans than
in sandstones formed above ground.

_Clay substance_ washed from the surface into the subsoil by rains
(chapter 10, p. 161) always helps materially to render the hardpan
impervious when afterwards cemented, a much smaller proportion of
the cementing material sufficing in that case to form a solid layer.
In such cases however the cement is rarely of a calcareous nature,
since lime prevents the diffusion and washing-down of the clay. It is
mostly siliceous or zeolitic; if the former, acid will have little or
no effect upon the solidity of the hardpan; while if zeolitic, acid
will pretty promptly disintegrate it. The presence of humus acids in
the cements, if not apparent to the eye, is readily demonstrated by
immersing the hardpan fragment in ammonia water or a weak solution of
caustic soda; when if humus acids are the main cementing substance the
fragment will fall to crumbs, or be softened to an extent corresponding
to the amount of the humus present. Calcareous hardpan is, of course,
readily recognized by its quick disintegration by dilute acid, with
evolution of carbonic gas.

In “alkali” soils containing sodic carbonate (“black alkali”) there
is commonly found at the depth of two or three feet an exceedingly
refractory hardpan resulting from the accumulation of puddled clay (see
above chapt. 4, p. 62) in the subsoil, or sometimes even on the surface
of depressed spots. This hardpan, easily destroyed by the use of gypsum
and water, is described more in detail in chapter 22, on alkali soils;
it blues red litmus paper instantly.

_The Causes of Hardpan._—The recognition of the _cause_ of hardpan is
of considerable importance to the farmer, because of the influence
of the nature of the cement and the causes of its formation upon the
possibility and methods of its destruction, for the improvement of the
land.

It may be said in general that inasmuch as the cause of the formation
of hardpan is a stoppage of the water in its downward penetration, the
re-establishment of that penetration will tend to prevent additional
induration; moreover, experience proves that whenever this is
accomplished even locally, as around a fruit tree in an orchard, the
hardpan gradually softens and disappears before the frequent changes in
moisture-conditions and the attack of roots. The use of dynamite for
this purpose in California has already been referred to; it seems to
be the only resort when the hardpan lies at a considerable depth. When
it is within reach of the plow, it may be turned up on the surface by
the aid of a subsoiler and will then gradually disintegrate under the
influence of air, rain and sun. But when the hardpan is of the nature
of moorbedpan, containing much humic acid and perhaps underlaid by bog
iron ore, the use of lime on the land is indicated, and will in the
course of time destroy the hardpan layer. This is the more desirable
as in such cases the surface soil is usually completely leached of its
lime content, and is consequently extremely unthrifty.

Woodlands of northern countries bearing beech and oak are especially
apt to be benefited by the action of lime on the “raw,” acid humous
soil and underlying hardpan, which is commonly underlaid by a
leaden-blue sandy subsoil (“Bleisand” of the Germans, “Podzol” of
the Russians) colored brown by earth humates and mostly too moist in
its natural condition to permit of adequate aeration. These soils
are usually of but moderate fertility, and are best suited to forest
growth unless somewhat expensive methods of improvement can be put into
practice.

“_Plowsole._”—An artificial hardpan is very commonly formed under the
practice of plowing to the same depth for many consecutive years. The
consolidated layer thus created by the action of the plow (hence known
as plowsole) acts precisely like a natural hardpan, and is sometimes
the cause of the formation of a cemented subsoil crust simulating
the natural product. This is most apt to occur in clayey lands, and
greatly increases the difficulty of working them, while detracting
materially from the higher productiveness commonly attributed to them
as compared with sandy lands. Of course it is perfectly easy to prevent
this trouble by plowing to different depths in consecutive years, and
running a subsoil plow from time to time. In this case, also, lime
will generally be very useful and be found to aid materially in the
disintegration of the “plowsole.”

It is hardly necessary to insist farther upon the need of the
examination of land to be occupied, for the existence of hardpan or
other faulty subsoil, which may totally defeat for the time being the
farmer’s efforts, or make him lose his investment in plantations after
a few years. Probing by means of the steel rod described above (p. 177)
or boring with a post-hole auger; or finally, if necessary, digging a
pit to the proper depth (from four to six feet in the arid region),
should precede every purchase of new or unexplored agricultural land.

_Marly Substrata._—Among the causes of failure occasionally found in
the case of the “going-back” of orchards, is the occurrence of strongly
calcareous or marly substrata, at depths which in the humid region
would not be reached by the roots, but in the course of a few years are
inevitably penetrated by the roots of trees in the arid region. Then
there appears a stunting of the growth, and sometimes a yellowing of
leaves, or chlorosis, due to the influence of excessive calcareousness
at the depth of four or five feet. For this of course there is no
remedy except the planting of crops which, like the mulberry, Texas
grapes, Chickasaw plum and others, are at home on such lands; which in
the Eastern states are naturally occupied by the crab apple, honey
locust and wild plums.




CHAPTER XI.

THE WATER OF SOILS.

HYGROSCOPIC AND CAPILLARY MOISTURE.


When it is remembered that from 65 to over 90% of the fresh substance
of plants consists of water, the importance of an adequate and regular
supply of the same to growing plants is readily understood. But it
seems desirable, before discussing the relations of water to the soil
and to plant life, to consider first the physical peculiarities which
distinguish it from nearly all other substances known. That it is
colorless, tasteless, inodorous, and also chemically neutral, alone
constitutes a group of properties scarcely found in any other fluid.
But its special adaptation to its functions in relation to vegetable
and animal life are much more fundamental, as is shown in the table of
its physical constants as compared with other well-known substances,
given below.

     PHYSICAL FACTORS OF WATER COMPARED WITH OTHER SUBSTANCES
                      (PER UNIT WEIGHT).
     ================================+===================================
     Capillary ascent in glass tubes |     Specific Heats.
           of one mm. diameter.      |
                                     |
     Water                     14 mm.| Water         1.000
     Alcohol                    6 mm.| Ice            .502
     Olive oil                  1 mm.| Steam          .475
                                     | Clay, Glass    .180-.200
                                     | Charcoal       .241
             HEAT RELATIONS.         | Wood           .032
                 Density.            | Gold, Lead     .032-.031
                                     | Zinc           .096
     Water at 0° C. (freezing        | Steel          .119
       pt.)                    .99988|
     Water at 4° (Maximum            |            Heat of fusion.
                   density)   1.00000| Water (Ice)              80   Cal.
     Water at 15° C. (ordinary       | Metals                   5-28  “
                  temperature) .990  | Salts, (incl. silicates) 40-63 “
     Ice at 0° (freezing pt.)  .92800|
     --------------------------------+-----------------------------------

    +===================================
    |   Heat of Evaporation.
    |
    | Water at  20°C.           613 Cal.
    |   “   “  100°C.           637  “
    | Alcohol                   209  “
    | Spirits of  Turpentine     67  “
    +-----------------------------------

Summarizing the meaning of the data given in the above table with
respect to organic life, we see, first, that water rises higher both
in the soil and in the tissues of the plant than any other liquid.
Second, that as its density decreases in cooling after a certain point
is reached, it freezes at the _surface_ instead of at the _bottom_, as
other liquids do; and as solid water (ice) is lighter than fluid water,
ice stays at the surface and is readily melted when spring comes.
Third, since its temperature changes more slowly than that of any other
liquid, it serves to prevent injuriously rapid changes of temperature
in plants and animals as well as in soils. Its high “heat of fusion”
also serves to prevent quick freezing of plant and animal tissues, so
that the brief prevalence of a low temperature may be more readily
borne. Finally, the large amount of heat absorbed in evaporation of
water serves to keep both plants and animals cool under excessive
external temperatures which would otherwise quickly destroy life.

_Capillarity or Surface Tension._—In this table it will be noted,
first, that water rises higher in fine (“capillary”) or hair tubes
than the other fluids mentioned, which fairly represent all others. No
other fluid approaches water in the height to which it will rise[66]
in either soils or plant tissues. Were its capillary factor no higher
than, _e. g._, that of oil or alcohol, trees could not grow as tall
as we find them, and the water supply from the substrata, and all
the movements of water in the soil, and hence plant growth, would be
similarly retarded. It is easy to verify these differences by immersing
a cylinder of clay soil (or a cotton wick) in water on the one hand,
and in oil or alcohol on the other. Notwithstanding the greater
fluidity of alcohol as compared with water, the latter will be found to
fill the porous mass much more quickly.

[66] Excepting only the water-solutions of certain salts, among which
common salt, kainit and nitrate of soda are of agricultural interest.
Common salt may increase the capillary rise to the extent of more than
five per cent.

    The smaller the diameter of the tube, the higher will the
    water rise in it, and the greater will be the curvature
    of its upper surface, to which the rise is sensibly
    proportional. But in the case of liquids which do not “wet”
    the walls of the tube (as in that of mercury and glass),
    the curve (meniscus) is convex, instead of concave, and the
    liquid is depressed instead of rising.

It is in its _relations to heat_, however, that water is specially
distinguished from other substances; and these differences are most
vital not only to living organisms, but to the entire economy of Nature.

_Density._—As regards the density or specific gravity of water (which
is by common consent assumed as the unit of comparison), it will be
seen from the “Density” table that whereas all other bodies contract
and become more dense as they grow colder, water has its point of
(fluid) “maximum density” at 4°C. (49°.2 Fahr.), and _expands_ as it
grows colder, until at 0°C. (32° Fahr.) it solidifies into ice. In
so doing it departs still farther from the rule obtaining with all
other bodies (excepting certain mixtures, such as type metal) and
again expands so as to decrease the density from .99988 to .92800;
thus causing ice to float on water at the freezing point. Hence water,
unlike all other fluids, solidifies first on the surface; and but for
this, the thawing of the winter’s ice, which would be formed at the
bottom of rivers and lakes, would be deferred until late in summer. The
expansion of water in freezing is forcibly illustrated in the bursting
of water pipes and pitchers in winter; in the soil, the ice forming in
the interstices serves to loosen the compacted land and give it better
tilth for the ensuing season.

_Specific Heat._—Considering next, the column showing the “specific
heat” of water as compared with other substances, we see that it
exceeds all other known bodies in the amount of heat required to change
its temperature; hence again, _its_ heat capacity is taken as the unit
to which all others are compared. The figures given in the table show
that even ice and steam require for equal weights only about half as
much heat (or burning of fuel) to change their temperature (_e. g._,
1 degree) as would liquid water. But earthy matters, such as clay or
soil and glass, require only one-fifth as much heat for a similar
change; charcoal only about one-fourth as much. But vegetable matter as
represented by wood on the one hand, and gold and lead on the other,
require only about one-thirtieth as much heat as an equal weight of
water; zinc about one-tenth as much, steel somewhat more.

It is thus plain that masses of water act powerfully, more than any
other substance, as moderators of changes of temperature by their
mere presence. The body of an animal or plant is protected against
violent changes by the presence of from 60% to 90% of liquid water,
the temperature of which can only be raised or lowered slowly; and
the presence of the sea tempers the climates of coasts and islands
as compared with the heat or cold occurring in the interior of the
continents.

_Ice._—Again, it is shown in the table that the heat required to _melt
ice_ is greater than in the case of any other substance, especially
the metals; which when once heated to the fusing point, require only a
very little more heat to become liquid. The fusion of salts (including
silicate rocks) requires more heat than does that of the pure metals.

_Vaporization._—In the amount of heat required for its _vaporization_
water is also especially pre-eminent, and potent in its influence upon
organic life. The table shows that the evaporation of water requires
six hundred heat units[67] as compared with alcohol, requiring only two
hundred; while spirits of turpentine, the representative of a large
proportion of vegetable fluids, needs but sixty-seven.

[67] A heat unit, or “calorie,” is the amount of heat required to raise
the temperature of a unit-weight (pound, kilogram, or gram) of water
one thermometric degree. According to the unit-weight and thermometric
scale used, the figures will vary, but in this text the basis is
understood to be kilograms and the centigrade scale.

The practical result is that evaporation of water from the surface
of animals and the leaves of plants, is exceedingly effective in
preventing excessive rise of temperature, the heat of the sun and air
being spent in evaporating the perspiration of animals and plants
before an injurious rise of temperature, such as would cause sunstroke
in animals, and wilting or withering in plants, can occur. But since
evaporation is most rapid in dry air, it follows that the cooling
effect will be the greater in the arid regions than in the humid. In
the latter, therefore, sunstroke is much more frequent than in the
fervid regions of the arid west, even though the temperature in the
latter may be higher by twenty or twenty-five degrees Fahrenheit.
White men who would soon succumb if they attempted to work in the
sun in Mississippi or Louisiana when the thermometer stands at 95°F.
will experience no inconvenience under the same conditions in the dry
atmosphere of the Great Valley of California.

_Solvent Power._—To the exceptional properties of water discussed
above, should be added another hardly less important one, viz., that
of being an almost universal solvent especially of mineral matters,
including even those which, like quartz, appear to be most insoluble
and refractory (see chapt. 3). The water of the soil is thus enabled
to convey to the roots of plants, in solution, all kinds of plant
food contained in the soil. It should be noted that distilled (hence
also rain-) water is a more powerful solvent, _e. g._, of glass, than
ordinary waters containing mineral matter, and even free acids.

Practically, plants take up _all_ their water supply from the soil
in the liquid form; and hence the soil-conditions with respect to
this supply are of the most vital importance to plant growth. The
most abundant supply of mineral plant food may be wholly useless,
unless the physical conditions of adequate soil-moisture, access of
air, and warmth, are fulfilled at the same time. On the other hand,
comparatively few plants are adapted to healthy growth in soils
saturated with water, or in water itself; and but few among these are
of special interest from the agricultural standpoint.

_Water-requirements of Growing Plants._—The amount of water contained
in any plant at one time, however large, is but a small proportion of
what is necessary to carry it through its full development. When we
measure the amount of water actually evaporated through the plant in
the course of its normal growth, we find it to be several hundred times
the quantity of dry vegetable substance produced; varying according
to the extent and structure of the leaf-surface, the number and size
of the breathing pores (stomata) of the leaves, and the climatic
conditions (including specially the duration of active vegetation, and
temperature during the same), from 225 to as much as 912 times the
weight of the mature, dry plant.

The following are extreme figures for water consumption of different
plants as reported by different observers, viz., Lawes and Gilbert in
England, Hellriegel in northern Germany, Wollny in Southern Germany
(Munich), and King in Wisconsin: Wheat, 225 to 359; barley, 262 to 774;
oats, 402 to 665; red clover, 249 to 453; peas, 235 to 447; mustard
and rape, 845 to 912 respectively; the latter figure being the maximum
thus far reported. The highest figures given are throughout very nearly
those of Wollny, working in the very rainy climate of Munich.

_Evaporation from Plants in Different Climates._—It might be expected
that in countries where the air is usually moist, the evaporation
will, other things being equal, be less than where it is commonly far
below the point of saturation. But the “guardian cells” (stomata) of
the leaf pores possess the power of regulating, to a certain extent,
the evaporation from the leaf-surface in accordance with temporarily
prevailing conditions, so as to allow free evaporation in moist air,
but to prevent the wilting and drying-up of the leaf in hot and dry
air, save in extreme cases. Moreover, plants adapted to arid conditions
are usually provided with additional safeguards in the form of thick,
non-conducting layers of surface cells, or long channels connecting the
interior tissue with the breathing-pores on the surface. Often hairy,
scaly or viscous coverings serve the same end. On the other hand, when
the air is very moist, so as to check evaporation, water is sometimes
found secreted in minute droplets around the breathing-pores of the
leaves, since its ascent is a necessary condition of nutrition and
development.

_Relation between Evaporation and Plant-growth._—There is not in all
cases any direct relation between the amount of evaporation and plant
growth; but experience, as well as numerous rigorous experiments
have shown that _under ordinary conditions of culture, and within
limits varying for different soils and crops, production is almost
directly proportional to the water supply during the period of active
vegetation_.

On the basis of Hellriegel’s results, showing that wheat uses (in
Germany) about 435 tons, or nearly four acre-inches of water in the
production of one ton of dry matter, and assuming the ratio of grain
to straw to be 1:1.5, King calculates the following table of probable
production under different moisture conditions (Physics of Agriculture,
page 140):

                           YIELD PER ACRE.
    =========+===========+===========+=========+=============
    Number of| Weight of | Weight of | Total   | Water used.
    Bushels. |  Grain.   |  Straw.   | Weight. | Acre-inches.
             |   Tons.   |   Tons.   |  Tons.  |
    ---------+-----------+-----------+---------+-------------
       15    |     .45   |  .675     |  1.125  |    4.498
       20    |     .60   |  .90      |  1.500  |    5.998
       25    |     .75   | 1.125     |  1.875  |    7.497
       30    |     .90   | 1.350     |  2.250  |    8.997
       35    |    1.05   | 1.575     |  2.625  |   10.495
       40    |    1.20   | 1.800     |  3.000  |   12.000
    ---------+-----------+-----------+---------+-------------

S. Fortier has made several series of tests to determine the actual
yield of grain crops under field conditions when supplied with
different amounts of water. Two of these were made at the Montana
experiment station in 1902 and 1903, (see reports of these years), in
large tanks placed in a field, level with the ground. The results of
the last year’s experiments are shown graphically in the figure below,
from which it will be seen that the yield increased quite regularly
with the amount of water supplied, up to the depth of 36 inches of
water.

[Illustration: FIG. 37.—Experiments on Cereal production with various
amounts of water (Fortier, Report Mont. Expt. Sta., 1903).]

It should be noted that in this case (and as usual) not only the
quantity but the quality of the grain was greatly improved as the
water-supply increased, it becoming larger and more uniform in size.

Of similar experiments made in the San Joaquin Valley, California, in
1904, Fortier says:[68]

    “In experimenting with barley last winter the natural
    rainfall, which amounted to 4½ inches during the period of
    growth, produced at the rate of nine bushels per acre, while
    the application of sixteen inches of water increased the
    yield to twenty-two bushels per acre. In the same case; of
    wheat, the rainfall, alone, produced straw, but no grain;
    four inches of additional irrigation water produced a yield
    at the rate of ten bushels, and sixteen inches of water
    increased the yield to thirty-eight bushels per acre.”

[68] “Water and Forest,” January, 1905. “The Use of Water,” by S.
Fortier.

It is thus obvious that, other things being equal and with conditions
sufficiently favorable for the growth of crops, the rule as formulated
above is verified in practice.

    Whitney (Bulletin 22, Bureau of Soils, U.S. Dept. Agr.),
    has carried this rule so far as to claim that in all soils,
    the moisture supply is the _only_ important factor,
    and that so long as this is provided for, soil fertility
    continues indefinitely without replacement of ingredients
    withdrawn. The latter conclusion is so thoroughly disproved
    by experience as well as experiment that it hardly requires
    discussion here.

Whether plants, especially cultivated ones, are capable of adapting
themselves to arid conditions so as to be capable of producing
satisfactory crops with less water than is actually consumed in the
humid region, has not been directly determined. Such is, however, the
impression produced by farming experience; and the fact that among
the common weeds of arid California are mustard and rape, cited by
Wollny[69] as requiring over three times as much water as does maize
for the production of one part of dry matter, lends color to the
supposition that in some manner these, and probably other plants, use
more water in humid than in dry climates (see this chapt. p. 212).

[69] See Wollny’s experiments, Forsch. Agr. Phys. Vol. 20, p. 58.

It is therefore impossible to assign a definite figure for the amount
of water required by vegetation _at large_; and even for one and the
same plant, only approximations conditioned upon climatic factors can
be given. We can in many cases, however, assign for one plant, or for
certain groups of plants, the amounts of water producing the best
results (“optimum”) and the least amount (“minimum”) compatible with
a paying crop, that must be furnished during the growing season, to
produce certain results. For when instead of fruiting, it is desired
that the crop should produce the largest possible amount of vegetable
substance, as in the case of forage crops, a larger amount of water
will usually be serviceable.

_Different conditions of Soil-Water._—Water may be contained in the
soil in three different conditions, viz.:

1. From absorption of water vapor; Hygroscopic water.

2. Liquid water held suspended between the soil particles so as to
exert no hydrostatic pressure; capillary water, or water of imbibition.

3. Liquid water seeking its level; bottom, ground or hydrostatic water.


HYGROSCOPIC WATER.

Soils artificially dried so as to deprive them of all their moisture,
when exposed to moist air absorb water vapor with great energy at
first; both the rapidity of absorption and the amounts absorbed, when
full time is given, varying greatly with their nature. Sandy soils,
broadly speaking, absorb the smallest amounts; while clayey soils, and
those containing much humus, or finely divided ferric hydrate, take up
the largest proportion.

The figure expressing the amount of aqueous vapor absorbed at the
standard temperature of 15° Cent., is called the _coefficient of
moisture absorption_. For one and the same substance, this coefficient
rises as the grain becomes finer, the surface being correspondingly
increased (see chapt. 6).

The table below indicates the effect of the three substances mentioned
in increasing moisture absorption as compared with a very sandy soil
from the pine woods of Mississippi, and a gray silt or “dust” soil
from Washington, very fine-grained but poor both in humus and ferric
hydrate. (For details of the physical composition of the Mississippi
soils see table in chapt. 6, p. 93). A highly ferruginous soil from
Oahu shows plainly the effect of that substance.

    TABLE SHOWING INFLUENCE OF SILT, SAND, CLAY, FERRIC HYDRATE,
               AND HUMUS ON MOISTURE ABSORPTION.
    ================================+======+======+=====+=========
                                    | 248  |  79  | 238 |   230
                                    |      |      |     |
                                    |Miss. |Wash’n|Miss.|  Miss.
                                    |Pine  | Dust |White|Flatwoods
                                    |Hills | Soil.|Pipe |  Clay
                                    |Sandy |      |Clay.|  Soil.
                                    |Loam. |      |     |
                                    |      |      |     |
                                    |  %   |   %  |  %  |   %
    --------------------------------+------+------+-----+---------
    Hygr. Moisture                  |  2.48|  4.92| 9.09|   9.33
    Clay                            |  2.94|  1.27|74.65|  25.48
    Ferric Hydrate                  |  1.64|      |  .15|
    Humus                           |   .55|   .44| 0.00|    .50
    Finest Silts (.01-.0250 mm.)    | 60.10| 45.04|23.15|  68.60
    Sands, f. and c. (.0250-.50 mm.)| 31.20| 42.40|  .20|   4.70
    --------------------------------+------+------+-----+---------
    ================================+===========+===========+=====+=====
                                    |    246    |           | 220 | 215
                                    |           |           |     |
                                    |   Misc.   |   Oahu    |Miss.|Miss.
                                    |Ferruginous|Ferruginous|Marsh|Marsh
                                    |   Clay    | Laterite. |Muck.|Soil.
                                    |   Soil.   |           |     |
                                    |           |           |     |
                                    |           |           |     |
                                    |     %     |     %     |  %  |  %
    --------------------------------+-----------+-----------+-----+-----
    Hygr. Moisture                  |   18.60   |   19.66   |21.00|15.40
    Clay                            |   28.15   |     ?     | Tr. | Tr.
    Ferric Hydrate                  |   12.10   |   41.00   |     |
    Humus                           |  little   |    3.33   |66.10|19.83
    Finest Silts (.01-.0250 mm.)    |   40.33   | } 45.66   |33.94| 8.70
    Sands, f. and c. (.0250-.50 mm.)|   15.61   | }         |     |70.18
    --------------------------------+-----------+-----------+-----+-----

It will be noted that the greater fineness of grain in the Washington
dust soil induces a higher absorption of moisture than occurs in the
sandy soil from Mississippi, although the latter contains more clay.
Comparison of the figure for the Mississippi pipe-clay and clay soil
with the ferruginous soils, from the same state and from Oahu, indicate
plainly the influence of the ferric hydrate in increasing absorption;
although in the latter case the clay determination was not made,
because of the excess of ferric hydrate. The influence of humus is
plainly shown in the case of the marsh muck and soil, neither of which
contain any appreciable amount of either clay, or ferric hydrate in
the finely diffused condition. The relatively slight difference in the
absorptions of muck and soil is due to the only partial humification of
the organic matter in the former, while in the soil the humification is
sensibly complete, and the sand forming the body of the material serves
to render it more loose.

These data, referring to natural materials, while not as complete
as could be desired, are sufficient to prove the facts, and seem
preferable to any artificially devised imitation of their kind.

_Influence of Temperature, and Degree of Air-Saturation._—The amount
of moisture absorbed varies materially both with the temperature,
and with the degree of saturation of the air to which the soil is
exposed. Schübler, Knop and other earlier observers, operating with
earth exposed to air only partly saturated, and with soil layers of
considerable thickness (in watch glasses), found that the absorption
decreased as the temperature increased, according to a law formulated
by Knop. The writer found that under the conditions established in the
experiments of Knop and others, the air was not nearly saturated,[70]
so that these determinations are marred by ineliminable faults, the
more as the soils used are only designated in general terms, as “garden
soil,” “loam,” “peaty land,” etc., without any definite indication of
their actual physical or chemical constitution. The writer therefore
undertook to correlate these coefficients, determined with respect to
completely saturated air, with the physical composition of certain
soils, as determined by means of the methods heretofore described.

[70] It should be understood that it is by no means easy to insure full
saturation in any considerable volume of air.

It has generally been considered sufficient to cover with water the
bottom of the space in which absorption was to occur. The writer found
that in order to insure uniform results, it was necessary to cover the
entire inner surface of the vessel with wet blotting paper, and even
then to exclude carefully all circulation of air by padding the joints
with such paper. When only the bottom of the box was covered, samples
placed at different levels above the water surface gave discordant
results. It was also observed that whenever the thickness of the soil
layer exceeded about one millimeter, a long time was required for
full saturation; during which inevitable changes of temperature would
bring about a deposition of dew on the soil, greatly exaggerating the
absorptive coefficient.

In the chamber used at the California station for soil saturation,
dimensions 12 × 18 × 19 inches high, the same soil was exposed on a
shelf close to the surface of the water, another midway up, a third
near the lower surface of the cover; liquid water being in the bottom
of the chamber, and the rest covered with wet blotters. It was found
that despite these precautions, the lowest soil layer absorbed in the
same time as much as ¾% more than the uppermost one.

    Some of the data so obtained are given in the table of
    physical soil composition on page 93, chapt. 6. They
    have since been extensively supplemented by additional
    determinations, but without materially changing the
    coefficients approximately corresponding to the several
    designations accepted in farm practice. Experiments
    conducted by the writer have conclusively shown that Knop’s
    law of decrease of absorption with rise of temperature not
    only is not true for _fully_ saturated air, but must be
    reversed; the fact being that the amount of water absorbed
    by the soil _increases in a fully saturated atmosphere_
    (i.e., in presence of excess of water) _as the temperature
    rises_, at least between 15 and 35 degrees Cent. Thus,
    fine sandy soil which at 15° absorbed 2% of moisture, took
    up 4% at 34°; while loam soil absorbing 7% at 15°, showed
    nearly 9% at 35°; an increase of 2% in each case. But in
    partially saturated air[71] it was found that, as stated by
    Knop, the amounts absorbed steadily decrease, though not
    according to the law announced by him. Taking as a unit
    the moisture absorbed at 15°, it was found that in air
    three-fourths saturated, ¾ of the unit was taken up by the
    soil; at half saturation, nearly the proportional amount;
    but at one-fourth saturation the earths absorb materially
    more than a similar proportion, being then capable of
    withdrawing moisture from greatly undersaturated air. Since
    air thus undersaturated occurs not uncommonly in the arid
    regions of the world, the fact that the soil cannot be
    farther dried by such air of the same temperature, is of
    some practical significance.

[71] The partial saturation to a definite extent was effected by means
of solutions of calcium chlorid of different degrees of concentration,
according to the determinations of Wüllner (Pogg. Ann.). These
solutions were placed in a wide, flat dish, over which a layer of soil
1 mm. in thickness was exposed, all being covered with a bell glass
lined inside with the same solution, so as to insure equal saturation.

In view of the highly variable composition of soils and of the
doubtless varying hygroscopic properties of their several physical
constituents, it is not to be expected that any one numerical law will
hold good exactly for all kinds of lands. Mineral powders, colloidal
clay, ferric hydrate, aluminic hydrate, the zeolites, humus, and other
hydrates known to occur, doubtless each follow a different law in the
absorption of moisture and gases; so as to modify the hygroscopic
properties[72] of the soil in accordance with their relative
predominance in each case. (See table of absorption of gases, chapter
14).

_Utility of Hygroscopic Moisture to Plant-growth._—The early
experimenters considered the hygroscopic moisture of the soil to be
of very great importance to the welfare of crops. Within the last
twenty-five years much doubt has been cast upon this claim, even to
the extent of stating that “the hygroscopic efficacy of soils must
be definitely eliminated from among the useful properties” (Mayer’s
Agriculturchemie, vol. 2, p. 131). Yet Mayer himself concedes the
cogency of the experiments made by Sachs, which proved that dry soil
immersed in a (probably not even fully) saturated atmosphere is capable
of supplying the requirements of normal vegetation; thus explaining the
obvious beneficial effects on vegetation of the summer fogs prevailing
in portions of the arid region, _e. g._; on the coasts of California
and Chile.

[72] E. A. Mitscherlich (Bodenkunde für Land-und Forstwirthe, p. 156
et al.) claims that all determinations of soil hygroscopicity thus far
made are grossly incorrect on account of the dew liable to be condensed
on the soil layer from fully saturated air, as the result of slight
changes of temperature. He therefore would have all such determination
made either in an air-vacuum, or over a 10% solution of sulfuric acid.

Such dew-formation, however, cannot happen to any appreciable extent
under the conditions maintained in the writer’s work, viz, absorption
within a thick-walled (two-inch) wooden box of the dimensions given
above, and sunk in the ground in a cellar in which the temperature
varies only a few tenths of a degree during 24 hours. The soil layer
of one millimeter thickness being put down in the morning, the 7 hour
absorption period falls at the time of slightly rising temperature, as
an additional precaution against dew-deposition. Mitscherlich fails,
moreover, to show that this source of error produces any wide or
serious discrepancies except under such long absorption periods as he
finds it necessary to use because of the great thickness of his soil
layers. It is doubtful whether the limits of errors in soil sampling
do not greatly exceed any of those involved in the writer’s method,
and whether such accuracy as is attempted by Mitscherlich is of any
practical significance.

    Mayer’s experiments relied upon to prove the uselessness
    of hygroscopic moisture to plant growth, were carried out
    in flower-pots, in which it was plainly shown that the
    plants wilted before even the visible liquid (capillary)
    moisture of the earth was entirely exhausted. But this
    simply proves that under such artificial conditions, plants
    cannot withdraw moisture from the soil _rapidly_ enough
    for their needs. In _nature_, and notably in the arid
    regions, the chief supply of water is received through the
    deep-going main roots, while the bulk of the active feeding
    roots of the plant may be surrounded by almost air-dry
    soil; under which conditions, as Henrici (Henneberg’s
    Journ., 1863, p. 280) has shown, slow growth and nutrition
    occurs even in such plants as the raspberry, a native of
    humid climates. But in the arid region this is the normal
    condition of the native vegetation through most of the
    rainless summer. That a higher moisture-coefficient does not
    necessarily imply that a larger amount of moisture can be
    withdrawn from the soil by the plants, is undoubtedly true
    in some, but not in all cases; for in soils rich in humus,
    the moisture is more freely shared with the roots than in
    non-humous, clay lands.

The higher moisture-absorption is however of the most unquestionable
service in the case of the occurrence of the hot, dry winds that so
frequently threaten the entire crops of some regions. In this case the
soil containing the greater amount of moisture requires a much longer
time to be dried, and heated up to the point of injury to the roots,
than in the case of sandy soils of low absorptive power, whose store
is exhausted in a few hours and then permits the surface to be heated
up to the scalding point, searing the stems and root crowns. That such
injury occurs much sooner in sandy lands than in well-cultivated clay
soils, is a matter of common note in the arid region.

_Summary._—The significance of hygroscopic moisture in connection with
plant growth may then be thus summarized:

1. Soils of high hygroscopic power can withdraw from moist air enough
moisture to be of material help in _sustaining_ the life of vegetation
in rainless summers, or in time of drought. It cannot, however,
maintain normal growth, save in the case of some desert plants.

2. High moisture-absorption prevents the rapid and undue heating of the
surface soil to the danger point, and thus often saves crops that are
lost in soils of low hygroscopic power.


CAPILLARY WATER.

The liquid water held in the pores of the soil, in the form of surface
films representing the curved surface seen in capillary tubes, and
therefore tending to cause the water to move upwards, as well as in
all other directions, until uniformity of tension is established, is
of vastly higher importance to plant growth than hygroscopic moisture.
It not only serves normally as the vehicle of all plant food absorbed
during the growth of the usual crops, but also, as a rule, to sustain
the enormous evaporation by which the plant maintains during the heat
of the day, a temperature sufficiently low to permit of the proper
operation of the processes of assimilation and building of cell tissue.

Comparatively few plants have roots adapted to healthy action while
submerged in water, excluding them from free access of the oxygen of
the air; and when such roots are formed by plants not naturally growing
in water or swampy ground, they differ so far from earth-roots in
their structure that when transferred to soil they usually die, normal
earth-roots being gradually formed instead. Conversely, there is for
all land plants a definite time-limit beyond which their roots cannot
live, or at least remain healthy, in submersion. Thus grain fields
will with difficulty recover from a week’s total submersion; while
young rice fields will resist considerably longer. When in the resting
(winter) condition vineyards will bear submergence for thirty-five and
even forty days, deciduous orchards about three weeks; but when in the
growing condition, injury is suffered much more quickly.

It follows that whenever the soil-pores remain completely filled with
water for a length of time, there is danger to the welfare of nearly
all plants commonly cultivated in the temperate zones. It is therefore
important to know how much water will bring about this undesirable
condition in the different kinds of soil.

To determine this point we may either employ the determination of pore
space by a comparison of the density of the soil constituents (see
chap. 7, p. 107) with the volume weight of the soil; or we may measure
directly the amount of water required to fill the pore-space. For the
latter purpose it is only necessary to measure the amount of water
(conveniently flowing from a graduated pipette) which, rising slowly
from below in a U-shaped tube so as to expel all the air before it, is
required to fill a definite weight or volume of the soil entirely full,
so as to rise to its surface. We thus ascertain the amount of empty
space existing within the soil,[73] which in the absence of water will
ordinarily be filled by air.

[73] Simple as this operation appears to be, it is found to be by no
means easy to expel with certainty every small air bubble without
resorting to means which would destroy the natural condition of the
soil; such as boiling, or the use of the air-pump. These determinations
cannot therefore lay claim to great accuracy.

In most cultivated soils, as already stated, the air-space constitutes
about 25% to 50% of their volume; and this space when filled with water
represents what is commonly termed their _maximum water capacity_ or
saturation point. It is of interest to know this, because it has been
ascertained from experience that in order that plants may reach their
best development, the capillary water present should not amount to
more than 60%, or less than 40% of its maximum water-holding capacity;
thus leaving about half the pore-space filled with air. This optimum,
however, varies somewhat for different plants, some, like celery, being
more tolerant of excess, and others being more tolerant of a deficiency
of moisture, as is the _e. g._, egg-plant, originally a desert growth.

_Capillary Ascent of Water in Soil Columns._—When a column of dry soil
(e. g., contained in a glass tube closed with muslin at the lower end)
is brought in contact with water, the latter is soon seen to ascend in
the soil, wetting it and thus changing its color so as to permit of
ready observation of its progress. At first the rise is comparatively
rapid, in some cases as much as an inch in one minute; but it soon
slows down and after a time ranging from a few days to many months,
reaches a maximum height beyond which the liquid water will not rise.
The ascent is most rapid, and stops soonest, in coarse sandy soils; it
rises most slowly, but in the end considerably higher, in heavy clay
soils. The most rapid continuous rise, and ultimately the highest,
occurs in salty soils containing but a small proportion of clay. The
maximum height of capillary rise thus far observed, viz. 10.17 feet,
was noted in the case of quartz tailings from a stamp mill, ranging
from .005 mm. to .016 mm. in diameter; but it took about 18 months’
time to reach this maximum. The excessively fine texture of clay
opposes great frictional resistance to the movement of the water, and
the same is true of the finest silts, which, like clay, remain almost
indefinitely suspended in water. But it must be remembered that while
pure grains of silt will in wetting remain unchanged in size, clay
particles, and the clay incrusting silt grains, will on wetting swell
greatly, and thus fill up the interstices, largely closing them up
against the passage of water.

These facts are exemplified and graphically illustrated below.

The soils selected for this illustration, from California localities,
are the following:

_No. 233._ Very sandy soil from near Morano, Stanislaus County. Typical
of the noted wheat-growing region of the lower San Joaquin Valley,
from northern Merced to Southern San Joaquin Counties; bench or plains
lands. First foot.

_No. 1197._ Sandy alluvial soil from near the confluence of the Gila
and Colorado rivers, near Yuma. Very deep, light and easily cultivated.
First foot, but almost identical to 15 feet.

_No. 168._ Silty alluvial soil from the old alluvium of the Santa Clara
River, near Santa Paula, Ventura County. Very deep, very easily tilled;
a typical alluvial loam of the arid region.

_No. 1697._ Black adobe or clay soil, from the experiment station
grounds, Berkeley. A heavy clay soil, originally a swamp deposit,
becoming very tenacious when wet. An excellent wheat soil.

The physical analyses of these soils are given below.

                    PHYSICAL ANALYSES OF TYPICAL SOILS.
    ==============================+=====+=========================+======
                                  |     |          Silt.          |Sand,
                                  |     +------------+------------+2.0 to
                                  |Clay.|   Fine,    |  Coarse,   |64 mm.
                                  |     |  <.25 to   |.5 to 2. mm.|h. v.
                                  |     |.5 mm. h. v.|   h. v.    |
    ------------------------------+-----+------------+------------+------
    No.  233. Morano sandy soil   | 2.82|    3.03    |    3.49    |89.25
    No. 1197. Gila bottom soil    | 3.21|    5.53    |   15.42    |72.05
    No.  198. Ventura silty soil  |15.02|   15.24    |   25.84    |45.41
    No. 1697. Berkeley adobe soil |44.27|   25.35    |   13.47    |13.37
    ------------------------------+-----+------------+------------+------

The most striking feature in this diagram is the very rapid[74] and
high ascent in the combination of sediments represented by the Gila
bottom soil. It outstrips at once both the sandy soil from Stanislaus,
which contains a trifle less of clay, and the silt soil from Ventura,
from which at first sight it does not seem to differ widely, but which
contains considerably more clay. It is doubtless the latter which so
greatly retards the motion of the water, as is still farther seen in
the case of the clay or adobe soil. It will be noted that on the second
and third days, the Gila soil had raised the water nearly twice as
high as the adobe, and that it took only 18 hours to raise it nearly
the same height as that attained by the Ventura silt in so many days.
But it ceased to rise after the 125th day, while the Ventura soil,
continuing for 195 days, finally rose 3 inches higher. The adobe also
continued its rise, but did not reach the same height as the Gila soil
by nearly two inches. There can be no doubt that the energetic and high
rise of the latter proves an important factor in the culture of these
lands.

[74] The ascent is of course most rapid, in the large tubes almost
instantaneous, when the capillary space is entirely clear; but in the
complex system of connected air spaces in soils, the curved paths and
the friction obstruct the movement.

The coarse sandy soil reached its highest limit, 16½ inches, within six
days, when the silty Gila soil stood at about double that height.

[Illustration: FIG. 38.—Columns showing heights to which water will
rise by capillarity in soils of different physical composition, and
rates of ascent.]

[Illustration: FIG. 39.—Capillary Rise of Water in Soil Sediments of
different Diameters.]

_Ascent of Water in uniform_[75] _Sediments._—Loughridge has
ascertained the rate of ascent of uniform sediments of different
grain-diameters, with the results shown in the diagram subjoined,
together with the maximum height reached by each. The diagram is very
eloquently illustrative of the great differences in the capillary
properties of granular sediments of the various grades; and it would
seem that it ought to be possible to deduce from it by a somewhat
complex formula the rate and height of ascent of water in any soil of
known physical composition. In nature, however, the presence of clay
and the greater or less degree of flocculation of mixed sediments will
always vitiate to a very great extent the results deducible from such
calculations; hence the data conveyed by the observations of Loughridge
must be considered applicable only to granular sediments free from clay
and entirely deflocculated.

[75] I. e., uniform between the narrow limits given.

It is curious that in this case the “clay” showed a rise markedly below
that of the finest granular sediment, despite the extreme fineness of
its particles. This proves plainly that the physical nature of colloid
clay is unlike that of the granular sediments; as has been repeatedly
mentioned above.

_Maximum and Minimum of Water-holding Power._—It is clear that at
the base of the columns of soils just considered, the maximum of
water-absorption of which the soil is capable will have been brought
about; while at the top of the same column, the minimum of possible
liquid absorption (continuous films of water) will exist. The same
minimum moisture-condition will be produced when a limited quantity of
water is placed with a large mass of soil; the moisture will spread
to certain limits, until the surface films of water have all acquired
uniform tension; and will then cease to extend, except by evaporation
and hygroscopic absorption.[76] It is clear that the same condition
will be brought about in the course of time at the top of a soil
column in which water has percolated from above; and hence the minimum
mentioned, aside from evaporation, represents approximately the usual
condition of the soil near the surface within a variable time after a
rain, or irrigation, when the descending water column has attained a
length corresponding to the height to which the water would have risen
from below in a tube arranged as shown on p. 205. It is therefore a
condition of very frequent occurrence in the arid region.

[76] Ad. Mayer (Agriculturchemie 2, p. 141) designates this minimum
content of liquid water as the “absolute” water capacity of the same;
but it is not obvious wherein this factor is better entitled to this
name than would be the maximum (see Wollny’s Forsch., 1892, p. 1.). M.
Whitney (Rep. Proceedings Ass’n Agr. Coll. & Exp’t St’ns, Nov. 1904)
gives as a new observation the fact that in soils approaching the
drought condition water “does not obey the ordinary physical laws as
we recognize them in capillarity.” This evidently refers simply to the
well-known phenomenon mentioned above.

_Capillary Water held at Different Heights in a Soil Column._—To
determine the amounts of water held in the different portions in
columns of soils in which water ascends by capillary rise, the
following plan was adopted by the writer in collaboration with
Loughridge (Rep. Calif. Sta. 1892-4, p. 99).

Instead of glass tubes the soils to be tested were placed in copper
tubes one inch in diameter, divided into segments six inches long,
and flattened on one side. In the flattened side a slot half an inch
wide was left, and glass plates, held in position by rubber elastics,
were cemented on the slotted side by means of paraffin, to prevent a
sifting-out of the soil. The short sections can be connected at the
ends like joints of stove-pipe, and the earths can be easily introduced
in proper, even condition. It was thus possible to gain access to any
portion of the column at any time, for the taking of samples.

        WATER CONTENTS OF SOIL COLUMNS AT VARIOUS HEIGHTS
                          ABOVE WATER LEVEL.
    ==================+===========+===============+================
           No.        |    233    |     1197      |      1679
    ------------------+-----------+---------------+----------------
    Height above Water|Sandy Soil,|Sandy Alluvium,|Adobe, Berkeley.
          Level.      |  Morano.  |     Gila.     |
    ------------------+-----------+---------------+----------------
        47 inches     |           |      4.33     |
        42 inches     |           |     10.26     |
        36 inches     |           |     11.99     |
        30 inches     |           |     15.26     |
        24 inches     |           |     21.39     |     10.26[77]
        18 inches     |           |     27.63     |     29.48
        12 inches     |    3.93   |     32.48     |     33.04
         6 inches     |   14.15   |     35.04     |     38.47
         3 inches     |           |               |     38.49
         1 inch       |   24.34   |      36.64    |     44.41
    ------------------+-----------+---------------+----------------

[77] This figure represents only a temporary condition; the full height
of 46 inches was not reached until the 195th day.

    Since gravity limits the capillary ascent in a progressive
    ratio, as shown in diagram 39, it is obvious that the true
    _maximum_ saturation can exist only in a very short
    (strictly speaking, an infinitesimally short) vertical
    column. The least practicable height for experimental work
    being about 1 cm. (⅖ in.), the writer has adopted for
    the purpose of rapid determination of this factor, the
    use of a brass cylinder 1 cm. high and of such width as
    to contain, for the sake of convenience, 25 or 50 cm. of
    soil. This cylinder has a finely perforated bottom, which
    may be covered with filter paper; after being filled with
    soil which has been struck level, and weighing, it is
    immersed to 1 mm. depth in distilled water and allowed to
    rest for an hour; then quickly dried outside and beneath
    with filter paper, and again weighed. The amount of water
    found by difference should for all practical purposes be
    referred to the volume, not to the weight, of the soil, so
    as to eliminate the error arising from the varying specific
    gravity of the latter.

In most cases the surface of the soil in the sieve cylinder remains
level after wetting; but sometimes it swells so as to rise above its
dry level, even to the extent of nearly 30% (see chapter 7, p. 114).
This happens especially in strongly ferruginous soils. In the case of
“black alkali” soils, in wetting an enormous _collapse_ sometimes takes
place (see chapter 22).

If it be desired to determine also the _minimum_ liquid absorption
(see below), the surface of the wet soil is first covered with air-dry
soil, to absorb the surplus moisture, and finally with soil previously
saturated with hygroscopic moisture; the added soil being each time
thrown off and finally the surface “struck” level with a tense silk
thread before weighing. Corrections must be applied for the usual
increase in weight, from the addition of soil, and for the hygroscopic
moisture.

While the _minimum_ of liquid absorption can thus be determined
quickly, without awaiting the capillary ascent of a water column,
and if sufficient time is given can also be determined in higher
columns, as proposed by Mayer (Wollny’s Forsch. Vol. 3), the _maximum_
cannot thus be determined without gross inaccuracy. In determinations
made by the writer it was found that the figures for the minima of
very different soils (clayey and sandy) of the arid region, differ
proportionally much less than do the respective maxima. In few of these
soils it was found to exceed about 10 per cent, and it scarcely fell
below 4 per cent even in very sandy soils. A very deep, sandy soil,
which had been irrigated in May, and upon which no rain had since
fallen, showed in July in the second foot, upon which rested ten inches
of fully air-dried soil free from vegetation, a water-percentage of
eight per cent.[78]

[78] Hall (The Soil, p. 66) gives for the minima in the case of soils
examined by him the following figures: coarse sandy soil, 22.2, light
loam, 35.4, stiff clay, 45.6, sandy peat, 52.8. These figures are very
much higher than for apparently similar materials used by the writer,
and the differences exceed those between the maxima given for the same.
This discrepancy I am unable to account for.

_Capillary Action in Moist Soils._—In the preceding discussion the case
of columns of air-dry soils, so common in the arid regions, has been
considered. It is obvious that a soil column holding the _minimum_ of
capillary water may be of any height; so that when, as happens in the
open field, the rain water soaks down beyond the range of capillary
rise in a given soil, the upper portions of the latter, above that
range, will remain at the minimum of moisture-content so long as it is
not depleted by evaporation. King has made extended observations on
soil columns ten feet high and moistened throughout the mass. Capillary
movement takes place in moist soils much more rapidly than in dry ones,
although when sufficient time is given the final adjustment will of
course be the same. King’s experiments showed that evaporation at the
surface of the tenfoot columns caused a sensible depletion of the water
content originally existing at the depth of ten feet, in the course of
314 days. While so slow a movement might not be of any benefit during
the growth-period of shallow-rooted annual crops, the fact shown is of
importance to permanent plantings, as of trees and vines.

Another and not so readily intelligible effect observed by King is that
when the surface-soil is wetted, moisture may be withdrawn toward the
surface from the lower layers. In one experiment he found that when
water was applied on the surface so as to add two pounds of water to
_each_ surface foot in several soils, at the end of 26 hours there had
been an increase of _three_ pounds in the same, and a loss of one and
three quarter pounds from the second and third feet. The cause of this
translocation is probably a “distillation” of the subsoil moisture
toward the cooled soil; the fact that it occurs is of practical
interest, since it seems to show that wetting the upper portion of the
soil by cold rain or irrigation may tend to raise additional supplies
from below. At the change of seasons we not uncommonly find, in digging
tree holes or wells, a wet streak at from 9 to 18 inches below the
surface, caused evidently by the condensation of subsoil moisture, at
the limit of a cold zone resulting from the penetration of unseasonable
temperature (“cold snap”) from above. Such movements of soil-moisture
by means of evaporation and recondensation within the soil can of
course take place even when the minimum of liquid absorption has been
reached and direct capillary movement has ceased. It is, as it were,
dew within the soil.

_Proportion of Moisture Available to Growing Plants._—Not all the
capillary moisture contained in soils is available to plants, as can
readily be seen from the fact that many plants, especially when growing
in pots, begin to wilt while the soil still appears visibly moist. The
limit of wilting differs greatly in different plants, and in the open
ground it is difficult to ascertain that limit, because the deeper
roots continue to supply moisture from moister substrata. Hence potted
plants wilt while the soil appears much moister than when the same grow
in the field. King[79] has determined the amounts of moisture down to
43 inches in a Wisconsin soil in which clover and corn were at the
wilting point, as in the following condensed table:

    ===============================+=========+========+========
                                   | Clover. | Maize. | Fallow
                                   |         |        | ground.
    -------------------------------+---------+--------+--------
    First 12 inches, clay loam     |   8.44  |   7.03 |  17.01
    Second 12 inches, reddish clay |  12.84  |  11.79 |  19.86
    24 to 30 inches, sandy clay    |  13.52  |  10.84 |  18.56
    40 to 43 inches, sand          |   9.53  |   4.17 |  15.90
    -------------------------------+---------+--------+--------

[79] Physics of Agriculture, p. 135.

It is plainly shown here that the roots of clover and corn were unable
to utilize the higher moisture-content of the subsoil-clay to the same
extent as the smaller amounts present in the surface foot, and in the
sandy substrata. Evidently the moisture in the clay soil was more
tenaciously retained.

This is doubtless due, as King shows, to the equal thinness of the
moisture film remaining on the soil grains in either case; the number
of grains, and therefore the aggregate surface holding these films,
being much greater in the clay than in sands; hence the higher water
content.

It is interesting to compare these figures given by King for clover
and maize at the wilting-point, and fallow ground adjacent, with those
given by Eckart (Rep. Expt. Sta. Haw. Sugar Planters’ Ass’n., 1903) for
those affording good growing conditions for sugar-cane on the (highly
ferruginous) soils of that station. The plots were irrigated at the
rate of one, two and three inches of water per week, allowance being
made for the rainfall. Two inches proved, on the whole, to give the
best average results for production. The moisture determination of
the soil under the two-inch regime gave an average moisture content
of 29.13% in the first foot of soil. It is not stated what was the
hygroscopic coefficient of that soil, but it was probably very high;
in the neighborhood of 21.5%, judging by the determinations made with
six Hawaiian soils at the California Station. This would indicate about
7.63% of free moisture as the optimum for sugar-cane.

_Moisture-requirements of Crops in the Arid Region._—Plants
(particularly broad-leaved ones) which have made a brash growth during
a period of abundant moisture, will wilt quickly when sunshine returns,
and take some time to adapt themselves to the drier conditions. On the
other hand, plants accustomed to dry air and scanty soil-moisture,
will not wilt or suffer under what would elsewhere be considered very
rigorous conditions. Loughridge[80] has made numerous determinations of
moisture in soils in which crops were beginning to suffer, and others
on similar soils that were growing normally, and found that in general,
not only were the differences in moisture content considerably less
than in the case above quoted from King’s observations, but that the
amounts of free moisture required by various crops in the arid climate
of California were surprisingly small.

[80] Rept. Cal. Expt. Sta. 1897-08, pp. 65-96.

The tables below show the results of observations made by Loughridge
during several drought years in California; so arranged as to show the
differences of moisture content for the same crop in different soils.
It will be observed that in all cases where a crop growing on a clay
soil could be compared with the same on a lighter soil, the moisture
required to keep the crop in good condition was very much greater
in the clay than in the loam or sandy soils. In the case of apples,
_e. g._, 8.3% of water was abundant to keep the trees in excellent
condition on a loam soil, while on a clay soil holding 12.3% the
condition was very poor. That this difference is due in the main to the
difference in the hygroscopic-moisture coefficient of the respective
soils, is plainly apparent in several cases. It is therefore not the
_total_ moisture content, but the free moisture present in excess of
what is held by hygroscopic absorption, that determines the welfare of
the plant.

By determining, first, the total moisture in the soils, as taken in
the field, then, after allowing them to become air-dry, determining
the maximum of hygroscopic moisture they would absorb (see p. 198),
Loughridge found by difference the amount of free moisture, or liquid
water which must be present in the soil to prevent the crops from
suffering. An exceptionally good opportunity for these observations
was offered by the dry season of 1898, during which crops suffering
and not suffering, on identical lands, could easily be found. The
determinations were always made for each foot of the upper four feet
of the land in the immediate neighborhood of the trees or among the
field crops. The first table exemplifies the method of procedure; the
second gives the summary of results for the several crops and trees, as
calculated from observations made during the season.

        TABLE SHOWING CONDITION OF CROPS ON VARIOUS SOILS UNDER
                     DIFFERENT MOISTURE-CONDITIONS.
     ============+=============+=========+==============================
                 |             |         |Per cent Moisture in four feet.
      Kind of    |  Kind of    |Condition+------+------------+-----+----
       Crop.     |   Soil.     |   of    |      |            |     |Tons
                 |             |  Crop.  |Total.|Hygroscopic.|Free.| per
                 |             |         |      |            |     |acre.
     ------------+-------------+---------+------+------------+-----+----
     Wheat       |Very sandy   |Poor     |  2.6 |     1.9    |  .7 |  56
       “         |Sandy loam   |Good     | 12.8 |     5.6    | 7.2 | 576
       “         |Clay         |Dead     | 14.1 |    10.5    | 3.6 | 288
     Maize       |Clay adobe   |Very good| 12.9 |     8.8    | 4.1 | 328
       “         |Sandy loam   |Fair     |  6.1 |     2.3    | 3.8 | 304
     Barley      |Black adobe  |Wilting  | 10.7 |     8.8    | 1.9 | 152
     Sugar Beets |Black loam   |Good     | 12.4 |     5.6    | 6.8 | 544
     Vines       |Loam         |Good     |  8.5 |     5.0    | 3.5 | 280
       “         |Sandy loam   |Poor     |  1.9 |     1.5    |  .4 |  32
     Almonds     |Loam         |Good     |  8.5 |     6.6    | 1.9 | 178
        “        |Same field   |Suffering|  7.9 |     6.9    | 1.0 |  80
     Apples      |Loam         |Excellent|  8.3 |     5.5    | 2.8 | 224
       “         |Clay         |Poor     | 12.3 |    10.8    | 1.5 | 120
     Apricots    |Loam         |Excellent|  6.3 |     3.3    | 3.0 | 240
        “        |Gravelly loam|Poor     |  6.9 |     5.0    | 1.9 | 152
     Figs        |Red loam     |Good     |  5.2 |     3.8    | 1.4 | 112
      “          |Heavy loam   |Wilting  |  8.6 |     8.6    | 0   |   0
     Olives      |Red loam     |Good     |  5.2 |     3.8    | 1.4 | 112
       “         |Sandy loam   |Suffering|  1.9 |     1.9    | 0   |   0
     Peaches     |Red loam     |Good     |  8.2 |     5.0    | 3.2 | 256
        “        |   “         |Poor     |  6.8 |     5.0    | 1.8 | 144
     Prunes      |Gray loam    |Excellent| 11.2 |     9.0    | 2.2 | 176
       “         |    “        |Poor     |  6.4 |     5.4    | 1.0 |  80
    Citrus fruits|Sandy loam   |Good     |  6.3 |     3.1    | 3.2 | 256
       “      “  |Sandy soil   |Leafless |  3.1 |     2.4    |  .7 |  56
     ------------+-------------+---------+------+------------+-----+----

    TABLE SHOWING DROUGHT-ENDURANCE OF VARIOUS CROPS IN ARID REGION.
    ===================+=========================+====================
    Free water in four |                         |
        feet of soil.  | Crops that did well in  |Crops that suffered
    ----------+--------+    lowest amount of     |in highest amount of
              |  Tons  |   moisture mentioned    |moisture mentioned
     Per cent.|   per  |    in first column.     |in first column.
              |  acre. |                         |
    ----------+--------+-------------------------+--------------------
    0 to 1.0  |    80 {|Apricots, Olives, Grapes,|Citrus, Pears,
              |       {|Peaches, Soy-bean.       | Plums, Acacia.
              |        |                         |
    1.0 to 1.5|   120  |Citrus, Figs.            |Almonds, Apples.
    1.5 to 2. |   160  |Almonds, Plums, Saltbush.|Barley.
    2   to 2.5| { 176  |Prunes.                  |
              | { 200  |Walnuts, Eucalyptus.     |Prunes.
    2.5 to 3  |   224  |Apples.                  |
    3   to 3.5|   288  |Pears.                   |
    3   to 4  |   322  |Hairy Vetch.             |Wheat.
    4   to 5  |   400  |Wheat, Maize.            |
    5   to 6  |   480  |Sugar beets, Sorghum.    |Sugar beets.
    ----------+--------+-------------------------+--------------------




CHAPTER XII.

THE WATER OF SOILS.—_Continued._

SURFACE, HYDROSTATIC AND GROUND WATER; PERCOLATION.


Since all the water of soils and plants is directly or indirectly
derived from the rainfall (including therein snow and hail), some
general points regarding this factor require first consideration. While
it is not the object of this work to discuss climatology in detail,
yet the times of the year and the manner in which precipitation comes,
acts upon and is disposed of in the soil under different climatic
conditions, must of necessity form an essential part of its subject
matter.

_Amount of rainfall._—The rain falling in the course of a year is
usually stated in the form of “inches” (or centimeters), implying
the height of the water column that would be shown at the end of the
year had it all been allowed to accumulate; or, the sum of all the
successive rains (including snow) observed during the year. Since
this amount ranges all the way from nothing, or a mere fraction of an
inch (as in portions of the Andes, and of the great African and Asian
deserts) to as much as 600 inches or fifty feet (Cherapundji in eastern
India), the adaptation of agricultural practice to the maintenance
of the proper moisture-supply to crops is largely a local question,
oftentimes of not inconsiderable difficulty. This is especially the
case where torrential rains, yielding several inches of rain in a few
hours, alternate with light, soaking rainfall, as is very commonly
the case in the interior of continents, and more especially in the
United States east of the Rocky Mountains. Westward of the same the
rainfall decreases so rapidly that at or about the one-hundredth
meridian (the longitude of Bismark and Pierre, Dakota, and Dodge City,
Kansas) we already reach the annual average of 20 inches, which is
commonly assumed to be the limit below which crops cannot safely be
grown without irrigation. The “cloudbursts” occasionally occurring
within these limits are usually confined to mountainous regions, and
the water they pour down on the dry soil is rarely of any direct
benefit to agriculture; hence they cannot be properly counted in the
general estimate of the effective rainfall. A region of high rainfall
(up to 100 inches and over), however, extends along the Pacific coast
from northern California through western Oregon and Washington across
British Columbia to Alaska, to seaward of the Sierra Nevada, Cascade,
and Alaskan coast ranges.

In the country east of the Mississippi river, the average annual
rainfall ranges from 30 inches in the region of the Great Lakes, and
45 to 50 inches on the north Atlantic coast, to 60 inches in Louisiana
and up to eighty in southern Florida. The average of the Mississippi
Valley and Atlantic coast States is usually stated at about 45 inches,
which is distributed more or less evenly throughout the year, excepting
usually from six to eight weeks of more scanty precipitation in the
latter part of August and in September—the “Indian summer” season; so
that the winter is the season of greatest total rainfall.

_Natural disposition of the Rain Water._—The rainfall is naturally
first disposed of in two ways, viz., a portion which is absorbed by
the soil, and another which is at once shed from the surface and
constitutes the “surface runoff.” The portion absorbed into the soil
is subsequently disposed of either by soakage downward into the
subdrainage and through springs and seepage[81] into the streams and
rivers; or by evaporation. The latter again occurs in two different
ways, viz., from the soil-surface itself, or through the roots and
leaves of plants. The importance of each of these modes is sufficiently
great to entitle each to detailed consideration.

_The Surface Runoff._—This portion of the disposal of rain may range
all the way from nothing to almost totality, according to the nature of
the soil and the condition of its surface.[82] Sandy soils, especially
when coarse, may absorb instantly even a very heavy rainfall. Heavy
clay soils when dry will at first also absorb quickly quite a heavy
precipitation; but as the beating of the raindrops compacts the
surface, the absorption quickly slows down, so that heavy downpours of
brief duration, while wetting thoroughly into a plastic mass the first
two or three inches of a clay soil, may leave all beneath dry, to be
very gradually moistened by the slow downward percolation against the
resistance of the air in the soil; while the greater part of the later
portion of the shower will drain off the surface in muddy runlets.
Certain soils classed as loams, having the property of crusting readily
by rain followed by sunshine (see chapter 7, p. 111), in heavy showers
behave hardly better than strong clay soils; shedding the water until
the soaked crust gives way, and is carried off in muddy streamlets.
Then begins the cutting-away of the soil that, in portions of the
Cotton States, as well as north of the Ohio river, has been the cause
of extensive devastation of once fruitful culture lands, the site of
which is now marked by “red washes” and gullies but too familiar to the
eye in many regions, especially of the southern United States.

[81] The quiet seepage from the banks and beds of streams plays a much
more important part in the increase of volume of flow than is commonly
supposed, because unperceived save by measurement of the tributaries
and comparison with the main streams. This is especially true of the
drainage in the arid region, where the deep and pervious soils favor
diffuse seepage as against definite spring flow.

[82] Toumey (Yearbook U. S. Dep’t Agr. 1903) states that in the San
Bernardino mountains in southern California, the first rainfall (in
December) was absorbed to the extent of 95% in forested areas, against
only 60% in the non-forested; but that later, after the soil had been
partially saturated, 60% only was absorbed in the forested land,
against 5% in the non-forested. While it is generally admitted that
forests diminish the runoff, Rafter (Relation of Rainfall to Runoff, U.
S. Geol. Survey Paper, No. 80, p. 53) contends that in New York State
the reverse is true.

[Illustration: FIG. 40.—Erosion in Mississippi Table Lands, causing
destruction of agricultural value both of Uplands and Valleys. (McGee,
12th Ann. Rept. U. S., 1890-91.)]

    _Washing-away and Gullying in the Cotton
    States._—Nowhere perhaps have these effects been so
    severely felt as in portions of northwestern and central
    Mississippi, and this case is so instructive as to deserve
    a more detailed description. In the regions in question the
    soil stratum consists of a yellow or brownish loam from
    three to seven feet in original thickness, constituting a
    very desirable class of gently rolling uplands, which at
    one time claimed to be the best cotton-growing portion of
    the State. It was originally covered with an open forest
    of oaks, with an abundant growth of grasses that afforded
    excellent pasture to deer and cattle; a natural park gay
    with flowers during most of the season.

    When these lands were taken into cultivation little or no
    attention was paid to the direction of the furrows and rows
    of corn and cotton; most commonly the plowing was done
    “up-hill and down,” so that the “dead-furrow” afforded
    a ready opportunity for the formation of washes cutting
    into the subsoil, during the torrential rains sometimes
    falling during the summers. Even when filled with soil by
    plowing, these washes would frequently re-open during rains,
    shedding the soil in a muddy flood upon the lower lands. The
    washing-away of the surface soil, thus brought about, of
    course diminished the production of the higher lands, which
    were then commonly “turned out” and left without cultivation
    or care of any kind. The crusted surface shed the rain water
    into the old furrows, and the latter were quickly deepened
    and widened into gullies—“red washes”—whose presence
    rendered any resumption of cultivation difficult. In the
    course of a few years the soil-stratum of brown loam was
    penetrated into the loose or loosely cemented sand which
    underlies it almost everywhere, and is very readily washed
    away. Soon the water, gaining yearly in volume, undercut the
    loam stratum so as to cause it to “cave” into gullies in
    huge masses, which with the sand were carried into the
    valleys adjacent, filling the beds of the streams so as to
    cause their flow to disappear under the flood of sand. As
    the evil progressed, large areas of uplands were denuded
    completely of their loam or culture stratum, leaving nothing
    but bare, arid sand, wholly useless for cultivation; while
    the valleys were little better, the native vegetation having
    been destroyed and only hardy weeds finding nourishment on
    the sandy surface.

[Illustration: FIG. 40a.—Erosion in Mississippi Table Lands, causing
destruction of agricultural value both of Uplands and Valleys. (McGee,
12th Ann. Rept. U. S. G. S., 1890-91.)]

    In this manner whole sections, and in some portions of the
    State whole townships of the best class of uplands have
    been transformed into sandy wastes, hardly reclaimable by
    any ordinary means, and wholly changing the industrial
    conditions of entire counties; whose county seats even in
    some instances had to be changed, the old town and site
    having, by the same destructive agencies, literally “gone
    down hill.” This destruction of lands was greatly aggravated
    by the civil war, during which, and for some time after,
    large areas of lands once under cultivation were left to the
    mercy of the elements.

_Injury in the arid regions._—In the arid regions, where the rainfall
frequently comes in heavy downpours or “cloudbursts,” immense damage
to pasture lands has been brought about by overstocking, in Arizona
and New Mexico; involving the destruction of the natural cover of
vegetation and the loosening of the surface especially by sheep; after
which a heavy rainfall will carry off the surface soil, the muddy water
being gathered largely in the trails made by cattle going to water.
Thus gradually gullies are formed, which enlarging more and more become
ravines and cut up the pasture slopes into “bad lands,” useless equally
for pasture and for agriculture.[83] California, eastern Oregon and
Washington, and Montana, offer striking and lamentable examples of the
same destructive agencies.

[83] Open Range and Irrigation Farming. R. H Forbes, in Forester, Nos.
7, 9, 1902.

_Deforestation._—The deforestation of hill and mountain lands has, the
world over, led to similar results; causing not only the destruction
of pasture and agricultural lands, but also the conversion of streams,
flowing from springs and seepage all the year, into periodic torrents,
flooding the lowlands during rains by the rapid running-off of the
water from the bare and hard-baked mountain slopes, and then running
dry within a short time, so as not even to afford drinking water to
pasturing cattle in summer. Thus for half a century the unsolved
problem of the “correction of the waters of the Jura mountains” was
before the Swiss and French governments; and the great and costly
public work involving re-forestation, deflection of torrents and
filling-in of deep ravines and gullies, is not even yet nearly
completed. In Spain, which in the time of the Roman occupation was
largely a forest country with abundant rainfall, the same results are
seen, notably in the South, in the wide, and mostly dry, sandy beds
of streams once running deep and clear; and in the scarred hill-and
mountain-sides, and scant vegetation of low shrubs (“chaparral”) that
replace the once abundant tree growth, _e. g._, in Old and New Castile.
Unfortunately the lessons taught by the bitter experience of the old
world seem to require actual repetition in the new, before means of
prevention are even thought of.

_Prevention of Injury to Cultivated Lands from excessive Runoff._—The
fundamental remedy for the injurious effects of excessive runoff from
the land surface is, of course, to facilitate its absorption into the
soil to the utmost extent possible, by deep tillage; or in cases where
this is undesirable (as when in rainy climates excessive leaching of
the land is feared), to so direct and control the surface drainage
that its flow shall nowhere be so rapid as to carry with it any large
amounts of earth, or to wash out the furrows. To this end its fall must
be diminished by “circling,” _i. e._, plowing nearly at right angles
to the slope instead of up-and-down, and on steep slopes especially
also by maintaining open furrows or ditches having a gentle fall only,
into which the water can shed and flow off quietly in case the furrows,
left in plowing, prove insufficient to retain and shed gradually the
water they cannot hold permanently. The early adoption of this simple
expedient would have wholly prevented the enormous waste of fine
agricultural lands referred to above.

The underdraining of lands liable to washing is a costly but highly
effective means of preventing denudation; and the laying of underdrains
in gullies already formed, to prevent farther deepening, is among the
most obvious means of arresting farther damage. The beneficial effects
of underdrainage in conserving moisture will be discussed farther on.


ABSORPTION AND MOVEMENTS OF WATER IN SOILS.

The phenomena and laws of capillary ascent of water in soils, as
discussed in the preceding chapter, serve best to demonstrate the
general behavior of liquid water within different soils and their
several grain-sizes; because measurably independent of the physical
changes that almost unavoidably accompany the percolation of water
from above downward; whether such water comes in the form of rain,
or irrigation, or even when applied with the utmost precautions in
the laboratory. The “beating” of rains quickly compacts the surface
to a certain extent, varying with the nature of the soil, its
condition of more or less perfect tilth, and the degree of violence
with which the rain strikes the surface. When the latter has been
compacted by a previous rain and then dried, “baking” or incrusting
the surface, the latter may almost wholly shed a rain of brief
duration, which, had the surface been loose, would have been wholly
absorbed, materially benefiting the crop. Such surface-crusting is,
therefore, injurious in preventing the absorption of water from above;
and in addition, it serves to waste, by evaporation, the moisture
contained in the underlying soil and subsoil. For the crust being of
a finer (single-grain) texture than the tilled portion beneath, it
will forcibly abstract from the latter, by absorption, its capillary
moisture, and evaporating it at the upper surface, continue to deplete
the land, to the great injury of crop growth, until destroyed by
cultivation.[84]

[84] This effect is well illustrated by the behavior of a dry brick
laid upon a wet sponge. It will quickly absorb all the liquid moisture
contained in the latter, while the sponge will be wholly unable to take
any moisture from a fully-soaked brick.

The flow of irrigation water produces the same compacting effect, but
to a less extent; the more as, unlike rain water, irrigation water
usually contains a certain amount of alkaline and earth salts, which
tend to prevent the diffusion of clay and of fine sediments, and
therefore the disintegration of the soil-floccules into single grains.
Nevertheless, it is in some soils as necessary to cultivate after
surface-irrigation as after rains, in order to prevent great waste of
moisture by evaporation.

_Determination of rate of percolation._—When water is allowed to
soak into an air-dry soil column without sensible shock or motion,
from a constant level, we obtain the nearest approach to a definite
determination of the relative permeability of soils to water under
the conditions usual in the arid region. A number of determinations
thus made is tabulated in the diagram given below, which embodies the
observations made by Mr. A. V. Stubenrauch[85] in connection with a
more extended investigation.

[85] Rep’t Calif. Exp’t Station for 1898 to 1901, p. 165.

    As these experiments were made with soils not in their
    field condition, but gently broken up with a rubber pestle,
    a standard of compactness was established by weighing the
    quantity which could conveniently be settled into a tube
    space of 100 centimeters capacity by tapping the sides and
    bottom of the tube, without touching the soil itself. In
    this way the following standards were established: For the
    University Adobe soil, 140 grams; for the Yuba loam soil,
    110 grams; for the Stanislaus sandy soil, 170 grams. Tubes
    1½ inches wide were used, and the soils were introduced
    in bulk, inside of a cylinder of stiff paper upon which
    previously to rolling it up the soils had been thoroughly
    mixed. After introducing the soil-filled paper roll it
    was gently withdrawn, leaving the soil column in the tube
    as uniform as before; a condition almost impossible of
    fulfilment when the soil is introduced piece-meal. The tubes
    were, of course, left open at the lower end, using a wire
    netting to keep the soil column in, so that the air could
    escape freely before the descending water column.

    The results thus obtained do not, of course, apply directly
    to the same soils undisturbed in place in the field; where,
    moreover, the air is confined by the wetting of the surface
    and thus directly opposes penetration of the water. Still,
    they doubtless give a correct idea of their _relative_
    permeability for water when in the tilled condition. The
    water level was automatically maintained at the depth
    of half an inch above the surface of the soil columns.
    Pore-spaces given are calculated from volume-weight and
    specific gravity.

This diagram shows plainly that there is no direct relation between
the total pore-space in a soil and the facility of water-penetration.
The highest pore-space, in the fine-grained alluvial loam, allows more
rapid percolation than the heavy clay or adobe soil, but is greatly
exceeded by the coarser sandy soil. In all it is very apparent that the
downward movement slows down as the water descends, doubtless because
the great friction in a longer column gradually diminishes the effect
of hydrostatic pressure. It may be presumed that at a certain distance
from the surface the downward movement becomes practically uniform, and
independent of the pressure from above.

[Illustration: FIG. 41.—Diagram showing differences in rates of
percolation through different soils.]

_Summary._—Two salient points are revealed by even a cursory inspection
of the preceding diagram, viz.:

1. The downward percolation is most rapid in the same soils in which
the capillary ascent is quickest, that is, in the coarse, sandy soil.

2. The rapidity of percolation decreases materially as the wetted soil
column increases in length.

    The first point is readily foreseen and needs no comment.
    As regards the second, it results from the fact that as
    the wetted column lengthens the frictional resistance
    increasingly counteracts the effects of the hydrostatic
    pressure from above, until the water’s descent becomes but
    little more rapid than would be its lateral diffusion,
    or its ascent at the end of a similar column supplied by
    capillary rise from below. In both cases the frictional
    resistance has so far counteracted the effect of gravity
    that the capillary coefficients of the soil-material become
    the controlling factors of the water movement.

_Influence of Variety of Grain-sizes._—King (Physics of Agriculture,
pp. 159, 160), compared the rapidity of the percolation of water
through definitely graded pure sands on the one hand, and a sandy loam
and a clay soil on the other. The materials were arranged in 8-foot
columns fully saturated with water at the outset, and then allowed
to drain freely. The following abridged table shows the tenor of his
results:

    TABLE SHOWING RELATIVE RAPIDITY OF PERCOLATION IN PURE
       SANDS AND SOILS, IN INCHES OF WATER DRAINED OFF.
    ===================+==================+==============+============
    Diameter of Uniform|       First      |    Second    |   Total
       Sand Grains.    |    30 minutes.   | 30 minutes.  |in one hour.
    -------------------+------------------+--------------+------------
         .475 mm.      |      10.25       |     4.68     |   14.93
         .155  “       |       5.67       |     4.52     |   10.19
         .083  “       |       1.21       |      .85     |    2.06
    ===================+===========+======+===+==========+============
                       |First 21-23| First 10 |Second 10 |  Total in
          Soils.       |  hours.   |   days   |   days   |   about
                       |           |following.|following.| 505 hours.
    -------------------+-----------+----------+----------+------------
    Sandy loam         |   2.64    |   5.07   |   .91    |    8.62
    Clay loam          |   1.96    |   2.11   |   .49    |    4.56
    -------------------+-----------+----------+----------+------------

This table is very instructive in showing the great difference in the
rapidity of percolation in materials of uniform, even-sized grains,
as compared with such as contain particles of many different sizes,
in which the interspaces of the larger ones are filled more or less
closely by the smaller sizes of particles (see chapter 7, p. 109).
While it is true that we have no definite physical analysis of the
soils here used, the differences are so great as to be sufficiently
striking. Compare the percolation through the sand of .155 mm. uniform
grain-size (a fine sand), during the first half hour, with that through
the sandy loam during the first 21 hours. Twice as much water has
passed from the sand as from the soil in one forty-second part of the
time. Comparing similarly the finest sand, .083 mm. in diameter, with
the clay loam, we find the difference to be as one to seventy-three.
It is thus evident that but for the variously assorted sizes of the
soil-particles, water would not be held long enough to supply plant
growth.

_Percolation in Natural Soils._—In artificial percolation experiments,
as well as during a fall of rain, the gradual settling of the fully
wetted soil-column produces a compacting of that portion of the mass,
that increasingly impedes the downward penetration. The effect of this
under natural conditions is readily seen in the fact that after the
first, rapid absorption of falling rain by the soil when in good tilth,
there is a gradual slackening of the process even when the rain is fine
and slow, causing a perceptible increase of the runoff until, should
the rain continue for some time, the absorption becomes so slow as to
cause all, or nearly all the water to drain off the surface. The soil
is then called “saturated,” having really arrived at that point right
at the surface, and to a depth varying according to the duration and
amount of rain, and the natural perviousness of the land.

When the rain ceases, the visible saturation of the surface usually
soon disappears in cultivated soils, and the zone of saturation begins
to descend. The progress of this descent may be very strikingly
observed in a series of holes (post-holes) dug or bored across a
ridge; as indicated in the subjoined schematic diagram, in which the
successive dotted lines represent the levels of the descending “bottom
water” at successive intervals, as derived from the observation of the
water levels in the several holes.[86]

[86] The exact record of these observations was unfortunately destroyed
by fire; the soil was a heavy clay, and it took ten days before the
water disappeared from the lowest hole.

[Illustration: FIG. 42.—Percolation in clay land after heavy rain.]

    It will be seen that while at first the upper surface of
    the zone of saturation coincides with the surface of the
    ground, in falling it descends most rapidly on the highest
    ground, while at the lower levels the holes may remain full
    or overflowing; the drainage taking place sideways as well
    as vertically. The curved surface connecting the levels in
    the several holes gradually flattens, rapidly at first, then
    progressively more slowly; the water disappearing entirely,
    first from the holes lying highest, then successively from
    those at lower levels; those located in valleys or drainage
    channels remaining full until surface-water ceases to run
    in such channels. But even after liquid water has ceased to
    be visible in the holes, the descent of the water continues
    within that portion of the soil, tending (unless more rain
    should come before that time), to establish the condition
    of equilibrium as existing in the soil columns shown in
    the diagram on p. 205, chapt. 11; such as results from the
    capillary ascent of water from below, but having above it a
    column of soil of minimum water-content, of greater or less
    height according to the length of time allowed for the water
    to descend. This is a very common state of things during the
    long summer droughts in the arid region, when neither rain
    nor irrigation has added to the water supply in the soil for
    many months, and yet ordinary deciduous fruit trees mature
    their normal crops. Frequently, however, before this state
    of equilibrium is reached, evaporation from the surface so
    draws upon the water supply within the first few feet, as to
    reduce the soil to undersaturation at the lowest point of
    the descending column, so stopping farther descent and soon
    reversing the direction of the movement. The latter is the
    usual condition of scantily irrigated ground.

_Ground or Bottom Water, Water Table._—During and after long-continued
and abundant rains, the zone of supersaturation continues to descend
until it finally reaches a more or less permanent level, varying
somewhat from season to season, but on the whole usually definable for
each region and locality; being the depth to which wells must be sunk
in order to secure a fairly permanent water supply. This is called
the water table, ground water, bottom water, or “first water.”[87]
The proportion of the rainfall that reaches the permanent water level
varies enormously, of course, in different soils and at different
times. With brief and moderate rains, in soils of high water-holding
power and slow percolation, it may never reach the bottom-water level;
this is very commonly the case in the arid regions. Where, as in the
humid regions, rains are frequent or much prolonged, one half and even
more may finally reach the permanent level; runoff and evaporation
disposing of the balance.

[87] In contradistinction to other levels or “streams” of water which
may usually be found lower down, separated from the first water by some
impervious stratum of clay, hardpan or rock, and very commonly under
sufficient pressure to rise somewhat higher than the point at which it
was struck, owing to connection with higher-lying sources of supply.
When such pressure is sufficient to cause an overflow at the surface of
the ground, we have “Artesian” water as commonly understood.

    _Lysimeters._—For the determination of the amount of
    water percolating to given depths, water-tight receptacles
    called lysimeters are usually employed. The best way to
    establish such receptacles is to isolate a unit-area
    (usually a square meter) by digging all around it to the
    depth desired, then surrounding it with a metal sheet
    soldered tightly at the cut edges, and finally driving in a
    sharp-edged, stiff metal sheet so as to form the bottom when
    soldered to the upright walls; leaving on one side an outlet
    for the percolating water, which is then received into a
    measuring receptacle somewhat like a rain gauge.

Hall (The Soil, p. 75) states that at Rothamstead, where an average
rainfall of 31.3 inches is distributed rather uniformly through the
season, and where the soil is a moderately clayey loam, a little less
than half percolates through 20 inches of soil, and about 45% through
60 inches.

_Surface of Ground Water; Variations._—The surface of the water table,
however, is rarely level except in level and very uniform ground, or
after long periods of drought. The undulations of its surface conform,
in general, to that of the ground surface, but are less abrupt; so that
the water lies nearer to the surface in low than in high ground, as is
indicated in the diagram above.

King[88] has shown, moreover, that the level of the ground water shows
sensible variations due to increased or diminished barometric pressure,
as well as to variations of temperature in the soil, which cause the
air in the pores to expand or contract to a degree sufficient to bring
about variations in the flow of springs and underdrains to the extent
of 8 and 15% respectively, in conformity with the daily changes of
temperature and pressure.

[88] Physics of Agriculture, p. 270.

_The Depth of the Ground Water most Favorable to Crops_ cannot be
stated in a general manner, as it depends materially upon the nature of
the crop, its root habit, and the nature of the soil. As has already
been said, the amount of soil-moisture most favorable to plant growth
is about half of the maximum it can hold; and this condition, as is
shown in the table in chapter 11, p. 208, is reached about the middle
of the maximum height to which the water can rise by capillarity from
the water level. Below this point the access of air to the roots
becomes too limited, and in case of continuous rains the root-ends
would soon begin to suffer from want of aeration. On “sub-irrigated”
land, therefore, which is generally considered desirable, crops must
be carefully selected with respect to their root habits. Thus while
alfalfa needs considerable moisture to do its best, its deep-rooting
habit renders it undesirable when the ground water is at less than five
feet depth; but red clover may be grown even with the water level at
three feet.

In clayey soils root-penetration is always less than in sandy lands;
and although in the former the capillary ascent of water goes higher
than in the latter, yet its movement in clays is so much slower than in
sandy materials that unless water is within comparatively easy reach,
the plants may suffer from drought. Experience has long ago fixed the
proper depth at which to lay underdrains limiting the rise of bottom
water, at from three to four and a half or even five feet in clay
soils; greater depths are only exceptionally used, partly because the
laying of drains then becomes too expensive.

A mass of four feet of clay-loam soil is commonly, then, considered
as sufficient to supply the needs of a crop; it being understood that
in the humid region at least, such soils are usually the richest in
plant food, so that a deeper range of the root system is not called
for. It is quite otherwise in the sandy soils of the same region, which
being usually poor in plant food, must afford a deeper penetration in
order that an adequate amount of the same shall be within reach of the
roots. Sandy lands, then, should be deep in order to repay cultivation;
and fortunately this is usually the case. But when this is otherwise;
when for instance a sandy soil four feet in depth is underlaid by
impervious clay, underdrains may be quite as necessary as in the clay
lands; since the depth of actually available soil mass would otherwise
be reduced to two or two and a half feet only, by the water stagnating
on the clay surface and rising from 16 to 24 inches in the sand. Soils
thus shallowed can with difficulty be maintained in good productive
condition even by the most energetic fertilization.

_Moisture supplied by tap roots._—In most cases, sandy lands do not
require underdraining; and in them, root-penetration may reach to
extraordinary depths in the case of certain plants, especially when
taprooted. Thus the roots of alfalfa (lucern) are very commonly found
to reach depths of twenty to twenty-five feet, and even sixty feet has
been credibly reported for the same plant in the arid region. It is
obvious that for such plants, a high level of bottom water is wholly
undesirable, since they are enabled to obtain their moisture supply
from great depths, and can thus utilize for their nutrition much larger
soil-masses than can shallow-rooted plants.

_Reserve of Capillary Water._—It must be remembered that it is not
only, nor usually, the bottom water that supplies moisture to plant
growth; for all soils of proper texture for cultivation retain within
them a certain amount of capillary moisture after the ground water has
reached its permanent level (see this chap. p. 226), and when the tap
or main roots are plentifully supplied with water, the upper and chief
feeding roots draw but lightly upon the moisture within their immediate
reach for the purpose of leaf evaporation. This fact can be plainly
observed in the arid region, when on the advent of the summer drought,
young plantlets whose tap roots have reached a certain depth continue
to flourish and develop, while others practically of the same age, but
slightly behind, quickly succumb, though the _feeding_ roots of both
may draw upon the same soil layer. It is especially in sandy soils that
moisture is naturally thus conserved in the upper layers, because of
the failure of the water to rise by capillary ascent so as to evaporate
from the surface layer. It is often surprising to find a good amount
of moisture in the sandy soils of desert regions at the depth of eight
or ten inches, when the surface is so hot as to scorch the fingers;
and this moisture continues very uniformly to great depths, probably
to bottom water lying twenty or more feet below the surface, which in
such materials may readily be reached by taprooted plants such as the
“sagebrush” (Artemisia tridentata), the saltbushes (Atriplex) and
others.

_Injurious Rise of Bottom Water resulting from Irrigation._—In the
deep, pervious sandy lands of the arid region, especially where the
rainfall is very low and can wet the soil annually only to two or three
feet depth, the substrata are sometimes found to be barely moist to
depths of thirty and forty feet, and the short-lived spring vegetation
carries off during its growth all the moisture supplied by the winter
rains. When such lands are subjected to irrigation and the ditches
carrying the water are simply dug into the natural sandy land, the
thirsty soil absorbs the water greedily, so that even a considerable
volume of water makes but slow progress toward the farther end of
the canals. Gradually, as the rapidity of absorption decreases, the
diminution of flow becomes less sensible, but still the loss thus
experienced may be a very considerable percentage of the whole supply.
Thus in the Great Valley of California, as well as in portions of
Wyoming (Bull. 61, p. 32), the permanent loss from seepage is in the
case of some extensive irrigation systems estimated at fully 50 per
cent. When such lands have a considerable slope, the injury commonly
ends with the loss of the water, which in many cases is again gathered
and utilized at a lower level. But when the lands have but a slight
slope, the drainage may become so slow as to permit of the gradual
rise of the seepage water in the substrata, until finally it may come
to within a few feet of, or actually to the surface.

_Consequences of the Swamping of Irrigated Lands._—The injurious
consequences of this swamping of the irrigated lands may readily
be imagined. The first effect is usually noted in the sickening or
dying-out of orchards and vineyards, consequent upon the submergence
of the deeper roots, which in such lands frequently reach to from
fifteen to twenty feet below the surface. But even where pre-existing
plantations are not in question, the shallowing of the soil- and
subsoil-strata from which the plants may draw their nourishment,
constitutes a most serious injury to the cultural value of the land. It
has become unsuited to deep-rooted crops; and where the natural soil,
alone, would have perpetuated fertility for many years, fertilization
becomes necessary within a short time. The injury becomes doubly great
when, as is frequently the case, the rising bottom water brings up
with it to the surface soil the alkali salts which previously were
distributed throughout many feet of substrata, frequently rendering
profitable cultivation impossible where formerly the most luxuriant
crops were grown.

Theoretically of course it is perfectly easy to avoid or remedy these
troubles. It is only necessary to render the ditches water-tight by
puddling with clay, cement, or otherwise. But the heavy cost of this
improvement forms a serious obstacle to its adoption by the ditch
companies who are not themselves owners of land. Thus, extensive areas
of lands which when first irrigated were among the most productive,
have in the course of eight or ten years become almost valueless to
their owners, to whom legislation thus far affords but distant promise
of relief; although the case seems in equity to fall clearly within the
limits of the laws governing trespass.

_Permanent Injury to Certain Lands._—In cases like those alluded to the
remedy usually available for higher ground-water does not always afford
relief, even when otherwise available. Long-continued submergence
produces in many soils effects which cannot easily, if at all, be
overcome by subsequent aeration. This is most emphatically true of
soils containing a large proportion of ferric hydrate in the finely
divided form in which it is usually present in “red” soils.

The first effect of the stagnation of water in such lands (as
already explained in a former chapter 3, p. 45) is to set up a
reductive (bacterial) fermentation of the organic matter of the soil,
transforming the ferric into ferrous hydrate, which in the presence of
the carbonic acid simultaneously formed, becomes ferrous carbonate,
readily soluble in carbonated water. That this compound is poisonous
to plant growth, has been stated (chap. 3, p. 46). The carbonates of
lime and magnesia are simultaneously dissolved by the same, as is
also calcic phosphate, the usual form in which phosphoric acid is
present in the soil. Under the influence of partial aeration from
the surface, the ferrous carbonate is slowly re-transformed into
ferric hydrate, aggregated in the form of spots or concretions of
“bog ore” (see chapter 5, p. 66). In this process the greater part of
the phosphoric acid of the soil is also abstracted from its general
mass and concentrated in the bog ore (chap. 5, p. 65), in which it is
wholly unavailable to vegetation, and cannot be made available while
in the ground, by any known process. The soil is therefore permanently
impoverished in phosphoric acid; it is also deprived of its content of
ferric hydrate, and is transferred from the class of “red” to that of
“white” soils, well known everywhere to be unthrifty and to require
early fertilization. Not only is this true, because of their almost
invariable poverty in phosphoric acid, but also usually in lime, which
like the iron, if not leached out, is aggregated into concretions
in the subsoil, leaving the surface soil depleted of this important
ingredient. The humus, also, is either destroyed or at least “soured”
at the same time.

    _Reduction of Sulfates._—Should such a soil contain
    any considerable amount of sulfates, especially in the form
    of gypsum or calcic (or magnesic) sulfate, the reductive
    process results in the formation of iron pyrite (ferric
    sulfid, chap. 5, p. 75); while at the same time the soil is
    often sufficiently impregnated with sulfuretted hydrogen as
    to be readily perceived by the odor, or by the blackening of
    a silver coin. This is very commonly the case in sea-coast
    marshes, where a hole made with a stick thrust into the mud
    will be found to give forth both carburetted and sulfuretted
    hydrogen, while a careful washing of the soil will reveal
    the presence of minute crystals of iron pyrite. Hence the
    need of prolonged aeration of marsh soils, effecting the
    peroxidation of the ferrous compounds, and the conversion of
    the pyrite first into ferrous sulfate, and subsequently into
    innocuous, yellow, insoluble ferric oxy-sulfate.

_Ferruginous Lands._—The injurious effect of the swamping of
ferruginous lands has been especially conspicuous in some of the
irrigated rolling lands of the Sierra Foothills of California, where
orchards planted in relatively low ground and in full bearing have
succumbed to the poisonous effects of the ferrous carbonate formed
in the subsoil, long before the water had risen so high that, had
the trees been grown afterwards, they would have adapted their
root system to the existing conditions and fared moderately well
at least. Underdrainage of the lower lands is, of course, the only
possible remedy for this state of things, although even then the
root-penetration is much more restricted, and therefore natural
fertility of much shorter duration, than would have been the case
without the rise of the irrigation water.

It is thus clear that in the practice of irrigation, the liability
of injury to the lower ground by “swamping” through the rise of the
ground water should always be kept in view; that, in fact, irrigation
and provision for drainage should always go hand in hand. The legal
provisions facilitating the rights-of-way for irrigation ditches should
be made equally cogent with respect to drainage.




CHAPTER XIII.

WATER OF SOILS (_Continued_).

THE REGULATION AND CONSERVATION OF SOIL MOISTURE.


In view of the commanding importance of an adequate supply of water to
vegetation, the possible and available means of assuring such supply
by utilizing to the best advantage both rainfall and irrigation water,
require the closest consideration.

_Loosening of the Surface._—The first thing needful, of course, is
to allow the water free opportunity to soak into the soil, so as to
moisten the land as deeply as possible. That to this end the surface
should be kept loose and pervious by tillage, breaking up crusts
that may have been formed by the beating of rains, has already been
discussed. In the case of heavy clay soils, however, this alone is not
always sufficient. The most effectual way to loosen the land to greater
depths than can be reached by tillage, is by means of underdrains laid
at the greatest depth that is practically admissible.

_Effects of Underdrains._—That drain tiles laid for the express purpose
of carrying off surplus water should help to conserve soil moisture,
seems at first sight to be a paradox. Yet the explanation of the fact,
which has been demonstrated by long experience, is not difficult. The
effect is most striking in clay soils, for sandy soils are commonly
naturally underdrained already.

In discussing the changes of volume which soils undergo in wetting
and drying, the fundamental points in the premises have already
been mentioned (see chap. 7, p. 112). Clay soils in drying shrink
considerably, and re-expand on wetting, but rather slowly; moreover,
some clays crumble when wetted _after_ drying, while others, very
plastic when wet, crumble on drying (see chap. 7, p. 116).

It follows that while a clay subsoil when kept permanently wet, will
form a uniform, pasty, difficultly penetrable mass: when subjected
to frequent alternate wetting and drying, it becomes fissured and
crumbly, so as to resemble in its texture a tilled soil. This frequent
alternation of wetting and drying is precisely what, in the course of
time, is brought about by underdrains; rendering clay subsoils pervious
both to air and water. The consequence is that even heavy rains can be
fully absorbed by the soil mass lying above the drains, the surplus
draining off readily in a short time. Roots therefore can not only
penetrate, but exercise their vegetative functions perfectly at the
full depth of the drains. They are still at liberty to penetrate as
much deeper as their demands for moisture may require; but the depth
of four to four and a half feet is already so much greater than in the
humid region would usually be reached by them in undrained clay soils,
that commonly the moisture successively retained within that mass is
as much as is required by them during the growing season. At the same
time, their feeding roots are so far below the surface, that ordinary
short droughts do not reach them at all; while the underdrains prevent
any injurious stagnation of water around them. It need hardly be added
that the entire task of cultivation is also greatly facilitated; not
only because drained soils can be plowed within a few hours after the
cessation of rains, as against the same number of days that would have
to elapse in the undrained areas; but because tillage is easier, and
less draft is required, even when it is carried to a much greater depth.

_Underdrainage_, then, must be counted as being among the most
effective means both of utilizing the rainfall so as to prevent loss
from runoff and injury from washing, and of creating a deep, loose,
pervious soil mass, well adapted to root penetration as well as to
the conservation of moisture; rendering possible timely tillage and
cultivation, and early development of crops fully supplied with
moisture and therefore secure against loss from drought. The safety
and improvement of crops thus secured corresponds in the humid region
to that brought about by the command of irrigation water in the arid
countries. But it by no means follows that underdrainage can therefore
be dispensed with in the latter, or irrigation in the former. Both
have their proper place in both regions; but from special causes
underdrainage, as has already been stated, should be widely used in
irrigation countries to prevent the injuries otherwise but too likely
to arise from over-irrigation (see chap. 12, p. 231).

_Winter Irrigation._—In many regions where irrigation is desirable but
not absolutely necessary in ordinary seasons, or where irrigation water
is scarce in summer, much advantage is gained by insuring thorough
saturation of the land during the latter part of winter, especially
when spring or summer crops are to be sown. The not inconsiderable time
required for water to reach its permanent level or the country drainage
in most soils, often insures the retention of a certain surplus over
what the soil can permanently hold, within the period when it can be
utilized by growing crops; whose roots moreover are more likely to
penetrate deeply in land where there is a steady increase of moisture
as they descend, than when the contrary condition is encountered. The
use of _winter flood-waters to saturate the land_ is therefore in many
cases the saving clause for a dry season.


METHODS OF IRRIGATION.[89]

The manner in which irrigation water is supplied to land and especially
to growing crops exerts such a potent influence not only upon the
welfare of the plants but also upon the condition of the land, that a
brief discussion of this topic seems necessary.

The following methods are in use to a greater or less extent:

    1. Surface sprinkling.
    2. Flooding.
           A. By lateral overflow from furrows or ditches.
           B. By the “check” system.
    3. Furrow irrigation.
    4. Lateral seepage from ditches.
    5. Basin irrigation.
    6. Irrigation from underground pipes.

[89] Only a general outline of the principles of this subject is
given in this volume; special works must be consulted for working
details. Among these the volume by King on “Irrigation and Drainage”
gives probably the most comprehensive presentation of the subject for
both humid and arid climates. Also bulletins of the U. S. Dep’t of
Agriculture.

_Surface Sprinkling._—This method seems to be the closest imitation of
the natural rainfall; and yet it is in practice about the most wasteful
and least satisfactory of all. It is difficult of application on any
large scale, from obvious causes; on the small scale, in gardens and on
lawns, its disadvantages become amply apparent. As usually practiced,
from a rose spout or spray nozzle, the water falls much more abundantly
than in the case of any desirable rain, within the short time allowed
by the patience of the operator. If continued for a sufficient length
of time to soak the soil to the desirable depth, it compacts the
surface of the ground so as to render subsequent tillage indispensable.
To avoid this, amateur gardeners usually restrict the time of
application, repeating the same at frequent intervals, sometimes daily.
The result is that the very slight penetration of the water either
fails to reach the absorbent roots, so that it is of little use to
them, and is evaporated by the next day’s sun or wind; or else it tends
to draw the roots close to the surface, where, unless the application
of water is actually made daily, they are sure to suffer from the first
intermission of the daily dose. In actual practice the sprinkling
method is therefore both inefficient and wasteful of water, and exposes
the plants to grave injury from any cessation of the water supply.

Flooding presupposes land either level or only slightly sloping
naturally, or rendered so artificially; usually by means of the plow
and horse scraper.

_Flooding_ by lateral overflow from large furrows, or ditches, is very
commonly practiced where the water supply is abundant and large areas,
such as alfalfa or grain fields, are to be irrigated. The overflow is
regulated by portable check-boards, proceeding from the highest points
to the lowest, and leaving each temporary dike in place until the
ground is adequately soaked or the water reaches the next furrow below.
In heavy ground the operation may have to be repeated to insure proper
depth of percolation.

_Check flooding_ necessitates more careful leveling, and the throwing
up of small dikes, either temporary or permanent. The costliness
of the earth-work restricts the use of this method materially, and
the inconvenience caused in tillage by the dikes is objectionable,
especially in large-scale culture. For the case of alfalfa fields,
which remain permanently set for a number of years, it is however
the largely preferred method. In the case of field cultures, the
consolidation of the surface that follows flooding on the heavier soils
renders subsequent tillage necessary in all but very sandy soils; and
hence it should always _precede_ broadcast sowing.

One disadvantage of the surface flooding system is the slow penetration
of the water caused by the resistance of the air in the soil to
downward displacement; its buoyancy acting directly contrary to the
percolation of the water. In close-grained, heavy soils this objection
is very serious, on account of the loss of time involved when the
irrigator’s time is limited. On sandy lands the air bubbles up quite
livelily at first, but this soon ceases and the air is compelled to
escape sideways as best it can.

_Furrow Irrigation._—By this method it is intended to soak the land
uniformly by allowing the water to flow through furrows drawn 3 to 8
feet apart, with a gentle slope from the supply or head ditch; the
flow being continued until the water has reached the far end of the
furrows, or longer according to the nature of the soil, especially
if another ditch to receive the surplus flow lies below. The furrows
should subsequently be closed by means of the plow or cultivator; but
even if left open they are much less a source of waste by evaporation
than would be a flooded surface. The water thus, in the main, soaks
downward and only reaches the surface by capillary rise, so that the
land between the furrows is not sensibly compacted when the furrows
have been made deep enough. Evidently this is a much more rational
procedure than surface flooding, as it tends to leave most of the
surface in loose tilth, while penetrating to much greater advantage,
because of the ready escape of the air from the soil. It is the system
naturally and almost exclusively used in truck gardens and orchards,
and generally where crops are grown in drills or rows sufficiently far
apart to permit of cultivation.

[Illustration: _Furrows 6 inches deep in Heavy Loam Soil_]

[Illustration: _Narrow and Wide Furrows in Sandy Loam Soil_]

[Illustration: _Furrows 5 and 10 inches deep in Sandy Loam Soil_

_Water running, in each, seven hours_

FIG. 43.—Profiles of Water penetration in Furrow Irrigation.]

The figure annexed[90] shows the manner in which water sinks and
spreads from furrows of various depths and widths, as actually
observed in the work of the Irrigation Division of the U. S. Dep’t of
Agriculture, under the direct supervision of Prof. R. H. Loughridge of
the California Station. The mode of percolation is shown for two soils,
a heavy loam and a sandy one, both in the vicinity of Riverside, Cal.

[90] Published by permission of the Department.

The upper section shows the variation in penetration in one and the
same soil with the same kind of furrow, the broken line indicating the
cessation of the flow in the furrows; after which there was a still
farther penetration of the water to from 6 to 9 inches deeper.

The second section from above shows the percolation of the water
respectively in wide and narrow furrows of the same depth. It is
evident at a glance how much more effective is the wide furrow in
utilizing the limited time during which the irrigator usually has the
flow at his command.

The third section shows several practically important points in favor
of the wide and deep instead of narrow and shallow furrow. It is seen
that in doubling the width and depth, the penetration has also nearly
doubled. Moreover, it is seen that in the deep furrow the water has not
in the course of seven hours reached the surface at all, being still
six inches away; so that in view of the diminishing ratio of capillary
ascent, it probably would not have reached the edge of the furrow, at
the surface, in less than thirty hours. Thus all surface evaporation,
which oftentimes causes the loss of 50 % of the water entering the
shallow furrows, would be prevented; and a dry furrow-slice might
be turned into the furrow immediately after the cessation of the
water-flow, effectually obviating the need of subsequent tillage
also. The cost of the latter, together with the saving in water, and
increased efficiency of the water by deeper penetration, will much more
than offset the additional cost and trouble of plowing deeper furrows.

There is therefore every reason for doing away with the wasteful,
easy-going practice of irrigating in numerous shallow furrows, by which
the irrigator loses up to half of the water paid for, by evaporation;
is compelled to wait for the soaked surface to dry before being able
to turn back a furrow-slice into the furrows to prevent the drying-out
of their moisture; and by losing penetration of the water, is obliged
to irrigate again within a much shorter time than will be necessary if
deep-furrow irrigation be used.

A similar experiment with deep and shallow furrows was made at the
Southern California station near Pomona in 1901, as reported in
Bulletin 138 of the California Station. The results as far as they
went were precisely similar, and upon the basis of these the writer
earnestly advocated deep-furrow irrigation, and had the satisfaction
of seeing it strongly approved by orange-growers at Riverside and
elsewhere, by putting it into practice.

In addition to the saving and better utilization of the water used,
this mode of application has the advantage of preventing the roots from
coming too near the surface; it will also largely eliminate “irrigation
hardpan” or plowsole.

The results produced by long-continued shallow plowing and irrigation
in shallow furrows is well illustrated in the last of the irrigation
profiles, which shows the observations made on the same land as the
others, but where rational cultivation and deep-furrow irrigation
had not yet been introduced. It will be seen that after applying,
and of course paying for, the water for three days, its average
penetration was only about eighteen inches; so that the trees of the
orchard received very little benefit, and were supposed to be needing
fertilization when in fact they were simply suffering from lack of
water at the lower roots.

One somewhat unexpected point is shown by these diagrams, viz., the
slight sidewise penetration of the water; the wetted areas having a
nearly vertical lateral outline. This means, of course, that unless the
furrows run very near the trees of an orchard, the soil immediately
beneath the trees will remain dry; thus inducing the roots to spread
sideways and losing depth of penetration and soil. It will be noted
especially in the lower figure that here again the deep furrow offers
a material advantage over the shallow, the sidewise spread being much
more pronounced than in the shallow furrow alongside.

_Distance Between Furrows and Ditches._—The distance between the
furrows must, of course, be proportioned to the readiness with which
the water penetrates, being less as the land is of closer texture.
The distance between head ditches must, on the contrary, vary in
the opposite sense, since if these are too far apart, the water near
the head ditch will in sandy lands be wasting into the subdrainage
before the end of the furrows is reached; so that the distribution
will be very uneven. The great differences observed between crops, and
especially trees, _below_ and _above_ the head ditches, are mainly due
to this unevenness in water distribution, caused by too great distance
between successive head ditches. Each farmer must himself, however,
determine by actual trial the proper distances between ditches as well
as furrows, for his particular case; since everything depends upon the
rapidity with which water will penetrate the soil and subsoil. Actual
tests to determine this point[91] should be the first step, before
laying off the system of ditches as well as furrows. It not uncommonly
happens that the failure to do this at first, compels a subsequent
total change of arrangements in this respect. (See page 253 below).

[91] Such tests can be readily made by any one, by digging a pit to
four or five feet depth, and supplying water to a shallow basin dug
into the surface 8 to 12 inches distant from the vertical wall of the
pit. The descent of the water is then readily observed on the vertical
side of the pit nearest to the water basin. Preliminary tests with soil
probe (see chap. 10, p. 177).

    Thus while in some very pervious land furrows may be six
    or even eight feet apart, in other cases, in certain
    finely pulverulent or silty soils such as the “dust soils”
    described in a former chapter; (see chapter 6, p. 104),
    furrows drawn three feet apart may fail to allow the water
    to penetrate so as to prevent grain on the middle foot
    from suffering from drought after the water has run for
    twenty-four hours.

_Irrigation by lateral Seepage._—Is in reality a mere modification
of furrow irrigation, practiced in the case of lands very readily
permeable, and where water is abundant. The fields are laid off in
“lands” twelve to twenty-five feet wide, with a deep furrow or narrow
ditch between, from which water percolates in a short time so as to
overlap from the two sides. In this case sometimes the water does not
reach the surface visibly at all; a very great advantage where alkali
exists, as surface evaporation, and the consequent accumulation of
alkali, is thus effectually prevented; while deep-rooting is favored to
the utmost.

_Basin Irrigation._—In this method of irrigation, practiced only in
the case of trees and sometimes vines, and when water is scarce, a
wide circular furrow or basin is excavated around each trunk and water
is run either from one to the other, or sideways from a furrow laid
along the rows. The water thus applied of course percolates immediately
around the trunk first, and in practice is found to follow also the
large roots; so that it goes precisely where it is most wanted, besides
forming a vertical body of moist soil reaching to considerable depth,
where it is most desirable that the root system should follow. By
this deep penetration to natural moisture in the depths of the soil,
comparatively small quantities of water produce very marked effects.

On the same principle, the grape vines which bear some of the choicest
raisins of Malaga on the arid coastward slopes, are made to supply
themselves with moisture, without irrigation, by opening around them
large, funnel-shaped pits, which remain open in winter so as to catch
the rain, causing it to penetrate downward along the taproot of the
vine, in clay shale quite similar to that of the California Coast
Ranges, and like the latter almost vertically on edge. Yet on these
same slopes scarcely any natural vegetation now finds a foothold.

Similarly the “ryats” of parts of India water their crops by applying
to each plant immediately around the stem such scanty measure of the
precious fluid as they have taken from wells, often of considerable
depth, which form their only source of water-supply. Perhaps in
imitation of these, an industrious farmer has practiced a similar
system on the high benches of Kern River, California, and has
successfully grown excellent fruit for years, on land that would
originally grow nothing but cactus. Sub-irrigation from pipes has been
applied in a similar manner.

A combination of the furrow- and basin-irrigation system is sometimes
practiced in southern California by drawing the furrow so as to bring
the tree within a square, one side of which is left closed. The same
result may be accomplished by plowing cross furrows at right angles
near the tree and then placing check-boards so as to force the water
along the rows, zigzagging, on three sides.

The basin irrigation of orchards was originally largely practiced in
California, but has now been mostly abandoned for furrow irrigation.
The latter has been adopted partly because it requires a great deal
less hand-labor, partly under the impression that the whole of the soil
of the orchard is thus most thoroughly utilized; partly also because of
the injurious effect upon trees produced at times by basin irrigation.

The explanation of such injurious effects is, essentially, that _cold_
irrigation water depresses too much the temperature of the earth
immediately around the roots, and thus hinders active vegetation to
an injurious extent, sometimes so as to bring about the dropping of
the fruit. This of course is a very serious objection, to obviate
which it might be necessary to reservoir the water so as to allow it
to warm before being applied to the trees.[92] In furrow-irrigation
the amount of soil soaked with the water is so great that the latter
is soon effectually warmed up, besides not coming in contact too
intimately with the main roots of the tree; along which the water
soaks very readily when applied to the trunk, thus affecting their
temperature much more directly. It is for the farmer to determine which
consideration should prevail in a given case. If the water-supply be
scant and warm, the most effectual use that can be made of it is to
apply it immediately around the tree, in a circular trench dug for the
purpose. When on the contrary, irrigation water is abundant and its
temperature low, it may be preferable to practice furrow irrigation, or
possibly even flooding.

[92] See below, chap. 17.

As to the supposed more complete use of the soil under the latter
two methods, it must be remembered that while this is the case in
a _horizontal_ direction, if irrigation is practiced too copiously
under the shallow-furrow system, it may easily happen that the gain
made horizontally is more than offset by a corresponding loss in the
_vertical_ penetration of the root-system. This is amply apparent in
some of the irrigated orange groves of southern California, where the
fine roots of the trees fill the surface soil as do the roots of maize
in a corn field of the Mississippi States; so that the plow can hardly
be run without turning them up and under. In these same orchards it
will often be observed, in digging down, that at a depth of a few feet
the soil is too water-soaked to permit of the proper exercise of the
root-functions, and that the roots existing there are either inactive
or diseased. That in such cases frequent irrigation and abundant
fertilization alone can maintain an orchard in bearing condition, is
a matter of course; and there can be no question that a great deal of
the constant cry for the fertilization of orchards in the irrigated
sections is due quite as much to the shallowness of rooting induced by
over-irrigation, as to any really necessary exhaustion of the land.
When the roots are induced to come to and remain at the surface,
within a surface layer of eighteen to twenty inches, it naturally
becomes necessary to feed these roots abundantly, both with moisture
and with plant-food. This has, as naturally, led to an overestimate of
the requirements of the trees in both respects. Had deep-rooting been
encouraged at first in the deep soils of the southern “citrus belt,”
instead of over-stimulating the growth by surface fertilization and
frequent irrigation, some delay in bearing would have been compensated
for by less of current outlay for fertilizers, and less liability
to injury from frequently unavoidable delay, or from inadequacy, of
irrigation.

_Irrigation by Underground Pipes._—Where economy in the use of
irrigation water is a pressing requirement, its distribution through
underground pipes affords the surest mode of accomplishing that end,
in connection with the application of the water in accordance with the
principles just discussed. The enormous saving of water effected by
its conveyance in cement-lined ditches or concrete pipes, as compared
with earth ditches, if additionally combined with its application to
individual trees or vines, presents the maximum of economy that can
be effected. The actual use of this method is unfortunately limited
in practice by the high first cost of piping; but as its use renders
unnecessary the digging of basins and plowing of furrows and their
subsequent closing-up, it is when once established by far the cheapest
system, both as to the use of water and of labor.

    The best results of this system are undoubtedly achieved
    by the use of iron pipes for the distribution in field and
    orchard, whatever may be the material used for the main
    conduits. The use of concrete and tile in small sizes proves
    in the end very expensive, because of frequent breakage,
    and leakage due to varying pressure in the supply pipes or
    reservoirs; as well as from even slight earthquake tremors,
    undermining by water or by the burrowing of animals, and
    many other accidents which do not affect an iron pipe
    system. The pipes must in any case, of course, be laid
    deep enough to be out of reach of the deepest tillage;
    therefore not less than one foot, and preferably eighteen
    inches. A proper construction of the outlets, permitting of
    exact regulation of the flow and ready operation from above
    ground, as well as preventing their being clogged by earth,
    rust, roots or burrowing animals, insects etc., is of course
    of the greatest importance. A variety of devices for this
    purpose is already on the market.


QUALITY OF THE IRRIGATION WATER.

_Saline Waters._—Considering the large amount of water annually used
in irrigation, among the most needful precautions to be observed by
the irrigator is in the testing of the quality of his water-supply.
First among the points to be noted is the possible content of
soluble “alkali” salts. While in most cases what is called the “rise
of the alkali” is due to the salts already contained in the soil
and subsoil, in but too many the evil is either brought about, or
greatly aggravated, by the excessive saline contents of the water
used in irrigation. The effects of the use of saline irrigation water
(containing in this case about 100 grains per gallon, or 1700 parts
per million) are shown in the accompanying plate. The predominant
ingredients of these alkali salts were common salt and carbonate of
soda. In the lands near Corona, Cal., where this case was observed, the
original alkali-content of the soil was about 2500 pounds per acre in
four feet depth, and had been just quadrupled, with the results shown;
viz., complete defoliation of the orange trees, while on the same
land, where the trees had been irrigated with good artesian water, the
orchard was in fine condition.

_Limits of Salinity._—It is not easy to assign a definite limit of
mineral content beyond which water should be considered unfit for
irrigation purposes; partly because of the differences in the kind of
the mineral salts, partly because the nature of the soil and the amount
of water at command, materially influence its availability.

[Illustration: FIG. 44.—Orange Trees Irrigated with Artesian Water.]

[Illustration: FIG. 45.—Lake Elsinore Water, Three Years.]

Forty grains per gallon is usually assigned as the limit for potable
as well as irrigation waters. But if most or the whole of such mineral
contents should consist of the carbonates and sulfates of lime and
magnesia, the water while unsuitable for domestic use may be perfectly
available for irrigation, since these salts are either beneficial or
harmless in the amounts likely to be introduced by the water. But
if most or the whole of such forty grains should consist of “alkali
salts” proper, viz., the sulfates, chlorids and carbonates of potash
and soda, or if they should contain even small amounts of the chlorid
of magnesium, they might render the water either wholly unsuitable for
irrigation, or if used it would be needful to take the mineral content
into consideration, by regulating its application accordingly.

It has been found in California that practically the upper limit of
mineral content for irrigation water _under the ordinary practice_ lies
below seventy grains per gallon in _all_ cases; for when this strength
is reached, even though such water may bathe the roots of almost any
plant with impunity, yet accidental concentration by evaporation is
so certain to happen, that injury to crops is practically almost
unavoidable.

    In South Dakota and other parts of the American semi-arid
    region, waters containing seventy grains and even more of
    alkali salts per gallon are annually used during the short
    irrigation season. This can be done harmlessly because the
    aggregate amount used is only small, and the more abundant
    rainfall of that region annually washes the salts out of the
    soil. But where almost the full amount of water required
    by crops must be supplied by irrigation, the total amount
    of salts thus introduced would speedily render the land
    uncultivable.

According to the observations of Means and other explorers[93] of the
U. S. Dep’t of Agriculture, waters of much higher mineral content are
used for irrigation both in Egypt and in the Saharan region, some
going as high as 8000 parts per million, or 214 grains per gallon. The
cultivators are said to be very skilful in the use of these waters,
applying them only to plants of known resistance, and in certain ways.
These ways include doubtless a good deal more time and patience than
American irrigators are ordinarily willing to bestow upon their work.
Much depends of course not only upon the character of the salts in the
water, but also upon the long experience had in the old irrigation
regions.

[93] Bull. No. 21, Bureau of Soils; also circular No. 10, ibid.

_Mode of using Saline Irrigation Waters._—The fact that abundant
growths of native as well as cultivated plants may sometimes be seen
on the margins of “alkali lakes” where water of over a hundred grains
of mineral salts per gallon continuously bathes the roots, while the
same plants perish at some distance from the water’s edge, points the
way to the utilization, in emergencies, of fairly strong saline waters;
viz., by the prevention of their concentration to the point of injury
_by evaporation_. It is clear that when such waters are used sparingly,
so as to penetrate but a few feet underground, whence the moisture
re-ascends for evaporation at the surface, a few repetitions of its
use will accumulate so much alkali near the surface as to bring about
serious injury. If, on the other hand, the water is used so abundantly
that the roots may be considered as being, like the marginal vegetation
of alkali lakes, bathed only by water of moderate strength, no such
injury need occur; and what does accumulate in consequence of the
inevitable measure of evaporation occurring in the course of a season,
may be washed out of the land by _copious winter irrigation_.

This, of course, presupposes that the land, as is mostly the case
in the arid region, is readily drained downwards when a sufficiency
of water is used. When this is not the case, _e. g._, in clay or
adobe soils, or in those underlaid by hardpan, waters which in sandy
lands could have been used with impunity, may become inapplicable to
irrigation use.

_Apparent Paradox._—The prescription to use saline waters _more
abundantly_ than purer ones, in order to avoid injury from alkali,
though paradoxical at first sight, is therefore plainly justified by
common sense as well as by experience, in pervious (sandy) soils;
while in difficultly permeable ones, their use may be either wholly
impracticable, or subject to very close limitation.

Sometimes the alternate use of pure and salt-charged water serves to
eke out a too scant supply of the former. But in all such cases, close
attention to the _measure_ of water that will wet the soil to a certain
depth, and “eternal vigilance” with respect to the accumulation of
alkali near the surface, must be the price of immunity from injury. In
all cases the farmer should know how much of alkali salts he introduces
into his land with the irrigation water, and watch that it does not
approach too closely, or exceed, the tolerance of his crops for alkali
salts, as given in chapter 26.

_Use of Drainage Waters for Irrigation._—When lands charged with alkali
salts are being reclaimed by drainage, the question sometimes arises
whether the drainage-water may not be used for irrigation, lower down.
This of course depends entirely upon the amount of alkali in the water,
the nature of the lands to be irrigated, and the manner of applying it.
In the Fresno drainage-district of California it has been shown that
some of the drainage-water contains not more than 25 to 30 grains per
gallon of objectionable salts, and such waters could of course be used
on pervious lands with the precautions above noted.

_“Black Alkali” Waters._—As regards, however, waters containing any
large proportion of carbonate of soda, it must be remembered that
even very dilute solutions of salsoda serve to puddle the soil and
thus render it difficultly tillable. When such waters are used it
is necessary to forestall injury either by the use of gypsum in the
reservoir or ditch, or by annually using on the land a sufficient
amount of gypsum to transform the carbonate of soda into the relatively
innocuous sulfate.

_Variations in the Saline Contents of Irrigation Waters._—When
irrigation waters are derived from deep wells, there is little if
any variation of their saline contents to be expected, and a single
analysis will serve permanently. But in the case of relatively
shallow wells, from which the water must be raised by pumping, it not
unfrequently happens that after a series of seasons of short rainfall,
saline waters are brought up by the pump and may seriously injure
crops and orchards. Again, in the case of streams and rivers whose
flow becomes very small in summer, the saline content may increase to
several times the amount carried at the time of high water. Both kinds
of cases occur in southern California, in Arizona,[94] New Mexico and
other states of the arid region. The Gila, Pecos and upper Rio Grande
are cases in point, and to a certain extent the Colorado of the West.

[94] Bull. Ariz. Exp’t Sta. No. 44.

_Muddy Waters._—In the latter as well as other streams of Arizona,
there is another point which sometimes creates difficulties to the
irrigator, together with some current expense. It is the amount of silt
or mud carried by the water, which while it is a benefit to the land
over which it is spread, (“warping”) as in the classic case of the
Nile, often clogs the irrigation ditches to such an extent as to cause
considerable inconvenience and expense in cleaning them out. This is
especially the case in the streams draining pasture lands that have
been overstocked, and where the destruction of the natural herbage
allows the rain water to run off rapidly, at first forming runlets and
then gullies and ravines that originally were simply cow-paths leading
toward the watering places.[95] The devastation of lands thus caused
in Arizona is almost as great as that which has occurred in the Cotton
states, as mentioned above chap. 12. p. 217.

[95] Bull. Arizona Exp’t Station Nos. 2, 38.

These variations in the character of the irrigation water must of
course be watched by the farmer who does not receive directly from
mountain streams, or from deep artesian wells water known to have a
constant content of saline matter.

THE DUTY OF IRRIGATION WATER.—The amount of water thought to be needed
for the production of satisfactory crops varies widely in different
regions, ranging all the way from about two feet to as much as eight
annually, within the United States; while in the sugar-cane fields
of the Hawaiian Islands as much as three inches per week, or over
twelve acre-feet in the course of the year, have been thought to be
beneficial, if not absolutely required for the best crop results.

As has been stated above (chap. 12, p. 215), the rainfall limit below
which irrigation becomes, if not absolutely essential, at least a
highly desirable condition for the safety of crops, is usually assumed
to lie at about 20 inches (500 milimeters). This general statement
is, however, subject to material modification according to the manner
in which the rainfall is distributed. Thus in central Montana with 24
inches of rainfall distributed throughout the year, irrigation is
indispensable; while in the Santa Clara valley of central California,
with an average rainfall of 15 inches falling through the winter and
spring, the growth of all ordinary field crops has for fifty years
not failed oftener than is commonly the case in the humid region of
the North Central states. This is because in California the winter
and spring are the growing seasons, while the rainless summers do not
stand in the way, for crops are already harvested; and the deep rooting
of trees and vines provides these with the needful moisture from the
depths of the substrata (see chap. 10, pp. 163 to 173).

It would thus seem that twenty inches of irrigation water properly
applied ought to be sufficient for all purposes, when added to the
natural rainfall, which is rarely entirely absent. Yet in actual
practice less than 24 acre-inches is rarely used, and much more is
the rule; 72 to 96 ins. being sometimes used in Arizona. Evidently
enormous losses occur in practice, and it is of the utmost importance
to discover the causes of these.

_Causes of Loss._—Since irrigation water is commonly measured at the
distributing weirs, loss from seepage and evaporation on the way to the
fields is an obvious source of an overestimate of the water actually
supplied to the farmer. In sandy districts the loss thus incurred is
reliably estimated at nearly 50% in many cases. The apparent duty of
the water is thus at once reduced to half its effect, and four instead
of two feet of water are supposed to have been used, and are charged
for.

_Evaporation_ resulting from surface flooding or use in shallow furrows
may, again, cause the loss of from 30 to 50% of the water that actually
reaches the land; so that in the latter case, between seepage and
evaporation the irrigator may lose the effect of three-fourths of the
water he pays for.

_Loss by Percolation._—Finally, the water may be wasted on the land
itself in leachy soils by over-use, _i. e._, it may percolate to a
large extent beyond the reach of the roots when the flow is continued
too long; as will always be the case when the head (supply) ditches are
laid too far apart, so that the water may be wasting into the country
drainage just below the upper ditch long before the water in the
furrow reaches the lower one; as illustrated in the upper one of the
subjoined diagrams. That this will not happen when the head ditches are
nearer together, is shown in the lower diagram.

[Illustration: FIGS. 46, 47.—Diagram showing loss by percolation when
head ditches are too far apart.]

The means of avoiding the mechanical losses have already been
discussed, and may be summarized thus: tightening of leaky ditches; use
of water in deep furrows; and ascertaining the rapidity of percolation
(see p. 242) so as to obtain a proper gauge for the time during which
water should run, and for the distances at which head ditches or
furrows should be placed.

The importance of thus diminishing the losses of water is obvious when
it is considered that if the duty of water can be reduced to twenty
instead of forty or fifty acre-inches, twice the area can be irrigated
with the same amount of water, or the cost of water correspondingly
reduced. It should be noted that when the land is leachy it may be pure
waste to continue the flow beyond a few hours; but the irrigation must
then be more frequently repeated.


EVAPORATION.

Alongside of and supplementary to the best possible utilization of
the rainfall and irrigation water, the prevention of unnecessary
evaporation has to be considered. Evaporation from the soil’s surface
implies not only unnecessary loss of water that should have remained
for the use of the crop, but also the depression of temperature which,
as a rule, is unfavorable to the best development of vegetation. It
is only in case of extreme stress from hot, drying wind that such
evaporation and the consequent depression of the temperature of the
surface soil can be of advantage to the farmer.

The amount of water evaporating either from a water-surface, or from a
wet or moist soil, varies greatly according to the climatic conditions,
and the state of the weather; also according to the condition of the
soil-surface. There are damp climates, and days or periods when,
the air being nearly saturated with moisture, evaporation even from
a water-surface will be almost insensible. On the other hand, with
dry air and a high temperature, enormous quantities of water may be
evaporated in the course of a day. The evaporation from water-surfaces
interests deeply those who supply, as well as those who are supplied
with, water from storage reservoirs; evaporation from the soil-surface
interests deeply all farmers, and more especially irrigators whose
water-supply is scanty, or is paid for by them by measurement. Light
rains, as well as light surface irrigations, may at times evaporate
almost wholly without any effect save a lowering of the temperature of
the soil. In the case of snow, it is a well-known fact in the northern
arid regions that a light snowfall may in winter evaporate entirely
without imparting any liquid moisture to the soil. A loss of 50% of the
water actually brought upon land by surface irrigation is of common
occurrence in some portions of the irrigated region.

The dependence of evaporation upon air-temperature under conditions
otherwise identical, is well illustrated by the experiments made in
1904 by S. Fortier[96] on the Experiment Station grounds at Berkeley,
California, at a time when under the influence of the sea breeze the
average saturation of the air might be assumed at about 70%. The tests
were conducted in six tanks sunk into the ground so as to place the
water-surfaces on a level with it, and the water-temperatures were
maintained in four of the tanks by means of ice or heating lamps. The
results are shown in the following table:

      SUMMARY OF AVERAGE WEEKLY LOSSES BY EVAPORATION,
    WITH VARYING TEMPERATURES OF WATER, AT BERKELEY, CAL.,
                IN JULY AND AUGUST, 1904.
              =======================+============
               Temperature of water. |  Weekly
                                     |evaporation.
              -----------------------+------------
              Degrees Fahrenheit:    |  Inches.
                  55.5               |   0.42
                  62.0               |   0.77
                  69.2               |   1.54
                  80.1               |   3.08
                  89.2               |   3.92
              -----------------------+------------

[96] Progress Report on Coöperative Irrigations in Calif.; Cir. No. 56,
Office Exp’t Stations.

A farther illustration is given in the subjoined table, showing maxima
and minima of monthly evaporation, as well the totals of one (seasonal)
year, in three California localities where the air-saturation is
considerably below that at Berkeley, ranging in summer from 50% to 20%
and even less (at Calexico in the Colorado desert):

     SUMMARY OF EVAPORATION-LOSSES FROM WATER-SURFACES, AT POMONA,
    TULARE, AND CALEXICO, CAL., FROM JULY 1, 1903, TO JULY 31, 1904.
    ============+=================+=================+=================
                |     Pomona.     |     Tulare.     |    Calexico.
                +---------+-------+---------+-------+---------+-------
                | Month.  |Inches.| Month.  |Inches.| Month.  |Inches.
    ------------+---------+-------+---------+-------+---------+-------
    Maximum     |Aug. 1903|  9.07 |July 1903| 12.34 |July 1903|  14.48
    Minimum     |Feb. 1904|  2.57 |Jan. 1904|  1.46 |Jan. 1904|   4.39
                |         | ----- |         | ----- |         | ------
    Totals for  |         | 66.92 |         | 74.68 |         | 108.23
         year   |         |       |         |       |         |
    ------------+---------+-------+---------+-------+---------+-------

Of these three stations, Pomona is located within reach of the ocean
winds, but distant 25 to 30 miles from the shore. Tulare is situated
in the upper San Joaquin valley, far in the interior; Calexico is in
the southern part of the Colorado desert, with extremes of temperature
ranging from 13° Fahr. in winter to 120° in summer.

_Evaporation in Different Climates._—The following table conveys some
general data regarding average evaporation from water-surfaces in
different climates. Evaporation from the soil-surface depends largely,
of course, upon the mechanical condition of the surface, the extent
to which it is wetted, and the rapidity with which moisture will be
supplied from the subsoil as the surface dries. A field plowed into
rough furrows will evaporate more water than when harrowed, because of
the larger surface exposed; and a harrowed field moderately compacted
by rolling will lose less water by evaporation than when unrolled,
other things being equal. On the other hand, a thoroughly compacted
surface, even if suffering less loss at first than a plowed or harrowed
field, will continue to lose moisture longer by withdrawing it from the
substrata by its superior capillary suction; while a loose surface,
once dried out, will prevent farther loss from the subsoil very
effectually, as stated below.

    TABLE SHOWING EVAPORATION, FROM WATER-SURFACE EXPOSED IN SHALLOW
                TANKS, NEAR WATER OR GROUND SURFACE.
    ===================================+========+====================
                                       | Years. |      Inches.
    -----------------------------------+--------+--------------------
    Rothamsted, England                |    9   | 17.80 (16.6 to 18.4)
    London,        “                   |   14   | 20.66
    Oxford,        “                   |    5   | 31.04
    Munich, Germany                    |    ?   | 24.00
    Emdrup, Denmark                    |   10   | 27.09
    Cambridge, Massachusetts           |    1   | 56.00
    Syracuse, New York                 |    1   | 50.20
    Logan, Utah                        |    1   | 52.39
    Tucson, Arizona                    |    1   | 75.80
    Fort Collins, Colorado             |   11   | 41.00
    Fort Bliss, Texas                  |    1   | 82.70
    San Francisco, California          |        | 45 to 50
    Sweetwater Reservoir,              |        |
           San Diego, California       |    1   | 57.6
    Peking, China                      |    ?   | 38.80
    Demerara, South America            |    3   | 35.12
    Bombay, East India                 |    5   | 82.28
    Petro-Alexandrowsk, West Turkestan |    ?   | 96.40
    Kimberley, South Africa            |    ?   | 98.80
    Alice Springs, South Australia     |    ?   |103.50
    -----------------------------------+--------+--------------------

This table, the data for which are taken from various sources,
exhibits clearly the enormous variations in evaporation in different
countries, and even in localities not very remote from each other. The
low evaporation near London is doubtless due to its foggy and hazy
atmosphere, but it is not clear why Rothamsted should show so low an
evaporation compared with Oxford. Tropical Demerara stands nearest to
Oxford in its evaporation; Bombay indicates its location on the hot
and arid west coast of India, despite its nearness to the sea. The
inland localities in the desert regions of South Africa, Australia and
Western Turkestan, show how enormous may be the losses from evaporation
of irrigation water, unless the latter is applied with special care
for their prevention. Thus, with the wasteful methods of irrigation
prevailing in portions of the American arid region, it is certain that
in many cases 50% and more of the water evaporates before it reaches
the crops.

_Evaporation from Reservoirs and Ditches._—The evaporation from
water-surfaces especially may, in many cases, exceed the rainfall of
the year, so as to materially diminish the available water-supply
in reservoirs. Thus the annual evaporation from the reservoir-lakes
forming part of the water-supply of the city of San Francisco, ranges
from 40 to 50 inches, while the rainfall averages less than 24 inches.
Were it not, then, for the prevention of evaporation by a covering
of dry earth during summer, no moisture would remain in the ground
to sustain vegetation. In the cool coast climate of Berkeley, Cal.,
directly opposite the Golden Gate and subject to its summer fogs,
evaporation from a water-surface maintaining the average climatic
temperature of 60°, was found to be ¾ inch during the month from
the middle of July to the middle of August, 1904. But at the high
temperatures and low degree of air-saturation prevailing in the great
interior valley, or in the Colorado desert, the evaporation from
water-surfaces is enormously increased, exceeding even the figure given
in the table for Bombay. Hence the great importance of preventing all
avoidable evaporation, particularly in the use of irrigation water.

_Prevention of Evaporation; Protective Surface Layer._—The loose
tilth of the surface which is so conducive to the rapid absorption of
surface-water, is also, broadly speaking, the best means of reducing
evaporation to the lowest possible point. For while it is true that the
floccules of well-tilled soil permit of the ready access of air, and
therefore of evaporation, it is also true that these relatively coarse
compound particles are incapable of withdrawing capillary moisture
from the denser soil or subsoil underneath; just as a dry sponge is
incapable of absorbing any moisture from a wet brick, while a dry
brick will readily withdraw nearly all the water contained in the
relatively large pores of the sponge (see chap. 11). A layer of loose,
dry surface-soil is therefore an excellent preventive of evaporation of
the moisture from soils, and may be regarded as the natural and most
available means to be used by the farmer, both for the prevention of
evaporation and to moderate the access of excessive heat and dryness to
the active roots.

As regards the desirable thickness of this protective layer of tilled
surface-soil, it should be kept in mind that in the humid region, where
rain can be expected at intervals of from one to three weeks, the
feeding roots may usually be found within a few inches of the surface;
while in the arid region, where irrigation is practiced at long
intervals or sometimes not at all, so that no water enters the soil
oftener than from two to six months, the roots necessarily vegetate at
lower depths, and hence the protective surface-layer can, and should
be, of greater thickness, to prevent the penetration of excessive heat
and dryness during the long interval.

The failure to appreciate this necessary difference often leads to
heavy losses on the part of newcomers to the arid region, who in this
as in other respects are apt to follow blindly the precepts familiar to
them in the East, until taught better by sore experience. In the East
and Middle West a depth of three inches is considered the proper one
for the protective surface-layer; and in the case of maize even this
is considered excessive in many cases. In the arid region this depth
should be at least doubled where irrigation is not practiced at least
every four to six weeks; and in some sandy soils even seven and eight
inches is not too much for effective protection.

_Illustrations of Effects of Surface Tillage._—The efficacy of loose
surface tilth in preventing evaporation, as compared with mere
superficial scratching or with the total omission of cultivation,
is well exemplified in a series of investigations conducted on this
subject during the extremely dry season of 1898, by the California
Experiment Station; the seasonal rainfall having during that year been
on an average from one-third to one-half only of the usual amount, so
as to test to the utmost the endurance of all growing plants. Some
of the details of this investigation have been given above (p. 214)
in connection with the question of moisture requirements of crops.
Loughridge[97] also investigated the moisture conditions in adjacent
orchards differently treated in cultivation. In one of these cases
two orchards of apricots were separated only by a lane, and the soil
identical; but one owner had omitted cultivation, while the other had
cultivated to an extra depth in view of the dry season apparently
impending. The results are best shown by the plates below, showing
representative trees and the annual growth made by each. The table
annexed shows the differences in the moisture-content of the two fields
to the depth of six feet, in July:

          MOISTURE IN CULTIVATED AND UNCULTIVATED LAND.
    ===================+=====================+==================
                       |     Cultivated.     |  Uncultivated.
      Depth in soil.   +------------+--------+---------+--------
                       | Per cent.  |Tons per|Per cent.|Tons per
                       |            | acre.  |         | acre.
    -------------------+------------+--------+---------+--------
    First foot         |    6.4     |  128   |   4.3   |   86
    Second foot        |    5.8     |  116   |   4.4   |   88
    Third foot         |    6.4     |  128   |   3.9   |   78
    Fourth foot        |    6.5     |  130   |   5.1   |  100
    Fifth foot         |    6.7     |  134   |   3.4   |   68
    Sixth foot         |    6.0     |  120   |   4.5   |   90
                       |    ---     |  ---   |   ---   |  ---
    Total for six foot |    6.3     |  756   |   4.2   |  512
    -------------------+------------+--------+---------+--------

[97] Rep. Calif. Expt. Sta. for 1897-98, p. 65.

The difference of 244 tons per acre of ground shown by the analyses is
quite sufficient to account for the observed difference in the cultural
result. The cause of this difference was that in the _uncultivated_
field there was a compacted surface-layer of several inches in
thickness, which forcibly abstracted the moisture from the substrata
and evaporated it from its surface; while the loose surface soil on the
_cultivated_ ground was unable to take any moisture from the denser
subsoil.

[Illustration: Cultivated.]

[Illustration: Uncultivated.

FIGS. 48, 49.—Apricot Trees, Creek Bench Land, at Niles, Cal.]

[Illustration: FIG. 50.—New Growth and Fruit on Trees, Cultivated and
Uncultivated. Creek Bench Land at Niles, Cal.]

The cultural results were that on the cultivated ground the trees made
about three feet of annual growth, and the fruit was of good, normal
size; while the trees in the uncultivated ground made barely three
inches of growth, and the fruit was stunted and wholly unsaleable. It
may be added that when, instructed by the season’s experience, the
owner of the “uncultivated” orchard cultivated deeply the following
season, his trees showed as good growth and fruit as his neighbor’s.


EVAPORATION THROUGH THE ROOTS AND LEAVES OF PLANTS.

Undesirable as is the evaporation from the surface of the soil, under
all but exceptional conditions the evaporation from the leaves of
plants is one of the essential functions of vegetable development. Not
only because water serves as the vehicle of the plant-food absorbed by
the roots and to be organized by and redistributed from the leaves,
and the aeration occurring in the latter must of necessity result in
a certain degree of evaporation; but largely because the conversion
of liquid water into vapor serves to prevent an injurious rise of
temperature in the leaves under the influence of hot sunshine and dry
air. It is undoubtedly for the latter purpose that the greater part of
the enormous amount of water required, as above stated (chap. 11) for
the production of one part of dry substance, is actually used. When
sufficient water to supply the required evaporation through the leaves
cannot be brought up from the soil, the plant begins to wilt; or in the
case of some plants with very thin and soft leaves the blade normally
begins to droop during the hottest hours of the day; thus escaping
excessive exposure to the sun’s rays, and recovering their turgor later
in the afternoon.

    The amount of water actually evaporated from orchard
    trees has unfortunately not been directly determined, the
    investigations made in this respect having borne mainly upon
    forest trees. The Austrian Forest Experiment Station made a
    series of elaborate investigations on this subject in 1878,
    and the following data (quoted from the Report of the U.
    S. Dep’t of Agriculture for 1889) convey some idea of the
    results.

    It was found that the surface-areas of the leaves do not
    give reliable results, but that these depend very largely
    upon the thickness (mass) of the leaves. The dry weight of
    the latter was found, as in the case of field crops, to
    correspond most nearly to the observations made directly.
    It was thus found that _e. g._ birch and linden
    transpired during their annual period of vegetation from
    600 to 700 pounds of water per pound of dry leaves; oaks
    200 to 300, while the figures for ash, beech and maple
    were in between. On the other hand the conifers—spruce,
    fir and pine—ranged, under the same conditions, from 30
    to 70 pounds of water only. In another year, these figures
    were increased for deciduous trees to from 500 to 1000,
    the conifers, 75 to 200 pounds. This great variability
    in different seasons, together with other elements of
    uncertainty, render these figures only roughly approximate;
    but it will be noted that the figures for deciduous trees
    are in general of the same order as those given above for
    field crops. Assuming the evaporation for citrus trees to
    be approximately the same as for the European evergreen oak
    (_Q. cerris_) viz. 500 pounds per pound of dry matter,
    and taking the weighings made by Loughridge of the leaves of
    a 15-year-old orange tree at Riverside as a basis (40 pounds
    of dry leaves), the water evaporated by each such tree would
    be about 20,000 pounds per year, or about 1000 tons per acre
    of 100 trees. This is equivalent to about 9 acre-inches of
    rainfall, out of the 35 inches commonly given.

Since different plants evaporate very different amounts of water
during a given time, according to their leaf-surface and the number
and size of their stomates, the maintenance of the equilibrium between
the soil-supply and the evaporation of the leaf-surface requires
correspondingly varying moisture-conditions in the soil. Therefore
desert plants, with their elaborate structural provisions against
leaf-evaporation, will develop normally, and without wilting, under
conditions which in the case of most culture plants would result in
severe injury or death. Since diminution of leaf-surface will in all
cases diminish evaporation, the heroic measure of cutting back the
twigs and branches of shrubs and trees in seasons of severe drought
is sometimes resorted to in order to save their life. In Nature this
diminution of leaf-surface may be observed in many cases of desert
plants, whose “fugacious” leaves are developed during the rainy season,
in winter and early spring; dropping off so soon as the dry season
begins, and leaving only the green surface of twigs, stems or spines to
perform the functions of the leaves.

The shading of the ground by leafy vegetation will, of course, greatly
diminish and sometimes suppress evaporation from the soil-surface; thus
very nearly fulfilling the same conditions referred to above (chap. 7,
page 111) in discussing the effect of natural vegetation in rendering
tillage unnecessary; the beating of rains, and the formation of surface
crusts, being alike prevented. This fact is of essential importance in
contributing to the welfare of crops sown broadcast, where subsequent
cultivation is impracticable.

_Weeds Waste Moisture._—The injurious effects of weedy growth among
culture plants are in most cases due quite as much to the appropriation
of moisture that should have gone to the crop, as to the abstraction
of plant-food, to which the injury is generally attributed. This is
much more obvious in the arid region, where during the dry summers
every pound of moisture counts, than where summer rains obscure this
influence. It has led orchardists in California almost to an excess of
clean culture, resulting in the burning-out of the humus from the bare
surface-soil during the long, hot summers, and an injurious compacting
impossible to remedy by the most careful tillage. It thus happens
that green-manuring, the natural remedy for this evil, cannot safely
be done there with summer crops, but must be accomplished with winter
crops, such as can be turned under before the dry season begins. The
same objection holds against the growing of summer crops between the
orchard-rows.


DISTRIBUTION OF MOISTURE IN THE SOIL AS AFFECTED BY VEGETATION.

The investigations of Wollny and others have long shown quantitatively
what common experience has taught the farmer, viz., that a field in
crops or grass is always drier within the soil-mass penetrated by the
roots than is a cultivated field bare of crops, unless perhaps when
heavily crusted on the surface. The depletion of moisture caused by
grass sward is the most easily observed because of the shallowness
of the root-system; and this is one cause at least why grass sward
does not occur naturally in the arid region, and when planted cannot
be maintained without irrigation repeated at short intervals.
Deeper-rooted plants of course deplete the soil at different and
varying levels; and where surface roots are few or absent it may
readily happen that the surface soil is moister than the subsoil.

    This was very strikingly shown by the investigations of
    Ototzky in the South-Russian steppes, in comparing both the
    moisture contents and the depth of bottom water as between
    forest land and the open plains. On the steppe near Chipoff,
    Government of Voronej, he found the ground water at from 3
    to 5 meters (10-16 feet) depth; under the forest in the same
    region and in identical underground formations, the water
    level stood at 15 meters. In the Black Forest near Cherson,
    the water is found at about 15 feet beneath the surface;
    under the steppe and in cultivated ground it stood at 10
    feet. At the same time the forest soil was moister in the
    upper two feet than the soil of the steppe, where surface
    evaporation (partly through shallow plant-roots, partly
    direct) was greater than under the shadow of the forest;
    under which, moreover, there were few shallow rooted plants
    to draw upon the moisture of the surface soil.

The great evaporation from forests is a matter demonstrated by actual
measurement; hence it is not surprising that certain shallow-rooted
trees should serve for the reclamation of wet ground, as has been
demonstrated on the large scale, _e. g._, in the use of the eucalyptus
in the Pontine Marshes of Italy, and of the maritime pine in the Landes
of western France. Thus the sanitation of swampy districts through
tree-planting has become one of the established measures in their
settlement. But this refers only to the evaporation from the trees
themselves; for in the shade of the forest, a free water-surface is
found to evaporate on the average only one-third as much as in open
ground. Of course there must be a correspondingly great difference in
the amounts of evaporation from the soil-surfaces in the respective
areas.

The great draft made by the _Eucalyptus globulus_ upon soil-moisture
has been also abundantly shown in California, where on account of its
rapid growth this tree has been largely used for windbreaks. It was
found that the trees deplete the fields of moisture for from twenty to
thirty feet on either side, so as to materially reduce crops within
that limit. For this reason the pine and cypress has of late found
greater acceptance for this purpose.

_Mulching._—Covering the soil with straw or similar loose materials
to prevent waste of moisture is a common garden practice everywhere,
although not usually applicable on the large scale. It may readily
however, be carried to excess, in preventing not only evaporation but
also the warming of the soil which is so needful to the thrifty growth
of plants. It must not therefore be done too early in the season; and
after cold rains it sometimes becomes necessary to remove the mulch in
order to allow the ground to become properly warmed. Mulching in early
spring is often used to retard blooming of trees where spring frosts
are feared.

In the arid region, sanding of the surface is sometimes resorted to
for the prevention of the evaporation which brings alkali salts to the
surface. But the necessity of repeating this dressing annually unless
cultivation can be omitted, restricts the use of this expedient to
narrow limits.

The sanding of the surface of cranberry plantations in swamps or bogs
in the northern parts of the humid region doubtless owes its efficacy
largely, if not chiefly, to the retention of moisture, while at the
same time it prevents the consolidation of the surface, so as to render
tillage unnecessary.




CHAPTER XIV.

ABSORPTION BY SOILS OF SOLIDS FROM SOLUTIONS. ABSORPTION OF GASES. AIR
OF THE SOILS.


ABSORPTION OF SOLIDS FROM THEIR SOLUTIONS.

Just as solids have the power of condensing gases upon their surfaces,
to an extent proportional to that surface, and therefore to the state
of fine division: so fine powders have the power of withdrawing from
solutions solids held in solution, to an extent varying with the
nature of the substance dissolved, and the absorbing solid. The most
commonly-known manifestation of this principle is that sea-water
filtering through the sands of the shore, will at a certain distance
become sensibly less brackish, and finally so nearly fresh as to be
capable of domestic use.[98] The extent to which this occurs is in a
measure proportional to the fineness of the sand, and to the amount of
clay present in it. This is a clearly physical effect, independent of
any chemical action whatever; for it occurs equally with quartz sand,
charcoal, glass, limestone, or other rock powders having no chemical
effect upon the substance dissolved or upon the liquid dissolving
it. Very large amounts of water are often required to remove all the
soluble matter thus “adsorbed.”

[98] In many cases this decrease of salinity is probably due to a slow
influx of fresh water from landward; but very often it cannot be thus
explained.

_Decolorizing Action._—One of the commonest applications of this
principle is the decolorization of colored solutions by means of finely
pulverized charcoal. This property of charcoal, as is well known, is
extensively utilized in the arts, and particularly in the refining of
sugar; the charcoal used in this case being preferably bone charcoal
(“bone black”), which on account of its state of extreme fineness, and
separation by the earthy particles with which it is associated, is more
effective than any other form. It is rendered still more effective,
however, by the extraction of these earthy particles (calcic carbonate
and phosphate) by means of acid; for by removal of the earthy
particles, the surface of the charcoal is greatly increased, and its
decolorizing as well as its absorbing power increases accordingly.

While in one and the same substance the decolorizing effect is more
or less directly proportional to the fineness of the particles,
corresponding to increased surface, it is nevertheless true that in
this case, as in that of the absorption of gases, there are specific
differences between different powders; so that for example no other
substance can replace charcoal in the decolorizing effect which it
produces upon colored solutions. It must not, however, be supposed that
there is any special reason why _coloring_ matters, as such, should be
taken up by preference. Coloring matters are of all kinds of chemical
composition, and have in common only the fact that a relatively small
amount produces a very strong coloring effect; hence their name, and
hence also the apparently extraordinarily strong effect produced upon
them by charcoal.

This effect is not, however, by any means greater than it is in the
case of many other compounds which are colorless.

_Complex Action of Soils._—The powdery ingredients of soils, of course,
share this power with all other powders. In the case of soils, however,
the action is almost always much more complex than in that of charcoal,
because solutions that are passed through the soil are apt to act
chemically upon one or the other of its ingredients, usually resulting
in a partial exchange of ingredients between the soil and the solution;
one or more of the constituents of the solution being retained by the
soil, while one or more of the (basic) soil constituents pass into the
solution, in combination with its acidic ingredients.

    Thus when a very dilute (½ or 1%) solution of potassic
    chlorid is filtered through almost any soil, the first
    portions passing through will be practically free from
    potash, but will contain the chlorids of calcium and
    magnesium. But as more of the solution is passed through,
    potash passes also ultimately without absorption. In
    addition to the zeolitic and clay portions of the soil, the
    humus is very effective in absorbing mineral ingredients
    from solution, and retaining them in such manner as to be
    readily available to plant growth. (See chap. 8, p. 124.)

In view of the almost invariable conjunction of physical and chemical
effects, it may be fairly said that no solution, at least of mineral
salts, can pass through the soil without being changed in its
concentration and chemical composition. It is sometimes difficult to
decide to which of the two classes of effects the several changes may
be due.

_Purifying Action of Soils._—The disinfecting action of dry soil,
absorbing offensive gases from manure piles and from earth closets, has
already been alluded to. Similarly it is a matter of common experience
that the colored and otherwise offensive drainage from manure piles,
tanneries, dyeworks, etc., is not only deodorized but also decolorized
when passed through a sufficiently thick layer of clay soil. The
filtration through fine sand by which the drinking waters of cities
are so commonly purified before delivery to the consumer are familiar
examples of the same effects.

Equally familiar, however, is the fact that this power of
decolorization and retention of offensive compounds is limited; that
after a while the filtering earth or sand becomes saturated, and
afterwards the water or drainage will pass through without any sensible
purification.

It is therefore clear that this purifying effect of earth cannot
be relied upon for the permanent protection of wells from the
surface-drainage from barnyard or house refuse. Even if fissures or
layers of sand or gravel should not intervene so as to permit of the
direct communication of surface-drainage with wells, it is certain that
in the course of a few years at most, the intervening earth will become
so far saturated with the noxious ingredients that the latter will pass
through unhindered, and may contaminate to a considerable extent the
domestic supply of drinking water.

_Waste of Fertilizers._—The same, of course, holds true in regard to
manure-water, or soluble fertilizers of any kind used on the soil of
a field. The soil will retain them to a certain extent; but beyond
that limit any surplus added will be quickly washed through into the
country drainage by the rains. Moreover, a soil once so saturated
will yield to rain water filtering through it, notable amounts of all
the ingredients absorbed in it; and, at least so far as the physically
condensed soluble ingredients are concerned, long-continued leaching
with pure water will inevitably result in the withdrawal of additional
amounts of absorbed ingredients, apparently dividing themselves up _pro
rata_ between the water and the soil.

It is obviously of the utmost importance to the farmer to know to what
extent the soil will retain manurial ingredients against the influence
of leaching rains; for unless this is taken into consideration, it may
readily happen that the fertilizer supplied before a rainy season will
be washed through beyond the reach of plant-roots, and so practically
become a dead loss.

_Absorptive Power Varies._—So far as the mere physical absorption is
concerned, it will readily be understood that a coarse sandy soil
exercises less retentive influence upon dissolved substances than clay
or humous soils. In the humid region, where sand is substantially
nothing but granular silica (see above, chap. 6, page 86), the same may
be measurably true as regards the chemical absorption also. In the arid
region, on the contrary, a great many sandy or silt soils, very poor in
clay, exert fully as much chemical absorption as clay soils, and are
no more liable to the washing-out of soluble fertilizers introduced
than are the latter. For the chemical absorption lies chiefly in the
zeolitic portion of the soil (see above chap. 3, p. 37), which in the
humid region accumulates in the clay, while in the arid it remains
encrusting the sand and silt grains.

_Generalities regarding Chemical Absorption and Exchange._—In regard to
the leaching-out and absorption or retention of substances important to
agriculture, the following general statement may be made:

The substances most likely to be leached out of soils are, of bases:
soda, magnesia and lime; of acidic constituents: chlorine, sulfuric
acid and nitric acid. Lime sometimes passes off with either of the
above acidic ingredients, and also in the form of carbonate.

Substances rather tenaciously retained in soils are: potash and ammonia
among the bases, and phosphoric acid among the acids.

Thus (as stated above) when a weak (one or two per cent) solution
of potassic chlorid or sulfate is poured upon a column of good soil
several inches thick, it will be found that the first portions passing
through are free from potash, but contain the chlorids or sulfates of
magnesium and calcium. If potassic nitrate be used, lime and magnesia
will pass off as nitrates; while in the case of potassic phosphate,
both ingredients will be retained. A solution of gypsum (calcic
sulfate) will usually cause the passing-off of some of the magnesia,
soda and potash contained in the soil, in the form of sulfates; but
the amount of potash thus dissolved soon diminishes to a mere trace.
Solutions of potassic or amnionic phosphates will be absorbed and
retained by the soil to a very considerable extent, before the soil
becomes saturated.

While it is true that the degree to which the soil retains the several
ingredients may serve in a very general way to indicate their richness
or poverty in the same, the attempt to make such experiments serve
to determine the agricultural needs of soils has met with but little
practical acceptance.

_Drain Waters._—The table on p. 22, chapter 2, illustrates forcibly the
working of the above principles, which are verified by the composition
of drain-waters. In all, the chief nutritive ingredients of plants,
except nitrogen, are present in traces only; chlorids, nitrates and
sulfates of sodium and magnesium form the bulk of the permanently
soluble matter, with usually a considerable proportion of calcic (and
magnesic) carbonate, depending upon the amount of the earth-carbonates
present in the soil, as well as upon that of oxidizable organic
matter from which carbonic acid can be formed. That calcic carbonate
filters readily through the soil has already been somewhat elaborately
discussed (see chap. 3, p. 41); one of the results being that the
surface soil is sometimes almost completely depleted of this important
substance, while it accumulates at a greater or less depth in the
subsoil, or in underdrains, as the case may be.

Of the ingredients appearing in the above list, the one of greatest
agricultural importance is nitric acid, since chlorine and sulfuric
acid, as well as soda, are required only in very small quantities
by most culture plants; so that they rarely need to be supplied
in fertilizers. Nitric acid, however, is not only one of the most
important fertilizers, but also the most expensive; hence the
passing-off of nitrates in drainage-water is of such serious concern
to the farmer, that the causes of its occurrence, and the means of
preventing such loss, should be fully understood. This subject will,
however, be more fully considered farther on.

_The above Distinctions not Absolute._—It should, however, be also
understood that while the above statements hold good in a general way,
yet the line drawn is by no means an absolute one. For just as in the
case of physical adsorption the long passing-through of distilled water
will gradually abstract the substances condensed on the surface of
the soil-grains, so an overwhelming amount of a solution of any one
kind will have a tendency to substitute its own ingredients for those
already present in the soil, removing the latter to a greater or less
extent, even in the case of potash and phosphoric acid.

    As an example in point, may be cited the case of the natural
    minerals Analcite and Leucite, which Lemberg was able to
    reciprocally transform from their natural condition of
    soda- and potash-alumina silicates merely by alternate
    treatment with solutions of potassium and sodium chlorids
    respectively. (See chap. 3, p. 37). The same is true in
    the case of the zeolitic matter of the soil. There is
    nevertheless a distinct preference in the direction of the
    retention of potash as against soda; so that in the case
    of alkali soils, a large excess of potash is found to be
    present in the zeolitic form, notwithstanding the presence
    of sometimes very large amounts of the chlorid, sulfate and
    carbonate of soda. This preferable retention of potash is,
    of course, of material advantage in the case of the use of
    soluble potash-fertilizers, as well as in preventing the
    waste of the potash of the soil itself.


ABSORPTION, OR CONDENSATION, OF GASES BY SOILS.

Like all bodies in a state of fine division, soils are capable of
absorbing a not inconsiderable amount of various gases. It may be said
that in general, other things being equal, the amount thus condensed on
the surface of the soil-grains is more or less directly proportional
to the facility with which the gas is condensed by either pressure or
cooling. Hence the very large amount of water-gas or vapor which may
be absorbed by soils, as shown in a preceding chapter. But excepting
perhaps the case of ammonia, moist soils are less absorbent of gases
than dry ones.

Oxygen and nitrogen, the main constituents of the atmosphere, being
difficultly condensable by either pressure or cold, are absorbed by
soils only to a relatively small, yet by no means unimportant extent.
The condensation of oxygen within the soil-mass is doubtless of
considerable importance in the processes of oxidation, as is shown by
its partial replacement by carbonic gas in the free air of the soil
(see chap. 2, p. 17). The intensifying of oxidizing action caused by
surface condensation is well illustrated in the case of finely divided
platinum, in which hydrogen is brought to rapid combustion when mixed
with oxygen; as well as by the effect of bedding tainted meat in
charcoal powder, when all odors of decay disappear, both by absorption
and oxidation, ammonia and carbonic gas alone ultimately escaping
through the powder.

_Carbonic dioxid and ammonia gases_, both normal constituents of the
atmosphere, and of high importance to plant nutrition, are more readily
condensable than either oxygen or nitrogen, and consequently may be
taken up by the soil in larger relative proportions. Especially is
this the case with ammonia gas, which is not only readily condensed by
pressure, but is also extremely soluble in water; so much so that it
rushes into a tube filled with this gas almost as quickly as though
it were a vacuum. Water will absorb at the ordinary temperature,
under normal pressure, about 700 times its volume of ammonia gas; but
inasmuch as the proportion of the latter in the atmosphere amounts
to only a few millionths, the actual amount taken up can only (as
in the case of all gases) be proportional to its proportion (or
“partial pressure”) multiplied into its coefficient of absorption.
Consequently, water exposed to the ordinary air can absorb at best only
a small fraction of a per cent of ammonia. Its presence in soils can
be readily demonstrated by passing through the warmed soil a current
of purified air, which is made to bubble through Nessler’s reagent
(potassio-mercuric iodid) solution.

_Absorption of gases by dry soils._—Perfectly dry soils are powerful
absorbers of ammonia, and their absorption of this gas, as well as of
carbonic gas, can readily be shown by the arrangement shown on the page
opposite.

    The two tubes shown to the left are filled with carbonic
    gas, those to the right with ammonia gas. After being
    immersed in a mercurial trough, there are introduced into
    each tube through the mercury small cylinders (conveniently
    one cubic centimeter in volume) consisting respectively of
    a very sandy soil or loose hardpan, a gray plastic clay,
    a gray clay soil or adobe, a very black “adobe” clay, and
    a highly ferruginous and humous soil (from Hawaii), which
    gives the highest absorption of all; next brown peat, and
    pine charcoal. The latter, and the ferruginous soil, were
    also exposed for the absorption of carbonic gas. All the
    absorbing cylinders are first heated for an hour to 110°C.
    (218°F) for the purpose of expelling from them moisture,
    air, and other absorbed gases. They are then quickly
    introduced into the tubes through the mercury and allowed to
    absorb the gases enclosed until the mercury columns cease to
    show any farther rise; in which condition they are shown in
    the figure.

It will be seen that this absorption is a different one, not only
for each of the different substances used, but is also differently
proportioned for the two gases. For it will be noted that while the
clay soil has absorbed a very much larger amount of ammonia than the
charcoal, and the sandy soil has remained far behind both: yet the
charcoal has absorbed a considerably larger _proportion_ of carbonic
gas than either the clay or the sandy soil, proving that charcoal has
a strong _specific_ absorptive power for carbonic gas, independently
of the relative size of clay and charcoal particles respectively.
The sandy soil shows, by its low absorption even of ammonia gas,
the coarseness of its particles and the scarcity of clay in its
composition. The highest absorption of all is shown by the ferruginous
soil from Hawaii, containing nearly 40% of ferric oxid together with
3⅓% of humus. The moisture-absorption of this soil at the ordinary
temperature is 19.7 per cent. The difference in the absorbing power
of the (non-humous) gray clay and gray adobe soil indicates the
strong influence of humus upon the absorption; which is still farther
emphasized by the difference between the gray and black adobe, the
latter containing 1.2% of humus. As to the peat, since its weight was
only .5 grams against an average of 2 grams for the soils employed, its
absorptive power _by weight_ doubtless exceeds all other substances.

[Illustration: FIG. 51.—Absorption of Carbonic and Ammonia Gases by
different Soils.]

While the experiment shown in the figure serves as a convenient and
striking demonstration for lecture purposes, it is of course not
adapted to a direct comparison of the absorbing powers of the several
substances, because of different heights of the mercurial columns
counteracting the atmospheric pressure. For direct comparative
measurement the tubes must be sunk in mercury so as to equalize the
levels inside and outside, since the corrected volumes obtained by
calculation would not serve the purpose.

According to special measurements made under normal atmospheric
pressure, the writer found that a black clay soil (“adobe”) absorbed
(at 60°F) over two hundred times its bulk of ammonia gas, while under
the pressure of one-fifth of an atmosphere (as shown in the photograph)
the absorption was one hundred and twenty-three times its bulk. This
energetic absorption of ammonia and related gases explains the marked
disinfecting effects which a covering of dry earth exerts in the case
of cemeteries, manure piles, and earth closets. But the difference
between the sandy soil and the clay soil in the amount of absorption
admonishes us that in all these cases, to secure disinfection the earth
to be used should contain as much clay as possible, and should not be
mere sand, as is sometimes the case. It also shows that the addition
of charcoal to such materials does not increase their efficacy, as has
been supposed, but that an equal bulk of clay would be more efficient.

Of course, so soon as the absorbing cylinders used for this experiment
are exposed to the atmosphere, the principle above stated in regard to
“partial pressure” asserts itself. The absorbed gases quickly begin
to be given off, and in some hours the equilibrium with the ordinary
conditions of the atmosphere is re-established. That the strong
absorptive power of soils for ammonia is to some extent effective
in maintaining the supply of this substance by absorption from the
atmosphere, cannot be doubted.

Boussingault, and later Stenhouse, determined the absorptive power of
wood charcoal for ammonia to be 90 and 98 volumes respectively.


THE COMPOSITION OF GASES ABSORBED FROM THE ATMOSPHERE BY VARIOUS SOLIDS.

In 1864 and 1865 Reichardt and Blumtritt[99] investigated elaborately
the composition of gases driven off by heat from various powders,
including soils, exposed to the atmosphere. All the substances examined
were therefore “air-dry,” therefore to a certain extent moist; and
the presence of this aqueous vapor of course modifies in a measure
the results that would have been obtained had the materials used been
exposed to dry air only. They found that, as had already been stated
by previous observers, the presence of capillary water _diminishes_
materially the absorption of gases, especially of those not as easily
absorbed by water as are carbonic gas and ammonia. Contrary to what
might have been expected from the more ready condensation of oxygen by
pressure or cold, in nearly all cases nitrogen is absorbed to a greater
extent than oxygen, and sometimes exclusively so; so that in some cases
the latter was found to be present only in traces, as will be perceived
from the subjoined table:

    COMPOSITION OF GASES ABSORBED FROM THE ATMOSPHERE
                   BY VARIOUS POWDERS
    ================================+========+=======
                                    |  100   |  100
                                    |  Grms  | Vol’s
               Substance.           |gave cc.| gave
                                    |  Gas.  | Vol’s.
                                    |        |  Gas.
    ------------------------------=-+--------+-------
    Charcoal, coniferous, air dry   |  16.21 |
       “    moistened and air-dried | 140.11 |  59.0
       “    Lombardy Poplar         | 466.95 | 195.4
    Peat                            | 162.58 |
    Garden Earth, moist             |  13.70 |  19.9
      “      “    air-dried         |  30.28 |  53.6
    River Silt, air-dried           |  40.53 |  48.07
      “    “    slightly moistened  |  24.12 |  29.2
      “    “    air-dried           |  26.52 |  30.05
    Clay, long exposed              |  25.58 |  39.05
     “    slightly moistened        |  28.62 |  35.08
    --------------------------------+--------+-------
    Ferric Hydrate, commercial      | 251.59 | 275.0
      “       “     freshly         | 375.54 | 308.6
                      precipitated, |        |
                       air-dried    |        |
    --------------------------------+--------+-------
    Ferric Oxid, ignited            |  39.4  |  52.4
    Aluminic Hydrate, air-dried     |  69.02 |  82.0
       “       “    dried at 100°C. |  10.83 |  13.6
    Prepared Chalk, 1864-65         |  43.48 |  52.4
       “       “    1865            |  38.98 |  48.0
    --------------------------------+--------+-------
    Calcic Carbonate, precipitated, |  65.09 |
                        1864-65     |        |
       “       “      precipitated, |  51.53 |  52.0
                         1865-66    |        |
    --------------------------------+--------+-------
    Magnesic Carbonate              | 729.21 | 124.9
    Gypsum, finely powdered         |  17.26 |
    --------------------------------+--------+-------
    ================================+==================================
                                    |     100 vol’s gas contained
                                    +--------+------+--------+---------
              Substance.            |        |      |        |
                                    |Nitrogen|Oxygen|Carbonic| Carbon
                                    |        |      | Dioxid |Monoxid
    --------------------------------+--------+------+--------+---------
    Charcoal, coniferous, air dry   | 100.00 |  0.0 |    0.0 |   0.0
       “    moistened and air-dried |  85.60 | 2.12 |   9.15 |   3.13
       “    Lombardy Poplar         |  83.60 |  0.0 |  16.50 |   0.0
    Peat                            |  44.44 | 4.60 |  50.96 |   0.0
    Garden Earth, moist             |  64.34 | 2.85 |  24.06 |   8.75
      “      “    air-dried         |  64.70 | 2.04 |  33.26 |   0.0
    River Silt, air-dried           |  67.69 |  0.0 |  18.61 |  13.70
      “    “    slightly moistened  |  67.34 |  0.0 |  30.56 |   2.10
      “    “    air-dried           |  67.40 | 9.09 |  16.07 |   7.44
    Clay, long exposed              |  70.17 | 4.71 |  25.12 |
     “    slightly moistened        |  59.59 | 6.39 |  34.02 |
    --------------------------------+--------+------+--------+---------
    Ferric Hydrate, commercial      |  33.26 | 1.43 |  65.31 |   0.00
      “       “     freshly         |  26.29 | 3.85 |  69.86 |   0.00
                      precipitated, |        |      |        |
                       air-dried    |        |      |        |
    --------------------------------+--------+------+--------+---------
    Ferric Oxid, ignited            |  82.87 |13.41 |   3.72 |   0.00
    Aluminic Hydrate, air-dried     |  40.60 | 0.00 |  59.40 |
       “       “    dried at 100°C. |  83.09 |16.91 |   0.00 |
    Prepared Chalk, 1864-65         | 100.00 | 0.00 |   0.00 |
       “       “    1865            |  74.49 |15.49 |  10.02 |
    Calcic Carbonate, precipitated, |  80.81 |19.19 |   0.00 |
                        1864-65     |        |      |        |
    Calcic Carbonate, precipitated, |  77.37 |15.09 |   7.54 |
                         1865-66    |        |      |        |
    Magnesic Carbonate              |  63.92 | 6.72 |  29.36 |
    Gypsum, finely powdered         |  80.95 |19.05 |   0.00 |
    --------------------------------+--------+------+--------+---------

[99] Journal für praktische Chemie, Vol. 98, p. 167.

    _Discussion of the Table._—It will be observed that
    in this table, the largest amount of total gas given off
    by equal weights of any one substance was in the case of
    carbonate of magnesia; but it is quite probable that in
    part, at least, this large amount of gas was due to the
    evolution of carbonic gas from the easily decomposable
    carbonate; the more as the analysis of the gases shows over
    29% of carbonic gas. But the highest absorption by equal
    _volumes_ of any substance is shown by the ferric
    hydrate; next to this by the light poplar charcoal, and
    next by the carbonate of magnesia. The high absorptive
    power here shown by the ferric hydrate is of great interest
    in connection with the facts already stated regarding the
    absorption of moisture and ammonia by ferruginous soils
    (see page 274, this chapter); and the fact that the larger
    proportion of the gas—as much as 70% in one case—consisted
    of carbonic gas, is particularly interesting in the same
    connection. Both in the amount of gas contained, and in
    the proportion of carbonic gas therein, the ferric hydrate
    exceeds even peat, the representative of humus in soils.
    It will, however, be noted that in the garden soil, also,
    the proportion of carbonic gas is very large, while that of
    oxygen is very low. It is curious to note that in very few
    cases the proportion of oxygen to nitrogen is the same as
    in the atmosphere; in most cases the nitrogen predominates
    considerably beyond its normal proportion, and in two cases,
    that of charcoal and of calcic carbonate (whiting), the gas
    was found to consist of pure nitrogen.

We are forced to conclude that the substances here enumerated, as a
rule, condense oxygen in smaller proportions than they do nitrogen, or
carbonic gas. As regards the carbon monoxid mentioned in the table, it
is doubtful that it was contained as such in the substance originally
examined; it may readily have been formed under the influence of the
heat required in expelling the gases from the substances containing
organic matter. Among the important results shown in the table, is the
comparative determination of the gases in moist, and in dry garden
earth, showing that in the moist earth the amount of gas absorbed
ranged from less than one-half down to almost one-fourth that absorbed
by the dry. The importance of these differences in the case of the
fallow can readily be appreciated.

    The changes in the absorptive power brought about by
    wetting and drying, as shown in the above table, are very
    insignificant. In the case of the charcoal, soil and silt
    the diminution may fairly be assumed to be caused by the
    deposition of soluble salts on the surface, partly clogging
    the pores. In the case of the clay as well as in that of
    the river silt, the inevitable content of organic matter
    in process of decomposition has doubtless influenced the
    result, as is suggested by the increase of carbonic gas.
    That prepared chalk should in one case contain exclusively
    nitrogen gas, in the other case mixed gases, seems to
    indicate a difference in the air to which it is exposed,
    or in the water employed in its preparation; the latter
    case agreeing substantially with the results obtained from
    the precipitated carbonate. In both (as well as in the
    carbonates of barium and strontium), the absorption of
    carbonic gas is very small, or _nil_.

It thus appears that for the condensation of carbonic dioxid gas,
ferric and aluminic hydrates are prepotent among mineral substances;
while clays, river silts and soils may always be expected to contain
relatively large proportions of this gas in absorption.


THE AIR OF SOILS.

_The Empty Space in Soils._—In dry soils the empty space, usually
amounting to from 35 to 50 per cent of its volume, is filled with
air;[100] in moist or wet soils the space unoccupied by water is
similarly filled. Hence when soils are in their best condition for
the support of vegetation (chap. 11, p. 202), about one half of their
interstices is filled with water, the other half with air. Actual
measurements of the amount of air contained in well-cultivated garden
soil have been shown by Boussingault and Levy to range between 10,000
and 12,000 cubic feet per acre, substantially agreeing, therefore, with
the above statement. In uncultivated forest soil, on the contrary, they
found only from somewhat less than 4000 to 6000 cubic feet of air per
acre. Extended observations since carried out by Wollny, Ebermayer, and
others have in general confirmed the earlier observations, while adding
greatly to their significance in respect to their relations to plant
growth, and to the process of humification and soil-formation.

[100] The normal composition of atmospheric air is given on p. 16,
chap. 2.

As a matter of course, when water evaporates from the soil in drying,
its place is taken by air so far as it is not filled by capillary water
drawn from below.

_Functions of Air in Soils._—That roots require for the performance of
their vegetative functions the presence of oxygen, has already been
discussed; but there can be no question that the higher productiveness
of well-cultivated soils is largely due to the greater and readier
access of air to the roots. Apart from this direct function, however,
the presence of oxygen in the soil serves other important purposes,
and among these doubtless the most dominant is the promotion of the
oxidation of the organic matter of the soil through the agency of
micro-organisms; and more particularly that of nitrification, which
chiefly governs the supply of nitrogen to non-leguminous plants. In
the case of leguminous plants, the presence of air as a furnisher of
nitrogen as well as oxygen is absolutely essential.

The injurious effects of insufficient aeration of the soil have been
repeatedly referred to already (pp. 45, 76). In water-logged soils
reductive fermentations are soon set up, and the nitrates of the
soils are reduced partly with the evolution of nitrogen gas, partly
to ammonia; while their oxygen is consumed to supply the demands of
the roots. Ferric oxid is reduced to ferrous carbonate, sulfates to
sulfids; thus deranging the whole process of plant-nutrition and
absorption of plant-food. If continued for any length of time these
conditions end in the death of the plant. Too much importance cannot
therefore be attached to the proper aeration of the soil and subsoil.

_Excessive Aeration; Compacting the Soil._—On the other hand, excessive
aeration of the soil may be injurious in causing a serious waste of
moisture; especially in arid climates, where the hot, dry winds may
readily destroy the germinating power of the swollen seed when the
seed-bed is too loose and open, and later may injure or destroy the
feeding roots. The abundant growth of grain often seen in the tracks
of a wagon carrying the centrifugal sower, when the stand in the
general surface is very scanty, is usually due to the consolidation of
the seed-bed, and suggests at once the well-known efficacy of light
rolling to insure quicker germination and a better stand. Similarly,
the rolling of grain fields in spring is often the saving clause for
a crop in dry years. But such needful consolidation must not, of
course, be carried to the extent of creating a surface crust which
would subsequently serve to waste the subsoil moisture. Hence, the
soil-surface should be rather dry when rolling is resorted to.

The pressing of the earth around transplanted plants, similarly, is
a needful precaution, not only with respect to the drying-out of the
soil, but also to insure close contact between the roots and the soil.

_The Composition of the Free Air of the Soil_ usually differs from
the air above, in that besides being saturated with moisture, its
nitrogen-content is slightly increased (by one-half to over one per
cent); the oxygen-content on the other hand, is diminished, being
in part (sometimes nearly to the extent of one-half of its volume)
replaced by carbonic gas, derived partly from its secretion by the
roots, partly from the oxidation of organic substances. It naturally
follows that the richer the soil in the latter, the more carbonic gas
will be formed under favoring conditions; so that in freshly-manured
land the amount of oxygen transformed into carbonic gas will be
greatest, while in the surface-soil of ordinary fields, carbonic gas
rarely reaches to as much as one per cent. In all cases, however, the
content of carbonic gas in the air of the soil is materially higher
than that of the air above it, and thus serves to intensify greatly
the solvent and disintegrating effect of the soil water upon the soil
materials (see chap. 2, p. 17). The soil-mass itself, however, retains
carbonic dioxid with considerable tenacity, so that it is not possible
to wash it out completely by filtering water through it. When water
containing carbonic gas in solution is filtered through the soil, the
gas is sometimes completely absorbed, the water passing off free from
gas.

The presence of free carbonic gas in soils is readily demonstrated by
passing through the warmed soil a current of air, which is then made to
bubble through lime water; a clouding of the latter, and the ultimate
formation of a precipitate of calcic carbonate, proves the presence of
the gas, and may also serve to measure its amount.

From the fact that the free air in normal soils may contain as much as
one-fortieth of its bulk of carbonic gas, besides what may be contained
in the condensed form, we may conclude that this gas is formed within
them with considerable rapidity; for otherwise, in view of the free
communication and diffusion with the outer air, such large amounts
could not be maintained in the surface-soil. Doubtless a considerable
proportion of the carbonic gas normally contained in the atmosphere is
thus supplied from within the soil itself.

_Relation of Carbonic Gas to Bacterial and Fungous Activity._—It has
been fully demonstrated by the researches of Koch, Miquel, Adametz,
Fuelles, Wollny and others, that the formation of carbonic gas in the
soil is not a purely chemical oxidation process, but is essentially
dependent upon the presence and life-activity of numerous kinds of
organisms, bacterial as well as fungous. The crucial proof of this fact
is that the presence of any antiseptic diminishes, and if exceeding
certain proportions completely suppresses, the formation of carbonic
gas; while on the other hand all conditions known to be favorable to
the life of such organisms, viz., the proper conditions of temperature
and moisture (varying with different kinds), increase the formation
of the gas. Such formation is of course, however, conditioned upon
the presence of oxygen. In the case of most bacteria, there is a
certain limit beyond which the presence of their own product exerts
an injurious or repressive effect upon their activity; so that if the
gas accumulates beyond that limit, the rate of its formation decreases
despite of otherwise favorable conditions.

It follows that the best life-conditions of these organisms (even
when anerobic) cannot be fulfilled below a certain limited depth in
the soil; and all observations show that their number decreases very
rapidly with increasing depth (see chap. 9, p. 142), varying with the
perviousness of the soil, but rarely exceeding four or five feet in
the humid regions; though doubtless found at greater depths in the
arid climates. It is also obvious that the use of any antiseptic or
poisonous materials on the field or in the manure pile will tend to
disturb and restrain the useful activity of these organisms.

_Putrefactive Processes._—Carbonic gas is formed also, but to a much
more limited extent, in _putrefactive processes_, occurring in the
absence, or with only limited access, of air or oxygen. These processes
likewise are conditioned upon the presence or activity of (largely
anerobic) bacteria; but they should not occur in normally constituted,
and especially in tilled soils, being as a rule inimical to the growth
of cultivated plants (see chap. 9, p. 145).




CHAPTER XV.

THE COLORS OF SOILS.


The natural coloration of soils forms a prominent part of the
characters upon which farmers are wont to base their judgment of
land quality; hence the origin and value of soil-colors deserve
consideration.

_Black Soils._—From the oldest times down to the present a “rich, black
soil” has commanded attention and approval. The black and brown-black
colors being almost invariably due to the presence of much humus (very
rarely to an admixture of carbon [graphite], of magnetic oxid of iron,
or sesquioxid of manganese), it is obvious that the farmers’ judgment
coincides with a high estimate of the agricultural value of humus.
A discussion of this point will be found in another place; but the
popular judgment is based quite as much upon the experience had in the
advantages that usually accompany the presence of humus. It largely
characterizes low grounds, and therefore alluvial lands, whose richness
is due to far more general causes. But the _shade_ of the blackness
seen in the soil deserves and usually receives close consideration.
If tending toward brown, acid humus or “sour” land is indicated;
unless indeed the surface soil should be bodily derived from decayed
wood, as in the primeval forests. Forest soils in general are usually
dark-tinted for some inches near the surface, owing to the presence of
leaf mold, and mostly have an acid reaction.

But the black tint is equally welcome to the land-seeker when seen
outside of alluvial and forest areas. Belts of “black lands” appear
on hillsides and plateaus; and these lands, though clearly not
alluvial, are also found to be preëminently productive; witness the
upland prairies of the western and southern United States. These black
soils are always characterized by the presence of a full supply of
lime in the form of carbonate, under the influence of which the most
deeply black humus is formed. In other words, the jet black tint is
indicative of calcareous lands; and these, as will be more fully shown
below, are almost always highly productive.

From both points of view, then, the favorable judgment passed upon
black soils by practical men is justified.

But it is not necessarily true that soils showing no obvious black tint
are poor in humus; for in strongly ferruginous or “red” soils its tint
is frequently wholly obscured, though when still visible it gives rise
to the laudatory name of “mahogany land,” which every farmer considers
a prize.

Of course then it would be wholly incorrect to judge of the
agricultural value of land from its humus-content alone; for its color
may be entirely imperceptible and yet its amount and nitrogen content
be fully adequate to the requirements of thrifty vegetation. Gray and
even whitish soils very frequently fall within this category in the
arid region.

The black tint is also favorable to the absorption of the sun’s heat,
and is therefore conducive to earlier maturity than is to be looked for
in light-tinted lands similarly located.

Wollny (Forsch. Agr. Phys. Vol. 12, 1889, p. 385), discusses the
influence of color on soils in relation to moisture and content of
carbonic acid. The results show in general simply the effects due
to increase of temperature when the soils are either darker-colored
throughout, or made so superficially.

_“Red” Soils._—Next to a black soil, a “red” one will usually command
the instinctive approval of farmers. The cause of this preference is
not as obvious as in the case of the black tints; but the general
consensus of opinion requires an examination of its claims. It is of
course easy enough to adduce examples of very poor “red” soils, derived
from ferruginous sandstones that supply little else than quartz and
ferric hydrate; the Cotton States supply cogent examples in point, as
do also the lower Foothills of the Sierra Nevada of California. It is
not, therefore, the iron rust or ferric hydrate that renders the land
productive; but its presence is a sign of some favorable conditions.
First among these is, that ferric hydrate cannot continue to exist in
badly drained soils; a “red” soil is therefore a well-drained one, and
this is probably one of the chief causes of the popular preference. The
“white land” sometimes seem in tracts otherwise colored with iron, is
distinctly inferior in production to the red lands; and examination
will generally show that from some cause, such white lands have been
subjected to the watery maceration which proves so injurious (see chap.
3, p. 46, chap. 12, p. 231).

That finely-diffused ferric hydrate has a very high power of absorbing
moisture as well as other gases of the atmosphere, has been shown in
the preceding chapter; it stands in this respect next to humus itself,
and hence highly ferruginous soils need not contain as much humus as
“white” soils from this point of view. Like humus, also, it renders
heavy clay soils more easily tillable.

_Origin of Red Tints._—Where crystalline rocks prevail, the red tint
usually indicates the derivation from the weathering of hornblende;
implying also, outside of the tropics, the presence of sufficient lime
in the land. Such lands are naturally preferred to those of lighter
tints derived from purely feldspathic rocks (see chap. 3, p. 32),
although they may be poorer in potash than the latter.

But the red tint has also its intrinsic advantages in the more ready
absorption of the sun’s heat by the colored than by a white surface.
This is probably the chief cause of the higher quality of wines
grown on red hillsides in the middle and northern vine districts of
Europe, where everything that aids earlier maturity is of the greatest
importance. The function of ferric oxid as a carrier of oxygen (chap.
4, p. 45) probably also aids nitrification.

“Yellow” lands owe their tint, of course, to smaller amounts of ferric
hydrate, but share more or less in the advantages of the “red.”

_White_ soils, or more properly those having very light gray tints, are
not usually looked upon with favor, especially in the humid region. The
causes of the unfavorable judgment current among farmers in respect to
white soils has already been partially explained in the discussion of
the black and red tints. The light color means the scarcity or absence
of both humus and ferric hydrate, and usually implies that the soil has
been subject to reductive maceration through the influence of stagnant
water; reducing the ferric hydrate to ferrous salts, oxidizing away
the humus, and accumulating in the form of inert concretions most or
all of the lime, iron and phosphoric acid of the soil mass (see chap.
3, p. 46, chap. 10, p. 184). The term “crawfishy,” so commonly applied
to white soils in the eastern United States, expresses well the usual
condition of the white soils of that region; which are very commonly
inhabited by crayfish, whose holes reach water a few feet below ground,
and are surrounded on the outside by piles of white subsoil mixed
with “black gravel” or concretions of bog iron ore. It is needless
to say why such lands cannot command the favorable consideration of
the farmer; they cannot as a rule be cultivated without previous
drainage, and even after that will usually prove unthrifty, “raw,” and
in immediate need of fertilization by green-manuring, and the use of
phosphates.

In the arid region, lands of this character are of rare occurrence,
while (as has been explained above, chap. 8, p. 135), the light gray
or “white” tints are there a very common characteristic of even the
very best soils. It is true that they are poor in humus and in finely
diffused ferric hydrate; but their “light” texture renders the presence
of humus for this purpose less needful, and as stated elsewhere (see
chap. 8, p. 135), the high nitrogen-content of arid humus renders
a smaller supply adequate for vegetative purposes. As to iron, its
presence being more important as a sign of good drainage and aeration
than directly, its absence from soils of great depth and loose texture
is of no consequence; especially when the heat-absorption which it
favors is not only not needed, but is usually already in undesirable
excess during the hot summers.

_White Alkali Spots._—In the valleys of the arid region, however,
_very_ white spots commonly indicate the prevalence of alkali salts,
and to that extent are an unfavorable indication; especially when
coupled with the occurrence of black rings or spots, which indicate the
presence of black alkali or carbonate of soda (see chap. 22).




CHAPTER XVI.

CLIMATE.


_Heat and Moisture_ are the main governing conditions of plant growth.
In a preceding chapter the relations of soils and plant growth to
water have been considered; in the present one the relations of both
moisture and heat to soils and plants will be discussed; and to do
this intelligibly to those not making a specialty of such studies, it
becomes necessary to introduce, first, a summary consideration of the
subject of climate.

Climatic conditions control, and to a great extent determine, the
industrial pursuits of every country; all the more so as the rapid
communication and transportation by means of modern appliances now
brings every part of the globe in competition with every other. The
question is not now what it may be intrinsically possible to do under
certain climatic and geographical conditions, but whether these things
can be done with a reasonable prospect of profit and commercial
success, in competition with other countries offering more or less
of similar possibilities. While it is true that the cost of labor
frequently enters most heavily into such problems, yet favorable or
unfavorable climatic or soil-conditions may in many cases turn the
scale. Thus the high price of labor and fuel on the Pacific coast of
the United States would at first blush seem to render competition
with Europe and the East in the production of beet sugar commercially
impossible; yet exceptionally favorable climatic and soil-conditions
concur to turn the scale in favor of California at least, so as to have
placed that state at the head of the sugar-producing states of the
Union. A general understanding of the climatic conditions which concern
the United States more or less directly, is therefore needful to an
appreciation of their agricultural possibilities.

_Climatic Conditions._—The factors usually mentioned as constituting
climate are temperature, rainfall and winds. Since the latter
two factors, however, are themselves merely the result of _heat_
conditions, it is proper to discuss from the outset the origin and mode
of action of heat.


TEMPERATURE.

The temperature of stellar space outside of the atmosphere is
known to be very low. The increasing cold as we ascend to greater
heights, is a fact familiar to all. Langley has calculated upon the
basis of observations made at the summit and foot of Mount Whitney
in California, that the temperature of space lies near 200° Cent.
(360° F.) below the freezing point of water; and this would be the
temperature near the Earth’s surface, were it not for the surrounding
atmosphere. The latter absorbs but a small amount of the sun’s direct
heat rays (which are of _high_ intensity), as they penetrate it to the
Earth’s surface. But as the earth’s surface is warmed, the heat rays of
_low_ intensity which it emits cannot pass back through the atmosphere
to the sun or to outer space; they are “trapped,” as it were, by the
dense air resting near the earth’s surface, which is then warmed partly
by the radiation from, partly by direct contact with, the soil. It is
to the existence of our atmosphere, then, that the possibility of our
animal and vegetable life in their present form is due; and a decrease
of the trapping effect on the sun’s heat rays makes itself quickly felt
when ascending, either in balloons or on high mountains. Moreover, it
is well known to mountain climbers that at great elevations the sun’s
heat is extremely intense at noon; even though the temperature may fall
to the freezing point at night, owing to the failure of the thin air
to prevent the radiation back into space of the heat absorbed during
the day. On the high plateaus of the Andes and of Asia, therefore, very
extreme climates prevail, on account of the great range of temperature
between day and night.

_Ascertainment and Presentation of Temperature-Conditions._—The proper
understanding of the temperature conditions of any locality or region
is by no means a simple matter. Shall we study the daily, monthly, or
annual changes of temperature, or the means deduced from either or all
of them, in order to gain a clear insight into the climatic conditions
that control crop production and health conditions?

_The Annual Mean Temperature not a Good Criterion._—Since one and
the same figure may result equally from the averaging of two widely
divergent data, and from such as are close together, it is clear that
the mean _annual_ temperature cannot be a proper criterion of the
agricultural adaptations of a country. Thus an average temperature of
60° F. might result, equally, from the averaging of 65 and 55 degrees,
or from taking the mean of 15 and 105 degrees; yet the respective
cultural adaptations would be widely different.

_Extremes of Temperature are Most Important._—It is, on the contrary,
rather the _extremes_ of temperature, more particularly of cold,
but frequently also of heat, together with the total amount of
heat available during the growing season, that determines such
adaptation so far as temperature is concerned; for no culture plant
can be successfully grown where the temperature during winter even
occasionally falls for more than a few hours below the point which
it can resist; and for each plant there is a certain aggregate
requirement of heat to carry it from germination to fruiting. Even
different varieties of one and the same plant differ materially in the
latter respect, so that it is very important that in the selection of
varieties to be grown, this factor should be taken into consideration.
It cannot be too strongly urged that the comparison of annual means of
temperature, so commonly made by promoters of colonization schemes,
must not be taken as a guide either in the estimate of cultures in
which the immigrant may desire to engage, or by those in search of a
climate adapted to their health-conditions.

_Seasonal and Monthly Means._—The statement of the mean temperatures of
the conventional four seasons—spring, summer, autumn and winter—afford
a much clearer conception of climatic adaptations; provided always
that the extreme temperatures be considered at the same time. With the
same understanding, the monthly means are still more instructive; but
here again, it is most essential that the distribution and amount of
rainfall in each be regarded at the same time, since the most desirable
temperature is of no avail without the moisture required for vegetation.

In some cases, _e. g._, that of California, it becomes necessary
for practical purposes to regard the “season,” and not the calendar
year, as the unit or reference for crop production. There the crops
depend upon what rainfall may occur from October to May, there being
no summer rains of agricultural significance, and outside of irrigated
lands, almost all vegetation save that of trees being in abeyance. In
India, there are two distinct growing seasons (“kharif” and “rabi”),
corresponding to the two “monsoon” seasons; and no matter how much rain
may fall during _one_, almost total failure may occur in other tropical
and arid sections of that country.

The _Daily Variations_ are of interest chiefly with respect to health
conditions, since most plants are more adaptable in this respect than
the average man.


RAINFALL.

_Distribution Most Important._—The summary statements of the annual
rainfall are almost equally as deceptive as are those of annual mean
temperature, since quite as much depends on the manner in which it
is distributed through the year, as upon its absolute amount; and
also upon the manner of its fall. Thus Central Montana has the same
aggregate annual rainfall as the country surrounding the Bay of San
Francisco, viz. about 24 inches; but while in the Franciscan climate
this amount of rain falls during one-half of the year, and that the
growing season, enabling crops to be grown without irrigation, in
Montana the rainfall is distributed over the entire season, so that
irrigation is absolutely essential for the successful production of
crops. This so much the more as, while the winter snowfall is very
light, the rains of summer are largely torrential, running off the
surface in muddy floods and giving little time for absorption into the
soil. Farther west, in Washington, where grain crops are largely grown
without irrigation, the sowing of winter grain is impracticable because
the dry summer is immediately followed by the very light snowfall of
winter, which falls on dry ground. Fall-sown grain would thus simply
lie dormant in the ground through the winter, with great liability
to injury from stress of weather in early spring, apart from the
depredations of birds and rodents. Hence grain is always sown there in
spring only.

These examples may suffice to show that summary statements of either
temperature or rainfall by yearly means are of little practical
interest to the farmer. What he needs to know is whether or not
sufficient rains to mature a full crop are likely to fall during the
time that the growing temperature prevails; and what are the minima and
maxima of temperature—heat and cold—that his crops will be called upon
to endure.


WINDS.

The third climatic factor mentioned, the winds, though proverbial for
their unreliability and inconstancy, are not only very incisive in
their action, but also to a considerable extent of very definite local
or regional occurrence and significance. Moreover, their occurrence,
direction, temperature and moisture-condition can, in regions whose
climatology has been reasonably well studied, be foretold with
sufficient accuracy to be of great use to the farmer.

_Heat the Cause of Winds._—As already stated, the primary cause of
all winds is heat, substantially on the principle according to which
draught is created in our domestic fires. The hot air rising creates an
indraught from all directions, especially from that which it can most
readily come; viz., from the ocean,[101] or from level lands, rather
than across mountain chains. Hence the sea-breeze in the after part
of the day, when the land has become heated; while the sea, requiring
a much larger amount of heat to change its temperature to a similar
extent, remains relatively cool. But at night the earth cools more
rapidly than the sea, by radiation; hence toward evening the sea-breeze
dies down, and toward and after sunset the land-breeze takes its place.

[101] A striking case in point is the regular wind which in summer
blows through the “Golden Gate,” a gap in the Coast Range connecting
the Pacific Ocean directly with the great interior valley of
California, along the bays of San Francisco, San Pablo, and Suisun. The
great interior valley and adjacent mountain slopes becoming intensely
heated during the rainless summer, the ascending air is replaced by
a steady indraught from the sea, which is bordered by a belt of cold
water causing fogs along the coast. The fogs are quickly dispelled on
reaching the edge of the valley near the middle of its length; whence
steady breezes blow northward and southward, up the valleys of the
Sacramento and San Joaquin respectively. These winds, popularly often,
but erroneously, called trade-winds, are really “monsoons” similar in
their origin to those of India, which, when coming from the sea cause
rains, but when from the heated land itself are hot and dry; as in the
case of the sirocco of Italy and North Africa, the terral of Spain and
the northers of California.

The principle of this local change of winds, together with the rotation
of the earth, the absorption of moisture by air, and the fact that the
latter becomes cooler when it expands on rising and warmer when it is
compressed by descending, serves to explain all the major phenomena
usually observed in connection with winds. The air of the equatorial
belt, heated throughout the year, necessarily rises and creates an
indraught from both north and south; but since the air thus flowing in
has a lower rotary velocity than the earth’s surface at the Equator,
it lags behind and so gives rise to northeast and southeast winds,
respectively, between the two tropics and the equatorial belt. These
regular winds, from the aid they give to commerce in passing from
continent to continent, are known as the _trade winds_. On the other
hand, the air that has risen from the hot equatorial belt, cooling by
expansion as it rises and flowing northward and southward from the
Equator, on descending as it mainly does into the temperate zones, has
a higher rotary velocity than the land-surface and so tends to give
rise to southwest and northwest winds in the northern and southern
hemispheres respectively. At sea, on coasts and in level inland regions
to windward of mountain chains, such winds are often quite regular
during a portion of the year.

_Cyclones._—But local disturbances arising from heated land areas or
mountain slopes, as well as wide atmospheric changes whose causes are
not fully understood, give rise to waves of alternating high and low
barometric pressure, largely converting rectilinear or slightly curved
wind-motion into whirls or “cyclones”[102] ranging from a thousand to
over two thousand miles in diameter. These in the case of low-pressure
waves or centers, _toward_ which the air flows from the _outside_,
revolve in the direction contrary to the movement of the hands of a
clock, and commonly produce rain in their east portion. A high-pressure
wave or center, _from_ which the air naturally flows toward the
_outside_, will usually bring about an “anti-cyclone” area with fair,
and in winter cold (“blizzard”) weather, the direction of the whirl
being, in this case, the reverse, or in the same direction as the hands
of a clock. Both cyclones and anti-cyclones move in North America from
west to east, mostly entering from the Pacific Ocean off the northwest
coast and traversing the continent with a slight southeast (or in the
case of cold weather almost south) trend, with a velocity of twenty
to thirty miles an hour; until upon reaching the region of the Great
Lakes they generally turn northeastward and pass into the Atlantic
Ocean from the New England and Canada coasts.—It is upon these general
facts, roughly outlined here, that the weather forecasts are in the
main based; taking into consideration, of course, the local or regional
conditions, topography, etc., which modify the application of the
general rules.

[102] This designation is popularly and incorrectly applied to the
comparatively limited, but very violent and destructive rotary storms
or whirlwinds which originate locally on the heated plains of the
Middle West of the United States, and are almost always accompanied
by violent electric phenomena. These should properly be called
_tornadoes_. At sea such whirlwinds give rise to waterspouts, in
deserts to sand storms.

In the southern hemisphere, the air-movements substantially correspond
to those observed in the northern, so far as not modified by mountain
chains; as is especially the case in South America.


INFLUENCE OF TOPOGRAPHY.

_Rains to Windward of Mountain Chains._—The surface features or
topography of the regions traversed by the air currents or winds may
materially modify both their direction and their physical condition,
especially as to moisture and temperature. Mountain chains may deflect
them, or, causing the air currents to rise on their slopes, and thus to
cool by expansion, the moisture these bring with them from the sea may
be partially, or sometimes almost wholly, deposited in the form of rain
or snow; chiefly on the windward slopes. Then, continuing across the
range, the air deprived of most of its moisture cannot readily yield
up more; hence the scarcity of rain—“arid climate”—under the lee of
mountain chains; as in the Great Basin between the Sierra Nevada and
Cascade ranges on the one hand and the Rocky Mountains on the other,
and also on the Great Plains under the lee of the latter. The abundant
rainfall between the Mississippi river and the Atlantic coast is due to
the moist winds coming from the warm waters of the Gulf of Mexico and
Caribbean sea, whose access is not interfered with by any cross-ranges
of mountains. But the Great Plains lying between the Mississippi and
the Rocky Mountains are not within the sweep of the Gulf winds, whose
trend is SW to NE; while they are equally out of reach of moisture
from the Pacific, all that having been successively deposited on the
intervening mountains; hence their deficient rainfall.

Northward of the temperate zone the rainfall generally decreases as we
approach the arctic regions; except where the influence of warm ocean
currents to windward creates comparatively local exceptions, as in the
case of Norway and Alaska.

[Illustration: FIG. 52.—Composite Curve showing distribution of
Rainfall in Europe, Africa and America projected on 100 Meridian W. L.]

_The general Distribution of Rainfall_ on the globe is well shown in
the annexed diagram, which is copied by permission of the author from
his treatise on the “Evolution of Climates,”[103] and represents the
mean deduced from data given in the Atlas of Meteorology by J. G.
Bartholomew. It is a composite curve derived from the consolidation
of four curves showing the distribution of rainfall, viz., on the
meridians of 20°E.L.; the west coasts of Europe and Africa; the 30th
meridian W.L., in the Atlantic Ocean; and the west coasts of North
and South America, projected on the plane of the 100th meridian W.L.
The latter curve corresponds with remarkable closeness to the mean
curve here given. “It is not intended that these curves should include
the rainfall upon meridians on which the distribution in belts is
interrupted by continental influences, and by the irregular oblique
belts of rain on the east coasts.” But it presents an admirable
generalization upon which, as a basis, the local disturbances may be
studied.

[103] “The Evolution of Climates”; by Marsden Manson, July, 1903; also
_Amer. Geologist_, Aug.-Oct. 1897.

It will be noted that the maximum of rainfall in the tropical rain-belt
lies several degrees to northward of the equator, owing doubtless to
the greater land area in the northern hemisphere. There is thus, on the
whole, a narrower belt of deficient rainfall or aridity between the
tropical and _northern_ temperate rain-belts, than in the _southern_
hemisphere. The southern temperate rain-belt touches only the extreme
ends of South America, Africa and New Zealand; elsewhere on the ocean
it has not been sufficiently observed as yet. The zones of rainfall and
aridity are, however, known to be subject to seasonal oscillations of
several degrees in latitude, owing to the obliquity of the plane of the
ecliptic, which shifts its position upon that of the equator.

_Ocean Currents._—Since water as a fluid is subject to the same
circulatory motions which cause winds, it is to be expected that
ocean currents should exist corresponding to those of the air, as
characterized in general above. But as water warms so much more slowly
than air, its circulation would be comparatively insensible were it
not for the effects produced by the air currents upon the surface of
the sea, combined, as in the case of the winds, with the effects of
the rotation of the earth. Without going into the details of the ocean
currents in the tropics, it may suffice to say that owing partly to the
moving and warming effects of winds, partly to the natural circulatory
motion of the water, two great warm currents flow from the tropics
northward, materially modifying what would otherwise be the climates of
the coasts they touch.

_The Gulf Stream._—The current most generally known is the Gulf Stream,
flowing partly from the Gulf of Mexico and the Caribbean Sea, partly
from outside of the same along the chain of the Lesser Antilles, along
the southeast coast of the United States (Florida, Georgia and South
Carolina); but owing to its greater rotational velocity it is soon,
like the winds of the same latitudes, deflected from a northward to a
NE. course, which carries it away from the American coast, to impart
some of its warmth, (probably mainly through the winds that blow over
it), to Great Britain and Ireland, Scandinavia, and Western Europe
generally; while the northern American coast is left to be bathed by
the icy polar current flowing from the Arctic through Baffin’s Bay,
which carries icebergs far to the south in the way of the transatlantic
traffic between the Eastern States and Europe, and causes a difference
in climate that is well exemplified in the comparison of the climate
of New York with that of Naples, both lying in the same latitude; and
similarly of the bleak coast of Labrador with that of Great Britain.

_The Japan Stream._—On the eastern Asiatic Coast, a warm current
originating in the Sunda seas, flows off the coasts of the Philippines
and of China and bathes the Japanese islands; hence it is known as
the Japanese Current, or Kuro-siro. It is partly this current which,
failing to pass into the Arctic through the shallow waters of Behring
strait, renders the coast climates of the northwest coast of America
so much milder and moister than is that in corresponding latitudes on
the east coasts of both continents. Alaska corresponds to Norway in
its moist, foggy and relatively mild coast climate; British Columbia,
Washington and Oregon participate in the benefits of the tempering
influence of the return current of the Kuro-siro. But as this return
(“Alaska”) current passes southward into the warmer seas off the
California coast, its influence is reversed; it becomes a cold current
in the warm waters of the Pacific, and the warm, moist air of the ocean
being carried by the westerly winds across this cold stream which flows
along the shore of California, in summer dense fogs are formed, which
render navigation difficult and produce a coast climate whose average
summer and winter temperatures (_e. g._ at San Francisco) may differ
by only a few degrees, viz., 15.5 and 13.0° C. (60 and 56° F.); so
that a change of clothing from season to season is hardly called for.
The Alaska Current leaves the immediate coast of California off Pt.
Conception near Santa Barbara, gradually losing itself southwestward,
but still tempering the tropical heat in the Hawaiian Islands. Hence
the coast climate is much warmer and less foggy in southern California;
but throughout the State in the interior valleys, screened from the
coast winds by the Coast ranges, the temperature in summer may rise
several degrees above 100° F. for days together; although, owing to the
dryness of the air, the heat is not oppressive.

_Contrasting Climates in N. W. America._—An even more striking
contrast, showing the effects of the warm ocean and air currents, when
intercepted by mountain chains, exists on the Pacific coast farther
northward, as already mentioned. In Oregon and Washington first the
low Coast ranges, and then the higher Cascade mountains, obstruct
the eastward progress of the westerly ocean winds. The result is a
very heavy rainfall to coastward of and within the Coast ranges, and
an almost equally heavy precipitation on the western slope of the
Cascades. Standing on the crest of the latter in summer, one may see to
westward a rolling sea of clouds, causing almost daily rains; while to
eastward the eye ranges over brown or whitish, dusty plains or rolling
lands, almost destitute of tree growth and quivering with heat, under a
deep blue sky untroubled by clouds for months.

A somewhat similar contrast is seen in the Hawaiian islands, which are
in the sweep of the subtropical northeast trade winds, and on their
windward (eastern) slopes have abundant rains; while on the leeward
slopes an almost arid climate prevails, calling for extended irrigation.

_Continental, Coast and Insular Climates._—From what has been said
above, the striking differences of climate caused by the position of
any region with reference to the sea or other large bodies of water
on the one hand, and to mountain chains on the other, can be readily
understood; provided of course that the direction of the winds and the
trend of the mountain chains be properly taken into consideration.
Western coasts in the temperate and subtropical regions will have a
relatively even, temperate and moist climate as compared with the
interior of continents, from which the tempering influence of the
sea is cut off by mountain chains. Where no such chains intervene
the coast climate may extend far inland. The latter case is that of
Europe, where the prevailing westerly winds, warmed by the Gulf Stream,
temper the climate as far east as the borders of Russia, and northward
to Norway; while to southward the warm waters of the Mediterranean and
Black seas temper both heat and cold in Spain, southern France, Italy
and the Mediterranean border generally. But to eastward, in Russia and
Siberia, the climate becomes “continental” to an extreme degree, with
very cold winters and very hot summers. The same is true of interior
North America, wherever the continental divide cuts off the tempering
influence of the westerly winds; Montana, the Dakotas and the Great
Plains states generally being examples. The climate of the Mississippi
valley, as stated before, is tempered by the winds blowing from the
Gulf of Mexico, but with occasional irruptions of the continental
climate (sometimes reaching as far east as the South Atlantic coast)
in the forms of cold “blizzards,” from which the coast climates of
the Pacific and of western Europe are practically free. The Atlantic
coast of North America (including the coast of the Gulf of Mexico),
moreover, not unfrequently suffers from the violent cyclonic storms
that originate in the Antilles and follow more or less the direction of
the Gulf Stream.

Islands, differing from continents mainly in their extent, and having
a relatively large proportion of coast, naturally have climates
controlled essentially by the surrounding ocean. The insular or oceanic
climates are therefore, as a rule, more temperate and even than are
those of the nearest mainland. It is often said that the climate of
western Europe is “insular”; and owing to its position under the lee of
the Gulf Stream, this is eminently true of Great Britain.

_Subtropic Arid Belts._—Where the surface features of the land in
relation to the ocean and prevailing winds do not interpose special
obstacles, we find to poleward of both tropics a climatic belt of
greater or less width, in which the annual, or at least the summer
rainfall is too small to maintain annual herbaceous vegetation
throughout the season, even when the temperature is favorable. These
two “arid” belts are best defined in Africa, where the northern one is
represented by the Sahara desert, lower Egypt and Arabia, while the
southern one is exemplified in the Kalahari desert, to northward of
the Cape of Good Hope. The northern belt is continued into Asia Minor,
Palestine, Syria and Persia, and is again manifest in northwestern
India; but to eastward is stopped by the influence of the great
Himalaya range. The plateau countries beyond, in Central Asia, are
extremely arid, largely by reason of their high elevation.

In Australia the southern arid belt is very strongly defined. In North
America, the arid belt is characteristically defined on the Pacific
Coast. It embraces all but the southernmost point of the peninsula of
Lower California, with about two-thirds of the State of California;
thence eastward across Sonora and Arizona to New Mexico and western
Texas. But here the influence of the mountain ranges and high plateaus
obscures the subtropical belt as such, the arid climate continuing,
east of the great Pacific ranges, through Nevada, Utah, Wyoming,
Montana, Idaho, and eastern Oregon and Washington nearly to the line of
British Columbia on the north, and with gradually decreasing aridity,
into Colorado, Kansas, Nebraska, and the Dakotas.

In South America the rainless seaward slopes of southern Peru and
northern Chile indicate the southern arid belt; but here, the great
chain of the Andes intervening, the dry pampas of Argentina, and the
Gran Chaco of southwestern Brazil, like the Nevada basin, though arid
would naturally be referred to the moisture-condensing influence of
the Andes chain, under the lee of which they lie. From this cause
the region of deficient rainfall, which on the western coast ends to
northward of Santiago de Chile, is east of the Andes continued much
farther poleward, as in North America; reaching into Patagonia.

_Utilization of the Arid Belts._—While, as already explained, the
distribution of the rainfall through the year is nearly as important
as its total amount, yet it is evident that even with the minimum
of twenty inches of total precipitation as the measure for crop
production, a very large proportion of the land of the arid region
cannot, even with the most elaborate system of water conservation,
be supplied with sufficient water for ordinary crops, and must be
otherwise utilized, mainly for pasture purposes. This is rendered
practicable to a much greater extent than might be expected, because
the rapid transition from the rainy to the permanent dry season cures
the standing herbage into hay, which affords good grazing during the
rainless season. Moreover, the use of drought-resistant, browsing
forage plants, both shrubs and trees, serves to supplement materially
any deficiency in the supply of “standing hay,” especially in case the
rains should toward the end be unduly delayed. The same is true of
the dried pods and seeds of native herbage, which in some cases (bur
clover, lupins, etc.,) afford highly nutritious additions to the leafy
forage.[104]

[104] See Rept. of the U. S. Commissioner of Agriculture for 1878, pp.
486-488; Bull. Nos. 16 and 42, Wyoming Expt. Station; Bull. No. 150
Calif. Expt. Station; Bull. No. 51, Nevada Expt. Station; South Dakota
Station Bulletins Nos. 40, 69, 70, 74; Kansas Expt. Station, Bulletin
No. 102; New Mexico Expt. Station, Bulletin No. 18; Montana Expt.
Station, Bulletin No. 30; and others.




CHAPTER XVII.

RELATIONS OF SOILS AND PLANT GROWTH TO HEAT.


_The Temperature of Soils._—The rapid germination of seeds, as well
as the development of plants to maturity, is essentially dependent
upon the maintenance of the appropriate temperature. The temperature
most favorable to germination or growth, as well as the degree of
tolerance of high and low temperatures, varies greatly with different
plants, governing mainly what is known as their climatic adaptation.
A knowledge of these points with reference to the several crops is
therefore of no mean importance to the farmer; for, to a certain
extent, he can control the temperature in the soil itself, and he can
mostly choose for sowing and planting, the time when the soil shall
have the proper temperature for rapid germination or maturity. As a
rule, it is not desirable to have either seeds or seedling plants in
the ground for any length of time when the temperature is too low for
active vegetation; for while they rest, other, lower organisms (fungi
and bacteria), adapted to low temperatures, may continue in full
activity at the expense of the vitality of the crop plant.

_Water exerts controlling Influence._—Since the capacity of water for
heat is approximately five times greater than that of the average
soil, equal weights being considered, it follows that the temperature
of soil-water must exert a controlling influence over that of the
soil. Taking the case of a cubic foot of loamy soil, fully saturated
with water, in which one-third of the volume may be assumed to be
water: the weight of the dry soil being about eighty pounds per cubic
foot, calculation shows that the amount of heat required to raise the
temperature of the water contained, one degree, will be fully twice
as great as for that required for the soil itself. It is thus obvious
that the control of soil-temperature is largely dependent upon the
control of the water-content of the same, which has been discussed in
a former chapter. Even in the condition of moisture known to be most
favorable to plants, viz., one-half of the maximum water capacity, the
influence of the water-content upon the temperature will still be as
great as that of the entire soil mass. This consideration emphasizes
the importance of such control.

_Cold and Warm Rains._—It is not surprising then that the occurrence
of cold or warm rains or the use of cold or warm irrigation water at
critical periods, may largely determine the success or failure of
the crop. It is well known that the occurrence of a cold rain after
vegetation has started actively in early spring, may not only destroy
the season’s fruit crop by preventing the setting, or thereafter
causing the dropping, of the fruit, but may even, if the suppression
of vegetative action be continued for some length of time, result
in serious injury to, or death of trees. Widely extended disastrous
experience of the kind was had in California in February and March,
1887, resulting in the death of tens of thousands of fruit trees and
vines during that and the following season. It is obvious that in
such a case as this the rapid draining-off of the cold water through
underdrains would have materially mitigated, if not wholly prevented,
such injury.

_Solar Radiation._—Aside, however, from such overwhelming influences
as the above, the soil temperatures are measurably controlled by the
extent to which they receive and absorb the sun’s heat rays, whether
directly or through the mediation of the air. The direct effect of the
sun’s rays upon the surface is, upon the whole, the most generally
potent, although warm winds may occasionally exert a very strong
influence. The varying influence of the sun’s rays depends primarily
upon the change of seasons, which themselves result from the varying
angles at which the sun’s rays strike the surface; as well as upon the
duration of the day. The greater or less cloudiness or fogginess of the
sky, of course, exerts a decided effect in this connection.

_The Penetration of the Sun’s Heat into the Soil._—In the temperate
regions of the earth the daily variations of temperature cease to be
felt at depths ranging from two to three feet, according to the nature
of the soil material and its more or less compacted condition. The
monthly variations, of course, reach to greater depths; while the
annual variations do not disappear in the temperate zone, _e. g._, at
Paris, Zürich and Brussels, at a less depth than seventy-five feet.
At these depths of constant temperature we find approximately the
same temperature as that which we can deduce from the thermometric
observations as the annual mean. From similar causes the mean annual
temperature of any place may be approximately deduced from the
observation of the water of wells and springs derived from moderate
depths. For below the level of constant annual temperature the latter
begins to ascend steadily as we progress downward, owing to the
interior heat of the earth.

_Change of Temperature with Depth._—The following table of observations
made at Brussels illustrates the decrease of annual range of
temperature with depth:

                    Average       Annual
    At feet:     temperature.     range.
                     (°C.)        (°C.)
      3.25            7.2          10.5
     15.6            13.5           4.5
     30.8            16.4           1.3
     75.0            17.0           0.0

It is interesting to compare with this record that of a well sunk by
Ermann at Yakutzk, Siberia, where the mean annual temperature is—9.7 C.
(14.6 F.). This temperature was found a few feet below the surface. At
50 feet the temperature was—7.2 C. (19° F.); at 145 feet—5° Cent. (23°
F.) at 350 feet—.9° C. (30.8° F.) showing that the ground was below the
temperature of freezing water for some distance farther down; so that
the search for liquid water was abandoned.

We thus see that in the Arctic regions, owing to the presence of water
in the form of ice, the melting of which impedes the access of solar
heat, the level of no variation is found at the distance of a few feet
below the surface, despite the great variations in temperature between
the short but hot summer and the extremely cold winter. In the tropics,
also, the annual temperature-variation disappears at a less depth than
2 feet, in consequence of the very slight difference between the two
seasonal extremes of temperature.

_Surface—Conditions that influence Soil-Temperature._ Among these,
_color_ has already been mentioned, and to a certain extent discussed.
While it is true that, broadly speaking, dark-colored soils absorb more
of the sun’s heat than light-colored ones, other things being equal, it
must still be understood, that the nature of the color-giving substance
exerts a very material influence upon the amount of heat absorbed. Thus
charcoal is among all known substances the one absorbing and radiating
the sun’s heat rays most powerfully, and all kinds alike; so much so,
that its absorbent power is taken as 100. But other substances which
to the eye appear equally black, have by no means the same absorbing
power. The heat absorption by black humus is high, though not quite
equal to that of charcoal; and many gray soils, though appearing to
the eye of rather light tint, really absorb more heat than others
which, to our perception, have the darker tint, but are colored by
other substances. Gardeners and especially vine growers in the colder
portions of Europe often take advantage of the powerful absorbing power
of carbon by spreading charcoal or black slate powder over the surface
of the soil where early maturity is specially desired; and slate powder
is similarly used by the peasants at Chamouni to hasten the melting of
the snow.

_Heat of High and Low Intensity._—It must also be kept in view that
the surfaces, and especially the colors that favor absorption of the
intense rays of the sun, may comport themselves quite differently
toward heat rays of low intensity, such as those thrown back from the
soil at night when it cools. Were this otherwise, a soil that absorbs
much heat in the daytime would lose it with corresponding rapidity at
night. But this is true only of charcoal; in the case of most other
substances, there is a material difference in favor of the retention of
the heat, of low intensity, by slower radiation into a “heat-trapping”
atmosphere.

_Reflection vs. Dispersion of Heat._—Theoretically, a smooth surface
reflects more heat than a rough one, and warms much more slowly by
absorption; as is strikingly shown by the use of polished metal screens
placed on walls to prevent their being overheated by a flue near by.
In the case of soils, also, the condition of the surface affects
materially the absorption of heat, but not in accordance with the
above rule so far as the result is concerned. For it is found that,
other things being equal, a loose or cloddy surface disperses in many
directions the heat it receives, and does not permit it to penetrate
by conduction to so great extent as would a more compact soil, whose
smooth surface would waste less of the heat received by radiation.

    King has called special attention to the difference of
    temperature existing between soils smoothed and compacted by
    a roller, and the unrolled soil having a loose surface. He
    found that the former at a depth of one and a half inches
    was as much as 5.5°C. (10°F.) warmer than the loose soil,
    and that even at a depth of three inches a difference as
    high as 3.5°C. (6.5°F.) existed between the two. He observed
    at the same time that the temperature of the air over the
    unrolled ground was considerably warmer than above the
    rolled, thus corroborating the differences observed in the
    soil itself. But at night the heat is given out more rapidly
    from the rolled than from the unrolled surface, the latter
    acting as a non-conductor and keeping the soil warmer than
    that of the more compact rolled land. King gives as the
    average difference observed between rolled and unrolled land
    on eight Wisconsin farms, 1.6°C. (3°F.) in favor of the
    rolled land between 1 and 4 p.m.

It will thus be seen that the loose tilled layer, while impeding the
penetration of the sun’s heat into the deeper portions of the soil
during the day, on the other hand serves to retain it at night better
than a more compact soil. This obviously places it within the power of
the farmer to exert considerable control over the soil-temperature at
critical times; restraining or favoring the access of the sun’s heat in
accordance with the requirements of the climate or season, as the case
may be.

_Influence of a Covering of Vegetation, and of Mulches._—A cover of
either living or dead vegetation depresses the temperature of the soil
as compared with the bare land, as elaborately shown by Wollny and
Ebermeyer. In the monthly averages these differences rarely exceed
.8° C. (1.5 degrees F.), and are mostly below .50° C. (1° F.), but
during different parts of the day they may rise to 2.2 to 2.5° C. (4
to 4.5° F.), at 4 inches depth. In summer they are greater than at
other seasons. Of course the density of the vegetation or the thickness
of the mulch influences them materially. Forests exert the greatest
cooling influence upon the soil, and next to these the dense herbaceous
crops, such as clover, and the legumes generally.

_Influence of the Nature of the Soil-Material._—Aside from the
surface condition, the nature of the material itself exerts a certain
influence, not only upon the rate of introduction of heat, but also
upon the amount taken up. Thus quartz sand having the highest density
(greatest weight per cubic foot) and also the highest capacity for
heat among the usual mineral soil-ingredients, will, mass for mass,
experience a smaller rise of temperature than would clay or loam soil,
of less density or volume-weight, and also of lower heat-capacity.
While this holds good theoretically, differences corresponding to
this consideration rarely occur in nature, for the reason that the
much greater influence of the mechanical condition of the soil mostly
overbalances these effects. Thus Wollny has shown that while quartz
is a better heat-conductor than clay, quartz cobbles or gravel will
materially increase the temperature of the soil in which they are
imbedded. Yet compact clay is a better conductor of heat than loose
sand; hence the latter, when exposed to the intense heat of the summer
sun in the desert, becomes intensely hot on the surface, yet allows of
the existence of abundant moisture at a depth of ten or twelve inches;
while clay in the same region, being usually in a compacted condition,
will show a lower surface-temperature and will be warmer and drier
at a depth at which sand will still retain abundant moisture, and be
comparatively cool (See chap. 13, p. 257.) So much indeed depends upon
the state of mechanical division and flocculation in which the several
soils may happen to be, that a hard-and-fast statement in regard to
their relations to heat cannot and should not be given, as it would
only lead to disappointments and practical mistakes; the more as in all
cases the moisture-condition exerts an influence predominating by far
over that of the dry material itself, and this moisture-condition is
subject to rapid changes, owing to intrinsic differences in the several
classes of soils. Wollny states as the result of his experiments, that
in summer sandy soils are warmest; then humous, lime and loam soils;
while in winter humous soils are warmest, then loams; and sand coldest.

_Influence of Evaporation._—In treating of the Conservation of
Soil Moisture (chapter 13), the effects, conditions and control of
evaporation from the soil have already been discussed from several
points of view; so that a summary review of the subject must suffice in
this place.

It has been stated above that in the case of an average loam soil
saturated with water, the heat required to raise the temperature of the
water one degree would be about twice that needed to so change the dry
soil material itself. But if it is required to _evaporate_ the same
amount of water from the soil, nearly ten (9.667) times that amount of
heat will be required; or in the case assumed, twenty times as much
as would suffice to raise the temperature of the dry soil through an
equal interval of temperature. While in a few cases the cooling of the
soil by evaporation is desirable, in the vast majority of cases it is
injurious to the progress of vegetation, and should be restricted as
much as possible by the means outlined in a former chapter.

_Formation of Dew._—There is, however, another aspect of evaporation
from the soil which has been long misunderstood, although the true
state of the case was partially recognized long ago. Dew is in common
parlance said to “fall,” it being supposed that, like rain, it is
derived from the atmosphere. While this is partially true, inasmuch
as from very moist, and notably from foggy air dew is frequently
deposited on grass and foliage generally, as well as on wood and
other strongly heat-radiating surfaces; yet as a matter of fact, in
by far the majority of cases, as shown by H. E. Stockbridge[105] and
confirmed by everyday observation, dew is formed from the vapor rising
from the warmer soil into a colder atmosphere, and condensed on the
most strongly heat-radiating surfaces near the ground, such as grass,
leaves both green and dry, wood, and other objects first encountering
the rising vapor. In manifest proof of this it will be noted that very
heavy dews may be seen on the ground, when the roofs of houses as well
as the higher shrubs and trees remain perfectly dry. In winter this
may be most strikingly seen in the deposition of hoar-frost in and
immediately around the cracks of plank sidewalks, whose surface remains
dry.

[105] “Rocks and Soils,” pp. 175-189.

_Dew rarely adds Moisture._—Candid observations will convince any one,
therefore, that in most cases the supposed addition to the moisture of
the soil from dews is an illusion. Whatever dewdrops fall on the ground
are in general simply the return to the soil of a part of what came
from it; while the dew that evaporates from the bedewed leaves or other
objects represents simply a delayed outgo of moisture from the soil,
which for a time retards evaporation direct from the soil, and thus
effects a slight saving of moisture.

    But while this is measurably true of inland and especially
    of continental areas like the great plains of North and
    South America, it is also true that in deep moist valleys,
    and on the rainy and foggy coast regions of continents,
    dews are found to both fall and rise, not uncommonly to
    such an extent as to be equivalent to a not inconsiderable
    aggregate precipitation. Thus in the moist coast belt of
    Oregon and Washington lying west of the Cascade range of
    mountains, the morning dews of summer are frequently so
    copious that the water falls in showers from the lower trees
    and shrubs, so as to necessitate the use of water-proof
    clothing when traversing the woods in the morning, quite as
    much as though rain was actually falling. In hilly and more
    especially in mountainous regions the cold air descending
    from above and flowing down in the ravines will often cause
    a heavy condensation of dew in these, while the bordering
    ridges, which rise above the cold currents, remain free from
    dew. These descending currents as a rule not only bring no
    surplus moisture with them, but in their downward course
    become warmer by contraction and therefore relatively drier.
    In these cases also, therefore, the dew is purely moisture
    derived from the ground, which in rising encounters the cold
    air and is thus condensed.

The fact that dew is most commonly derived from the soil could have
been foreseen from the other fact, long ascertained and known, that
during the night the soil is as a rule warmer than the air above it; as
has been shown by the earlier observers, as well as more specifically
by Stockbridge.

_Dew within the Soil._—It is obvious that whenever dew is formed above
the surface of the soil, the air within the latter must be at or near
its point of saturation with vapor, as in fact is usually the case
a few inches below the surface. It follows that when a depression
of temperature occurs within the soil, _e. g._, at night, dew must
be deposited within the soil down to the depth to which the nightly
variation reaches, increasing at that depth as the vapor from the
warmer soil below rises, to be in its turn condensed. There is thus
formed at that level a zone of greater moisture, which may sometimes
be noted in digging pits, by a deeper tint, without any corresponding
variation in the nature of the soil. The daily repetition of this
process, at varying depths, and its greater or less recurrence at or
near the limit-levels of monthly and even annual variations, must
exert a not inconsiderable influence upon the vertical distribution
of moisture in the soil; which instead of being usually found in
horizontal bands or zones of varying moisture-contents, is usually
remarkably uniform for considerable depths, despite the fitfully
recurrent additions from rains. It is at least probable that this
process of dew-formation within the soil materially assists capillarity
in effecting a measurably uniform vertical distribution of moisture.
(See also page 207, chapter 11).

_Plant-development under different Temperature Conditions._—In the
arctic regions the ground, frozen in winter to unknown depths, may
thaw to only three to five feet during the summer, notwithstanding
the great length and continuous sunshine of the arctic day. The
shallow-rooted arctic flora develops very rapidly under the influence
of the continuous daylight and heat, in the course of from five to
eight weeks. The seeds of these plants must, of course, be capable of
germinating at very low temperatures; and as a matter of fact, we find
that both in the arctic regions and in the higher mountains, certain
plants are found growing and blooming on slopes flecked with snow; each
plant surrounded by a small circle of bare ground, where the snow has
been melted under the influence of the dark-tinted earth and leaves.
It is clear that here germination has occurred, the foliage has been
formed, and the roots have been exercising their vegetable functions,
in ground soaked with water practically ice-cold.

_Germination of Seeds._—While wild plants of special adaptation may
thrive in very low (or high) temperatures, it is also true that few of
our cultivated plants will germinate, and still less grow thriftily,
at such low temperatures. The limit below which most cultivated plants
may be considered as remaining practically inactive lies between
4.4 and 7.2° C. (40 and 45° F.). Few tropical plants will germinate
much below 23.8° (75° F.) and in some cases not below 35° Cent. (95°
F.). Even maize and pumpkins, according to Haberlandt, germinate
most rapidly between 35 and 38.3° C. (95 and 101° F.), while for
wheat, rye, oats and flax the best temperature for germination lies
between 21.1 to 26.1 (70 to 79°). Under the most favorable conditions
of temperature and moisture, some small seeds which readily absorb
moisture will germinate in from twenty-four to forty-eight hours, while
at a lower temperature they may require from three days to two weeks.
Thus Haberlandt found that while oats would germinate in two days at
a temperature of 17.2 to 17.5° C. (63° to 63.5°), it took a full week
for germination when the temperature was only 5° C. (41° F.). It is
obvious that seeds remaining inert in the soil for such lengths of time
will be subject to a variety of vicissitudes that may injure or destroy
their vitality. There are many bacteria and fungous parasites which at
low temperatures are perfectly capable of attacking and destroying the
water-soaked seed. There is thus for each plant, from the lowest to the
highest, a certain temperature most favorable to development; and both
above and below this, the vegetative activity is seriously interfered
with or wholly checked. A knowledge of these limits is manifestly of
the utmost practical importance.

    The influence of too high a temperature in preventing the
    germination of cinchona seed from India, was curiously
    exemplified when it was subjected to a supposedly favorable
    steady temperature of 23.8° C. (75° F.) under otherwise most
    favorable conditions. Not a single one came up in the course
    of six weeks, and the box in which it had been sown was put
    away outside of the hothouse as a failure. Within two weeks
    a full stand of seedlings was obtained, at temperatures
    ranging between 12.7 and 15.5° C. (55° and 60° F.). The fact
    that the cinchona is a tree of the lower slopes of the Andes
    (three to five thousand feet) although at home strictly
    within the tropics, explains the apparent anomaly.




PART THIRD.

CHEMISTRY OF SOILS.




CHAPTER XVIII.

THE PHYSICO-CHEMICAL INVESTIGATION OF SOILS IN RELATION TO CROP
PRODUCTION.


The chemical constituents of soils have been incidentally mentioned
and discussed above, both in connection with the processes of
soil-formation, and with the minerals that mainly participate therein.
The manner of their occurrence and their relations to plant life, so
far as known, must now be considered more in detail.


HISTORICAL REVIEW OF SOIL INVESTIGATION.

While the obvious importance of the physical soil-conditions has long
ago rendered them subjects of close study by Schübler[106] Boussingault
and others, the chemistry of soils was very generally neglected for a
considerable period, after the hopes at first entertained by Liebig
that chemical analysis would furnish a direct indication and measure
of soil fertility, had been sorely disappointed in respect to the only
soils then investigated, viz., the long-cultivated ones of Europe. The
results of chemical analysis sometimes agreed, but as often pointedly
disagreed, with cultural experiences; so that after the middle of the
nineteenth century, but few thought it worth while to occupy their time
in chemical soil analysis.

[106] The early work of Schübler on soil physics, published at Leipzig
in 1838 under the title of “Grundsätze der Agrikulturchemie” and now
almost inaccessible outside of old libraries, is remarkable as having
anticipated very definitely much that has since been brought forward
and elaborated anew. He is really the father of agricultural physics.

_Popular Forecasts of Soil Values._—In newly-settled countries, and
still more in those yet to be settled, the questions of the immediate
productive capacity, and the future durability of the virgin land are
the burning ones, since they determine the future of thousands for
weal or woe. This need has long ago led to approximate estimates made
on the part of the settler, by the _observation of the native growth,
especially the tree growth_; and where this consists of familiar
species, normally developed, such estimates on the part of experienced
men, based on previous cultural experience, are generally very
accurate; so much so that in many of the newer states they have been
adopted in determining not only the market value, but also the tax rate
upon such lands, their productiveness, and probable durability being a
matter of common note.

    Thus in the long-leaf pine uplands of the Cotton States, the
    scattered settlements have fully demonstrated that after
    two or three years cropping with corn, ranging from as much
    as 25 bushels per acre the first year to ten and less the
    third, fertilization is absolutely necessary to farther
    paying cultivation. Should the short-leaved pine mingle with
    the long-leaved, production may hold out for from five to
    seven years. If oaks and hickory are superadded, as many
    as twelve years of good production without fertilization
    may be looked for by the farmer; and should the long-leaved
    pine disappear altogether, the mingled growth of oaks and
    short-leaved pine will encourage him to hope for from twelve
    to fifteen years of fair production without fertilization.

Corresponding estimates based upon the tree growth and in part also
upon minor vegetation, are current in the richer lands also. The
“black-oak and hickory uplands,” the “post-oak flats,” “hickory
bottoms,” “gum bottoms,” “hackberry hammocks,” “post-oak prairie,”
“red-cedar prairie,” and scores of other similar designations, possess
a very definite meaning in the minds of farmers and are constantly used
as a trustworthy basis for bargain and sale, and for crop estimates.
Moreover, experienced men will even after many years’ cultivation be
able to distinguish these various kinds of lands from one another.

_Cogency of Conclusions based upon Native Growth._—Since the native
vegetation normally represents the results of secular or even
millennial adaptation of plants to climatic and soil-conditions, this
use of the native flora seems eminently rational. Moreover, it is
obvious that if we were able to interpret correctly the meaning of such
vegetation with respect not only to cultural conditions and crops, but
also as regards the exact physical and chemical nature of the soil, so
as to recognize the _causes_ of the observed vegetative preferences; we
should be enabled to project that recognition into those cases where
native vegetation is not present to serve as a guide; and we might
thus render the physical and chemical examination of soils as useful
practically, _everywhere_, as is, locally, the observation of the
native growths. To a certain extent, such knowledge would be useful
in determining the salient characters of cultivated soils, also; and
would be the more useful and definite in its practical indications the
more nearly the cultural history of the land is known, and the less the
latter has been changed by fertilization. For, so soon as the first
flush of production has passed, the question of how to fertilize most
effectually and cheaply demands solution.

It was from this standpoint, suggested by his early experience in the
Middle West and subsequently most impressively presented to him in the
prosecution of the geological and agricultural survey of Mississippi,
that the writer originally undertook, in 1857, the detailed study of
the physical characters and chemical composition of soils. It seemed
to him incredible that the well-defined and practically so important
distinctions based on natural vegetation, everywhere recognized and
continually acted upon by farmers and settlers, should not be traceable
to definite physical and chemical differences in the respective lands,
by competent, comprehensively-trained scientific observers, whose field
of vision should be broad enough to embrace concurrently the several
points of view—geological, physical, chemical and botanical—that must
be conjointly considered in forming one’s judgment of land. Such
trained observers should not merely do as well as the “untutored
farmer,” but a great deal better.

_“Ecological” studies._—Yet thus far we vainly seek in general
agricultural literature for any systematic or consistent studies of
these relations. We do find “ecological” lists of trees and other
plants, or “plant associations,” growing in certain regions or land
areas, described in some of the general terms which may refer equally
well to lands of profuse productiveness, or to such as will hardly
pay for taxes when cultivated. Or when the productive value is
mentioned, the probable cause of such value is barely alluded to, even
conjecturally, unless it be in describing the “plant formations” as
xerophytic, mesophytic or hydrophytic, upon the arbitrary assumption
that moisture is the only governing factor; wholly ignoring such
vitally important factors as the physical texture of the soil, its
depth, the nature of the substrata, and the (oftentimes abundantly
obvious) predominant chemical nature of the land. And on the other
hand, we find even public surveys proceeding upon the basis of physical
data alone, practically ignoring the botanical and chemical point of
view, and inferentially denying, or at least ignoring, their relevancy
to the practical problems of the farm.[107]

[107] Bull. 22, Bureau of Soils, U. S. Dept. of Agriculture.

_Early Soil Surveys of Kentucky, Arkansas and Mississippi._—Among
the few who during the middle of the past century maintained their
belief in the possibility of practically useful results from direct
soil investigation, were Drs. David Dale Owen and Robert Peter, who
prosecuted such work extensively in connection with the geological and
agricultural surveys of Kentucky and Arkansas; and the writer, who
carried out similar work in the states of Mississippi and Louisiana,
with results in many respects so definite that he has ever since
regarded this as a most fruitful study, and has later continued it in
California and the Pacific Northwest. This was done in the face of
almost uniform discouragement from agricultural chemists until within
the last two decades; with occasional severe criticisms of this work as
a waste of labor and of public funds.

_Investigation of Cultivated Soils._—All this opposition was largely
due to the prejudices engendered by the futile attempts to deduce
practically useful results from the chemical analysis of _soils long
cultivated_, without first studying the less complex phenomena of
_virgin_ soils; and these prejudices persisted longest in the United
States, even though in Europe the reaction against the hasty rejection
of chemical soil work had begun some time before; as is evidenced by
the methods employed at the Rothamsted Experimental Farm in England,
the Agricultural College of France, the Russian agronomic surveys, and
at several points in Germany. In none of these cases, however, more
than the purely chemical or physico-chemical standpoint was assumed;
although in Russia at least, virgin soils were easily obtainable and
their native growth verifiable; and were actually in part made the
subject of chemical investigation.

In the course of their work, Owen and Peter always carefully recorded
the native vegetation of the soils collected; but neither seems to
have formulated definitely the idea that such vegetation might be made
the basis of direct correlation of soil-composition with cultural
experience. Owen repeatedly expressed to the writer his conviction that
such a correlation could be definitely established by close study; but
early death prevented his personal elaboration of the results of his
work. Peter likewise stoutly maintained to the last his conviction that
soil analysis was the key to the forecasting of cultural possibilities;
but not being a botanist he did not see his way to put such forecasts
into definite form.


_Change of Views._

In the United States as well, the ancient prejudices have now gradually
given way before the urgent call for more definite information than
could otherwise possibly be given to farmers by the experiment
stations, most of whose cultural experiments, made without any definite
knowledge of the nature of the soil under trial, were found to be
of little value outside of their own experimental fields. Even the
multiplication of culture stations in several states, unaccompanied
by soil research, is found to be a delusive repetition of the same
inconclusive, random experimenting, since it takes into consideration
only the climatic differences, but leaves out of consideration the
potent factors of soil quality and soil variations. At most these were
usually mentioned by them in such indefinite terms as “a clay loam,”
“a coarse sandy soil,” “gray sediment land,” and the like; frequently
not even with a statement of the depth and character of the subsoil and
substrata, much less of their geological derivation or correlations.
Thus any one not happening to be personally acquainted with the land
in question would be wholly without definite data to correlate the
results with his own case. It is quite obvious that even if only to
make possible the identification of new lands with others that have
already fallen under cultural experience, and can therefore afford
useful indications to the new settler, a close physical and chemical
characterization of lands should be made the special object of study by
the experiment stations and public surveys, particularly in the newer
states.

_Advantages for Soil Study offered by Virgin Lands._—Among the special
advantages, then, offered by virgin soils for the study of the
correlations of soils and crops, the usual existence of a native flora,
representing the results of secular adaptation, is of fundamental
importance. As it is at this time still historically known of most
lands west of the Alleghenies what was their original timber growth,
it is clear that their original condition can still be ascertained
by comparison with uncultivated lands of similar growth, usually not
very far away; and as their cultural history also is largely within
the memory of the living generation, the behavior of such lands under
cultivation is known or verifiable. Foremost among the data thus
ascertainable is the _duration of satisfactory crop production, and
its average amount_. To ascertain these surviving data by inquiry
among the farming population should be among the foremost duties of
those connected with soil surveys; and persons temperamentally unable
to enlist the farmer’s sympathy and interest in such inquiries must
be considered seriously handicapped, no matter what their scientific
qualifications may be. In no quest is it more literally true that there
is no one from whom the earnest inquirer may not learn something worth
knowing.

_Practical Utility of Chemical Soil-Analysis; Permanent Value_ vs.
_Immediate Productiveness_.—In many existing treatises so much
emphasis is given to the alleged proofs of the inutility of chemical
soil examination in particular, that a special controversion of these
arguments seems necessary, in connection with a detailed statement
of what can, and in part has been, done in that direction. Hence the
often-repeated allusion, in the sequel, to points bearing on this
question. Hence, also, the detailed discussion of many points which in
most agricultural publications are given only passing notice.

_In all these discussions the difference between_ the ascertainment of
the _permanent-productive value of soils_, as against that of their
_immediate producing capacity_, must be strictly kept in view. The
former interests vitally the permanent settler or farmer; the latter
concerns the immediate outlook for crop production, the “Düngerzustand”
of the Germans. The methods for the ascertainment of these two factors
are wholly distinct, even though the results and their causes are
in most cases intimately correlated. The failure to observe this
distinction accounts for a great deal of the obloquy and reproach that
has in the past so often been heaped upon chemical soil-analysis and
its advocates.


PHYSICAL AND CHEMICAL CONDITIONS OF PLANT GROWTH.

While it is true that plants cannot form their substance or develop
healthy growth in the absence or scarcity of the chemical ingredients
mentioned on page xxxi of this volume, it is also true that they cannot
use these unless the physical conditions of normal vegetation are first
fulfilled. Both sets of conditions are intrinsically equally important
and exacting as to their fulfilment; and the farmers’ task is to bring
about this concurrence to the utmost extent possible. The chemical
ingredients of plant-food can, however, be artificially supplied in
the form of fertilizers, should they be deficient in the soil; but as
has been shown in the preceding pages, it is not always possible to
correct, within the limits of farm economy, physical defects existing
in the land. Hence, however important is the natural richness of the
soil in plant-food, _the first care should always be given to the
ascertainment of the proper physical conditions in the soil, subsoil
and substrata_. Without these, oftentimes, no amount of cultivation,
fertilization and irrigation is effective in assuring profitable
cultural results.

_Condition of the Plant-food Ingredients in the Soil._—But even
the abundant _presence_ of the plant-food ingredients, as shown by
analysis, will not avail, unless at least an adequate portion of
the same exists in a form or forms accessible to plants. Of course
this condition would seem to be best fulfilled by the ingredients in
question being in the _water-soluble_ condition. But in the first
place, plants are quite sensitive to an over-supply of soluble mineral
salts, as is evidenced by the injurious effects produced by the latter
in saline and alkali lands. Furthermore, substances in that form
would be very liable to be washed or leached out of the soil by heavy
rains or irrigation, and would be lost in the country drainage. It is
therefore clearly desirable that only a relatively small proportion
of the useful soil-ingredients should be in the water-soluble or
physically absorbed condition, but that a larger supply should be
present in forms not so easily soluble, yet accessible to the solvent
action which the acids of the soil and of the roots of plants are
capable of exercising. This _virtually_ available supply we may
designate as the _reserve food-store_.

Finally, there is practically in all soils a certain proportion of
the _soil-minerals in their original form_, as they existed in the
rock-materials from which the soil was formed. These minerals being
usually in a more or less finely divided or pulverulent condition, they
are attacked much more rapidly by the chemically-acting “weathering”
agencies, viz., water, oxygen, carbonic and humus acids, than when
in solid masses; and thus, transformation of the inert rock-powder
into the other two classes of mineral soil-ingredients progresses in
naturally fertile soils with sufficient rapidity to produce, in a
single season, sensible and practically important results, known as the
effects of _fallowing_.

_The Reserve._—The nature of these processes has been discussed
in chapters 1 to 4; and it will be remembered that two of their
most prominent results are the _formation of clay_, and of
_zeolitic-compounds_, the latter being, as heretofore stated (pp. 36
ff) hydrous silicates of earths and alkalies, easily decomposable by
acids, and also capable of exchanging part or the whole of such basic
ingredients with solutions of others that may enter the soil. These
zeolitic compounds therefore fulfil two important functions in the
premises, viz.: a ready yielding-up of part of their ingredients to
acid solvents, and a tendency to fix, by exchange, a portion or the
whole of the soluble compounds that may be set free in, or brought
upon the land. The first-mentioned property is of direct avail in that
the soil-humus forms, and the roots of plants exude, acid solvents on
their surface, and can thus draw upon the reserve store of food; the
second tells in the direction of preventing the waste of water-soluble
manurial ingredients supplied to, or formed in the soil. (See above,
chapter 3, page 38).

The reserve food-store may then be placed under the following heads:

_Hydrous or “zeolitic” silicates_, from which dilute acids can take up
the bases potash, soda, lime and magnesia. These silicates may be in
either the gelatinous or powdery form; in the former case they may also
occlude water-soluble substances.

_Carbonates of lime and magnesia_, which are readily dissolved by
carbonated water as well as by the vegetable acids.

_Phosphates of lime and magnesia_, not very readily soluble in
carbonated water, but more readily attacked by the acids of the soil
and of plant roots; thus supplying phosphoric acid to plants. The more
finely divided they are the more readily they are dissolved; some soils
containing only crystalline needles of apatite (see chap. 5, p. 63)
only are nevertheless poor in available phosphoric acid.

The natural phosphates of iron and alumina are practically insoluble
in all solvents at the disposal of vegetation and though present in
considerable amounts in some soils, (see chapter 19, page 355), may be
considered as being permanently inert, and therefore not to be counted
among the soil resources for plant nutrition. As yet no artificial
process by which their phosphoric acid can be made available within the
soil, has been discovered.

_Water-soluble Ingredients._—As regards these it has already been
explained that they are largely retained in the condition of purely
physical adsorption, as in the case of charcoal or quartz sand, through
which sea water filters and is thereby partially deprived of its
salts. But these can be gradually withdrawn by washing with pure water
alone, and still more easily when stronger solvents are used. Since
the soil-water is always more or less charged with carbonic acid, and
the roots themselves secrete carbonic as well as stronger acids in
their absorption of mineral plant-food, there is no difficulty about
explaining the manner in which such physically condensed ingredients
are taken up.[108]

[108] Whitney (Bull. 22, U. S. Bureau of Soils) claims on the basis
of a large number of (three-minute) extractions of soils made with
distilled water, that these solutions are essentially of the same
composition in all soils; that all soils contain enough plant-food to
produce crops indefinitely; and that the differences in production
are due wholly to differences in the moisture supply, which he
claims is, aside from climate, the only governing factor in plant
growth. The tables of analytical results given in Bull. 22 fail to
sustain the first contention; the second is pointedly contradicted
both by practical experience, and by thousands of cumulative culture
experiments made by scientific observers; the third fails with the
second, except of course in so far as an adequate supply of moisture
is known to be an absolute condition both of plant growth, and the
utilization of plant-food. It is moreover well known that it is not
water alone, but water impregnated more or less with humic and carbonic
acids, that is the active solvent surrounding the plant root.

_Recognition of the Prominent Chemical Character of Soils._ In a former
chapter the soils formed from the several minerals and rocks have been
discussed in a general manner. We can as a rule obtain some insight
into the nature of any soil which we can trace to its parent rock or
rocks, if we are acquainted with the composition of the latter.

Similarly, but in a much more direct manner, we can obtain a
strong presumption as to the nature of any soil by determining the
undecomposed minerals present in it. In all ordinary cases the
presumption must be that the decomposed portion of the soil has been
derived from the minerals still found in it. Of course it may happen
in the case of lands derived from widely distinct and distant regions
that no such characteristic minerals can be found; this is very
commonly true of the soils forming the deltas of large rivers, in
which sometimes the only remaining recognizable mineral is quartz in
its several forms, with occasional grains of such hardy minerals as
tourmaline, garnet, etc. Apart from such cases, the hand lens or the
microscope permits us to recognize in most soils the minerals that have
mainly contributed to their formation, thus also gaining a clew to
their prominent chemical nature.

Such recognition sometimes involves, of course, a somewhat intimate
knowledge of mineralogy; yet a little practice will enable almost
any one to identify the more important soil-forming minerals, under
the lens or microscope, according to the degree of abrasion or
decomposition they may have undergone. The details of such researches
lie outside of the limits of this treatise, but some general directions
on the subject are given farther on.[109]

[109] See Appendix B.

_Acidity, Neutrality, Alkalinity._—A test never to be omitted is that
of the reaction of the soil on litmus or other test paper, to ascertain
its acid, neutral or alkaline reaction. Should the latter occur quickly
(by the prompt blueing of red litmus paper), “black alkali” would be
indicated; but a blueing after 20 to 30 minutes means merely that
a sufficiency of lime carbonate is present. An acid reaction (the
reddening of blue litmus paper) of course indicates a “sour” soil (see
chap. 8, page 122).

_Chemical Analysis of Soils._—When the observations mentioned above
give no very decisive results or inferences as to the soil’s chemical
character, the more elaborate processes of qualitative and quantitative
chemical analysis may be called in. It would seem at first sight that
these ought to yield very definite results to guide the cultivator; yet
such is by no means always the case. Both the previous history of the
land, and the method of analysis, influence materially the practical
utility of the results of chemical soil analysis.

The cause of this uncertainty becomes obvious when we consider the
three groups of ingredients outlined above, viz., the insoluble or
unavailable, wholly _undecomposed rock minerals_; the “_reserve_,”
consisting of compounds not soluble in water but soluble in or
decomposable by weak acids; and the _water-soluble portion_, either
actually dissolved in the water held by the soil, or held by the soil
itself in (physical) absorption. While the latter portion is directly
and immediately available to plants, the amounts thus held are usually
quite small, and (outside of alkali lands) would rarely suffice for
the needs of a crop during a growing season.[110] This demand must be
materially supplemented by what can be made available from the soil
minerals and the “reserve” by weathering, conjoined with the direct
action of the acids secreted from the plant’s root-hairs upon the soil
particles to which they are attached. It is obvious that the greater
or less _abundance_ of the plant-food in the soil-material upon which
these processes may be brought to bear, must essentially influence the
adequacy of the plant-food thus supplied. Moreover, the greater or less
extent to which these sources may have been drawn upon previously in
the course of cultivation, will similarly influence that adequacy, on
account of the diminution of the readily available supply.

[110] The investigations of King (On the Influence of Soil Management
upon the Water Soluble Salts in Soils and the Yield of Crops, Madison,
1903) show that from some soils at least, a sufficiency of plant-food
ingredients for a season’s crop may be dissolved by distilled water
alone, if the soil be repeatedly leached and dried at 110°. Whether
such a supply can be expected under field conditions, remains to be
tested.

_Water-soluble and Acid-soluble Portions most Important._—It thus
seems that while the undecomposed rock minerals are indicative of the
nature of the soil, but not directly concerned in plant nutrition, the
most direct interest attaches to the _water-soluble portion_, and the
_acid-soluble_ reserve. Both of these can, of course, be withdrawn from
the soil by treatment with acids of greater or less strength; and it
would seem that if we knew just what is the kind and strength of the
acid solvent employed by each plant, we could so imitate their action
as to determine definitely whether or not the soil contains an adequate
or deficient supply of actually available food for the coming crop.

_We Cannot Imitate Plant-root Action._—In this, however, we encounter
serious difficulties. The acids secreted by the plant roots are not the
only solvents active in the dissolution of plant-food; as yet we know
the nature of only a few; and even these, instead of acting for a long
time (season) on a relatively small number of soil particles touched by
the root-hairs, can in our laboratories only be allowed to act for a
short time on the entire soil-mass. Clearly, the results thus obtained
cannot be a direct measure of the amount of plant-food which a plant
may take up in a given time; we can only gain comparative figures.
These, however, can be utilized by comparison with actual cultural
experience obtained in similar cases.

_Cultural experience_ must, of course, be the final test in all these
questions; and it is generally more fruitful to investigate the causes
underlying such actual practical experience, than to attempt to supply,
artificially, the supposed conditions of plant growth. The latter are
so complex and so difficult of control, that the results obtained
by synthetic, small-scale experiments are constantly liable to the
suspicion that they are partly or wholly due to other causes than those
purposely supplied by the experimenter.

    _Analysis of Cultivated Soils._—It is also clear that
    in view of the inevitable complexity of the conditions
    governing vegetable growth, we should whenever feasible
    proceed from the more simple to the more complex. The
    failure to conform to this rule in soil investigation has
    been the cause of an enormous waste of energy and work
    bestowed, at the very outset, upon the most complex problem
    of all, viz., the investigation of soils long cultivated
    and manured; lands which, having been subject perhaps
    for centuries to a great and wholly indefinite variety
    of crops and cultural practices, had thereby become so
    beset with artificial conditions that without a previous
    knowledge of what constitutes the normal regime in natural
    soils, the correlation of their chemical constitution, as
    ascertainable by our present methods, with their production
    under culture, became as complex a problem as that of
    motions of three mutually gravitating points in space.
    Neither can be solved by the ordinary processes of analysis,
    chemical or mathematical. Nevertheless, though it was at
    one time contended that the minute proportion of plant-food
    ingredients withdrawn from soils by cultivation could not
    be detected by quantitative analysis, numerous examples
    have shown that with our present more delicate methods this
    can in most cases be done, though not always after a single
    year’s crop.

    _Methods of Soil Analysis._—The more or less incisive
    solvent agents used in extracting a soil for analysis will
    of course produce results widely at variance with each
    other. When fusion with carbonate of soda, or treatment
    with fluohydric acid is resorted to, we obtain for each
    soil-ingredient the sum of all the amounts contained in
    each of the three categories—the unchanged minerals, the
    zeolitic “reserve,” and the water-soluble portion. It was
    early recognized that the results of such analyses bear no
    intelligible relation to the productive capacity of soils;
    for pulverized rocks of many kinds, or volcanic ashes
    freshly ejected and notoriously incapable of supporting
    plant growth, might be made to give exactly the same
    composition. The amounts of plant-food ingredients thus
    shown might be several hundreds or thousands of times
    greater than what one crop would take from the soil, and
    yet not an ear of grain could be produced on the material.
    The only case in which any useful information could be thus
    obtained would be that of the absence, or great scarcity, of
    one or more of the plant-food ingredients.

    The next step was to use in soil analysis acids of such
    strength as to dissolve all the zeolitic (and water-soluble)
    portion, leaving the unweathered soil minerals behind; it
    being assumed that the prolonged action of the roots and
    soil-solvents would in the end act similarly to the acids
    employed, such as chlorhydric or nitric acids.

    But here also the results of analysis very commonly failed
    to correspond to cultural experience in the case of
    _cultivated_ soils; which frequently failed utterly
    to produce satisfactory crops even when the acid-analysis
    had shown an abundance of plant-food ingredients. Upon
    this evidence, this method of soil investigation was also
    condemned as being of little or no practical utility; and
    this has ever since been a widely prevalent view.

The preferable investigation of cultivated soils was due to the fact
that they are practically the only ones _available_ in the countries
where the study of agricultural science was then being prosecuted;
and the paucity of useful results there achieved discouraged the
undertaking of similar researches where, as in the United States,
the materials for the investigation of the simpler cases—those of
unchanged, natural or virgin soils—were readily accessible. It was
not apparent on the surface that the indefinitely varied conditions
introduced by long culture would inevitably cause this lack of definite
correlation between the immediate productive capacity of a soil and
the composition of its acid-soluble portion, and that yet the same
might not be true of natural, uncultivated soils, which have all been
subjected, alike, only to the natural processes of weathering, and to
the annual return of nearly the whole of the ingredients withdrawn by
plant growth.

Following the failure of the treatment with strong acids to yield
with cultivated soils results definitely correlated with cultural
experience, numerous attempts were made to gain better indications
by the employment of weaker acid solvents. The pure arbitrariness of
such diluted solvents was equaled by the total indefiniteness and
irrelevance of the results with different soils. Only two rational
alternatives seem to remain, viz., either to push the extraction to
the full extent beyond which action becomes so slow as to clearly
exclude any farther effective action of plant acids; or else to use the
latter themselves at such strengths as by actual experiment is found
to exist in their root sap. The first alternative aims to ascertain
the _permanent productive values_ of soils; the latter to test
their _immediate productive capacity_. Both alternatives are purely
empirical, and derive their only claim to practical value from their
accordance with practical experience (see chapter 19).


THE SOLVENT ACTION OF WATER UPON SOILS.

The almost universal solvent power of pure water has already been
alluded to in chapter 2 (see p. 18), and illustrated by the analyses
of drain and river waters. While these convey a general idea of the
chief substances dissolved and carried off, the direct investigation
of the solutions actually obtainable from the soil by longer treatment
and with no more water than is compatible with the welfare of ordinary
crops, necessarily gives somewhat different results. For when drains
flow during or after heavy rains the water has not time to become
saturated. The following data afford a clearer insight into the actual
and possible solvent effects of water in the soil, and its possible
adequacy to plant nutrition unaided by acid solvents.

_Extraction of Soils with Pure Water._—Eichhorn and Wunder treated
soils from Bonn, and from Chemnitz (Saxony) respectively for ten days
and four weeks with about one-third of their weight of water; the
solutions thus obtained contain in 1,000,000 parts:

    ==========================+=======+===========
                              | Bonn. | Chemnitz.
    --------------------------+-------+-----------
    Silica                    |  48.0 |   25.7
    Potash (K₂O)              | 115.4 |    7.5
    Soda (Na₂O)               |  11.0 |   30.4
    Lime (CaO)                | 128.0 |   83.6
    Magnesia (MgO)            |  38.4 |   37.4
    Peroxid of Iron (Fe₂O₃)   | Trace |   11.7
    Alumina (Al₂O₃)           |   ?   |    ?
    Phosphoric acid (P₂O₅)    |  31.0 |  Trace
    Sulfuric acid (SO₃)       | 100.2 |
    Chlorid of Sodium (NaCl)  |  58.6 |   47.6
    ---------------------------+-------+-----------

These figures differ widely in most respects from those given for drain
and river waters. Potash especially is far more abundantly present in
the Bonn-soil solution than in the drain water, and so is phosphoric
acid; while lime is not widely different. Eichhorn therefore calculates
that with a reasonably adequate supply of water, these ingredients
would fully suffice for a full crop of wheat. The Chemnitz soil, on the
other hand, does not yield enough plant-food for more than a very small
crop upon the same assumptions.

_Continuous Solubility of Soil-ingredients._—It seems to be impossible
to exhaust a soil’s solubility by repeated or continuous leaching with
water. This was demonstrated in 1863 and 1864 by Ulbricht[111] and
by Schultze;[112] their general conclusions have quite lately been
corroborated by King,[113] as the result of extended and very careful
investigations.

Schultze experimented on a rich soil from Mecklenburg, by continuous
leaching with distilled water for six days, one liter passing every
twenty-four hours, with the following results.

[111] Vers. Stat. V. p. 207.

[112] Ibid. VI. p. 411.

[113] Proc. Ass’n Prom. Agr. Sci. 1904.

            RICH SOIL FROM MECKLENBURG (Schultze.)
            1,000,000 PARTS OF EXTRACTS CONTAINED:
    ==============+============+===========+==========+==========
                  |Total matter|Organic and|Inorganic.|Phosphoric
                  | dissolved. | volatile. |          |  acid.
    --------------+------------+-----------+----------+----------
    First extract |    535.0   |   340.0   |   195    |    5.6
    Second   “    |    120.0   |    57.0   |    63    |    8.2
    Third    “    |    261.0   |   101.0   |   160    |    8.8
    Fourth   “    |    203.0   |    83.0   |   120    |    7.5
    Fifth    “    |    260.0   |    82.0   |   178    |    6.9
    Sixth    “    |    200.0   |    77.0   |   123    |    4.4
                  |  -------   |   -----   |   ---    |   ----
        Total     |  1,579.0   |   740.0   |   839    |   41.4
    --------------+------------+-----------+----------+----------

It thus appears that while the first extraction removed the main
portion of the organic matter, the inorganic matters dissolved were
not greatly diminished in subsequent leachings; and that phosphoric
acid continued to come off to the last. The rich soil used in this case
gave results corresponding in general to these from the Bonn soil, in
the previous table. From a poorer soil similarly treated by Ulbricht,
described by him as a ferruginous sand from Dahme, the leaching of
which was continued for thirty days in periods of three days each, with
a total of forty times its weight of water, the results were as follows:

          SOIL OF LOW PRODUCTION FROM DAHME (Ulbricht).
        THE SEVERAL EXTRACTS CONTAINED IN 1,000,000 PARTS:
    ============+========+========+========+========+========+========
                | First  | Second | Third  | Fourth | Fifth  | Sixth
                |Extract.|Extract.|Extract.|Extract.|Extract.|Extract.
    ------------+--------+--------+--------+--------+--------+--------
    Potash      |    7   |    6   |    7   |    7   |        |    3
    Soda        |   41   |   11   |   26   |   17   |        |    8
    Lime        |   96   |   70   |   55   |   48   |   62   |
    Magnesia    |   14   |   10   |    9   |    7   |    8   |
    Phosphoric  |        |        |        |        |        |
         acid   | trace  |    2   | trace  |    1   |        |
                + -----  +   --   + -----  +   --   |   --   +   --
      Totals    |  158   |   99   |   97   |   80   |   70   |   11
    ------------+--------+--------+--------+--------+--------+--------

It will be seen that there is a considerable difference both in the
total amounts of matters dissolved and in the phosphoric acid taken out
by the water, as compared with the rich soil treated by Schultze. The
uniformity of the amounts of potash removed at the successive leachings
is remarkable.

_King’s Results._—The same general features are again strikingly
illustrated by King’s results, as given in the following table. King’s
first leachings were always made by shaking up the soil with ten times
its dry weight of water for three minutes, then after subsidence
filtering the solutions through a Chamberland (porcelain biscuit)
filter, and then (without evaporation) determining the ingredients
dissolved, by very delicate, mostly colorimetric methods. Subsequent
leachings were made by packing the soil around the filters and washing
with five times the weight of water, taking about fifteen minutes
each time; but drying the soil at 120 degrees C. between successive
leachings.

         WATER EXTRACTION OF SOILS OF LOW AND HIGH PRODUCTION,
                        BY F. H. KING.
                      PARTS PER MILLION.
    =======================+=======+=======+=========+===========
                           |Potash,| Lime, |Magnesia,|  Nitric
             Extractions   |  K₂O. |  CaO. |  MgO.   |   Acid,
                           |       |       |         |   N₂O₅.
    -----------------------+-------+-------+---------+-----------
                                SOILS OF LOW PRODUCTION.
    Sassafras sandy      1 |  12.62|  74.39|  17.82  |   18.03
      soil.             11 | 218.25| 135.35| 147.45  |   21.76
    -----------------------+-------+-------+---------+-----------
      Norfolk, North     1 |  21.17|  58.30|  22.91  |   30.64
    Carolina sandy soil 11 | 166.08| 162.98| 125.00  |   27.11
                Average.   | 192.60| 149.20| 136.23  |   24.44

                               SOILS OF HIGH PRODUCTION.
      Janesville, Wis.   1 |  25.35| 135.30|  51.72  |   55.10
    Loam.               11 | 313.70|1120.30| 500.60  |   51.42
    -----------------------+-------+-------+---------+-----------
      Hagerst’wn, Pa.    1 |  21.73| 165.25|  76.88  |   25.72
    Clay loam.          11 | 301.55| 967.80| 463.15  |   96.04
               Average.    | 307.60|1044.05| 487.88  |   73.73
    -----------------------+-------+-------+---------+-----------

    =======================+==========+========+========+========+======
                           |Phosphoric|Sulfuric|Carbonic|Chlorin,|Silica
             Extractions   |   Acid,  | Acid,  | Acid,  |  Cl₂.  | SiO₂.
                           |   P₂O₅.  |  SO₃.  |   CO₂. |        |
    -----------------------+----------+--------+--------+--------+------
                                  SOILS OF LOW PRODUCTION.
    Sassafras sandy      1 |    7.41  |  53.84 |  13.94 |   1    |  5.60
      soil.             11 |   64.16  | 203.96 | 221.33 |   2    | 70.20
    -----------------------+----------+--------+--------+--------+------
      Norfolk, North     1 |   10.15  |  42.82 |  20.42 |   1    |  8.24
    Carolina sandy soil 11 |   80.34  | 172.42 | 148.52 |   2    |122.20
                Average.   |   72.25  | 126.13 | 184.93 |   2.-  |146.20

                                 SOILS OF HIGH PRODUCTION.
      Janesville, Wis.   1 |   16.96  | 125.43 |  29.31 |  2.67  | 40.28
    Loam.               11 |  418.85  | 592.75 | 472.95 |  0.00  |414.50
    -----------------------+----------+--------+--------+--------+------
      Hagerst’wn, Pa.    1 |   11.51  | 187.59 |  97.09 |  1.67  | 21.17
    Clay loam.          11 |  136.21  | 502.82 | 620.00 |  0.00  |283.80
               Average.    |  277.03  | 547.79 | 546.48 |  0.00  |349.15
    -----------------------+----------+--------+--------+--------+------

King’s observations show strikingly both the continuous solubility of
the soil, and the differences between the solutions derived from soils
of low and high productiveness; wholly negativing the contention of
Whitney that the solutions from different soils are of practically the
same composition.[114] King also calls attention to the fact, shown in
other experiments made in the extraction of soils without intermediate
dryings, that the amounts extracted were very much less in subsequent
than in the first extraction; doubtless because the evaporation from
the soil particles had carried a large proportion of soluble matters
to the surface, whence it was readily abstracted by the first touch
of the solvent water. At each drying not only are the soluble matters
again drawn to the surface, but heating a soil even to 100° renders
additional amounts of soil ingredients soluble both in water and in
acids. It can scarcely be doubted that the intense heating which desert
soils undergo during the warm season is similarly effective; and thus
the great productiveness of these soils under irrigation, and the
marvelously rapid development of the native vegetation when rains
moisten the parched soil, is in part at least accounted for by this
immediate availability of a large supply of plant-food.

[114] Bulletin No. 22, Bureau of Soils, U. S. D. A.

_Composition of Janesville loam._—In connection with the above data
given by King, it is interesting to note the composition of the soil
in the above table yielding the highest proportions of soluble matter,
when analyzed according to the method practiced by the writer (see
chap. 19, p. 343). This analysis was made under the supervision of
Professor Jaffa in the laboratory of the California Experiment Station
by Assistant Charles A. Triebel.

    _Loam Soil from Janesville, Wisconsin_; sample sent by
    Prof. F. H. King, Madison, Wis.

    This soil is a light friable loam, resembling the northern
    Loess in color and texture; it is highly productive. It is
    underlaid at 5 feet by the drift gravel of that region,
    enclosing much calcareous material, which evidently has had
    a large share in the formation of this soil, just as is the
    case in southern Michigan.

    The soil, when dried at 110° C, consisted of

      CHEMICAL ANALYSIS OF FINE EARTH.

    Insoluble matter               69.35
    Soluble silica                 10.89
    Potash (KO₂)                     .59
    Soda (Na₂O)                      .04
    Lime (CaO)                       .83
    Magnesia (MgO)                   .51
    Br. ox. of Manganese (Mn₃O₄)     .08
    Peroxid of Iron (Fe₂O₃)         3.60
    Alumina (Al₂O₃)                 5.26
    Phosphoric acid (P₂O₃)           .06
    Sulfuric acid (SO₃)              .10
    Water and organic matter        8.72
                                  ------
                Total             100.03

    It will be noted that in accordance with the interpretation
    of analyses of soils as given in the next chapter, this is
    a high-class soil in every respect, except that its content
    of phosphoric acid is only just above the lower limit of
    sufficiency. But as is also shown below, in presence of a
    large supply of lime even lower percentages of phosphoric
    acid are adequate for long-continued production (see chap.
    19, pp. 354, 365) by rendering the substance more freely
    available; and that this is true in this case is shown by
    the result of King’s leachings, in which this soil yields
    a maximum of 419 parts per million as against 80 and 64
    parts in the poor soils, which at the same time yield only
    one fourth as much of lime. Unfortunately we have no full
    analyses of these other soils for comparison; although
    they have served as a basis of comparison for years in the
    Washington Bureau of Soils.

_Solubility of Soil Phosphates in Water._—The solubility of the
phosphate contents of soils has been elaborately investigated by
Th. Schloesing fils.[115] He found in the case of a number of soils
investigated by him that the amount of phosphoric acid P₂O₅ in the
soil-solution ranged from less than one millionth (or one milligram
per liter of water) in a poor soil, to over three milligrams in a rich
one. He also found that for one and the same soil the amount so found
was constant, if about a week’s time were allowed for saturation. He
calculates that while in general the amount of phosphoric acid capable
of being supplied to the crop during a growing season of twenty-eight
to thirty weeks would suffice for but few crops, the supply so afforded
is in no case a negligible quantity, frequently amounting to more than
half of the crop-requirements. Experiments with various crops prove
that these dilute solutions are utilized by all of them, sometimes to
the extent of completely consuming the content of the solution. The
much smaller content of phosphoric acid in drain waters is accounted
for by the lack of time for full saturation during the time that the
flow lasts. Whitney, (Bureau of Soils, Bulletin 22) has extracted
the soil-solution by means of the centrifuge from several soils; the
contents of phosphoric acid thus found are in general of the same order
as those shown in the preceding table by King, but much in excess of
Schloesing’s figures; notwithstanding the fact that Whitney’s soils had
been in contact with water for only twenty-four hours. The cause of
this wide discrepancy is not clear.

[115] Ann. de la Sci. Agron., 2de série tome I, pp. 416-349; 1899.

_Practical Conclusions from Water Extraction._—As regards the
practically useful conclusions to be drawn from the extraction of soils
with pure water, the data given above, and especially the results
obtained by King, seem to prove that there is a more or less definite
correlation between the immediate productiveness of soils and the
amount and kinds of ingredients dissolved; especially in the case of
phosphoric acid, the adequacy of the supply of which for immediate
production is assumed to be thus demonstrable by many French chemists.
Moreover, a number of King’s results, tabulated in curves, exhibit
a remarkable general parallelism of the curves showing totals of
plant-food extracted by water, and actual crop production. This is the
more remarkable since it is known to be, not pure water, but such as is
more or less impregnated with carbonic acid at least, that is actually
active in soil-solution and plant-nutrition. The farther development of
this method may, it would seem, lead to definite conclusions at least
in respect to the immediate productive capacity of cultivated, and
perhaps also of virgin soils. But it is not likely to give any definite
clew as to the _durability_ of such lands.


ASCERTAINMENT OF THE IMMEDIATE PLANT-FOOD REQUIREMENTS OF CULTIVATED
SOILS BY PHYSIOLOGICAL TESTS. PHYSIOLOGICAL SOIL-ANALYSIS.

As has already been stated, the quantitative analysis of cultivated
soils by means of strong acids affords a presumptive insight into their
immediate productiveness, and the kind of fertilizer needed to improve
it, only in case of the extreme deficiency of one or several of the
chiefly important plant-foods. The limits of deficiency of these in
virgin soils have been discussed above; but since in cultivated soils
amounts of soluble plant-food so small as to be beyond the limits
of ordinary analytical determinations, when distributed through an
acre-foot of soil may, when rightly applied, nevertheless produce very
decided effects, the indications thus obtainable are not absolute. Thus
a dressing of 150 lbs. of Chile saltpeter, containing only about 24
lbs. of nitrogen, is capable of causing the production of a full crop
of wheat where otherwise, even under favorable physical conditions,
only a fraction of a crop would have been harvested; _provided_, that
all the other requisite ingredients were present to a sufficient
extent and in available form. Yet the amount of nitrogen thus added
would amount, in one acre-foot of soil to only .0008%, say eight
ten-thousandths of one percent; which, with the amounts of substance
usually employed in soil analysis, would be an unweighable quantity,
and might easily be overlooked.

Since the amounts of potash and phosphoric acid actually taken out of
the soil by one crop are in general of the same order of magnitude as
the above, what is taken out by one or two crops will usually fall
within the limits of analytical errors, especially of those incurred
in sampling the soil. Yet that the changes caused by a number of
successive crops can be proved, even by the ordinary methods, has been
abundantly verified. For it seems that the losses of soil ingredients
in cultivated lands exceed considerably those calculated from the
actual drain represented by the crops.

_Plot Tests._—There is, however, an obvious and apparently simple
method by which every farmer might make his own fertilizer tests, on
a small and inexpensive scale, the results of which may afterwards be
put in effect on his entire land. It is to apply in proper proportions
on plots (of say from one twentieth to one fortieth of an acre), the
several plant-food ingredients usually supplied in fertilizers, singly
as well as conjointly with each other, leaving check unfertilized plots
around as well as among them. By comparison with these, the cultural
results should at once determine which of the fertilizers can most
advantageously be applied to the land. Such tests when carried out with
all the proper precautions are often very decisive and practically
successful. But they so frequently suffer from seasonal influences
(such as scanty or excessive rainfall, cold or heat, etc.), inequality
of soil conditions, failure to apply the fertilizers at the right time,
or in the right way, the depredations of insects and birds, and other
causes, that it generally takes several seasons’ trial to obtain any
definite results. On level lands of uniform nature and depth, they are
most likely to be successful; while on undulating or hill lands it is
not only very difficult to secure uniformity of soil and subsoil on
areas of sufficient size, but also to prevent the washing of fertilized
soil, or fertilizer in solution, from one plot to the other by the
influence of heavy rains or irrigation; thus wholly vitiating the
experiments. In very many cases, especially in the arid region, the
results of such trials have been practically _nil_, for the reason that
physical defects of the soil, and not lack of plant-food, were the
cause of unsatisfactory production.

[Illustration: Scheme for Plot-tests of Fertilizers.]

A full examination of physical conditions, as outlined in previous
chapters, should _in all cases precede_ the application of fertilizers;
such examination will at the same time serve to determine the greater
or less uniformity of soil-conditions, which is of first importance
to the cogency of fertilizer tests. As a matter of fact, few farmers
possess the necessary qualifications to carry out such tests
successfully, since their execution requires a certain familiarity
not only with the principles and methods of experimentation, but also
the faculty and practice of close and reasoning observation; which,
unfortunately, is not as yet a part of instruction in our schools.
The experience so often had in co-operative work between experiment
stations and farmers is cogent on this point.

Those desiring to do such work, however, can make use of something like
the plan given above; it being understood that in the case of clay
soils, the unplanted paths left between the plots should be at least
two feet in width; in the case of sandy soils the distance should be
not less than three feet, and more if the plots are located on a slope.
The crop from each plot should if possible be weighed as a whole; but
if the plot be large and the crop measurably uniform, an aliquot part,
such as one fourth, may be weighed instead. In regular experimentation
the crops are weighed both in the green (freshly cut) condition, and
after drying. Since the dry matter is the real basis of value in the
case of most field crops, its weight is the most important; as the
water-content of green crops may vary considerably. But in the case
of vegetables as well as fruit crops, not only must the weight of the
fresh crop be determined, but it should be sorted into the “marketable”
and “unmarketable” sizes and qualities. Failure to do this may vitiate
the entire experiment for practical purposes.

_Pot Culture Tests._—The uncertainty attending plot culture tests
on account of the difficulty of controlling seasonal and other
external conditions, has resulted in the extended adoption of indoor
culture tests, usually conducted in zinc or “galvanized” cylinders
of a size sufficient to contain from twelve to twenty or more
pounds of soil. These are kept in a green-house whose temperature
and moisture-condition can be regulated at will, and where the
soil-moisture is wholly under control. For investigations of the
effects of various kinds of plant-food upon vegetable development, this
method has served most satisfactorily and effectually, and striking
photographs of results thus obtained are seen on all hands: for which
reason, to save space, they have not been introduced into this volume.
It seems at first sight that the same method should serve admirably to
determine the manure-requirements of soils under controlled conditions.

It must, however, be remembered that the field conditions as regards
subsoil, evaporation, ascent of moisture from below, penetration and
spread of roots, etc., in other words, all the physical conditions
so vitally concerned in crop production, except the temperature and
moisture-condition of the soil, are wholly left out of consideration in
this method. Hence the application of the results so obtained to actual
field conditions can only be made with great caution, and are often
widely discrepant with actual experience.

    The method has of late been carried to an extreme by the
    U. S. Bureau of Soils in the proposition to supplant
    the large soil-pots heretofore used by small paraffined
    wire-cloth baskets, 3 × 3 inches in size, in which the
    soil to be tested is sown with seeds which are allowed to
    develop only for three to five weeks; it being claimed
    that the development occurring during that time is quite
    sufficient to indicate what will be the ultimate outcome
    in crop production. But practical experience has long ago
    demonstrated that these early stages of growth cannot be
    relied upon to show the crop results to be expected. Yet if
    this minute scale of pot-culture should, on further test,
    prove to give truthful forecasts even in a mere majority of
    cases, the facility with which it may be carried out will
    entitle it to favorable consideration. A great deal more
    proof is needed on this point than the confident claims of
    the Bureau indicate.


CHEMICAL TESTS OF IMMEDIATE PRODUCTIVENESS.

_Testing chemical soil-character by crop analysis._—Another method for
the determination of immediate soil requirements has been elaborated
by E. Godlewski.[116] The principle upon which this method rests is
that plants growing in a soil deficient in available plant-food of
any one kind will in their ash show a corresponding deficiency, or at
least a minimum proportion of the same; and that in many cases, the
nature of the deficiency manifests itself in the form or development of
the plant, so clearly as to render chemical analysis unnecessary (see
below, chapter 22).

[116] Zeitschr. Landw. Vers. Oesterr., 1901.

To a certain extent the latter idea has been and is constantly being
utilized in practice. It is essentially involved in the habit of
judging of land by its natural vegetation; and by agricultural chemists
and intelligent farmers, when they check excessive growth of stems and
leaf (indicating excess of nitrogen) by the use of lime or phosphates;
or prescribe the use of nitrogenous manures when a superabundance of
small, unmarketable fruit is produced. From the coincidence of such
indications with the results of the analyses of soils and ashes,
very definite and permanently valuable indications as to the proper
fertilization and other treatment of the land may be deduced.

    Godlewski insists strongly, and with a good deal of
    plausibility, upon the importance of making such trials in
    the open field and not merely in pots. While this is true,
    it is also true that such field experiments suffer from the
    same liability to imperfection as the “plot fertilizer-test”
    plan just described; viz., that the season may exert a much
    more powerful influence than the fertilization, and the
    tests may lead to wholly erroneous conclusions unless the
    experiments are continued for a number of years, and under
    skilled supervision. But when once the normal ratio between
    the ash ingredients for a particular soil and climatic
    region have been ascertained, the data will be of lasting
    benefit to agriculture there, and perhaps, other things
    being equal, to the world at large.

H. Vanderyst has discussed the entire subject of physiological soil
analysis elaborately in the Revue Génerale Agronomique of Louvain,
1902-3 (Exp’t St. Record, April 1904, Vol. 8, page 757) and shows in
detail the conditions under which it may be successful. Among these he
reckons as full a knowledge of the chemical characteristics of a soil
as can be obtained by chemical analysis.

_Chemical Tests of Immediately Available Plant-food._—It is scarcely
doubtful that plants differ considerably in the energy of their action
upon the “reserve” soil ingredients; hence no one solvent used by
the analyst could represent correctly the action of plant-roots in
general upon the soil, even if we could give that action the same
time (a growing season) and opportunity afforded them in nature by
the root-surface. We are forced to proceed empirically; and among the
numerous solvents suggested for the purpose of soil extraction, that of
Dyer, already mentioned, viz., a one per cent solution of citric acid,
making allowance for such neutralization as may occur in the soil, has
seemed to the writer to give results most largely in agreement with
cultural experience. Walter Maxwell has recommended aspartic acid in
lieu of citric, as approaching nearer to practical results, at least
with sugar-cane.

According to the investigations of Dyer, on Rothamstead soils of
known productiveness or manurial condition, it appears that when the
citric-acid extraction yields as much as .005% of potash and .010% of
phosphoric acid, the supply is adequate for normal crop production,
so that the use of the above substances as fertilizers would be, if
not ineffective, at least not a profitable investment. These figures
refer to the ordinary field crops of England and to soils originally
fertile and well supplied with lime. It can readily be foreseen that
under other climatic and soil conditions, different figures may have to
be established. So far as the writer’s experience goes, however, the
above figures are very nearly valid for the arid climates as well; only
the figures obtained for arid soils are usually far in excess of the
above minimum postulates. Figures for lime and nitrogen are given in
chapters 8 and 19. But the results obtained with the highly ferruginous
soils of Hawaii show that under such conditions, figures far exceeding
the minimum ones established by Dyer nevertheless coexist with need of
phosphate fertilization.




CHAPTER XIX.

THE ANALYSIS OF VIRGIN SOILS BY EXTRACTION WITH STRONG ACIDS.


As stated already, the analysis of soils by extraction with strong
acids is intended to enlighten us, not in regard to their _immediate_
productiveness (the “Düngerzustand” of German agricultural chemists),
but as to their _permanent value or productive capacity_. As has been
seen in the preceding chapter, the efforts to unite investigators
upon a generally applicable and acceptable method for the testing of
immediate productiveness have not been very successful, and the number
of methods employed in different countries and by different chemists
within the same country are widely at variance, with no immediate
prospect of agreement. Moreover, in most cases the effort is to combine
both problems—_temporary_ and _permanent_ productive capacity—in _one_
method or operation; which still farther confuses the issue.

Convinced that the only way to unification lies in the direction of
falling back upon a method that is based upon a natural limitation
about which there can be no difference of opinion, the writer has,
in following the lead of Owen and Robert Peter, endeavored to settle
definitely _the natural limit of the action of a suitable acid upon
soils, and the time and strength of acid producing the maximum effect_.

_Loughridge’s Investigation._—Systematic work on these points was
undertaken, at his suggestion, by Dr. R. H. Loughridge in 1871 and
1872. The results of this work were published in the succeeding year
in the Amer. Journal of Science, and in the proceedings of the A. A.
A. S. for 1873. They seem to be of sufficient general interest to be
reproduced here.

The soil selected for this purpose was a very generalized one,
representing large areas in the states of Kentucky, Tennessee,
Mississippi and Louisiana, bordering on the east the immediate valley
of the Mississippi river, and known locally as the “Table lands;” a
noted cotton-producing upland region. The brown or yellow, moderately
clayey loam is of great uniformity throughout its region of occurrence,
and is evidently derived from such widely-spread sources that it
represents no special rock or complex of rocks. Its natural growth is
a mixture of oaks and hickories, strong and well-developed trees, such
as any land-seeker would at once approve for settlement. Its cotton
product when fresh was a 400-pound bale of cotton lint per acre. It may
therefore well be considered a typical generalized soil of the humid
upland of the Mississippi valley. Its physical analysis is given in
chapter 6, it being No. 219 of the table on p. 98.

_Strength of Acid used._—Three different strengths of acid were
simultaneously employed, viz., chlorhydric of 1.10, 1.115 and 1.160
density. With these the soil was digested at steam heat in porcelain
beakers covered with watch glasses for five days each, then evaporated
and analyzed as usual. The results were as follows:

          ANALYSIS WITH ACID OF DIFFERENT STRENGTHS.
    =========================+=========================
          Ingredients.       |     Sp. G. of Acid.
    -------------------------+--------+--------+-------
                             |  1.10  |  1.115 | 1.160
                             +--------+--------+-------
    Insoluble residue        |  71.88 |  70.53 | 74.15
    Soluble silica           |  11.38 |  12.30 |  9.42
    Potash                   |    .60 |    .63 |   .48
    Soda                     |    .13 |    .09 |   .35
    Lime                     |    .27 |    .27 |   .23
    Magnesia                 |    .45 |    .45 |   .45
    Br. ox. Manganese        |    .06 |    .06 |   .06
    Ferric Oxid              |   5.15 |   5.11 |  5.04
    Alumina                  |   6.84 |   8.09 |  6.22
    Sulfuric acid            |    .02 |    .02 |   .02
    Volatile matter          |   3.14 |   3.14 |  3.14
                             | ------ | ------ | -----
                             | 100.02 | 100.69 | 99.29
                             |        |        |
    Amount of soluble matter |  24.00 |  27.02 | 22.27
    Amount of soluble bases  |  13.50 |  14.70 | 12.83
    -------------------------+--------+--------+-------

It will be noted that the strongest acid produced the smallest amount
of decomposition of the soil silicates, _e. g._ the silica soluble in
carbonate of soda solution being 3% less than in the case of the acid
of medium strength; a result possibly due to some difficultly-soluble
compound formed on the surface of the soil grains. The weakest acid had
a stronger solvent power; but the maximum effect was produced by the
acid of 1.115 density. This being also the most readily obtainable, by
simple steam distillation of acid of any other strength, the writer
adopted it as best suited to the purposes of soil analysis.

To ascertain the time required for the desired action, viz., the
solution of the plant-food ingredients to the extent likely to be of
any avail to growing plants, digestions of the same soil were made in
the same manner for periods of 1, 3, 4, 5 and 10 days, with the acid of
1.115 density. The results were as follows:

            ANALYSIS AFTER DIFFERENT TIMES OF DIGESTION.
    =========================+==================================
                             |     No. of Days’ Digestion.
          Ingredients.       +------+------+------+------+------
                             |  1   |  3   |  4   |  5   |  10
    -------------------------+------+------+------+------+------
    Insoluble Residue        | 76.97| 72.66| 71.86| 70.53| 71.79
    Soluble Silica           |  8.60| 11.18| 11.64| 12.30| 10.96
    Potash                   |   .35|   .44|   .57|   .63|   .62
    Soda                     |   .06|   .06|   .03|   .09|   .28
    Lime                     |   .26|   .29|   .28|   .27|   .27
    Magnesia                 |   .42|   .44|   .47|   .45|   .44
    Br. Ox. Manganese        |   .04|   .06|   .06|   .06|   .06
    Ferric Oxid              |  4.77|  5.01|  5.43|  5.11|  4.85
    Alumina                  |  5.15|  7.38|  7.07|  7.88|  7.16
    Phosphoric acid          |   .21|   .21|   .21|   .21|   .21
    Sulfuric acid            |   .02|   .02|   .02|   .02|   .02
    Volatile matter          |  3.14|  3.14|  3.14|  3.14|  3.14
                             +------+------+------+------+------
        Total                | 99.63|100.68|100.55|100.69| 99.80
                             |
    Amount of soluble matter | 19.67| 24.88| 25.57| 27.02| 24.87
    Amount of soluble bases  | 11.05| 13.68| 13.91| 14.49| 13.68
    -------------------------+------+------+------+------+------

While these results pointed clearly to the five-day period as being
sufficiently effective so far as the plant-food ingredients are
concerned, it was not easy to understand why a ten-day digestion should
be less incisive than a five-day one. Instead of repeating the ten-day
experiment, it was thought preferable to re-treat the residue from the
five-day digestion for five days more. The result was that only more
silica and alumina went into solution—in other words, additional clay
was alone being decomposed. This being of no interest in the matter of
plant nutrition, the five-day period was definitely adopted by the
writer for his work; and it, together with the acid of 1.115 density,
is the basis of all the results given in this volume, except where
otherwise stated. There appeared to him to be no good reason for the
acceptance of the arbitrary method of soil-extraction suggested by
Kedzie and since adopted by the Association of Official Agricultural
Chemists; the more as to do so would throw out of comparison all the
previous work done by Owen, Peter, and himself and his pupils, which
had already been definitely correlated with the natural conditions and
with cultural experience.[117]

[117] While regretting to thus “secede” from the fellowship of his
colleagues, the writer cannot but regret equally their voluntary
decision to do over again, or lightly reject, all that had been
done before in correlating soil-composition and plant-growth. He
still thinks that it is idle to expect any unification, national or
international, of methods of soil analysis based upon purely arbitrary
prescriptions, unless previously shown to be definitely correlated with
natural and cultural conditions; as is measurably the case with Dyer’s
method.

_Virgin Soils with High Plant-food Percentages are Always
Productive._—In strong contrast to the contradictory evidence deduced
from the analysis, by any method, of cultivated soils when compared
with cultural experience, it seems to be generally true that _virgin
soils showing high percentages of plant-food as ascertained by
extraction with strong acids_ (such as hydrochloric, nitric, etc.),
_invariably prove highly productive_: provided only that extreme
physical characters do not interfere with normal plant growth, as is
sometimes the case with heavy clays, or very coarse sandy lands.—_To
this rule no exception has thus far been found._ The composition of
some representative soils falling within this category is given in
the annexed table, which at the same time conveys some idea of the
proportion of acid-soluble ingredients usually found in the best class
of natural soils.

           TABLE EXEMPLIFYING HIGH PLANT-FOOD PERCENTAGE IN SOILS.

     (A) = Buckshot soil. Yazoo Bottom.
     (B) = Black Prairie. Rankin County.
     (C) = Loamy Sediment. Houma, Terrebonne parish.
     (D) = Rio Grande Bottom. Sandy Sediment.
    ============================+===================+==========+=========
                                |    Mississippi    |Louisiana.|  Texas.
                                +---------+---------+----------+---------
                                |         |         |          |
                                |    (A)  |   (B)   |    (C)   |   (D)
                                |   (Heavy Clay).   |  (Loam). |
    ----------------------------+---------+---------+----------+---------
             Number of Sample   |   390   |   188   |    240   |    37
    ----------------------------+---------+---------+----------+---------
     Chemical Analysis of       |         |         |          |
          Fine Earth.           |         |         |          |
    Insoluble matter            | 51.06   | 69.95   | 35.48    | 36.04
                                |    71.77|    74.40|    56.24 |    53.30
    Soluble silica              | 20.70   |  4.46   | 20.76    | 17.26
    ----------------------------+---------+---------+----------+---------
    Potash (K₂O)                |  1.10   |   .90   |  1.03    |  1.31
    Soda (Na₂O)                 |   .33   |   .24   |   .13    |   .22
    Lime (CaO)                  |  1.35   |  1.04   |   .72    | 14.43
    Magnesia (MgO)              |  1.67   |   .91   |   .88    |  1.53
    Br. ox. of Manganese (Mn₃O₄)|   .12   |   .12   |  .014    |   .07
    Peroxid of Iron (Fe₂O₃)     |  5.82   |  4.77   |  7.10    |  4.09
    Alumina (Al₂O₃)             | 10.54   |  7.25   | 15.45    |  9.11
    Phosphoric acid (P₂O₅)      |   .30   |   .47   |   .15    |   .20
    Sulfuric acid (SO₃)         |   .02   |   .16   |   .25    |   .04
    Carbonic acid (CO₂)         |         |         |          |  9.91
    Water and organic matter    |  7.37   | 10.74   | 18.52    |    ?
    ----------------------------+---------+---------+----------+---------
        Total                   |100.38   |101.01   |100.48    |100.22
    ----------------------------+---------+---------+----------+---------

     (E) = San Diego Co. Colorado Bottom. Silt Sediment.[118]
     (F) = Riverside Co. Palm Valley. Micaceous Sandy Soil.
     (G) = Tulare Co. Experiment Station. Plains Loam.
     (H) = Solano Co. Putah Valley. Dark Loam.
     (I) = San Luis Obispo Co. Arroyo Grande Dark Loam.
    ============================+=========================================
                                |              California.
                                +---------+------+------+--------+--------
                                |         |      |      |        |
                                |  (E)    | (F)  | (G)  |   (H)  |  (I)
                                |         |      |      |        |
    ----------------------------+---------+------+------+--------+--------
             Number of Sample   |   506   | 1092 | 1159 |   110  |  2061
    ----------------------------+---------+------+------+--------+--------
     Chemical Analysis of       |         |      |      |        |
          Fine Earth.           |         |      |      |        |
    Insoluble matter            | 58.57   | 71.45| 72.98|67.33   |53.43
                                |    63.90|      |      |   71.00|   72.43
    Soluble silica              |  5.33   |  5.50|  6.60| 3.67   |19.00
    ----------------------------+---------+------+------+--------+--------
    Potash (K₂O)                |  1.18   |  1.42|  1.20|  .93   |     .67
    Soda (Na₂O)                 |   .16   |   .18|   .52|  .12   |     .18
    Lime (CaO)                  |  8.67   |  2.20|  1.86|  .77   |    2.11
    Magnesia (MgO)              |  2.97   |  2.09|  1.81| 2.29   |    2.26
    Br. ox. of Manganese (Mn₃O₄)|   .03   |   .05|   .08|  .11   |     .06
    Peroxid of Iron (Fe₂O₃)     |  4.14   |  6.68|  6.86| 8.01   |    5.23
    Alumina (Al₂O₃)             |  8.40   |  5.78|  5.66| 9.16   |    7.40
    Phosphoric acid (P₂O₅)      |   .13   |   .35|   .10|  .11   |     .71
    Sulfuric acid (SO₃)         |   .15   |   .01|   .03|  .12   |     .22
    Carbonic acid (CO₂)         |  7.82   |   .18|      |        |    1.82
    Water and organic matter    |  3.34   |  4.29|  2.54| 7.12   |    6.63
    ----------------------------+---------+------+------+--------+--------
        Total                   |100.89   |100.18|100.24|99.74   |   99.72
                                |         |      |      |        |
    Humus                       |         |      |      |        |    3.06
    Nitrogen in humus           |         |      |      |        |   22.00
    Nitrogen in soil            |         |      |      |        |     .67
    Hygroscopic moisture.       |         |      |      |        |
              absorbed at 15°C  |         |      |      |        |   10.70
    ----------------------------+---------+------+------+--------+--------
                                                 Available phos. acid  .14
                                                 Available potash      .14

[118] The Rio Grande and Colorado bottom soils contain amounts of lime
carbonate largely in excess of requirements, 2 to 3% of that compound
being all that is needed to insure all the advantageous effects of lime
in any soil (see this chapter, page 367).

_Discussion of Table._—It will be noted in this table that while the
total of the matters soluble in acids (inclusive of silica) ranges from
a little below 50 to over 77 per cent, the total of directly important
mineral plant-food ingredients (potash, lime, magnesia and phosphoric
acid), constitute in moderately calcareous soils only from about 2.5
to somewhat over four per cent of the whole. Yet if all these were in
available form, the supply would be abundant for many hundreds and even
thousands of crop years. For, one-tenth of one per cent in the case
of the clayey soils of the preceding table would amount to about 3500
pounds per acre-foot, and to 4000 in the case of the sandy ones. Hence
the amount of phosphoric acid in _e. g._, the Mississippi delta soil
from Houma would suffice for the production of about 440 crops of wheat
grain (at 20 bushels per acre) if only one foot depth were drawn upon;
but as the roots of grain easily penetrate to twice and half and three
times that depth even in the humid region, the number might be tripled.
As a matter of fact, however, that soil has produced full crops for
from forty to fifty years only; yet this is considered an exceptionally
long duration of profitable production without fertilization.

    The first and last soils in the above list represent
    probably the highest types of productiveness known. The
    Yazoo bottom soil has produced up to one thousand pounds of
    cotton lint per acre when fresh, and is still producing from
    four to five hundred pounds after thirty years’ culture.
    The Arroyo Grande soil of California with its extraordinary
    percentages of phosphoric acid and nitrogen, as well as
    exceptionally high proportion of available phosphoric acid
    and potash, has made such a record of productiveness, and
    high quality of the seeds produced, that it has for a
    number of years been excluded from competition for prizes
    offered by seed-producers elsewhere, in order to give other
    sections a chance. Both these soils are rather heavy clays,
    but readily tillable in consequence of their abundant
    lime-content. The remarkably high content of acid-soluble
    silica, indicating the presence of much easily available
    zeolitic matter, is doubtless connected with the exceptional
    productiveness.

Experience, then, proves that lands showing such high plant-food
percentages will yield profitable harvests for a long time without
fertilization, or with only such partial returns as are afforded by
the offal of crops. Also that when fertilization comes to be required,
instead of supplying _all_ the ingredients usually constituting
fertilizers, only one or two of these will as a rule be actually
needed, and even these in smaller amounts than in “poor” lands; thus
materially reducing the expense of fertilization. The high production
and durability of such lands therefore amply justify their higher
pecuniary valuation; for which there would be no rational permanent
ground if they required fertilization to the same extent as poor lands.
In other words, if the entire amount of soil-ingredients removed by
crops had had to be currently replaced equally in _all_ cases (as
is implied in the hypothesis, advanced by some, that the chemical
composition of soils is of no practical consequence), the high prices
which from time immemorial have been paid for black prairie and rich
alluvial lands as against meagre uplands and barrens, would have been
so much money wasted.

The explanation of these advantages evidently lies largely in the
larger amounts of soil ingredients annually rendered available in
rich soils by the fallowing effect of the atmospheric agencies,
because of the generous totals present. The actual _amounts_ of soil
ingredients thus rendered accessible to plants, other things being
equal, are evidently more or less directly _proportional to the totals
of acid-soluble plant-food ingredients present_. And if this is true
in cultivated lands, the inevitable conclusion is that the same must
be true of virgin lands; _whose productive capacity and duration can
therefore be forecast by such analyses_. It will be observed that the
above data, which could be indefinitely increased by corroborative
analyses, seem to establish the fact that about one per cent of
acid-soluble potash, one of lime, the same, or less, of magnesia, and
.15% of phosphoric acid, are thus shown to be “high” percentages of
these ingredients in virgin soils.

It is not easy to see how the above conclusions can be successfully
controverted; they are, moreover, thoroughly in accordance with
cultural experience. Difficulties of interpretation arise mainly in
the case of medium soils, which show neither very high nor very low
percentages of plant-food; and which raise the question of what amount
or percentage constitutes “adequacy” of each of the several substances.

_Low Percentages._—On the other hand, whenever in virgin soils
acid-analysis shows the presence of but a _very_ small proportion
of one or several of the essential ingredients, we have a valuable
indication as to the one of these that will first be required to be
added when production slackens.

_What are “Adequate” Percentages of Potash, Lime, Phosphoric Acid and
Nitrogen?_—It is evident that a very critical discussion of cultural
experience can alone answer this question; and at first sight such
experience often appears very contradictory when compared with the
results of analysis.

    One of the chief causes of such apparent discrepancies is
    readily intelligible when we consider the differences in
    root-development of the same plant in different soils. In
    “light” or sandy lands the roots may penetrate to several
    times the depth attained by them in heavy clay soils. Having
    thus within their reach a soil-mass several times larger,
    and aerated to a much greater depth, it is but reasonable
    to expect that in deep, sandy lands plants would do equally
    well with correspondingly smaller percentages of plant-food
    than would suffice in clay soils, in which the root-range
    is very much more restricted. The well-known fact that the
    production of heavy clay lands may be increased by their
    intermixture with mere sand, adding nothing to their store
    of plant-food, emphasizes this expectation and elevates it
    into a maxim. On this ground alone, therefore, it is evident
    that the mere consideration of plant-food _percentages_
    found, can be a true measure of productiveness only in the
    case of virgin soils with _high_ percentages.

_Soil Dilution Experiments._—The extent to which dilution with mere
“lightening” materials can be carried without impairing production,
can of course be determined for concrete cases only; but the following
experiment made at the California Station is a case in point:

One kilogram of the heavy but highly productive black clay soil of
the experimental grounds of the University of California was used
in each of five experimental cultures, each made in duplicate, in
cylindrical vessels of zinc-covered (“galvanized”) sheet iron, all
proportioned alike in height and diameter, but containing respectively
one, two, four, five and six volumes of total soil. In the smallest
was placed one kilogram of the undiluted, original soil, in the others
successively the same amount of the soil thoroughly mixed with one,
three, four, and five volumes of a dune sand fully extracted with
chlorhydric acid, and washed with distilled water. The water capacity
of each of the mixtures was determined and the earth in the pots kept
at the point of half-saturation generally admitted to be the optimum
(best condition) for plant growth. Each pot was sown with ten seeds of
white mustard, subsequently reduced to five plants selected for their
vigor.

[Illustration: FIG. 53.—Natural Adobe Clay Soil.]

[Illustration: FIG. 54.—Adobe Soil diluted with sand, 1 to 1.]

[Illustration: FIG. 55.—Same, diluted 1 to 3.

DEVELOPMENT OF ROOTS OF WHITE MUSTARD IN CLAY SOIL, DILUTED WITH
VARIOUS PROPORTIONS OF PURE SAND.]

The (“galvanized”) vegetation pots were made as nearly as possible of
similar proportions in depth and width for each dilution, so as to give
opportunity for the proportional development of the root systems. The
photographs show the latter as nearly as practicable in their natural
form, restored after washing off the adherent soil. It was of course
extremely difficult to preserve intact the extreme circumferential
rootlets and hairs; yet the general development is correctly shown.

[Illustration: FIG. 56.—Adobe Soil diluted 1 to 4.]

[Illustration: FIG. 57.—Adobe Soil diluted 1 to 5.]

[Illustration: FIG. 58.—Soil-dilution Experiment: Photograph showing
Mature Plants.]

The following table shows the percentage composition of the original as
well as the diluted soils, while the photographs show the development
of the plants in their successive stages, so far as these could be
observed; the continued attacks of mildew and plant lice preventing
full maturity being attained.

                COMPOSITION OF BLACK ADOBE AND SAND DILUTIONS.
    ============================+========+==============================
                                |Original|        Dilutions.
    Chemical analysis of        | soil.  |
         fine earth.            |  1:0   |  1:1     1:3     1:4     1:5
    ----------------------------+--------+-------+-------+-------+------
    Insoluble matter            |  54.50 | 77.25 | 88.62 | 90.00 | 92.42
    Soluble silica              |  19.60 |  9.50 |  4.75 |  3.80 |  3.17
    Potash (K₂O)                |    .73 |   .36 |   .18 |   .15 |   .12
    Soda (Na₂O)                 |    .20 |   .10 |   .05 |   .04 |   .03
    Lime (CaO)                  |   1.15 |   .57 |   .29 |   .23 |   .19
    Magnesia (MgO)              |   1.08 |   .54 |   .27 |   .22 |   .18
    Br. ox. of Manganese (Mn₃O₄)|    .04 |   .02 |   .01 |   .01 |   .01
    Peroxid of Iron (Fe₂O₃)     |   8.43 |  4.22 |  2.11 |  1.68 |  1.40
    Alumina (Al₂O₃)             |   7.92 |  3.96 |  1.98 |  1.58 |  1.32
    Phosphoric acid (P₂O₅)      |    .19 |   .10 |   .05 |   .04 |   .03
    Sulfuric acid (SO₃)         |    .04 |   .02 |   .01 |   .01 |   .01
    Carbonic acid (CO₂)         |        |       |       |       |
    Water and organic matter    |   6.54 |  3.27 |  1.64 |  1.31 |  1.09
    Loss in analysis            |   1.18 |   .09 |   .04 |   .03 |   .03
                                +--------+-------+-------+-------+------
                      Total     | 100.00 |100.00 |100.00 |100.00 |100.00
                                |        |       |       |       |
    Humus                       |   1.21 |   .60 |   .30 |   .24 |   .20
      “   Ash                   |    .94 |   .47 |   .23 |   .19 |   .16
      “   Nitrogen, p. cent     |        |       |       |       |
                      in Humus  |  18.58 | 18.58 | 18.50 | 18.58 | 18.58
      “      “      p. cent.    |        |       |       |       |
                      in soil   |    .203|   .10 |   .05 |   .04 |  .034
    ----------------------------+--------+-------+-------+-------+------

    The restricted volume of soil occupied by the roots in
    the undiluted adobe soil, together with the very abundant
    development of root-hairs, is very striking. A marked
    change in these respects is manifest in the first dilution,
    and increasingly so as dilution increases; the paucity of
    root-hairs is very marked in the last (greatest) dilution,
    in which, as the photograph of the plants shows, the
    development was decidedly behind that in the pot containing
    dilution 1:4. The latter in fact showed the best development
    not only in this case, but in two other series of tests
    conducted at the same and subsequent times; and strangely
    enough, also in the pulverulent, “sandy loam” soil of the
    southern California substation tract. In the latter series,
    which for lack of space cannot be figured here, the main
    difference was that in the undiluted soil the roots filled
    the entire soil mass, instead of remaining near the surface,
    as in the pure adobe. It is possible that the latter was
    too wet when given the full half of its water-capacity,
    although, as the figures show, the water was slowly
    introduced from below by means of glass tubes, ending within
    a shield to prevent puddling.

_Limitation of Root Action._—These results, representing five soils of
different percentage-composition and physical character, but identical
chemical composition and ratios between the several ingredients,
and similarly acted upon by the atmospheric agencies in the past,
illustrate strikingly the impossibility of judging correctly of a
soil’s productiveness from _percentages_ of chemical ingredients
alone. It is clear that the physical characters of the land as well
as its depth, must be essentially taken into account. But there is
obviously a certain limit beyond which greater perviousness and
root-penetration cannot make up for deficiency in the absolute amounts
of plant-food within possible reach of the plant; for in the case of
excessive dilution these are rendered partially inaccessible within the
time-limits of a season’s growth.

It is hardly necessary to say that these experiments require repetition
with the aid of the experience acquired in these first trials, not
only in the laboratory but also in the field. It will be especially
interesting to compare with the results obtained in these strongly
calcareous soils, the effects of dilution in such soils as those
of Florida, mentioned below; the probability being that where lime
is naturally deficient, the effects of dilution will be much more
pronounced in diminishing production, because of the absence of
the previous favorable action of lime upon the availability of the
soil-ingredients.

_Lowest Limit of Plant-food Percentages and Productiveness found
in Virgin Soils._—The subjoined table shows some of the very low
plant-food percentages found in natural soils, all being of a sandy
character:

    ============================+====================================
                                |         MISSISSIPPI SOILS.
                                +----------+--------+--------+-------
                                |Homochitto| Shell  |  Pine  | Pine
                                | Bottom.  |Hammock.| Woods. |Flats.
    ----------------------------+----------+--------+--------+-------
          Number of Sample.     |    68    |   83   |  206   |  214
    ----------------------------+----------+--------+--------+-------
    CHEMICAL ANALYSIS OF        |          |        |        |
        FINE EARTH.             |          |        |        |
    ----------------------------+----------+--------+--------+------
    Insoluble matter            |          |        |        |
                                |   92.16  |  96.08 |  93.23 | 95.59
    Soluble silica              |          |        |        |
    ----------------------------+----------+--------+--------+------
    Potash (K₂O)                |     .15  |    .05 |    .26 |   .06
    Soda (Na₂O)                 |     .04  |    .06 |    .07 |   .05
    Lime (CaO)                  |     .12  |    .10 |    .12 |   .02
    Magnesia (MgO)              |     .21  |    .12 |    .18 |   .07
    Br. ox. of Manganese (Mn₃O₄)|     .28  |    .05 |    .15 |   .05
    Peroxid of Iron (Fe₂O₃)     |    1.18  |    .52 |   1.25 |   .46
    Alumina (Al₂O₃)             |    3.22  |    .46 |   2.36 |   .85
    Phosphoric acid (P₂O₅)      |     .08  |    .10 |    .03 |   .02
    Sulfuric acid (SO₃)         |     .05  | Trace  |    .02 | Trace
    Carbonic acid (CO₂)         |          |        |        |
    Water and organic matter    |    2.70  |   3.02 |   2.33 |  2.28
                                +----------+--------+--------+--------
          Total                 |  100.19  | 100.56 | 100.00 | 99.45
    ----------------------------+----------+--------+--------+--------

    ============================+====================
                                |   FLORIDA SOILS.
                                +--------------------
                                |    Pine Lands.
                                |-------+------------
                                | First |   Second
                                |Class. |   Class.
    ----------------------------+-------+------------
          Number of Sample.     |   6   |     7
    ----------------------------+-------+------------
    CHEMICAL ANALYSIS OF        |       |
        FINE EARTH.             |       |
    ----------------------------+-------+------------
    Insoluble matter            | 94.46 | 95.63
                                |       |       96.51
    Soluble silica              |  1.67 |   .88
    ----------------------------+-------+------------
    Potash (K₂O)                |   .19 |   .12
    Soda (Na₂O)                 |   .04 |   .06
    Lime (CaO)                  |   .07 |   .06
    Magnesia (MgO)              |   .04 |   .04
    Br. ox. of Manganese (Mn₃O₄)|   .06 |   .05
    Peroxid of Iron (Fe₂O₃)     |   .32 |   .22
    Alumina (Al₂O₃)             |   .92 |   .47
    Phosphoric acid (P₂O₅)      |   .11 |   .09
    Sulfuric acid (SO₃)         |   .09 |   .06
    Carbonic acid (CO₂)         |       |
    Water and organic matter    |  1.88 |  1.81
                                +-------+------------
          Total                 | 99.85 | 99.49
    ----------------------------+-------+------------

The average of plant-food percentages in all these soils is quite
low, and at first sight there seems to be little choice between them.
Yet two of them—Nos. 68 and 88, from Mississippi—are not only quite
productive at the outset, but also fairly durable. This becomes
measurably intelligible when it is known that both are of great depth,
and so well drained that roots can descend for many feet; while the
composition of the soil-material is almost identical for three or four
feet. On the other hand, both Nos. 206 and 214 are quite shallow,
being underlaid by sand almost devoid of plant-food at about two feet.
In addition, both have extremely low percentages of phosphoric acid;
while the rest show near .10% of that ingredient, an amount which, as
will be seen hereafter, is considerably above the recognized limit
of deficiency. The two Florida soils however bear only pine; they are
underlaid by almost clean sand at two or three feet, and are therefore
quickly exhausted. It will also be noted that their lime-percentage is
only about half of that of the two first-named Mississippi soils, both
of which bear a strong growth of deciduous timber trees, grape vines,
and other vegetation indicating the presence of lime carbonate.

It is noteworthy, also, that the popular classification of the
two Florida soils corresponds exactly with the differences in the
percentages of plant-food; those in the “second-class” soil being
uniformly lower than those in the one designated as first-class. This
indicates, again, that _as between soils of similar character and
origin, the production and durability are sensibly proportional to the
plant-food percentages_ when the latter fall below a certain limit; a
point more fully illustrated farther on.

In the light of the above experiment and tables, it becomes pertinent
to consider what _are_ the lowest percentage limits of each of the more
important plant-food ingredients compatible with profitable production.


LIMITS OF ADEQUACY OF THE SEVERAL PLANT-FOODS IN VIRGIN SOILS.

It is obvious that the lower limits of adequacy of the critical
plant-food ingredients are best ascertained in the case of virgin soils
containing very small amounts of some _one_ ingredient, while fairly
or fully supplied with the rest. In such cases, which are not at all
infrequent, the use of the deficient ingredient as a fertilizer should
produce a very marked effect so soon as the first flush of production
(always noted in fresh soil) is over. This first productiveness may,
even in poor lands, range from one to three years, when there is a
sudden decline.

_Lime a Dominant Factor._—When we investigate the cases of such lands,
it soon becomes apparent that besides the low percentage of any one
ingredient, the _proportions_ of others present require consideration.
Among these, _lime_ in the form of carbonate stands foremost. Its
presence exerts a dominant and beneficial influence in many respects,
as is readily apparent from the prompt change in vegetation whenever it
is introduced into soils deficient in it. In discussing the results of
soil analysis, its consideration is of first importance in forecasting
correctly the adequacy or inadequacy of other soil ingredients (see
chapter 20, page 379). For in general, we find that _lower percentages
of potash, phosphoric acid and nitrogen are adequate, when a large
proportion of lime carbonate is present_.—This has already been
referred to in connection with the table of soils of low percentages,
given above. In the interpretation of results obtained by analysis this
point must always be kept in view; and in the numerical statements
made below, it must be understood that they refer to virgin soils
sufficiently supplied with lime to assure a constant excess of lime
carbonate, maintaining the conditions of nitrification and insuring the
absence of acidity. (See chapter 9, page 146).

_Potash._—In respect to potash, the writer was led by his early
investigations in the State of Mississippi to conclude that less than
one-fourth of one per cent (.25) of potash constituted a deficiency
likely to call for early fertilization with potash salts; while as much
as .45% of the same seemed to cause the land to respond but feebly to
such fertilization. He has not found it necessary to revise materially
that early conclusion, whether from his own work or from that of
others. Within the last decade, Prof. Liebscher of Göttingen[119] has
arrived at this identical figure from analyses made of soils upon which
he had conducted a seven-year series of fertilizer tests; he having
found that potash fertilization produced no sensible, or at least no
paying results on land giving that figure, and otherwise well provided
with plant-food. The different (lower) figures given by Schloesing,
Risler and other French chemists in discussing the soils of France are
doubtless due to the weak acid and short period of digestion employed
in the analysis; an unfortunate discrepancy of methods which precludes
any direct comparison of results.

[119] Untersuchungen über die Bestimmung des Düngerbedürfnisses
der Ackerböden und Kulturpflanzen, von G. Liebscher; Journal für
Landwirtschaft 43 (1895), Nos. 1 & 2, pp. 48-216.

    These figures apply both to the arid and the humid regions
    in the temperate zones. In the tropics we find very much
    lower percentages quoted as adequate; thus in the laterite
    soils of India and Samoa, according to Wohltmann, in the
    soils of Jamaica according to Fawcett, and in those of
    Madagascar according to Müntz and Rousseaux.[120] There,
    potash-percentages over .10% seem to be high, and in
    Madagascar some lands in fair production range as low as
    .01%. The soil-extractions have however in these cases been
    made with a weaker acid than above specified, so that some
    increase of the figures (perhaps 33 to 50%) have to be
    allowed for. But even then there can be no question that a
    far less amount of potash, as determined by acid-extraction,
    is found sufficient for crop production in the tropics;
    doubtless because of the very intense decomposing
    (“fallowing”) effect of the continuous heat and moisture,
    tending also to a rapid decomposition of organic matter and
    a proportionally rapid formation of carbonic and nitric
    acids. Such soils are of course constantly kept in a leached
    condition, as a result of the heavy and continuous rainfall.

[120] _La Valeur Agricole des Terres de Madagascar._ Ann. de la Science
Agronomique, 2’me série, tome 1, 1901.

_Phosphoric Acid._—As regards the lower limit of adequacy of phosphoric
acid, there is a remarkable agreement in the investigations made
everywhere. It was placed at .05% by the writer as long ago as 1860,
as the result of investigations made in the State of Mississippi; and
the same figure has since been arrived at independently by agricultural
chemists in France, Russia, Germany and England. The cause of this
remarkable agreement is undoubtedly the readiness with which the
phosphates that come under consideration at all for the nutrition
of plants, are dissolved by almost any acid treatment likely to be
used in soil analysis. Almost the same agreement exists in regard to
the “adequacy” of .1% of P₂O₅; while all soils showing percentages
between .1 and .05% are considered weak on this side, and liable to
need phosphate fertilization soon. One-fourth of one per cent is
an unusually high percentage in most countries; .30% and over is
exceptional in non-ferruginous soils. But as stated on a previous page,
a high percentage of lime carbonate may offset a smaller percentage of
phosphoric acid, apparently by bringing about greater availability; and
a similar effect seems to result from the presence of a large supply of
humus.

On the other hand, very large percentages of finely divided ferric
hydrate may, especially in the absence of lime carbonate, render even
large supplies of phosphoric acid inert and useless, by the formation
of the totally insoluble ferric phosphate. Aluminic hydrate probably
acts in a similar manner. The following table gives examples in point,
as regards ferric hydrate.

       HAWAIIAN SOILS SHOWING HIGH CONTENTS OF FERRIC OXID.
               (Rept. Cal. Exp. Sta. 1894-5, page 27.)
    =============================+===============+=======================
                                 |     Oahu.     |        Hawaii.
    -----------------------------+-------+-------+-------+-------+-------
            NUMBER OF SAMPLE.    |No. 21.|No. 22.|No. 24.|No. 26.|No. 27.
    -----------------------------+-------+-------+-------+-------+-------
    Coarse Materials. 0.55ᵐᵐ     |  2.00 |  2.50 |  4.00 |  3.00 |  5.00
    Fine Earth                   | 98.00 | 97.50 | 96.00 | 97.00 | 95.00
                                 |       |       |       |       |
        CHEMICAL ANALYSIS OF     |       |       |       |       |
             FINE  EARTH.        |       |       |       |       |
    Insoluble matter             | 15.84 | 14.49 | 26.99 | 28.66 | 21.07
    Soluble Silica               | 14.07 | 30.37 | 10.26 |  7.35 |  2.68
    Potash (K₂O)                 |   .45 |   .26 |   .40 |   .61 |   .44
    Soda (Na₂O)                  |   .14 |   .08 |   .26 |   .17 |   .25
    Lime (CaO)                   |   .26 |  1.04 |   .52 |   .68 |   .28
    Magnesia (MgO)               |   .65 |   .80 |   .96 |  1.04 |   .60
    Br. ox. of Manganese (Mn₃O₄) |   .05 |   .03 |   .21 |   .20 |   .07
    Peroxid of Iron (Fe₂O₃)      | 39.05 | 19.68 | 19.10 | 18.23 | 30.10
    Alumina (Al₂O₃)              | 14.61 | 18.29 | 21.41 | 20.18 | 14.38
    Phosphoric acid (P₂O₅)       |   .19 |   .32 |   .64 |   .70 |   .97
    Sulfuric acid (SO₃)          |   .03 |   .09 |   .32 |   .21 |   .29
    Carbonic acid (CO₂)          |       |       |       |       |
    Water and organic matter     | 14.18 | 14.59 | 18.60 | 21.65 | 28.60
                                 +-------+-------+-------+-------+-------
        Total                    | 99.52 |100.04 | 99.67 | 99.61 | 99.73
                                 |       |       |       |       |
    Humus                        |  3.35 |  3.24 |  4.84 |  5.43 |  9.95
      “  Ash                     |  3.12 |  2.22 |  2.76 |  3.56 |  6.70
      “  Nitrogen, p.c. in Humus |  3.30 |  9.800|  2.800|  3.100|  1.71
      “     “    , p.c. in soil  |   .112|   .314|   .134|   .168|   .17
    Phosph. acid in humus ash    |   .110|   .166|   .580|   .500|
      Soluble in 2% Citric acid  |   .004|   .020|   .035|   .037|  .025
      in Nitric acid, 1.20 sp. g.|   .190|   .320|   .640|   .700|  .970
      in Chlorhydric acid        |       |       |       |       |
       (1.115 sp.g.)             |   .430|   .350|  1.600|  1.280|
    Hygroscopic moisture 15°C.   | 18.50 | 21.25 | 23.07 |  23.14| 23.81
    -----------------------------+-------+-------+-------+-------+-------

    _Unavailability of Ferric Phosphate._—It will be noted
    that in the soils from Oahu with an overwhelming amount
    of ferric oxid (mostly in the form of hydrate or rust)
    the citric acid has taken up only an insignificant amount
    of phosphoric acid; nitric acid took up 40 to 50 times as
    much, and chlorhydric doubled even this. In the much less
    ferruginous Hawaiian soils, though containing more alumina,
    the citric acid extracted nearly ten times as much; proving
    that it is chiefly ferric oxid, and not the alumina as has
    been supposed, that causes the insolubility of phosphoric
    acid in soils and doubtless also in fertilizers. The
    very unusually high content of phosphoric acid in the
    Hawaiian soils, exceeding all others on record, so far
    as known to the writer, emphasize the effects of ferric
    hydrate upon soluble phosphates; while the fact that these
    very soils are greatly benefited by the use of phosphate
    fertilizers, proves that the Dyer (citric acid) method for
    the determination of available phosphoric acid which in
    soils Nos. 21 to 26 yielded results largely in excess of the
    established limit in European soils, cannot be successfully
    applied to these highly ferruginous soils. It should also
    be noted that the amounts of phosphoric acid found in the
    humus extracted by the Grandeau method is in the first
    two Hawaiian soils over ten times the amount extracted by
    citric acid, but that while they rise and fall together, no
    definite quantitative ratio exists between the two.

It is obvious that in such soils, fertilization with water-soluble
phosphates would be likely to result in the quick partial withdrawal of
the same from useful action, and that any excess not promptly taken up
by the crop, is likely to become inert and useless. It will evidently
be desirable to use the phosphates in the form of bone meal or basic
slag (Thomas Phosphate), which because of their difficult solubility
will be acted upon but very slowly, if at all, by the ferric and
aluminic hydrates.

_Nitrogen._—In determining the nitrogen-content of the soil, a great
variety of methods has been followed. Some include all that can be
obtained by the combustion of the organic matters of soil and from
the nitrates present in the same; while others, the writer among the
number, believe that the mainly important source of nitrogen to the
plant being the nitrification of the humus-nitrogen, the determination
of the humus by the method of Grandeau, and of the nitrogen contained
in it, should be the standard; the unhumified vegetable matter being of
no definitely ascertainable value, and the nitrates varying from day
to day and being liable to be lost by leaching at any time; therefore
forming no permanent feature of the soil. Considering the variety of
methods, the unanimity with which about one-tenth of one per cent (.10)
has been assumed as the ordinarily adequate percentage is remarkable.
In view of the extremely variable amount of nitrogen in the humus
(ranging from 1.7 to nearly 22%), the amount of the latter cannot,
of course, afford even an approximation to the nitrogen-content;
except that as in the humid region, the nitrogen-percentage is not
known to exceed about 5 or 5.5%, an approximate estimate can be
made on that basis. In the arid region, according to location, the
nitrogen-percentage may be from three to six times greater for a
similar amount of humus. (See chap. 8. p. 135). In the writer’s
experience, a nitrogen-percentage of .1% in the arid region is a very
satisfactory figure, indicating that the need of nitrogen-fertilization
is not likely to arise for a number of years.

_Nitrification of the Organic Matter of the Soil._—In order to test the
question whether or not the nitrogen of the unhumified debris existing
in surface soils is directly nitrifiable, the writer selected a soil
which in its natural condition sustains intense nitrification, so that
at some points it contains as much as 1200 pounds of sodic nitrate
per acre. The composition of this soil, representing the land of the
“ten-acre tract” of the southern California substation, is as follows:

          SOIL FROM “TEN-ACRE TRACT,”
      SOUTHERN CALIFORNIA SUB-STATION, NO. 1284.

    Coarse Materials > 0.55ᵐᵐ              1.00
    Fine Earth                            99.00
                                         ------
                                         100.00
    CHEMICAL ANALYSIS OF FINE EARTH.

    Insoluble matter                      62.62} 70.92
    Soluble silica                         8.30}
    Potash (K₂O)                            .95
    Soda (Na₂O)                             .50
    Lime (CaO)                             5.07
    Magnesia (MgO)                          .84
    Br. ox. of Manganese (Mn₂O₄)            .06
    Peroxid of Iron (Fe₂O₃)                6.43
    Alumina (Al₂O₃)                        3.88
    Phosphoric acid (P₂O₅)                  .21
    Sulfuric acid (SO₃)                     .06
    Carbonic acid (CO₂)                    3.66
    Water and organic matter               6.02
                                          -----
    Total                                 99.70

    Water-soluble matter, per cent.         .137
    Sodic nitrate, per cent.                .020

    Humus                                  1.99
      “ Ash                                 1.13
      “ Nitrogen, per cent. in Humus        10.30
      “    “ , per cent. in soil            .203
    Total Nitrogen in soil                  .330
      “      “  in unhumified matter        .127
    Available Potash            {citric}    .03
    Available Phosphoric acid   {method}
    Hygroscopic Moisture
        absorbed at 15° C.                 5.81

It will be noticed that this is a rather strongly calcareous soil,
(nearly 9% of calcic carbonate), slightly impregnated with alkali,
of which about one-ninth is saltpeter. One portion of this soil was
thoroughly leached with distilled water until not a trace of nitrates
could be detected in the leachings. Another portion was treated for the
removal of humus according to the Grandeau method (see chapter 8, page
132); the extracted soil showed under the microscope an abundance of
vegetable debris, some slightly browned as from incipient humification.

The calcic and magnesic carbonates withdrawn in the humus-extraction
were then restored to the soil in the form of finely divided
precipitates and thoroughly mixed in, first in the dry and then in the
wet condition; the extracted soil being repeatedly wetted with turbid
water from the leached soil, in order to replace and reinfect it with
the nitrifying bacteria. Both soils were then spread out in flat glass
dishes and placed in a wooden box containing also a similar flat dish
with distilled water, upon which played the draught from the inlet
pipe opening into the outer air, with outlet-holes in the cover at the
opposite end; thus keeping the air within fairly moist. In addition,
the soils themselves were moistened with distilled water every three
days and restored to a loose condition by stirring. The whole was
placed so as to maintain, during the greater part of the 24 hours, a
temperature of from 30 to 35 degrees C. At intervals the samples of
both soils were leached and color-titrated for their nitrate content by
the picric-acid test. The results, calculated as sodic nitrate, during
two years were as follows:

    =====================+============+==============+==========
    Nitrate formed during|Four months.|Twelve months.|Two years.
    ---------------------+------------+--------------+----------
    Leached natural soil |    .012    |    .0420     |   .061
    Extracted soil       |    None.   |    .0030     |   .0042
    ---------------------+------------+--------------+----------

It will be noted that in the course of four months, nitrification had
not sensibly set in the extracted soil; while in the leached natural
soil the nitrate-content had reached to three-fifths the amount
originally present, and in the course of a year the nitrate-content
of the latter was more than double that of the original (unleached)
soil; while that in the extracted soil had only reached one-seventh
of the same. At the end of two years we find a still farther increase
of nitric nitrogen in both, the ratio between the two remaining about
the same (1:14). At the same time the ratio of increase attained at
first had materially diminished in the water-leached soil, probably on
account of the accumulation of the niter itself.

It thus appears that although the nitrogen of the unhumified organic
matter constituted about 40% of the total in the original soil, it
would during the entire year have contributed only to an insignificant
extent to the available nitrate-supply; while the fully humified
“matière noire” contributed fourteen times as much. During the ordinary
growing-season of four or five months the unhumified organic matter
would have yielded practically nothing to the crop.

_Functions of the unhumified Vegetable Matter._—The chief utility
of the unhumified matter in the soil consists of course in its
gradual conversion into true humus, in the course of which it evolves
carbonic gas to act on the soil minerals; while at the same time
it helps to render the soil more porous and thus facilitates the
action of the aerobic bacteria, for which it serves as food. Hence
the addition of vegetable matter to soils not already too “light”
is always advantageous, so long as it does not introduce injurious,
non-humifiable ingredients, like turpentine in the sawdust of resinous
pines. But it is always advisable to first use such matter as litter
for stock, in order to better prepare it for the processes of
humification, under the influence of ammoniacal fermentation, such as
occurs in the decay of green plants or animal matter. A portion of the
ash ingredients also is quickly utilized by solution in the soil-water.

_Matière Noire the Only Guide._—According to these results it is
clear that in order to gain any tangible indications with respect to
crop-bearing, it is the nitrogen in the humus proper, the _matière
noire_ only, that should serve as the basis; and that as a current
source of nitrogen to the plant, the unhumified matter is hardly
entitled to more consideration than the “insoluble silicates.” For,
the favorable conditions for nitrification under which the above
experiment was conducted, will very rarely be even approached under
field conditions.

_What are the Adequate Nitrogen Percentages in the Humus_?—The
nitrification of the _matière noire_ being, apparently, the main source
of plant-nutrition with that element under ordinary conditions, the
question naturally arises as to what may be considered an adequate
nitrogen-content of that substance, so as to permit a full supply of
nitrates to the crop.

The data extant on this subject are rather scanty, and thus far
have all been obtained at the California Experiment Station.[121]
But they seem to be very cogent in proving that the growth of crops
removed from the soil causes a rapid depletion of the nitrogen in the
humus-substance, and that _so soon as the nitrogen-percentage in the
same falls below a certain point, the soil becomes “nitrogen-hungry;”_
so that the application of nitrogenous fertilizers is needed and is
very effective. The data in the table below, as well as the figure of a
culture experiment (No. 52 below), illustrate this point.

         ADEQUACY AND INADEQUACY OF NITROGEN CONTENTS OF HUMUS.
     =========+=======+==================+=========+=========+===========
    Collection|Kind of|     Locality.    |Per cent.|Per cent.| Per cent.
      Number. | Soil. |                  |Humus in |Nitrogen | Nitrogen
              |       |                  |  Soil.  |in Humus.|in Soil.
              |       |                  |         |         |  [122]
     ---------+-------+------------------+---------+---------+-----------
           6  | Black |Near Stockton, San|         |         |
              | Adobe.| Joaquin Co., Cal.|   1.05  |  18.66  |   .196
     ---------+-------+------------------+---------+---------+-----------
        1679  |   “   |Virgin Soil,      |         |         |
              |       | University       |         |         |
              |       | Grounds, Berkeley|   1.20  |  18.58  |   .203
     ---------+-------+------------------+---------+---------+-----------
        1842  |   “   |Ramie plot, Univ. |         |         |
              |       | Grounds, 10 years|         |         |
              |       | cultivated       |   1.80  |   4.17  |   .075
     ---------+-------+------------------+---------+---------+-----------
        1841  |   “   |Grass plot, Univ. |         |         |
              |       | Grounds, 10 years|         |         |
              |       | cultivated       |   1.65  |   3.40  |   .056
     ---------+-------+------------------+---------+---------+-----------
          29  | Dark  |Sugar-cane land,  |         |         |
              | loam. |  Maui, H. T.     |  10.90  |   3.15  |   .347
     ---------+-------+------------------+---------+---------+-----------
          27  | Dark  |Guava-land hills, |         |         |
              | loam. | near Hilo, Hawaii|   9.95  |   1.71  |   .170
              |       | Island           |         |         |
     ---------+-------+------------------+---------+---------+-----------

[121] _The Supply of Soil Nitrogen_, Rep. Cal. Expt. Station, 1892-93,
page 68; ibid., 1894-95, page 28; _The Recognition of Nitrogen
Hungriness in Soils_, in Bull. 47, Div. of Chemistry, U.S. Department
of Agriculture, 1895; Landw. Presse, No. 53, July 1885. See also for
detailed data chapter 8, page 135.

[122] Calculated upon the true humus substance (matière noire), _not_
by determining total (incl. unhumified) nitrogen in the soil.

    Nos. 6 and 1679 show the usual humus-and
    nitrogen-percentages in the “black adobe” or “prairie”
    soils of California. Nos. 1842 and 1841 represent the same
    soil as 1679, upon which, however, ramie and ray grass had
    respectively been growing, without fertilization, for about
    ten years; showing that while the _humus-content of the
    soil has increased, the nitrogen-content of the humus has
    decreased_ in the case of ramie by 72.78%, in that of
    the grass by 76.78%; reducing the land to figures commonly
    found in the humid region. In the case of the ramie, the
    partial return through the leaves has resulted in a higher
    humus-content, together with higher nitrogen-percentage,
    than in the case of the grass, which in the several cuttings
    annually made, caused a greater depletion in nitrogen and a
    smaller accession of humus. The grass was very weak in its
    growth and partially dying out.

    No. 29, the sugar-cane land from Maui, was still in fair
    production, but beginning to weaken as against its first
    production. No. 27, the guava land from Hawaii, originally
    bore a luxuriant cover of wild guava, but after bearing one
    fair crop of seed-cane and one of ratoons, the cane planted
    on it “spindled up” and died so soon as the seed-cane
    planted was exhausted. Both the island soils, originally
    derived from the weathering of the black basaltic lavas
    of the region, were well supplied with mineral plant-food
    (see above, page 356), and the humus-content in both was
    exceptionally high; and neither was in an acid condition.
    The difference in their nitrogen-content, both in the totals
    and in the humus itself, suggested that notwithstanding
    the relatively high total of nitrogen in No. 27, it might
    be nitrogen-hungry, in view of the low percentage of the
    nitrogen in the humus.

[Illustration: FIG. 59.—Growth of Wheat on Guava Soil from Hawaii
Island.]

_Confirmatory Experiment._—A pot-culture with wheat, the results of
which are shown in the figure below, fully confirm this suspicion. One
kilogram of soil was used in each of two pots, one being fertilized
with half a gram of Chile saltpeter. The experiment could not be
carried to full completion on account of the overwhelming invasion
of mildew; but the figures speak for themselves. Moreover, a field
trial made on the island with saltpeter, in pursuance of the writer’s
recommendation, resulted in a luxuriant growth of the cane.

_Data for Nitrogen-adequacy._—It appears from the facts shown above,
that for the growth of grasses a nitrogen-percentage in the humus of
1.7 is wholly inadequate, no matter how much humus may be present. A
percentage of 3.15 in the Maui soil, No. 29, containing nearly 11%
of humus, gave only a fair crop of sugar-cane; on the Berkeley grass
plot, with 3.40% and only 1.65 of total humus, the ray grass was barely
maintaining life. The ramie, with 4.17% of nitrogen in the soil-humus,
was still doing fairly well.

It is doubtless impossible to give one and the same absolute figure for
nitrogen-deficiency for all plants and soils. Where the conditions of
nitrification are favorable, as in the presence of much of the earth
carbonates, a smaller percentage may suffice for the same plants that
elsewhere suffer; and it is highly probable that different minima will
be found for plants of different relationship and root-habits. But
there is every reason to believe that _in the nitrogen-percentage of
soil-humus, considered in connection with other chemical and physical
conditions and soil derivations_, we have a means of ascertaining the
needs of plants with respect to nitrogen-fertilization, if proper study
be given to the subject. Broadly speaking, it appears to be necessary
_to keep the nitrogen-percentage of soil-humus near 4% to insure
satisfactory production_.

It having been suggested that the frequent and disastrous crop failures
on the noted tchernozem or black-earth soils of Russia might be due in
part at least to nitrogen-depletion of the humus, the writer obtained
through the courtesy of Prof. P. Kossovitch of St. Petersburg soil
samples from the center of the Black-earth region, both cultivated and
uncultivated. These samples are in appearance exactly like some of the
dark alluvial soils of Louisiana and California, and approach them very
nearly in the essentials of composition, as will be seen from the table
below:

                            ANALYSES OF BLACK SOILS,
     =============================+=================+====================
                                  |   Tchernozem    |      Alluvial
                                  |    (Russia.)    | Black clay lands.
                                  +------+----------+---------+----------
                                  |      |          |Louisiana|California
                                  |      |          | No. 240.| No. 1167.
                                  |Virgin|Cultivated+---------+----------
                                  |      |          |Back-land|Black-land
                                  |      |          |  Houma. |  Tulare.
     -----------------------------+------+----------+---------+----------
     CHEMICAL ANALYSIS OF         |      |          |         |
          FINE EARTH.             |      |          |         |
    (No coarse material in soils.)|      |          |         |
                                  |      |          |         |
     Insoluble matter             | 48.38|  55.09   |   35.48 |   62.43
     Soluble silica               | 13.21|  12.28   |   20.76 |   16.99
     Potash (K₂O)                 |   .72|    .52   |    1.03 |    1.09
     Soda (Na₂O)                  |   .20|    .13   |     .13 |     .77
     Lime (CaO)                   |  1.51|   1.31   |     .72 |    1.46
     Magnesia (MgO)               |   .73|    .75   |     .88 |    1.44
     Br. ox. of Manganese (Mn₃O₄) |   .05|    .03   |     .01 |     .06
     Peroxid of Iron (Fe₂O₃)      |  7.12|   4.80   |    7.10 |    4.98
     Alumina (Al₂O₃)              |  5.22|   4.73   |   15.45 |    6.87
     Phosphoric acid (P₂O₅)       |   .14|    .13   |     .15 |     .12
     Sulfuric acid (SO₂)          |   .07|    .08   |     .25 |     .02
     Carbonic acid (CO₂)          |      |          |         |
     Water and organic matter     | 22.78|  19.94   |   18.52 |
           Total                  |100.13|  99.79   |  100.48 |  100.59
                                  |      |          |         |
     Humus                        |  5.11|   5.54   |    5.07 |    1.33
       “   Ash                    |  1.80|   1.40   |     .91 |     .36
       “   Nitrogen, per cent.    |      |          |         |
                       in Humus   |  4.63|   4.22   |         |
       “      “      per cent.    |      |          |         |
                        in soil   |   .27|    .24   |         |
     Available Potash             |      |          |         |
        (citric acid method)      |  .014|    .010  |         |
     Available Phosph. acid       |  .011|    .008  |     .08 |     .01
        (citric acid method)      |      |          |         |
     Hygroscopic Moisture         |      |  12.07   |   18.82 |    5.38
        absorbed at               |      |   17°C   |   13°C  |   15°C
     -----------------------------+------+----------+---------+----------

It will be seen that the Russian soil is of high fertility according
to the standards given above, and that the nitrogen-content of the
abundant humus is amply within the limits of adequacy suggested by the
experience in California and Hawaii. The humus-content of the arid
California soils is characteristically low as compared with the Russian
tchernozem as well as with the Houma backland of humid Louisiana; but
its nitrogen-content is doubtless at least three times that of the
latter, as is that of the humus of similar lands in which it has been
determined.


INFLUENCE OF LIME UPON SOIL FERTILITY.

Assuming as substantially correct the numerical data given above in
respect to the three leading ingredients of plant-food—phosphoric acid,
potash and nitrogen,—the dominant role of lime in soil fertility,
already mentioned, requires some farther illustration and discussion.

“_A Lime Country is a Rich Country._”—The instant change of vegetation
when we pass from a non-calcareous region to one having calcareous
soils, has already been alluded to. (See this chapter, p. 354). But it
is not necessary to be a botanist to see the change in the prosperity
of the farming population as one enters a lime district. The single
log-cabin with, probably, a wooden barrel terminating the mud-plastered
chimney, is replaced, first by double log-houses, then by frame, and
farther on by brick buildings, with the other unmistakable evidences of
prosperity. Thus this is seen in passing from the mountain region of
Kentucky into the “blue-grass” country, which is throughout underlaid
by calcareous formations; and thus, likewise, in crossing the strike
of the formations of Alabama, Mississippi and Louisiana, or any other
region where underlying calcareous formations have contributed to the
formation of the soils, as compared with some adjacent district where
this is not the case. The calcareous loess areas bordering on the
Mississippi river and some of its chief tributaries, are conspicuous
cases in point, as are also the prairies of Illinois and Indiana.

_Effects of High Lime-content in Soils._—The table below illustrates
the fact that in the presence of high lime-percentages, relatively
low percentages of phosphoric acid and potash may nevertheless
prove adequate; while the same, or even higher amounts, in the
absence of satisfactory lime-percentages prove insufficient for good
production.[123]

[123] This statement appears contradictory of the observations of
Schloesing upon the solubility of phosphoric acid in presence of lime
carbonate (Am. Sci. Agron., tome 1, 1899), but the natural conditions
seem to justify fully the above conclusion.

                  SOILS SHOWING LOW PHOSPHORIC ACID PERCENTAGE.
    ============================+======================================
                                |             HIGH LIME.
                                +-----------+---------+----------------
                                |Mississippi|Louisiana|   California
                                +-----------+---------+------+---------
                                |   Kemper  |  Vernon | Yuba |  Amador
                                |   County  | County  |County|  County
    ----------------------------+-----------+---------+------+---------
         Number of Sample.      |    139    |    171  | 499  |   1113
    ----------------------------+-----------+---------+------+---------
     Chemical Analysis of       |           |         |      |  Slate
         Fine Earth.            |           |         |      |   Soil
    ----------------------------+-----------+---------+------+---------
    Insoluble matter            |           |  53.19  | 78.79| 49.96
                                |  67.08    |    74.29|      |    64.92
    Soluble silica              |           |  21.10  |  3.80| 14.96
    ----------------------------+-----------+---------+------+---------
    Potash (K₂O)                |   .70     |   .33   |   .25|  1.48
    Soda (Na₂O)                 |   .14     |   .06   |   .04|   .43
    Lime (CaO)                  |  1.37     |  1.40   |  1.02|   .60
    Magnesia (MgO)              |  1.00     |   .74   |   .40|  2.21
    Br. ox. of Manganese (Mn₃O₄)|   .25     |   .15   |   .02|   .05
    Peroxid of Iron (Fe₂O₃)     |  6.75     |  4.52   |  5.81| 11.52
    Alumina (Al₂O₃)             | 13.07     | 11.36   |  6.28| 12.31
    Phosphoric acid (P₂O₅)      |   .03     |   .05   |   .04|   .05
    Sulfuric acid (SO₃)         |   .08     |   .12   |   .02|   .02
    Carbonic acid (CO₂)         |           |         |      |
    Water and organic matter    |  9.45     |  7.27   |  3.64|  6.63
                                +-----------+---------+------+---------
             Total              | 99.91     |100.29   |100.19|100.22
                                +-----------+---------+------+---------
    Hygroscopic Moisture        | 11.45     | 18.11   | 4.80 |  5.74
      absorbed at           °C  |  8.0      |  25.5   | 15.0 | 15.0
    ----------------------------+-----------+---------+------+---------

    ============================+=======================================
                                |          LOW LIME.
                                +-----------------+---------------------
                                |  Mississippi    |      California
                                +---------+-------+-----------+---------
                                |Chickasaw|Carroll|  Shasta   |Humboldt
                                | County  |County |  County   | County
    ----------------------------+---------+-------+-----------+---------
            Number of Sample.   |   164   |  48   |    559    |  207
    ----------------------------+---------+-------+-----------+---------
     Chemical Analysis of       |         |       |           |
         Fine Earth.            |         |       |           |
    ----------------------------+---------+-------+-----------+---------
    Insoluble matter            |93.62    |       | 76.27     | 65.35
                                |    94.98| 89.39 |      80.38|    72.24
    Soluble silica              | 1.36    |       |  4.10     |  6.90
    ----------------------------+---------+-------+-----------+---------
    Potash (K₂O)                |  .09    |   .19 |   .50     |   1.13
    Soda (Na₂O)                 |  .07    |   .08 |   .04     |    .28
    Lime (CaO)                  |  .07    |   .08 |   .10     |    .11
    Magnesia (MgO)              |  .13    |   .07 |   .40     |   3.33
    Br. ox. of Manganese (Mn₃O₄)|  .02    |   .12 |   .01     |    .12
    Peroxid of Iron (Fe₂O₃)     | 1.09    |  1.21 |  6.67     |   6.99
    Alumina (Al₂O₃)             | 1.47    |  4.37 |  8.48     |  10.24
    Phosphoric acid (P₂O₅)      |  .03    |   .05 |   .04     |    .17
    Sulfuric acid (SO₃)         |  .01    |   .05 |   .01     |    .02
    Carbonic acid (CO₂)         |         |       |           |
    Water and organic matter    | 2.00    |  4.09 |  3.97     |   5.63
                                +---------+-------+-----------+---------
                 Total          |99.94    | 99.70 |100.62     | 100.24
                                +---------+-------+-----------+---------
    Hygroscopic Moisture        | 1.80    |  4.66 |  5.05     |   7.87
      absorbed at           °C  |11.0     |  11.0 |  17.0     |   13.0
    ----------------------------+---------+-------+-----------+---------

    Nos. 139 and 171 are heavy black prairie soils of high
    productive capacity, whose production had, at the time of
    sampling, lasted almost undiminished for over twenty years.
    Nearly the same is true of the two California soils, Nos.
    499 and 1113; which, however, are ferruginous loams of only
    moderate clay-content. In all, the percentage of phosphoric
    acid shown by the analysis is at or below the recognized
    limit of deficiency, while the lime-content of all is as
    high as is required for the welfare of any soil, however
    constituted. The potash-percentage also is low in all except
    the “red foothill soil,” No. 1113.

    Passing to the soils of low lime-content, we find the two
    Mississippi soils, poor in both potash, lime and phosphoric
    acid, so low in production as to be wholly unprofitable in
    cultivation without previous fertilization; No. 559, from
    California, produced two fair crops of barley and then no
    more. No. 207, is the soil of Eel river bottom, California;
    profusely productive at first, by virtue of its high content
    of both potash and phosphoric acid; but “giving out” under a
    few years’ culture of clover or alfalfa (which draw heavily
    upon lime), and quickly restored to productiveness under the
    influence of dressings of quicklime. In this case the soil
    had become acid, a condition which always militates against
    the success of culture plants, and more especially against
    those of the leguminous relationship.

_What are Adequate Lime Percentages?_—We have in the presence or
absence of the natural vegetation peculiar to calcareous soils
(“calciphile”) an excellent index of the presence or absence of such
amounts of lime carbonate as fulfil the conditions of its beneficial
effects. Lists of such plants for the United States are given farther
on; they agree almost throughout with such plants as are everywhere
recognized by American farmers as indicating productive soils.

All soils bearing such vegetation show with red litmus paper, when
wetted, a neutral reaction at first, which after the lapse of twenty or
thirty minutes turns to a blue alkaline one; such as is given under the
same conditions by the carbonates of lime and magnesia.

But the reverse is not necessarily true; for we occasionally find soils
containing considerable amounts of lime carbonate that yet fail to bear
lime vegetation. This is the case of extremely heavy clay soils, as
exemplified in the table below in the case of the last three soils;
while the first, No. 220, exemplifies a case where although potash is
exceptionally high, only scrubby oak growth is produced in presence of
an amount of lime that in sandy lands would show profuse lime growth.

              TABLE ILLUSTRATING THE NEED OF HIGH LIME-PERCENTAGES
                            IN HEAVY CLAY SOILS.
    ============================+===========================+===========
                                |        Mississippi.       |California.
                                +---------+-------+---------+-----------
                                |Flatwoods|  Hog- |  Ridge  |  Yellow
                                | Pontotoc| wallow| Prairie,|  ridge,
                                |   Co.   | Jasper|Smith Co.|Alameda Co.
                                |         |  Co.  |         |
    ----------------------------+---------+-------+---------+-----------
    No. Sample.                 |   230   |  242  |   203   |     4
    ----------------------------+---------+-------+---------+-----------
        CHEMICAL ANALYSIS OF    |         |       |         |
             FINE EARTH.        |         |       |         |
    ----------------------------+---------+-------+---------+-----------
    Insoluble matter            |   77.85 | 76.76 |   51.75 |   86.00
    Soluble silica              |         |       |         |
    ----------------------------+---------+-------+---------+-----------
    Potash (K₂O)                |     .75 |   .53 |     .53 |     .19
    Soda (Na₂O)                 |     .11 |   .19 |     .22 |     .15
    Lime (CaO)                  |     .18 |   .42 |     .48 |     .48
    Magnesia (MgO)              |     .83 |   .67 |    1.01 |     .45
    Br. ox. of Manganese (Mn₃O₄)|     .17 |   .56 |     .10 |     .04
    Peroxid of Iron (Fe₂O₃)     |    5.90 |  4.12 |   23.79 |    4.01
    Alumina (Al₂O₃)             |   10.30 | 10.06 |   10.85 |    5.53
    Phosphoric acid (P₂O₅)      |     .05 |   .06 |     .15 |     .06
    Sulfuric acid (SO₃)         |     .03 |   .06 |     .02 |     .02
    Carbonic acid (CO₂)         |         |       |         |
    Water and organic matter    |    3.69 |  5.73 |   11.39 |    4.05
                                +---------+-------+---------+-----------
             Total              |   99.86 | 99.17 |  100.29 |  100.99
                                +---------+-------+---------+-----------
                                |         |       |         |
    Hygroscopic Moisture        |    9.3  |  6.8  |   19.7  |
      absorbed at            °C |   22.0  |air-dry|   17.0  |
    ----------------------------+---------+--------+---------+-----------

All of the soils in this table are heavy clays, very difficult to till;
in all, the lime-percentage falls below .5%; and none bear any lime
vegetation, the Mississippi soils having a stunted growth of black
jack and post oaks, such as is universally known to indicate soils too
poor for profitable cultivation. The California soil bears stunted
live-oak (_Q. agrifolia_); but not being as heavy as its brethren from
Mississippi, though unthrifty, is more readily improved.

    Comparison with the two first sandy soils in the table on
    p. 352 shows, that with plant-food percentages equal to, or
    even much below those here shown, not only was vigorous lime
    growth present, but crop-production was good and even high.

We are thus led to the conclusion that the greater the clay percentage
in a soil, the more lime carbonate it must contain in order to possess
the advantages of a calcareous soil; and that while in sandy lands lime
growth may follow the presence of only .10% of lime, in heavy clay
soils not less than about .6% should be present to bring about the
same result. This is apparent to the eye in that the dark-tinted humus
characteristic of truly calcareous lands, does not appear in clay soils
until the lime-percentages rise to nearly 1%; while in sandy lands a
much smaller amount (say .2%) will produce this effect.

    _European Standards._—It is of interest to consider,
    in connection with preceding discussions, the estimates
    given by Maercker of Halle, of the practical value of soils
    corresponding to chemical composition as ascertained by
    analysis with strong acids, substantially in accordance with
    the methods adopted by the writer.

    PRACTICAL RATING OF SOILS BY PLANT-FOOD PERCENTAGES ACCORDING TO
                 PROF. MAERCKER, HALLE STATION, GERMANY.
    =====================+==========+==========+====================
                         |          |          |       Lime.
       Grade of Soil.    | Potash.  |Phosphoric+----------+---------
                         |          |   Acid.  |   Clay   |  Sandy
                         |          |          |   Soil.  |  Soil.
    ---------------------+----------+----------+----------+---------
    Poor                 |Below 0.05|Below 0.05|Below  .10|Below .05
    Medium               | 0.05-0.15| .05 - .10|  .10- .25|  .10-.15
    Normal               | 0.15-0.25| .10 - .15|  .25- .50|  .15-.20
    Good                 | 0.25-0.40| .15 - .25|  .50-1.00|  .20-.30
    Rich                 |Above 0.40| Above .25|Above 1.00|Above .30
    =====================+==========+==========+==========+=========
    Av’age for California|   0.70   |   0.08   |        1.08
      “     “  Arid Reg. |    .73   |    .12   |        1.36
      “     “  Humid Reg.|    .22   |    .11   |         .11
    ---------------------+----------+----------+--------------------
                         |          |
       Grade of Soil.    |   Total  |  Humus
                         | Nitrogen.| Nitrogen.
                         |          |
    ---------------------+----------+----------
    Poor                 | Below .05|
    Medium               |   .05-.10|
    Normal               |   .10-.15|
    Good                 |   .15-.25|
    Rich                 | Above .25|
    =====================+==========+
    Av’age for California|          |  .102
      “     “  Arid Reg. |    .11   |   (?)
      “     “  Humid Reg.|    .12   |  .166
    ---------------------+----------+----------

    It will be observed that according to Maercker’s valuation,
    the average California soil is “rich” in potash and lime,
    but only “medium” as regards its contents of phosphoric
    acid and nitrogen. In this respect, and almost throughout,
    Maercker’s ratings are in remarkable agreement with those
    made by the writer as far back as 1860.[124] It also appears
    that Maercker’s figures for “normal” soils correspond to
    those of the American humid regions; the “arid” figures for
    potash and lime being “abnormally” high.

[124] See discussions of analyses of Mississippi soils in the Report
on the Agriculture and Geology of Mississippi, 1860; same in Rep. On
Cotton Production, Tenth Census, 1880, Vol. 5; also Appendix to the
Report on the Experiment Stations of the University of California,
1890, p. 163.

Unfortunately neither Maercker’s method of preparing the soil extract,
nor his ratings as given in the table, are accepted by all soil
chemists even in Germany. As will be seen by reference to Wohltmann’s
work on the soils of Samoa and Kamerun (chap. 21, p. 404), his methods
and numerical estimates differ widely from those given by Maercker,
and also from those adopted by the Prussian soil surveys. Reference
to the analyses of the soils of Madagascar by Müntz and Rousseaux,
given in the same chapter, page 406, shows still another different
method, although as it happens their numerical estimates do not differ
very widely from those of Wohltmann. In both cases, a special, more
incisive extraction is made for the determination of potash. Why the
same more energetic action is not used for the other ingredients also,
is not stated, and is obscure. Fortunately, in all cases the action
is at least sufficiently strong to secure the dissolution of all the
lime existing in the form of carbonate, and of all, or nearly all, the
phosphoric acid not securely locked up as ferric phosphate; the latter
being inert, is of no special interest (see Analyses of Hawaiian Soils,
this chapter, page 256).




CHAPTER XX.

SOILS OF THE ARID AND HUMID[125] REGIONS.


_Composition of Good Medium Soils._—In the preceding tables examples
have been given of rather extreme types of soils, both rich and poor
throughout, and also of such as are deficient in one or several of
the important ingredients. In the table below are given the analyses
of some of the good average farming lands; uplands of several states,
both of the humid and arid regions. In the former, the representative
timber trees of such lands are the black, red, white and (less
characteristically) the post, black-jack, Spanish, overcup and locally
some other oaks; grading higher in proportion to the presence of more
or less hickory, and lower as the latter is replaced by pine. In the
states south of Ohio, the “oak and hickory uplands” are what the farmer
usually looks for, outside of the valleys or bottoms.

[125] In the discussion in this chapter the “humid region” referred
to is always that of the temperate zones, unless expressly otherwise
stated. The most humid region of all—the tropics—is treated under a
special head.

_Criteria of Lands of the Two Regions._—In the country west of the
Rocky Mountains, the timber, while locally very characteristic,
cannot be as broadly used as a criterion, partly on account of
its scarcity, partly because the dominant factor in the growth of
trees is _moisture_, which is measurably independent of chemical
soil-composition. The latter, moreover, on account of climatic
conditions, already alluded to (chapter 16), does not vary as
materially in the arid as the humid region, on account of the almost
universal presence of larger proportions of lime carbonate; the
variations of which in the humid region govern largely the vegetative
changes. For we there find _the timber growth of the lowlands ascending
into the uplands so soon as the latter becomes decidedly calcareous_;
as is abundantly exemplified in the loess or “bluff” formations
bordering the Mississippi, Ohio, and Missouri rivers, where the black
walnut, tulip tree, ash, honey-locust, together with the lowland oaks,
hickories and cane usually characterizing the stream bottoms, grow
abundantly and with luxuriant development on the adjoining steep hill
country as well (see below, chapters 24, 25).

                      UPLAND SOILS OF HUMID REGION.
    ====================================================================
                       OAK UPLANDS WITH HICKORY AND WALNUT.
    -----------------------+----------+---------------+--------+--------
             State.        |Tennessee.|  Mississippi. |Alabama.|Georgia.
                           |          |               |        |
    -----------------------+----------+--------+------+--------+--------
             County.       |Rutherford|Pontotoc|Benton|Cherokee|  Polk
    -----------------------+----------+--------+------+--------+--------
        Number of Sample.  |    7     |   226  |  216 |  110   |   502
    -----------------------+----------+--------+------+--------+--------
    ANALYSIS OF FINE EARTH.|          |        |      |        |
                           |          |        |      |        |
    Insoluble matter       |75.35     |83.27   |      |78.73   |72.32
                           |   82.66  |   88.83|83.35 |   84.77|   76.55
    Soluble silica         | 7.31     | 5.56   |      | 6.04   | 4.23
    Potash (K₂O)           |  .26     |  .37   |   .55|  .26   |  .73
    Soda (Na₂O)            |  .26     |  .22   |   .08|  .12   |  .17
    Lime (CaO)             |  .34     |  .28   |   .25|  .33   |  .29
    Magnesia (MgO)         |  .30     |  .23   |   .48|  .40   |  .26
    -----------------------+----------+--------+------+--------+--------
    Bro. ox. of Manganese  |          |        |      |        |
                 (Mn₃O₄)   |  .04     |  .28   |   .76|  .22   |  .18
    -----------------------+----------+--------+------+--------+--------
    Peroxid of Iron (Fe₂O₃)| 5.18     | 2.39   |  4.80| 3.71   | 6.29
    Alumina (Al₂O₃)        | 5.57     | 4.51   |  6.28| 5.08   | 7.10
    Phosphoric acid (P₂O₅) |  .08     |  .08   |   .07|  .09   |  .26
    Sulfuric acid (SO₃)    |  .08     |  .02   |   .06|  .10   |  .11
    Carbonic acid (CO₂)    |          |        |      |        |
    -----------------------+----------+--------+------+--------+--------
    Water and organic      |          |        |      |        |
            matter         | 5.50     | 3.11   |  4.20| 5.15   | 6.60
    -----------------------+----------+--------+------+--------+--------
        Total              |99.77     |100.32  |100.88|100.23  |99.54
    -----------------------+----------+--------+------+--------+--------
    Humus                  |          |        |      |        |
      “   Ash              |          |        |      |        |
                           |          |        |      |        |
    Hygroscopic Moisture   |  7.29    | 4.08   |  6.84| 4.50   | 8.71
      absorbed at      °C  |  22°C    |        |      |        | 18°C
    -----------------------+----------+--------+------+--------+--------
                                  SHORT-LEAVED PINE.
    -----------------------+--------+---------+-----------------+---------
             State.        |  North |  South  |  Mississippi    |Louisiana
                           |Carolina|Carolina |                 |
    -----------------------+--------+---------+--------+--------+---------
             County.       |Cabarrus|Spartan- | Sumner |Franklin|Morehouse
                           |        |  burgh  |        |        |
    -----------------------+--------+---------+--------+--------+---------
        Number of Sample.  |    9   |     5   |   142  |   71   |   232
    -----------------------+--------+---------+--------+--------+---------
    ANALYSIS OF FINE EARTH.|        |         |        |        |
                           |        |         |        |        |
    Insoluble matter       |78.79   | 43.74   |90.23   |88.75   |81.70
                           |   86.19|    49.61|   92.55|   90.56|   87.45
    Soluble silica         | 7.40   |  5.87   | 2.32   | 1.81   | 5.75
    Potash (K₂O)           |  .13   |   .21   |  .24   |  .14   |  .44
    Soda (Na₂O)            |  .01   |   .09   |  .09   |  .09   |  .27
    Lime (CaO)             |  .34   |   .03   |  .09   |  .07   |  .10
    Magnesia (MgO)         |  .31   |   .21   |  .20   |  .19   |  .24
    -----------------------+--------+---------+--------+--------+--------
    Bro. ox. of Manganese  |        |         |        |        |
                 (Mn₃O₄)   |  .05   |   .01   |  .07   |  .08   |  .39
    -----------------------+--------+---------+--------+--------+--------
    Peroxid of Iron (Fe₂O₃)| 4.99   | 11.70   | 1.84   | 2.41   | 3.55
    Alumina (Al₂O₃)        | 4.02   | 26.54   | 1.86   | 2.20   | 4.87
    Phosphoric acid (P₂O₅) |  .14   |   .13   |  .09   |  .08   |  .10
    Sulfuric acid (SO₃)    |  .08   |   .01   |  .01   |  .01   |  .08
    Carbonic acid (CO₂)    |        |         |        |        |
    -----------------------+--------+---------+--------+--------+--------
    Water and organic      |        |         |        |        |
            matter         | 3.88   | 11.60   | 2.83   | 4.31   | 2.54
    -----------------------+--------+---------+--------+--------+--------
        Total              | 100.14 |100.22   | 99.87  | 100.14 |100.03
    -----------------------+--------+---------+--------+--------+--------
    Humus                  |        |         |        |        |
      “   Ash              |        |         |        |        |
                           |        |         |        |        |
    Hygroscopic Moisture   | 3.65   | 11.21   | 3.57   | 4.4    | 5.47
      absorbed at      °C  | 21.8°C | 21.8°C  | 21.8°C |        |
    -----------------------+--------+---------+--------+--------+--------

                     UPLAND SOILS OF ARID REGION.

     ======================+============================================
                           |                   CALIFORNIA.
                           +----------+---------+------------+----------
                           |Placer Co.|San Diego| Ventura Co.|Riverside
                           | Auburn.  |   Co.   |            |    Co.
                           |          |National |            |Arlington.
                           |          |  City.  |            |
     ----------------------+----------+---------+------------+----------
       Number of Sample.   |    51    |   47    |     182    |   1406
     ----------------------+----------+---------+------------+----------
    ANALYSIS OF FINE EARTH.|          |         |            |
     ----------------------+----------+---------+------------+----------
     Insoluble matter      |          |         | 74.91      |  76.41
                           |  69.52   |  86.21  |    82.84   |     84.61
     Soluble silica        |          |         |  7.93      |   8.20
     ----------------------+----------+---------+------------+----------
     Potash (K₂O)          |    .38   |    .48  |   .62      |    .87
     Soda (Na₂O)           |    .07   |    .14  |   .16      |    .29
     Lime (CaO)            |    .96   |    .36  |   .95      |   1.57
     Magnesia (MgO)        |   1.09   |    .54  |   .96      |   1.33
     ----------------------+----------+---------+------------+----------
     Br. ox. of Manganese  |          |         |            |
                (Mn₃O₄)    |    .39   |    .10  |   .04      |    .04
     ----------------------+----------+---------+------------+----------
     Peroxid of Iron(Fe₂O₃)|  12.42   |   3.69  |  5.07      |   4.20
     Alumina (Al₂O₃)       |  10.97   |   5.12  |  5.94      |   5.30
     Phosphoric acid (P₂O₅)|    .16   |    .23  |   .13      |    .14
     Sulfuric acid (SO₃)   |    .01   |    .03  |   .04      |    .01
     Carbonic acid (CO₂)   |          |         |            |
     ----------------------+----------+---------+------------+----------
     Water and organic     |   5.14   |   2.60  |  2.67      |   1.60
                 matter    |          |         |            |
     ----------------------+----------+---------+------------+----------
                 Total     | 101.10   |  99.50  | 99.41      | 100.05
     ----------------------+----------+---------+------------+----------
     Humus                 |   1.14   |    .56  |  1.06      |    .20
       “   Ash             |   1.12   |   1.04  |  1.00      |    .64
     Hygroscopic Moisture  |          |   2.30  |  6.59      |   1.77
       absorbed at      °C |          |   15    |     15     |     15
     ----------------------+----------+---------+------------+----------
     ======================+=======================++==================
                           |      WASHINGTON.      ||      MONTANA.
                           +---------+-------------++---------+--------
                           |  Bunch  | Bunch grass || Judith  | Near
                           |  grass  |Selah Valley,||  Gap.   | Bozeman
                           |Ritzville|   Rolling   ||         | Allen’s
                           |  Ridge. |   Upland.   ||         | Ranch.
     ----------------------+---------+-------------++---------+--------
       Number of Sample.   |    46   |     37      ||    371  |   387
     ----------------------+---------+-------------++---------+--------
    ANALYSIS OF FINE EARTH.|         |             ||         |
     ----------------------+---------+-------------++---------+--------
     Insoluble matter      |76.71    | 77.18       || 74.17   |67.28
                           |   82.00 |    81.69    ||    78.80|   73.83
     Soluble silica        | 5.28    |  4.59       ||  4.61   | 6.54
     ----------------------+---------+-------------++---------+--------
     Potash (K₂O)          |  .72    |   .62       ||  1.07   |1.20
     Soda (Na₂O)           |  .09    |   .24       ||   .16   |  .21
     Lime (CaO)            | 1.04    |  1.32       ||   .71   | 2.92
     Magnesia (MgO)        |  .94    |   .92       ||  1.16   | 1.44
     ----------------------+---------+-------------++---------+--------
     Br. ox. of Manganese  |         |             ||         |
                (Mn₃O₄)    |  .05    |   .05       ||   .97   |  .62
     ----------------------+---------+-------------++---------+--------
     Peroxid of Iron(Fe₂O₃)| 5.14    |  5.62       ||  4.20   | 4.63
     Alumina (Al₂O₃)       | 5.74    |  5.24       ||  7.08   | 8.09
     Phosphoric acid (P₂O₅)|  .16    |   .13       ||   .12   |  .18
     Sulfuric acid (SO₃)   |  .01    |   .05       ||   .02   |  .01
     Carbonic acid (CO₂)   |         |             ||  1.76   |
     ----------------------+---------+-------------++---------+--------
     Water and organic     |         |             ||         |
                 matter    | 4.58    |  3.52       ||  6.54   | 5.37
     ----------------------+---------+-------------++---------+--------
                 Total     |100.48   | 99.37       || 99.94   | 99.69
     ----------------------+---------+-------------++---------+--------
     Humus                 |  .90    |   .48       ||         |
       “   Ash             |  .42    |   .32       ||         |
     Hygroscopic Moisture  | 5.60    |  4.84       ||  9.77   |10.37
       absorbed at      °C |   15    |     15      ||     15  |   15
     ----------------------+---------+-------------++---------+--------

_Soils of the Humid Region._—Taking a view, first, of the table
showing the soils of the _humid_ region, it appears that the change of
vegetation from walnut and hickory to the short-leaved pine bears no
visible relation to the increase or decrease of potash or phosphoric
acid, but is plainly governed mainly by the amount of lime present.
Where the short-leaved pine prevails the soil is almost always either
neutral or shows the alkaline reaction in the course of half an hour;
but where the long-leaved pine predominates the soil has almost always
an acid reaction. The latter is also usually found in bottoms in which
the loblolly pine (_P. taeda_) prevails, and where, although the soil
may show a fair proportion of lime in the analysis, it does not exist
in the form of carbonate.

_The examples here given are from lands not derived from, or underlaid
by, limestone formations._ Where the latter exist the percentage of
lime is usually materially increased; as it is also in the lowlands or
bottoms when compared with adjacent uplands (see above, chapter 10, p.
162; chapter 18, p. 331); as well as in the delta lands of rivers.

_Soils of the Arid Region._—Even a cursory comparison of the soils
of the arid regions of the Pacific slope with those of the humid, as
given in the above tables, shows some striking points of difference.
The most obvious is the uniformly high percentage of lime, and usually
also of magnesia, in the arid soils, and that quite independently of
underlying formations, calcareous or otherwise. This occurs despite
the fact that while limestone formations are very prevalent east of
the Rocky Mountains, they are quite scarce west of the same. The red
(Laramie) sandstones of Wyoming, the slates of the foothills of the
Sierra Nevada, the clay shales, granites and eruptives of the Coast
Ranges of California, Oregon and Washington, and the varied black rocks
of the great lava sheet of the Pacific Northwest, all alike produce
soils of high _lime_ content as compared with Eastern soils not derived
from calcareous formations. This fact has already been referred to, but
is more fully illustrated in the table below.

Aside from the lime-content, however, it will be noted in the preceding
table that the _potash_-content of the arid soils is on the average
considerably higher than in those of the humid region. In fact it is
hard to find west of the Rocky Mountains (except where high elevation
causes a humid climate) any soils as poor in potash as are many of the
commonly cultivated lands of the Eastern United States.

Other ingredients do not show such marked differences from the purely
chemical standpoint: yet, as will be shown below, the forms in which
silica and alumina occur are also not inconsiderably modified.

GENERAL COMPARISON OF SOILS FROM THE ARID AND HUMID REGIONS OF THE
UNITED STATES.[126]—In order to verify the conclusions just mentioned
upon the broadest basis possible, the following table has been compiled
from all available sources; partly published, partly in manuscript
only, having remained in the writer’s hands since the cessation of
the Northern Transcontinental Survey, prosecuted from 1880 to 1883,
under the auspices of the Northern Pacific Railroad, in Washington
and Montana. The published data are derived partly from the records
of State surveys, partly from the soil work connected with the Tenth
Census; partly also from those of Experiment Stations. In most cases
it has of course been necessary to restrict the comparison to such
analyses as have been made by substantially identical methods, for
reasons already given; but in the cases of some states from which
numerous analyses made by the Kedzie method, adopted by the Association
of Official Chemists, were available, the average has been given but
the name of the state starred, to indicate that the percentages,
excepting phosphoric acid, are lower than they would be if made by
the method adopted by the writer, particularly as regards potash. The
adoption of the one-millimeter mesh for the fine-earth sieve instead of
the half-millimeter size also creates an unfortunate and ineliminable
discrepancy.

[126] Abstracted and revised from Bulletin No. 3, U. S. Weather Bureau,
1893.

In order to exhibit clearly the influence of climate as distinct from
other local conditions, it was also necessary to eliminate, in both the
arid and humid regions, the soils directly derived from, or connected
with calcareous formations; such as the prairies of the Southwestern
States, the Bluegrass region of Kentucky, etc. This rule having been
applied impartially to the soils of both climatic regions, it can
hardly be questioned that the conclusions flowing from a discussion of
the results of the comparison are entitled to as much weight as are
those of any comparison based on large numbers of observations made,
not with reference to the special point under consideration, but with
a practical object of which the governing conditions were more or less
uncertain, and required to be ascertained by a process of elimination.

    The table gives, first, the averages for each ingredient for
    each of the states represented, the number of analyses from
    which the averages are derived being given in each case.
    These averages are given separately for the states of the
    humid and the arid regions respectively; and at the base
    of each group the grand average is shown in two forms. The
    first gives the figures as derived from the aggregate number
    of soil analyses in each great group, being 696 for the
    humid, 178 for the transition region and 573 for the arid,
    divided into the totals resulting from the summation of each
    ingredient for the whole 696, 178 and 573, respectively.

    The second form is that in which the soils of each state
    are considered as representative of the general character
    of such state, as the result of intentional selection; such
    as actually occurred in the cases of those included in the
    census work of 1880. The figures given here are therefore
    the result of a summation of the _state averages_
    as such, and of their division by the number of states
    represented.

    It will be noted that while these two modes of presentation
    do change the figures a little, yet in either form the
    same general result is outlined with striking accuracy.
    It is also notable that notwithstanding the less complete
    extraction of soil-ingredients in the starred (★)states,
    the general ratios between arid and humid soils remain
    substantially the same. For Western Oregon, local calcareous
    formations compel omission of three lime figures from the
    averages.

                AVERAGE COMPARISON OF SOILS IN THE HUMID AND ARID REGIONS
                                    OF THE UNITED STATES.
    (A) = Number analyzed.
    (B) = Insoluble Residue.
    (C) = Soluble Silica.
    (D) = Sum of Insoluble Residue and Soluble Silica.
    (E) = Potash
    (F) = Soda.
    (G) = Lime.
    (H) = Magnesia.
    ===========================+===+=====+=====+=====+===+====+====+====
                               |   |     |     |     |   |    |    |
                               |(A)| (B) | (C) | (D) |(E)| (F)| (G)| (H)
                               |   |     |     |     |   |    |    |
    ---------------------------+---+-----+-----+-----+---+----+----+----
                               |   |     |     |     |   |    |    |
    HUMID REGION.              |   |     |     |     |   |    |    |
                               |   |     |     |     |   |    |    |
    Rhode Island ★            |  7|82.41| 1.69|84.10|.15| .09| .43| .27
    North Carolina             | 20|81.63| 3.50|85.13|.15| .06| .09| .08
    South Carolina             | 30|85.54| 3.39|88.93|.12| .07| .07| .12
    Georgia                    | 40|86.07| 2.89|88.96|.15| .07| .08| .10
    Florida                    | 10|85.33| 1.33|86.66|.07| .03| .09| .03
    Alabama                    | 50|81.58| 4.89|86.47|.23| .07| .17| .21
    Mississippi                | 97|85.87| 4.39|90.26|.28| .11| .15| .31
    Louisiana                  | 35|81.12| 3.54|84.66|.19| .09| .16| .23
    Arkansas                   | 38|     |     |88.54|.17| .06| .08| .43
    Kentucky                   |185|     |     |86.72|.20| .10| .08| .19
    Ohio ★                    |140|     |     |87.00|.26| .35| .28| .44
                               |   |     |     |     |   |    |    |
    Oregon (W. of Cascades)    | 44|64.82| 5.38|70.20|.23| .19| .83| .73
    Average for Humid Region   |696|84.17| 4.04|88.21|.21| .14| .13| .29
       “    by States          |   |81.59| 3.45|85.04|.18| .11| .11| .26
                               |   |     |     |     |   |    |    |
    TRANSITION REGION.         |   |     |     |     |   |    |    |
                               |   |     |     |     |   |    |    |
    Minnesota. ★--Semi-humid  |144|76.60| 8.74|85.34|.30| .23| .65| .43
    North Dakota. ★--Semi-arid| 34|68.44| 7.28|75.72|.42| .73| .91| .64
    Average for region         |178|75.04| 8.46|83.50|.33| .32| .70| .47
                               |   |     |     |     |   |    |    |
    ARID REGION.               |   |     |     |     |   |    |    |
                               |   |     |     |     |   |    |    |
    Montana                    | 59|70.98| 4.17|75.15|.87| .27|1.03|1.36
    Idaho                      | 17|75.34| 5.22|80.56|.56| .26| .85|1.11
    Wyoming ★                 | 23|76.86| 2.25|79.11|.64| .41|1.91|1.31
    Colorado ★                | 16|77.70| 7.10|84.80|.44| .44|1.43| .81
    Utah ★                    | 38|     |     |81.04|.98| .53|1.77| .73
    Arizona                    | 20|64.58|13.78|78.36|.82| .43|2.37|1.89
    Nevada ★                  | 22|71.77| 5.95|77.72|.54| .93|2.04| .96
    California                 |262|66.28| 9.79|76.07|.61| .29|1.25|1.50
    ---------------------------+---+-----+-----+-----+---+----+----+----
    Oregon         East of     |  7|72.10| 9.68|81.78|.54| .26|1.23| .73
    Washington     Cascades    |109|71.60| 6.09|77.69|.65| .36|1.25| .96
    ---------------------------+---+-----+-----+-----+---+----+----+----
    Averages for Arid Region   |573|69.16| 6.71|75.87|.67| .35|1.43|1.27
       “     by States         |   |71.91| 7.11|79.02|.67| .42|1.61|1.14
    ---------------------------+---+-----+-----+-----+---+----+----+----

    (I) = Br. oxide Manganese.
    (J) = Peroxid of Iron.
    (K) = Alumina.
    (L) = Phosphoric Acid.
    (M) = Sulfuric Acid.
    (N) = Water and Organic Matter.
    (O) = Hygroscopic Moisture.
    (P) = Humus.
    (Q) = Nitrogen in Humus.
    (R) = Nitrogen in Soil.
    ===========================+===+====+====+===+===+=====+====+====+=====+===
                               |   |    |    |   |   |     |    |    |     |
                               |(I)| (J)| (K)|(L)|(M)| (N) | (O)|(P )| (Q) |(R)
                               |   |    |    |   |   |     |    |    |     |
    ---------------------------+---+----+----+---+---+-----+----+----+-----+---
                               |   |    |    |   |   |     |    |    |     |
    HUMID REGION.              |   |    |    |   |   |     |    |    |     |
                               |   |    |    |   |   |     |    |    |     |
    Rhode Island ★            |.04|3.59|3.66|.09|.10| 7.43|5.23|2.59|     |
    North Carolina             |.07|4.72|5.71|.12|.06| 3.98|4.18|    |     |
    South Carolina             |.05|2.47|4.59|.11|.06| 3.39|4.22| .42|     |
    Georgia                    |.09|2.75|4.02|.11|.10| 3.62|3.54|    |     |
    Florida                    |.08| .60|1.17|.08|.05| 1.89|1.72|    |     |
    Alabama                    |.12|3.81|2.70|.13|.05| 4.04|7.07|    |     |
    Mississippi                |.14|2.64|4.07|.09|.03| 3.33|5.41|    |     |
    Louisiana                  |.02|3.12|4.24|.10|.05| 4.26|6.66|    |     |
    Arkansas                   |.21|3.10|3.51|.15|.05| 3.70|2.47|    |     |
    Kentucky                   |.20|6.01|3.52|.11|.04| 3.69|1.85|    |     |
    Ohio ★                    |   |3.11|3.36|.11|.04| 5.04|    |    |     |
    Oregon (W. of Cascades)    |.09|  11.64  |.23|.16| 3.20|    |1.55|     |
    Average for Humid Region   |.13|3.88|3.66|.12|.05| 4.40|    |1.22|     |
       “    by States          |.10|3.26|3.68|.12|.06| 3.96|    |1.52|     |
                               |   |    |    |   |   |     |    |    |     |
    TRANSITION REGION.         |   |    |    |   |   |     |    |    |     |
                               |   |    |    |   |   |     |    |    |     |
    Minnesota. ★--Semi-humid  |   |2.83|4.43|.22|.01| 7.30|    |2.91| 6.53|.19
    North Dakota. ★--Semi-arid|   |3.62|5.17|.19|.05|13.85|    |4.67| 7.28|.34
    Average for region         |   |2.08|4.57|.21|.02| 8.55|    |3.24| 6.67|.22
                               |   |    |    |   |   |     |    |    |     |
    ARID REGION.               |   |    |    |   |   |     |    |    |     |
                               |   |    |    |   |   |     |    |    |     |
    Montana                    |.37|4.28|6.81|.22|.06|     |7.14|    |     |
    Idaho                      |.02|3.85|6.38|.16|   | 5.01|2.00|1.68| 5.95|.10
    Wyoming ★                 |   |3.05|6.61|.18|.11| 5.48|    |    |     |
    Colorado ★                |   |3.82|4.98|.23|.03| 3.57|2.31|    |     |.03
    Utah ★                    |.03|3.08|5.50|.22|   | 6.27|2.37|    |     |
    Arizona                    |.06|4.92|6.43|.13|.06| 3.57|    |1.65|     |
    Nevada ★                  |.32|5.67|5.00|.32|.14| 5.93|    |    |     |.12
    California                 |.06|6.61|8.44|.10|.06| 4.74|6.09|0.91|15.23|.14
    Oregon     } East of       |.08|  10.75  |.11|tr.| 4.40|    | .67|     |
    Washington } Cascades      |   |5.37|6.31|.21|.03| 5.77|5.14|    |     |
    Averages for Arid Region   |.11|5.48|7.21|.16|.06| 5.15|5.46|1.13|12.50|.13
       “     by States         |.13|4.74|6.27|.19|.07| 4.71|5.34|1.03|10.59|.10
    ---------------------------+---+----+----+---+---+-----+----+----+-----+---

_New Mexico._—Few analyses of New Mexico soils have been made, but
the average results of six partial determinations made by Goss, and
one full analysis made by Hare according to the method of the writer,
and given below, show substantial accord with the averages of the
above table. The averages of Goss’ determinations are: Potash .780,
Phosphoric acid .221, Nitrogen .108 per cent.

       CHEMICAL ANALYSIS OF RIO GRANDE SILT
           (_by Prof. R. F. Hare._)

         Deposited on land by irrigation.

    Insoluble matter                      63.70
    Potash (K₂O)                           1.06
    Soda (Na₂O)                             .22
    Lime (CaO)                             4.97
    Magnesia (MgO)                         2.43
    Br. ox. of Manganese (Mn₃O₄)            .14
    Peroxid of Iron (Fe₂O₃)                5.80
    Alumina (Al₂O₃)                        6.86
    Phosphoric acid (P₃O₅)                  .16
    Sulfuric acid (SO₃)                     .13
    Carbonic acid (CO₂)                    7.45
    Water and organic matter               9.98

    Humus                                  1.17
      “   Nitrogen                        11.11
      “      “     per cent. in soil        .13

    Hygroscopic Moisture absorbed at °C    2.63


DISCUSSION OF THE TABLE.

_Lime._—Considering in this table, first, _lime_, a glance at the
columns for the two regions shows a surprising and evidently intrinsic
and material difference, approximating in the average by totals to
the proportion of 1 to 11; in the average by states, 1 to 14½. This
difference is so great that no accidental errors in the selection
or analysis of the soils can to any material degree weaken the
overwhelming proof of the correctness of the inference drawn upon
theoretical grounds, viz., that the soils of the arid regions must be
richer in lime than those of the humid countries. For the differences
in derivation would, in view of the wide prevalence of limestone
formations in the humid regions concerned, produce exactly the reverse
condition of things from that which is actually found to exist; and
if further proof were needed it can readily be found in the detailed
discussion of the analyses of the soils of the arid areas forming the
contrast. This shows that for instance, in Washington highly calcareous
soils are directly derived from the black basaltic rocks; while
similarly, calcareous lands are found in California to be the outcome
of the decomposition of granites, diorites, lavas, clay-shales and
sandstones.

It is not easy to overrate the importance of this feature of the
soils of the arid region, as it is intimately connected with other
theoretically and practically important facts, in part already
mentioned.

_Summary of Effects of Lime Carbonate in Soils._—It is best to
summarize, briefly, at this point, the advantages (and possible
disadvantages), resulting from the presence of a proper amount of lime
carbonate in soils, so far as these are at present understood.

_Physically_, even a small amount of lime carbonate, by its solubility
in the carbonated soil-water, will act most beneficially in causing
the flocculation of clay and in the subsequent conservation of the
flocculent or tilth condition, by acting as a light cement holding
the soil-crumbs together when the capillary water has evaporated;
thus favoring the penetration of both water and air, and of the roots
themselves. It should be added that according to the experience of the
writer, amounts of lime carbonate in excess of 2% do not add to the
favorable effects, except as would so much sand.

As to chemical effects, among the most important are:—

1. The maintenance of the neutrality of the soil, by the neutralization
of acids formed by the decay of organic matter, or otherwise.

2. The maintenance, in connection with the proper degrees of moisture
and warmth, of the conditions of abundant bacterial life (see above,
chapter 9, p. 146); more especially those of nitrification, thus
supplying the readily assimilable form of nitrogen. Also in favoring
the development and activity of the root bacteria of legumes, and of
the other nitrogen-gathering bacteria, such as Azotobacter (ibid. p.
156).

3. The rendering available, directly or indirectly, of relatively small
percentages of plant-food, notably phosphoric acid and potash; as shown
in the preceding pages.

4. The prompt conversion of vegetable matter into black, neutral
humus, and (as shown in the case of the soils of the arid region) the
concentration of the nitrogen in the same; while accelerating the
oxidation of the carbon and hydrogen, as shown by S. W. Johnson and
others.

6. It counteracts the deleterious influence of an excess of magnesia in
the soil, as first shown by Loew,[127] and verified by his pupils in
Japan.

7. In alkali soils, according to Cameron and May, it counteracts the
injurious action of the soluble salts upon the growth of plants,
not only in the form of carbonate, but also in those of sulfate and
chlorid.[128]

[127] Bull. No. 1, Div. Veget. Physiol, and Plant Pathol. U. S. Dept.
Agr.; et al.

[128] Loeb, Publications of the Spreckel’s Physiological Laboratory of
the University of California, has shown a similar protective influence
of the lime salts in sea-water, against the other salts, in the case of
the lower marine organisms.

8. As a matter of experience, both in the case of grapes and orchard as
well as wild fruits, an adequate but not excessive supply of lime in
the soil will produce sweeter fruit than when lime is in small supply.

9. An excess of carbonate of lime in soils (from eight to twenty per
cent and more), constituting “marliness,” tends to seriously disturb
the nutrition and general functions of many plants (calcifuge), and to
produce a suppression or diminution of the formation of chlorophyll and
starch; as in the case of grape vines, citrus fruits and others, which
nevertheless flourish best in lands moderately calcareous.

Among the points thus enumerated the third and fourth require some
comment. Without pretending to define exactly how lime acts in
rendering other ingredients more available to plant assimilation,
attention may be called to the fact that lime carbonate may be
considered as acting similarly to, albeit more mildly than, caustic
lime, in the displacement of other bases from their compounds. It
doubtless acts thus in liberating potash from its zeolitic compounds.
As to phosphoric acid, the connection of the effect of lime carbonate
with the remarkable availability of that substance when present in the
form of tetra-basic salt, in the case of phosphate slag, is at least
possible.

    As to the action of lime carbonate in forming humus,[129] no
    one who has observed the characteristic dark black tint of
    our calcareous “prairie soils” can question the fact; which
    moreover is perfectly explicable upon the analogy already
    alluded to, with caustic lime, which, together with caustic
    alkalies (potash and soda), is known to act powerfully in
    the conversion of vegetable matter into humus. That instead
    of liberating the nitrogen in the form of ammonia, as do the
    caustic hydrates, the milder carbonate should only cause
    the formation of humic amides, is quite intelligible. That
    such is really the case, has been conclusively proved by
    the investigations of the writer made conjointly with M.
    E. Jaffa (Rep. Sta. Cal. Agr. Expt. 1892-4); the general
    result being that while in the humid region the average
    nitrogen-content of soil-humus is less than 5%, in the
    upland soils of the arid region (where _all_ soils
    are calcareous) that percentage rises as high as 22.0%,
    with a general average of between 15 and 16%. That such
    highly nitrogenous material can be more readily attacked
    by the nitrifying bacteria than when a large excess of
    other oxidable matter is present, is at least a legitimate
    presumption, especially in view of the very active
    nitrification known to take place in the arid regions
    everywhere. So long as a large excess of carbohydrates
    is present, the oxidation of these will naturally take
    precedence over that of the relatively inert nitrogen. The
    accumulation of the latter in the humus-substance of the
    arid region, where oxidation of the organic matter of the
    soil is very active, points strongly to this view of the
    case.

[129] “Black Soils;” Agric. Science, January, 1892.

_Magnesia._—While the differences in respect to the proportions of
lime are the most prominent and decided, yet the related substance,
magnesia, shows also a very marked and constant difference as between
the soils of the humid and arid regions. It will be observed that the
general average for magnesia in the soils of the Atlantic Slope is
about double that of lime; Florida and Rhode Island being the only
states in which the average is lower for magnesia than for lime. In
the arid region, on the contrary, magnesia on the general average is
nearly the same as lime; in the average by states, somewhat less; thus
bringing the ratio for the two regions for magnesia up to one to six
or seven. This also is so decisive a showing that no accident could
bring it about. We must conclude that climatic influences have dealt
with magnesia similarly as with lime; which from the standpoint of
the chemist is just what might be expected, since magnesia carbonate
behaves very much like that of lime toward carbonated waters.

That magnesia is a very important plant-food ingredient is apparent
from its invariable and rather abundant presence in the seeds
of plants, where it takes precedence of lime. Its functions in
plant nutrition have been specially investigated by O. Loew,[130]
particularly with respect to its relations to lime. As already
stated in connection with the soil-forming properties of magnesian
minerals (see chapter 2), soils containing large proportions of
magnesia generally are found to be unthrifty, the lands so constituted
being frequently designated as “barrens.” Loew finds that certain
proportions of lime to magnesia must be preserved if production is to
be satisfactory, the proportion varying with different plants, some
of which (_e. g._ oats) will do well when the proportion of lime to
magnesia is as 1:1, while others require, that that ratio should be as
2 or 3 is to 1, to secure the best results. In general it is best that
lime should exceed magnesia in amount.

[130] Bull. No. 18, Div. Vegetable Physiology and Plant Pathology;
Bull. No. 1, Bureau of Plant Industry, U. S. Dept. of Agr.; Bull.
College of Agriculture, Tokyo, Vol. 4, No. 5.

    Loew explains the injurious action of magnesium salts thus:
    The calcium nucleo-proteids of the organic structures are
    transformed in presence of soluble salts of magnesium
    into magnesium compounds, while the calcium of the former
    enters into combination with the acid of the magnesium
    salt. By this transformation the capacity for imbibition
    will change, which must result in a fatal disturbance of
    functions. The presence of soluble lime salts will prevent
    that interchange. Thus certain algæ perished in a solution
    containing 1 per 1000 of magnesium nitrate, but remained
    alive when .3 per 1000 of calcium nitrate was added.

Magnesia seems to be specially concerned in the transfer of phosphoric
acid through the plant tissues, in the form of dimagnesic-hydric
phosphate, which is rather soluble in the acid juices of plants. It is
probable that, apart from the relations just referred to, such excess
of lime as is known to produce chlorosis in plants interferes with the
transfer of the magnesic phosphate. Some plants, as already stated,
dispose of an excess of lime by depositing it in the form of oxalate,
while others (such as the stone crops) excrete it on the surface of
leaves and stems in the form of carbonate. But others seem to possess
this power to a limited extent only.

In the case of soils containing much magnesia the proper proportion
between it and lime may easily be disturbed by the greater ease with
which lime carbonate is carried away by carbonated water into the
subsoil, thus leaving the magnesia in undesirable excess in the surface
soil. Hence the great advantage of having in a soil, from the outset,
an ample proportion of lime. From this point of view alone, then, the
analytical determination of lime and magnesia in soils is of high
practical value.

    Aso, Furuta and Katayama (Bull. Coll. Agr. Tokyo, Vol. 4
    No. 5; Ibid. Vol. 6), have by direct experiment determined
    the most advantageous ratio of lime to magnesia in several
    crop plants. They find for rice and oats 1:1, for cabbage
    2:1, for buckwheat 3:1; there being apparently a connection
    between the extent of leaf-surface and lime requirement,
    since leaves contain predominantly lime, while in the fruit,
    magnesia predominates.

_Manganese._—A decided difference in the manganese content of the arid
as against the humid soils appears in the table, the ratio being about
11:13 in favor of the humid soils. Manganese has not been regarded as
being of special importance to plant growth in general, although, as
already stated, some plants contain a relatively large proportion of
manganese in their ashes; thus, _e. g._, the leaves of the long-leaved
pine of the cotton states.[131] But no definite data showing the
importance of this element to crops were available until Loew and
his co-workers at Tokyo[132] established its stimulating action in a
number of cases, in which crop production was materially increased by
the use of protoxid salts of manganese. Aso[133] applied manganous
chlorid to an experimental plot of thirty square meters, at the rate of
twenty-five kilos of Mn₃O₄ per acre, and thus obtained a yield of rice
one-third greater than on the control plot, at a cost of about $2.00,
while the value of the increase of the product was nearly $68.00.
More experimental evidence on this subject is required to establish
the _general_ value of the large-scale use of the salts of manganese;
which are obtained in large quantities as a comparatively valueless
by-product of the bleaching industries.

[131] Rep. Agr. and Geology of Mississippi, 1860, p. 360.

[132] Bull. Agr. Coll. Tokyo, Vol. V., Nos. 2 and 4.

[133] Ibid. Vol. 6.


_The “Insoluble Residue.”_

Remembering, in discussing the facts shown by the table, that the
fundamental difference between the regime of the humid and arid regions
is the presence in the latter of an almost continuous leaching process,
in which the carbonated water of the soil is the solvent; remembering,
also, that the least soluble portion of rocks and soils is quartz or
silica (sand, as usually understood), it would be predicable that this
ingredient should in the humid region be found to be more abundant in
soils than in the arid. This portion is represented by the “insoluble
residue” of the table.

Inspection shows that both in the averages of the single states, and in
both of the general averages, this difference between the soils of the
humid and the arid regions of the United States is strongly pronounced;
the ratio being substantially as 69% in the arid region to 84% in the
humid.

We must then conclude that the leaching process must have influenced
materially other soil ingredients than lime, which have remained behind
in such amounts as to depress the percentage of insoluble residue in
the soils. It remains to be shown what are the substances so retained.


_Insoluble and Soluble Silica and Alumina._

The ingredient most nearly correlated with the insoluble residue is the
free silica which remains behind with it when the acid with which the
soil has been treated is evaporated to dryness. The silica is separated
from the practically insoluble, undecomposed minerals by boiling with a
strong solution of sodic carbonate. The amount of this “soluble silica”
is obviously the measure of the extent to which the soil-silicates have
been decomposed in the treatment with acid.

The most prominent of these is usually supposed to be clay—the hydrous
silicate of alumina that in its purest condition forms kaolinite or
porcelain earth. Any alumina found in the usual course of soil analysis
is generally referred to this mineral, which contains silica and
alumina nearly in the proportion of 46% to 40%.

In very many cases, however, the reference of these two ingredients
to clay is manifestly unjustified. This is clearly so when (as not
unfrequently happens) the amount of alumina found exceeds that which
would form clay with the ascertained percentage of soluble silica; it
is almost as certainly so when, in addition to the alumina, other bases
(notably potash, lime and magnesia), are found in proportions which
preclude their being in combination with any other acidic compounds
present. The only possible inference in such cases is that these
bases, together with at least a portion of the alumina, are present in
the form of hydrated, and therefore easily decomposable silicates or
zeolites.

    The subjoined analysis by R. H. Loughridge, of a clay
    obtained in the usual process of mechanical soil analysis
    (by precipitating with common salt the turbid water
    remaining after 24 hours subsidence in a column of 200
    millimeters) from a very generalized soil of northern
    Mississippi, shows one of the many cases in which the
    numerical ratios of the several ingredients are incompatible
    with the assumption that silica and alumina are present in
    combination as clay (kaolinite) only:

           ANALYSIS OF COLLOIDAL CLAY.

    Insoluble matter                     15.96
    Soluble silica                       33.10
    Potash (K₂O)                          1.47
    Soda (Na₂O)                           1.70
    Lime (CaO)                             .09
    Magnesia (MgO)                        1.33
    Br. ox. of Manganese (Mn₃O₄)           .30
    Peroxid of iron (Fe₂O₃)               18.76
    Alumina (Al₂O₃)                      18.19
    Phosphoric acid (P₂O₅)                 .18
    Sulfuric acid (SO₃)                    .06
    Carbonic acid (CO₂)                    .00
    Water and organic matter              9.00
                                        ------
              Total                     100.14

    If in this case we assign all alumina to silica, as required
    for the composition of kaolinite or pure clay, there yet
    remains a trifle over twelve (12.17) per cent of silica to
    be allotted to the other bases present. Deducting from this
    the ascertained amount of silica soluble in sodic carbonate,
    pre-existing in the raw material (.38 per cent), we come to
    11.79 per cent as the amount of silica which must have been
    in combinations other than kaolinite, viz., hydrous
    silicates, or soil zeolites, formed either with the
    bases other than alumina shown in the analysis or, more
    probably, containing some of the alumina itself in essential
    combination.

    We are thus enabled to obtain from the determination of the
    soluble silica an estimate of the extent to which these
    soil zeolites, that form so important a portion of the soil
    in being the repositories of the reserve of more or less
    available mineral plant-food, are present in the soils of
    the several regions. A glance at the table shows that the
    general average of soluble silica is very much greater in
    the soils of the arid regions than in those of the humid,
    approximating one to two in favor of the arid division.[134]

[134] Looking at the details of the several states, we find that on the
arid side Washington has a relatively low figure for soluble silica,
which in the average, however, is overborne by the high figures for
California and Montana. The explanation of this fact probably lies
in the derivation of the majority of the Washington soils examined,
from lake deposits brought down gradually from the humid region at the
heads of the Columbia drainage, where sandy beds are very prevalent;
while the country rock—the basaltic eruptives—are very basic, and
moreover slow to disintegrate. In California and Montana the rocks
are infinitely varied, and the general outcome of their weathering is
plainly a predominance of complex hydrous silicates in the soils, as
compared with humid regions.

_Differences in the Sands of the Arid and Humid Regions._—In chapter
5 mention has been made of the fact that while in the humid regions,
“sand” as a rule means quartz grains, mostly with a clean surface and
very frequently rounded and polished, in the arid regions even the
coarse sand grains consist of, or are covered with, a great variety
of minerals in a partially decomposed condition. This is owing to the
absence of the abundant rainfall which in humid climates continually
washes down the finely divided, half-decomposed mineral matter into
the subsoil; while in arid climates the light rains cannot produce any
such washing effect and hence the sand grains remain incrusted with the
products of either their own decomposition, or of that of neighboring
particles; it being therefore not concentrated in the finer portion
only, viz., the clay and finest silts. This fundamental difference,
which is illustrated in the analytical table below, at once explains
why in the arid regions generally, sandy soils are found so highly
productive that, owing to their easy cultivation they are preferred to
the clayey lands, in which tillage and irrigation are more difficult.
It is a well-known fact that on the “sands of the desert” when either
irrigated, or wetted by rain, vegetation at once springs up with
remarkable luxuriance, even on sand drifts; and this productiveness
appears to be quite as lasting as that of “strong” clay soils of the
humid regions.

    This difference is curiously illustrated on the southern
    edge of the “black adobe” or prairie soil area which
    surrounds Stockton, Cal. Here we find on the opposite sides
    of a small stream (French Camp slough) the two extremes,
    of heavy clay and the sandy soils which for many years
    made Stanislaus county the “banner” county for wheat. The
    grain product of both banks ranked alike in quantity and
    quality in average years; but in extreme seasons sometimes
    one, sometimes the other failed, according to the weather
    conditions which favored one or the other soil. No one would
    think of sowing wheat on so sandy a soil in the humid States.

    Table Illustrating Difference in Sands of the Humid and Arid Regions.

    ===============================+========+=======+=====+=========
                                   |Per cent|Potash.|Lime.|Magnesia.
           Clay.                   |in Soil.|       |     |
    -------------------------------+--------+-------+-----+---------
    Mississippi[135]               |  21.64 |  .32  | .03 |   .29
    California 1281 Chino[136]     |   7.60 |  .16  | .14 |   .17
        “      Jackson[137]        |  16.43 |  .13  | .12 |   .08
    Silt .06-.016 mm. diam.        |        |       |     |
    Mississippi                    |  35.10 |  .41  | .15 |   .36
    California (Chino)             |  18.53 |  .24  | .53 |   .29
        “      Jackson             |  34.90 |  .10  | .04 |   .08
    Silt .016-.025 mm. diam.       |        |       |     |
    Mississippi                    |  13.67 |  .12  | .09 |   .10
    California, Chino              |   5.49 |  .05  | .11 |   .02
        “       Jackson            |   9.96 |  .08  | .04 |   .10
    Silt .025-.036 mm. diam.       |        |       |     |
    Mississippi                    |        |       |     |
    California, Chino              |   3.92 |       |     |
        “       Jackson            |   7.68 |  .06  | .02 |
    Silt .036-.047 mm. diam.       |        |       |     |
    Mississippi                    |        |       |     |
    California, Chino              |   6.40 |  .05  | .18 |
        “       Jackson            |   8.21 |  .04  | .01 |
    Coarse Silt .047-.072 mm. diam.|        |       |     |
    California, Chino              |   7.92 |  .06  | .23 |
        “       Jackson            |   5.91 |  .01  | .01 |
    Fine sand .072-.12 mm. diam.   |        |       |     |
    California, Chino              |  11.87 |  .06  | .26 |
        “       Jackson            |   4.03 |  .01  | .01 |
    Sand .12-.50 mm diam.          |        |       |     |
    California, Chino              |  36.11 |  .11  | .69 |
        “       Jackson            |  10.10 |       |     |
    -------------------------------+--------+-------+-----+---------

    ===============================+==========+=======+=================
                                   |Phosphoric|Soluble|Alumina.|  Summ-
           Clay.                   |  Acid.   |Silica.|        |  ation.
    -------------------------------+----------+-------+--------+--------
    Mississippi[138]               |   .04    |  7.17 |  3.97  |   11.82
    California 1281 Chino[139]     |   .04    |  1.70 |  1.35  |    3.56
        “      Jackson[140]        |   .05    |  2.83 |  2.13  |    5.34
    -------------------------------+----------+-------+--------+--------
    Silt .06-.016 mm. diam.        |          |       |        |
    Mississippi                    |   .07    |  2.87 |  1.36  |    5.22
    California (Chino)             |   .06    |  4.96 |  1.76  |    7.84
        “      Jackson             |   .02    |  2.50 |  2.44  |    5.18
    -------------------------------+----------+-------+--------+--------
    Silt .016-.025 mm. diam.       |          |       |        |
    Mississippi                    |   .02    |   .32 |   .17  |     .82
    California, Chino              |   .01    |   .80 |   .51  |    1.50
        “       Jackson            |   .007   |  1.01 |  1.01  |    2.25
    -------------------------------+----------+-------+--------+--------
    Silt .025-.036 mm. diam.       |          |       |        |
    Mississippi                    |          |       |        |     .36
    California, Chino              |          |       |        |    lost
        “       Jackson            |   .006   |  0.82 |   .74  |    1.70
    -------------------------------+----------+-------+--------+--------
    Silt .036-.047 mm. diam.       |          |       |        |
    Mississippi                    |          |       |   .55  |   trace
    California, Chino              |   .01    |   .80 |   .64  |    1.66
        “       Jackson            |   .001   |   .43 |        |    1.12
    -------------------------------+----------+-------+--------+--------
    Coarse Silt .047-.072 mm. diam.|          |       |        |
    California, Chino              |   .02    |   .89 |   .59  |    1.79
        “       Jackson            |   .003   |   .42 |   .30  |     .77
    -------------------------------+----------+-------+--------+--------
    Fine sand .072-.12 mm. diam.   |          |       |        |
    California, Chino              |   .03    |   .98 |        |    1.43
        “       Jackson            |   .003   |   .28 |   .09  |     .40
    -------------------------------+----------+-------+--------+--------
    Sand .12-.50 mm diam.          |          |       |        |
    California, Chino              |   .04    |  2.43 |  1.59  |    4.98
        “       Jackson            |          |       |        |Not detd
    -------------------------------+----------+-------+--------+--------

[135] Analyses by R. H. Loughridge.

[136] Analyses by L. M. Tolman.

[137] Analyses by E. H. Lea.

[138] Analyses by R. H. Loughridge.

[139] Analyses by L. M. Tolman.

[140] Analyses by E. H. Lea.

It thus appears that while in the Mississippi soil, solubility of
plant-food practically ceased at grain-diameter of .036 mm, in the
arid California soils, as large an amount was found in the sand-grain
sizes between .12 and .50 millimeters as in the fine silt .016 to .025
mm. in Mississippi.

_Hydrous Silicates are More Abundant in Arid than Humid Soils._—This
predominance of hydrous silicates in the soils of the arid regions
should not be a matter of surprise when we consider the agencies which
are brought to bear upon these soils with so much greater intensity
than can be the case where the solutions resulting from the weathering
process are continually removed as fast as formed, by the continuous
leaching effect of atmospheric waters. In the soils of regions where
summer rains are insignificant or wanting, these solutions not only
remain, but are concentrated by evaporation to a point that, in the
nature of the case, can never be reached in humid climates. Prominent
among these soluble ingredients are the silicates and carbonates of
the two alkalies, potash and soda. The former, when filtered through
a soil containing the carbonates of lime and magnesia, will soon be
transformed into complex silicates, in which potash takes precedence
of soda, and which, existing in a very finely divided (at the outset
in a gelatinous) condition, serve as an ever-ready reservoir to catch
and store the lingering alkalies as they are set free from the rocks,
whether in the form of soluble silicates or carbonates. The latter have
another important effect: in the concentrated form at least, they,
themselves, are effective in decomposing silicate minerals refractory
to milder agencies, such as calcic carbonate solution; and thus the
more decomposed state in which we find the soil minerals of the arid
regions is intelligible on that ground alone.

    It must not be forgotten that lime carbonate, though
    less effective than the corresponding alkali solutions,
    nevertheless is also known to produce, by long-continued
    action, chemical effects similar to those that are more
    quickly and energetically brought about by the action of
    caustic lime. In fact, the agricultural effects of “liming”
    are only in degree different from those produced by marling
    with finely pulverized carbonate; and in nature the same
    relation is strikingly exemplified in the peculiarly black
    humus that is characteristic of calcareous lands, but which
    can be much more quickly formed under the influence of
    caustic lime on peaty soils.

    In the analysis of silicates we employ caustic lime for the
    setting-free of the alkalies and the formation of easily
    decomposable silicates, by igniting the mixture; but the
    carbonate will slowly produce a similar change, both in
    the laboratory and in the soils in which it is constantly
    present. This is strikingly seen when we contrast the
    analyses of calcareous clay soils of the humid region with
    the corresponding non-calcareous ones of the same. In the
    former the proportions of dissolved silica and alumina are
    almost invariably much greater than in the latter, so far as
    such comparisons are practicable without assured absolute
    identity of materials. That is, calcareous clays or clay
    soils are so sure to yield to the analyst large precipitates
    of alumina, that experience teaches him to employ smaller
    amounts for analysis than he would of non-calcareous
    materials, in order to avoid unmanageably large bulks of
    aluminic hydrate. It is but rarely that even the heaviest
    non-calcareous soils yield to the acid usually used in soil
    analysis more than 10 per cent of alumina; while heavy
    calcareous clay (prairie) soils commonly yield between 13
    and 20 per cent.[141] It would be interesting to verify this
    relation by artificial digestions of one and the same clays
    with calcic carbonate at high temperatures, as it must
    always be extremely difficult to insure absolute identity of
    all other conditions in natural materials.

    In most of these cases, what is true of alumina is also true
    of the soluble silica. But since the latter is constantly
    liable to be dissolved out by solutions of carbonated
    alkalies, it is not surprising that this relation is not
    always shown.

[141] Report of the Tenth Census, Vols. 5 & 6; see especially the
analyses of soils from Mississippi and Alabama. Also the Reports of the
California Experiment Station.

_Aluminic Hydrate._—In numerous cases, the amount of alumina
dissolved in analysis is greatly in excess of the soluble silica,
so as to force the conclusion that a portion of the latter must be
present in a different form from that of clay (kaolinite); the only
choice being between that of complex hydrous silicates (none of
which, however, could contain as large a percentage of alumina as
clay itself) and _aluminic hydrate_. The latter is alone capable of
explaining the presence of more alumina than silica in easily soluble
form;[142] and the visible occurrence of “gibbsite” and “bauxite” in
modern formaations renders this a perfectly simple and acceptable
explanation. Since these minerals are known to be incapable of
crystallization, we are moreover led to the presumption that it will as
a rule be found in the finest portions of the soil, viz., in the “clay”
of mechanical analysis.

[142] Excepting the relatively rare minerals of the Allophane,
Kollyrite, and Miloshite group.

    Some illustrations of these conditions are given below, for
    soils from Mississippi and California. The soluble silica
    being all assigned to kaolinite, the rest of the alumina
    must be assumed to be present as hydrate, since no other
    compound could fulfil the stoichiometrical requirements.[143]
    The table therefore shows the differences between the
    amounts of alumina found by analysis, and those assignable
    to kaolinite, calculated to the mineral bauxite—the
    most abundant, as well as the one containing the medium
    proportion of water, among the three naturally occurring
    aluminic hydrates.

    TABLE SHOWING EXCESS OF ALUMINA OVER SILICA IN SOILS;
               CALCULATED AS BAUXITE.

    (A) = Total soluble in HCl.
    (B) = Corresponding to Bauxite.
    (C) = Other Soluble Matters.
    Miss. = Mississippi.
    Cal.  = California.
    -------+---------------+--------+------+-----+----+-----+-----+-----
    Number.| Name of Soil. |County. |State.| (A) |SiO₂|Al₂O₃| (B) | (C)
    -------+---------------+--------+------+-----+----+-----+-----+-----
      195  |Prairie        |Alcorn  | Miss.|28.57| 3.6| 14.4|14.12| 2.92
      346  |Dark Loam      |Chicasaw|   “  |10.32| 6.6| 11.2| 6.91|  .86
      288  |Flatwoods Clay |Pontotoc|   “  |26.94| 5.0| 11.3| 8.75| 3.48
      676  |Red Volcanic   |Lake    |  Cal.|41.00| 5.9| 22.6|21.90| 2.00
      332  |Mojave Desert  |Kern    |   “  |24.82| 5.0|  9.2| 6.10| 5.13
      191  |Red Foothill   |Merced  |   “  |23.32| 4.5|  8.8| 6.20| 3.05
    -------+---------------+--------+------+-----+----+-----+-----+-----
      705  |Red Chaparral  |Shasta  |   “  |28.75| 5.5| 14.4|12.10| 1.12
      706  | “      “      |        |      |     |    |     |     |
           |       Subsoil |  “     |   “  |28.40| 4.7| 17.4|16.70| 1.32
    -------+---------------+--------+------+-----+----+-----+-----+-----
      573  |Tulare Plains  |Tulare  |   “  |29.27| 3.4|  8.7| 7.20|11.16
      701  |Dry Bog        |  “     |   “  |27.29| 4.3| 12.4|10.90| 5.04
     1004  |“Slickens“ Sed.|Butte   |   “  |30.80| 8.0| 14.2| 9.20| 1.95
      656  |    “       “  |Yuba    |   “  |22.23| 3.0| 10.4| 9.80| 2.19
      517  |Brownish Loam  |Butte   |   “  |29.80| 4.8| 12.0| 9.80| 4.42
      561  |Black Loam     |  “     |   “  |30.21| 3.2| 13.0|12.80| 4.67
    -------+---------------+--------+------+-----+----+-----+-----+-----
      563  |Sacramento     |        |      |     |    |     |     |
           |     Alluvium  |  “     |   “  |23.46| 2.7| 10.4|10.90| 4.58
    -------+---------------+--------+------+-----+----+-----+-----+-----
      863  |Red Foothill   |Nevada  |   “  |56.80|11.0| 36.4|33.60| 1.22
      861  | “     “       |  “     |   “  |45.46|11.5| 22.0|14.10| 3.97
    -------+---------------+--------+------+-----+----+-----+-----+-----

    It is apparent from this table that if, as is probable, the
    aluminic hydrate accumulates in the “clay” of the analysis,
    it will in some cases form a very considerable percentage
    of the same, and detract to that extent from its plastic,
    adhesive and other properties. But it must be remembered
    that the assumption upon which this table is calculated,
    leaves out of consideration the zeolitic portion, which as
    the 6th column shows, is frequently quite large as measured
    by the bases found, to which no other form of combination
    can be assigned. Since some of the alumina undoubtedly
    takes part in the formation of such zeolites, the silica
    must to that extent be withdrawn from the estimate made
    for kaolinite. While it is impossible to make any definite
    numerical allowance for this fact, it clearly will tend in
    many cases to increase materially the amount of alumina that
    must be assigned to the hydrate condition. It will be noted
    that in most cases given, the alumina per cent is rather
    large.

[143] Since any complex zeolite would contain _less_ alumina than
kaolinite, this assumption more than covers the possible zeolitic
alumina.

The relatively large number of such cases shown in the table for
California soils is not a matter of accident; for even a cursory glance
at the columns of analyses of California (and Washington and Montana)
soils, shows that the cases in which the alumina exceeds the silica
in amount are rather predominant, while the reverse is the case in
the humid region.[144] But it must not be inferred that the reverse
relation is not also frequently observed even in the arid region; it
occurs in fact in close proximity to the localities where some of the
most striking instances of excess of alumina over soluble silica have
been found.

[144] See for comparison the data given in vols. 5 and 6 of the report
of the Tenth Census of the United States.

    Thus Nos. 861 and 863 from the neighborhood of Grass Valley,
    which show this excess most strikingly, occur within 15
    miles of localities which show almost the reversal of the
    numbers given for the two former, and at a level of about a
    thousand feet lower. It would seem, on the whole, that the
    excess of alumina occurs most frequently in connection with
    soils formed from eruptive rocks; in the case referred to,
    from volcanic ash. It will require more detailed study to
    detect the causes of these marked differences.

    _Retention of Soluble Silica in Alkali Soils._—It is
    somewhat surprising that, contrary to the expectation one
    would naturally entertain, the alkali lands, so frequently
    rich in the carbonates of the alkalies that would dissolve
    free silica, on the contrary, show most frequently an
    excess of soluble silica over alumina. This is probably to
    be explained from the very liberal opportunities afforded
    in the alkali soils for the formation of complex zeolitic
    masses by the retention in soil of the soluble alkali
    salts, and the abundance of lime always present in them. As
    already stated, we usually find in alkali soils a very large
    proportion of both alkaline and earthy bases in acid-soluble
    silicate combinations. But much farther research is needed
    to explain fully the marked discrepancies observed in
    this respect between soils not only occurring in closely
    contiguous localities, but also showing marked similarities
    in their general composition.

_Ferric Hydrate._—There is no obvious reason, from the chemical
standpoint, why iron, that is, ferric hydrate or iron rust, should be
more abundant in the soils of the arid regions, as the averages given
in the table suggest; moreover, the fact does not impress itself upon
the eye, since the orange or reddish tints are by far more common in
the humid than in the arid regions of the United States at least.
The California average is considerably influenced by the very highly
ferruginous soils from the foothills of the Sierra Nevada, and by the
black (magnetite) sand so commonly present; that of Oregon by the
black, highly ferruginous country rock (basalts), from which they are
partly derived. The average for Montana is not higher than that of
three states of the humid region, and less than that of Kentucky. We
might imagine a cause for depletion of iron in the soils of the humid
areas in the frequency with which humid moisture and high temperature
will during the summers concur toward the bringing about of a reducing
process in the soil, which by getting the iron into proto-carbonate
solution would make it liable to be leached into the subsoil, as is
frequently the case; yet the resulting “black gravel” or bog ore, in
its various forms, is of not infrequent occurrence in the arid regions
also. A constant quantitative difference due to climatic conditions
does not appear to be shown by the data thus far at command, but the
_finer distribution_ of the ferric hydrate in the humid temperate as
well tropical regions is obvious to the observer, from the frequent
redness of humid and tropical soils.

_Manganese._—An unexpected and apparently well-defined contrary
relation appears to be shown as regards the related metal _manganese_;
the average percentage of which is in all cases less in the arid than
in the humid region. The cause of this relation is altogether obscure;
it is too frequent to be accidental.

_Phosphoric Acid._—As regards that highly important soil ingredient,
_phosphoric acid_, the indication in the table that there is no
characteristic difference in the average contents in soils of the arid
and humid regions, respectively, is doubtless correct. This substance
is so tenaciously retained by all soils that there is no obvious reason
why there should be any material influence exerted upon its quantity
by leaching, or by any of the differences in the process of weathering
that are known to exist between the two climatic regions. Moreover, it
is apparent that the average for the arid region is made up out of very
widely divergent figures; that of California exceptionally low (lower
than any of those for the states of the humid regions), while those
for Washington and Montana are exceptionally high. The latter is due
to country rocks (“basalts”) showing abundance of microscopic crystals
of apatite, which in some cases raise the contents of the soils in
phosphoric acid to nearly twice the average given for the states.

The forecast that for most California soils, fertilization with
phosphates is of exceptional importance, has already been abundantly
confirmed by cultural experience. Few definite data are as yet
available from other arid states, where fertilization is thus far
sporadic and unsystematic. But it is predictable that in view of the
presence of an excess of lime carbonate in the arid soils, and the
unfavorable effect of this compound on the _rapid_ solubility of
tri-calcic phosphate demonstrated by Schloesing, Jr.,[145] by Böttcher
and Kellner[146] and Nagaoka,[147] fertilization with readily available
phosphate fertilizers will be found necessary among the first, all over
the arid region, especially in view of the scarcity of humus in arid
soils. [145] Ann. Sci. Agronomique, tome 1, 1899.

[146] Landw. Presse, 1900, No. 52; ibid. 1901, Nos. 23 and 24.

[147] Bull. Univ. Tokyo, Vol. 6, No. 3. Production was diminished
to less than one half when lime was used with bone meal, and actual
assimilation of phosphoric acid to one fifth.

    A curious instance of the effects of continued warm
    maceration in rock decomposition is afforded by the
    highly ferruginous soils derived from the black basaltic
    lavas of the Hawaii Islands. These lavas, like the basalt
    sheet of the Pacific Northwest, contain a large amount
    of crystallized phosphate minerals, notably apatite and
    vivianite. A correspondingly large proportion of phosphoric
    acid is found in the soils derived from these rocks, up to
    nearly two per cent.[148] But almost the entirety of this
    substance is present in the form of an insoluble, basic iron
    compound, difficultly soluble even in acids, and rendering
    it wholly unavailable to vegetation. So that actually the
    most pressing need of most of these soils is phosphate
    fertilization. The same is probably true of some of the
    highly ferruginous soils of California and of the Cotton
    States.


[148] See table, chapter 19, p. 256.

_Sulfuric Acid._—From the absence of the leaching process in the soils
of the arid region, we should expect that sulfates would be more
abundant in them than in the soils of the humid. This is certainly
true in the case of the alkali soils, which are characteristic of the
regions of deficient rainfall. See below, chapter 22.

    Hence the showing made in the general table, indicating that
    sulfates are equally abundant in the soils of the humid than
    in those of the arid regions, is surprising in view of the
    efflorescences of alkali sulfates so frequently observed
    in the latter. This is obviously due to the fact that the
    majority of such alkali soils has, on account of their
    local nature and usually heavy lime content, been excluded
    from the comparison; which otherwise would have made a very
    different showing.

_Potash and Soda._—The compounds of the alkali metals potassium and
sodium, being on the whole much more soluble in water, even without the
concurrence of carbonic acid, than those of calcium and magnesium, the
leaching process that creates such pronounced differences in the case
of the two earths must affect the alkali compounds very materially.
Comparison of the soils of the two regions in this respect shows,
indeed, very great differences in the average contents of potash and
soda. For potash the ratio is .216 to .670 per cent on the general
average, and .187 to .670 per cent, in the average by states; for soda,
.140 per cent to .350 per cent on the general average, and .110 per
cent to .420 per cent in the average by states. For both, therefore,
the general average ratio is as one to between three and four for the
humid as against the arid region.

It is curious that an approximation to the ratio of one to two, or
somewhat less, is maintained in the average proportion of soda to
potash in both regions; but this does not by any means hold good in
detail, very high potash-percentages being often accompanied by figures
for soda very much below the above ratio. This is the result of an
important difference in the chemical behavior of the two alkalies,
which has already been alluded to in connection with the discussion of
the zeolites. (See chapter 3, p. 38).

The process of “_kaolinization_,” being that by which clays are formed
out of feldspathic minerals and rocks such as granite, syenite,
trachyte, etc., results in the simultaneous formation of solutions of
carbonates and silicates of potash and soda. These coming in contact
with the corresponding compounds of lime and magnesia, also common
products of rock decomposition, are partly taken up by the latter,
forming complex, insoluble, hydrous silicates (zeolites). In these,
however, potash whenever present takes precedence of soda; so that
when a solution of a potash compound is brought in contact with a
zeolite containing much soda, the latter is partially or wholly
displaced and, being soluble, tends to be washed away by the rainfall
into the country drainage. Hence potash, fortunately for agriculture,
is tenaciously held by soils, while soda accumulates only where the
rainfall or drainage is insufficient to effect proper leaching, and
in that case manifests itself in the formation of what is popularly
known as “alkali soils;” namely those in which a notable amount of
soluble salts exists, and is kept in circulation by the alternation of
rainfall and evaporation, the latter causing the salts to accumulate
at the surface and to manifest themselves in the form of saline crusts
or efflorescenses. Alkali lands are a characteristic feature of all
regions of scanty rainfall, and are found more or less on all the
continents. The substances composing the alkali salts are retained not
only in their soluble form, but by their continued presence influence
profoundly, in several ways, the processes of soil formation. A more
detailed discussion of this important subject is given in chapters 22
and 23.

_Arid Soils are Rich in Potash._—One of the most important practical
conclusions flowing from the comparison of the potash contents of the
humid and arid soils respectively is that while in the former, potash
is usually among the _first_ substances to be supplied by fertilization
when production languishes, in the arid regions it will as rule come
_last_ in order among the three ingredients commonly so furnished.
Aside from the water-soluble potash salts always forming part of the
salts of the alkali lands proper, which in many cases will alone
hold out for many years under the demands of cultivation,[149] they
rarely contain much less than one per cent of acid-soluble potash;
occasionally rising as high as 1.8 per cent. That in such lands
potash-fertilization is uncalled-for and ineffective, hardly requires
discussion; while on the other hand, phosphates are commonly required
for full production after ten or fifteen years of cultivation without
returns. Nitrogen usually comes next in order, but sometimes is the
first need.

[149] In the light alkali lands of the southern California Experiment
Substation at Chino, the average content of water-soluble potash in ten
acres amounts to the equivalent of 1,200 pounds of potash sulphate per
acre. Outside of this the acid-soluble potash of the soil is .95%.,
equal to 38,000 pounds per acre-foot.

    The constant indiscriminate purchase and use of all
    three ingredients, so urgently recommended by fertilizer
    manufacturers because of their success in the humid Eastern
    States, is therefore very poor economy for the farmers
    of the arid region. Excepting cases of very intense
    culture, _e.g._ of vegetables or berries, the use of
    potash salts is but rarely remunerative, and therefore
    uncalled-for, in arid soils for a number of years.

_Humus._—The figures shown in the table for the average
humus-percentages in the soils of the two regions do not adequately
represent the very important differences actually existing; partly
because of the inadequate number of determinations made by the
same method (Grandeau’s), partly because of the differences in the
composition, and especially in the nitrogen-content of this substance,
which render direct comparison delusive. A detailed discussion of
the marked differences existing between the humus of arid and humid
soils in this respect has already been given (chapter 8, p. 135);
showing that the high nitrogen-percentage in the arid humus probably
compensates largely the lower humus-percentage, while rendering
nitrification more rapid, because the oxygen is not consumed by
overwhelming amounts of carbon and hydrogen; which, as is already
known, take precedence of nitrogen in the oxidation of humus
substances. Nitrates are almost always more abundant in the soils of
the arid region than in those of the humid, sometimes to the extent of
influencing injuriously the quality of certain crops, such as tobacco
and sugar beets. Nevertheless, nitrogen is ordinarily, in the arid
region, the substance requiring replacement next to phosphoric acid.
And when considered in connection with the small humus-content, so
liable to burning-out, this places _green-manuring with leguminous
plants_ among the first and most vital improvements to be employed
there.

_The Transition (semi-humid or semi-arid) Region._—The sloping plains
country lying between the Rocky Mountains and the Mississippi, quite
arid at the foot of the mountains, but with rainfall increasing more
or less regularly to eastward, form a transition-belt between the arid
and humid region of which but a small portion has been systematically
studied in respect to its soil formations. The analyses made of soils
of the two adjacent states of Minnesota and North Dakota, have been
placed in the general table (p. 377) to show how far in their general
relations their soils correspond to the generalizations deduced from
the comparison of the decidedly arid and humid soil areas chiefly
represented in the table. Although it has not been possible, for lack
of detailed data, to eliminate the soils originating from calcareous
formations, it will be seen that those of semi-arid Dakota differ
from those of more humid Minnesota, almost throughout, as would be
anticipated from the studies of the extremes, given in this chapter.




CHAPTER XXI.

SOILS OF ARID AND HUMID REGIONS (_Continued_).


SOILS OF THE TROPICS.

Within the ordinary limits of atmospheric temperatures, and in
the presence of adequate moisture, chemical processes active
in soil-formation are intensified by high and retarded by low
temperatures, all other conditions being equal. We can usually
artificially imitate, and produce in a short time by the application
of relatively high temperatures, most of the chemical changes that
naturally occur in soil-formation. While it is true that the changes
of temperature are nearly as great in the tropical as in the temperate
climates, these changes all occur at a higher level and within the
limits favoring bacterial and fungous action.

This being true we should expect that the soils of tropical regions
should, broadly speaking, be more highly decomposed than those of
the temperate and frigid zones, and that the intensified processes
continue currently. This fact has not been as fully verified as
might be desirable, by the direct comparative chemical examination
of corresponding soils from the several regions, owing to the want
of uniformity in methods and the fewness of such investigations in
tropical countries. Yet the incomparable luxuriance of the natural as
well as artificial vegetation in the tropics, and the long duration
of productiveness that favors so greatly the proverbial easy-going
ways and slothfulness of the population of tropical countries, offers
at least presumptive evidence of the practical correctness of this
induction.

In other words, the fallowing action, which in temperate regions
takes place with comparative slowness, necessitating the early use of
fertilizers on an extensive scale, is much more rapid and effective
in the hot climates of the equatorial rainy belt; thus rendering
currently available so large a proportion of the soil’s intrinsic
stores of plant-food, that the need of artificial fertilization is
there largely restricted to those soils of which the parent rocks
were exceptionally deficient in the mineral ingredients of special
importance to plants, that ordinarily form the essential material of
fertilizers. Quartzose, magnesian, and other soils resulting from
the decomposition of “simple” rocks will, of necessity, be poor in
plant-food everywhere.

_Humus in Tropical soils._—Another inference from the climatic
conditions of the tropics is that the _properly_ tropical soils are
likely to be rich in humus, as a result of the luxuriant vegetation
which in the decay of its remnants must leave abundant humic residues.
This seems to be generally verified wherever the interval between rainy
seasons is not too long; for otherwise, under the great and constant
heat of the tropics a rapid burning-out of the humus, such as is known
to occur in the arid regions, must also take place. A good example
illustrating the inter-tropical regime as regards humus is given in the
table in chap. 8, p. 137, showing the humus-content of some Hawaiian
soils. Both are of the same order as in the soils of the temperate
humid region, though the nitrogen-content evidently can, consistently
with productiveness, range lower than has thus far been observed in
temperate climates. This again forms a striking contrast with the soils
of the arid regions.

It is greatly to be regretted that not even approximate determinations
of the organic matter, much less of the humus-substance proper, have
been made by any of those who have analyzed tropical soils; excepting
those made of Hawaiian soils at the California Experiment Station.

The “loss by ignition” is of course always very largely water, mostly
referable to ferric hydrate and clay substance, the latter presumably
essentially in the form of kaolinite. When, therefore, ferric oxid and
alumina have been determined, we may approximate to the amount of total
organic matter by making allowance for ferric hydrate at the rate of
about 14% of the ferric oxid, for kaolinite at that of 34.92% of the
alumina found. Deducting these amounts of water from the total “loss by
ignition,” we may obtain at least an approximate idea of the organic
matter, and the probable availability of the nitrogen determined by the
analysis. See chapter 19, p. 357.

While the continuous heat and moisture of the tropics concur toward
rapid rock decomposition, it must be remembered that the copious
rainfall is equally conducive to an _intense leaching effect_.
Striking examples of this action occur in the Hawaiian Islands, in
the highly ferruginous soils resulting from the decomposition of the
black (pyroxenic and hornblendic) lavas that are so characteristic
of the volcanic effusions of that region. The soils formed from
these rocks are sometimes so rich in ferric hydrate (iron rust)
that they might well serve as iron ores elsewhere. But these soils
are very unretentive, and though very productive at first they are
soon exhausted, the abundant rains having sometimes deprived them of
almost every vestige of lime, and of most of the potash contained in
the original rock. At the same time the abundant phosphoric acid of
the original rock has been reduced to almost total unavailability by
combination with ferric oxid, just as in the case of the bog ore of the
temperate climates; so that phosphate fertilization is urgently needed
in these lands, though showing high percentage of phosphoric acid.
(Chap. 19, p. 356.)

Soils highly colored by ferric hydrate occur rather frequently in
the tropics, and have received the general name of “laterite” soils.
Curiously enough, the intense reddish tint mostly shown in these soils,
and which is emphasized in the “terra roxa” of the Brazilians, and
the general “red” aspect of Madagascar, and of the Malabar and Bengal
coasts, is by no means always accompanied by markedly high percentages
of iron oxid; but the latter is very finely diffused, so as to be very
effective in coloration. The plant-food percentages of tropical soils
are generally quite low, so that in the temperate humid regions such
lands would be adjudged to be rather poor. Yet they mostly prove quite
productive and lasting, even without fertilization.

    This is doubtless to be explained by the continuous and
    rapid rock and soil-decomposition which goes on under
    tropical climatic conditions, already alluded to; so as to
    supply enough available plant-food for the demands of each
    season’s vegetation, analogously to the proverbial “nimble
    penny.” This is supplemented also by the rapid decay and
    leaching-out of the ash ingredients of the rapidly decaying
    and dying vegetation. Nitrification must likewise, of
    course, be very active under the continual heat and
    moisture, and the humus formed under these circumstances is
    likely to be quite poor in nitrogen. On this latter point,
    however, definite data are almost wholly wanting.

_Investigations of Tropical Soils._—The most extended chemical
investigations of properly tropical soils have been made by Wohltmann
in his investigations of the soils of India, German Southwest and
Southeast Africa, and Samoa;[150] and by Müntz and Rousseaux of soils
collected under Government auspices in Madagascar. Leather, Bamber
and Mann have also analyzed a large number of soils of India. But we
find in many of these cases a failure to specify distinctly the local
climatic conditions, and even the depth to which the samples have been
taken; so that the investigator is obliged to examine laboriously
the local climates, and especially the amount and distribution of
rainfall, before being enabled to discuss intelligently the data given.
Even Wohltmann, in his discussion of North African and Saharan soils,
classes these distinctly arid types among the tropical ones.

[150] Samoa Erkundung, by F. Wohltmann, Kolonial-Wirthsch. Komitee,
Berlin, 1904.

Again, the dry seasons intervening between the tropical rains, varying
in length and from locality to locality, obscure somewhat the relations
of the soils to the climatic conditions. Under the lee of mountains,
even of slight altitude, we find xerophytic (arid-land) vegetation,
as has been noted by many observers in Brazil, even near the Amazon;
in Hawaii, in Jamaica, and in Madagascar. Unless, therefore, a close
discrimination is exercised by field observers, many contradictory
results will appear in analyses of soils of inter-tropical countries.
This is naturally the case in India, where the topographic surface
conformation and seasonal climatic conditions are so complicated and
contrasted. On the whole, the results obtained in Samoa, Kamerun and
Madagascar seem, of those available, to be the most characteristic
of true tropical conditions. In comparing these with the soils
of low plant-food percentages in the temperate humid region (see
chapter 19, p. 352), it must be remembered that those mentioned as
being productive are so by virtue of great depth and relatively
high proportions of lime; while in the tropics, the intense leaching
process prevents lime from reaching any high absolute or relative
percentages, save where limestone formations prevail. Moreover, the
mode of preparation of the soil extracts for analysis by Wohltmann,
and by Müntz and Rousseaux, differ so widely from that forming the
basis of discussion of soil-composition in this volume, that it becomes
necessary to make separate allowances in each case; since some of the
ingredients, phosphoric acid, lime and magnesia, are fully dissolved
by the weaker treatments, while others,—_e. g._, potash—are not, and
are therefore not directly comparable with the data obtained in the
writer’s work. The analyses made in India by Leather and others have
apparently been made substantially in accordance with the author’s
methods and may be considered directly comparable.


SOILS OF SAMOA AND KAMERUN.

Wohltmann has investigated the soils of Samoa, notably those
of the main island of Upolu, under the auspices of the German
“Kolonial-Wirthschaftliche Komitee” in 1903, and gives the results
of his observations and analyses in a report published at Berlin in
1904. The analyses are quite numerous, but unfortunately are made by a
special method which renders them only partly comparable with those of
any other analyst.

    Wohltmann’s method is this: “450 grams of fine earth (below
    2 millimeters diameter) is treated for 48 hours with 1½
    liters of cold chlorhydric acid of 1.15 density. Another
    portion, designed for a fuller determination of potash, is
    treated for one hour with the same acid, boiling hot. Potash
    was determined in both soil extracts; the hot extract gave
    from one-third to twice the amount obtained in the cold
    extraction.”[151]

    Wohltmann justifies this method by the statement that it has
    yielded him results more nearly in accord with experience
    than any other tried, both with tropical and European soils.

    Under these conditions only a few of the determinations in
    Wohltmann’s analyses are directly comparable with those upon
    which the discussions in this volume have been based. The
    figures for nitrogen and phosphoric acid may be assumed to
    be fully comparable; that of lime will in general represent
    fully only that which is present in the forms of carbonate,
    sulfate and humate, and a part of that existing in zeolitic
    or hydrous silicate form. Of the two potash determinations
    only the one made in hot extraction will be even remotely
    comparable, being probably at least 30% lower than would
    have been obtained by the writer’s method.

[151] Wohltmann states that the hot extraction sometimes yielded as
much as five times more than the cold; but no such case appears in his
reports on Samoa and Kamerun.

Even thus, however, Wohltmann’s results are highly instructive. He
gives the following summary of his mode of interpreting such analyses:

    ==================+============+=======+=============
                      | Very rich. | Good. | Inadequate.
    ------------------+------------+-------+-------------
    Potash            |     .2     |  .1   |     .05
    Lime and Magnesia |    1.0     |  .4   |     .07
    Phosphoric acid   |     .2     |  .1   |     .06
    Nitrogen          |     .2     |  .1   |     .05
    ------------------+------------+-------+-------------

It will be observed that the figures of this table differ materially
only in the matter of potash from those given in chapter 19, p. 354;
for the latter substance they would have to be multiplied by from 2 to
4, according to the lime-content and other conditions.

With this understanding a number of Wohltmann’s analyses of soils from
Samoa and Kamerun are given below, the potash determinations made with
hot acid being placed in parentheses after the other.

    _Soils of Samoan Islands._—A discussion of these
    analyses shows, from the writer’s point of view, a very low
    content of potash and lime, with the peculiarity that both
    are somewhat higher at the depth of a meter than in the
    surface ten-inches. This is probably to be accounted for
    from the very high content of organic matter (humus), which
    is apparent from the high “loss by ignition,” a very large
    proportion of which must be credited to the burning of the
    organic matter. That this humus reaches to the lowest depths
    examined, is clear from the nitrogen-content given for these
    samples. Wohltmann, whose estimate of these soils agrees in
    most respects with the writer’s, attributed to them a very
    satisfactory nitrogen-content. This would be true of the
    total; but as he has not determined either the true humus or
    its nitrogen-content, it remains uncertain whether or not a
    sufficiency is in an available form, and whether their case
    may not be like that of the Hawaiian soil mentioned above
    (chapter 19, p. 362), in which despite 10% of humus and
    .17% of nitrogen, the land was found to be nitrogen-hungry.
    Again, as regards the phosphoric acid, which Wohltmann
    considers satisfactory to high, it is questionable to what
    extent it is rendered unavailable by the very high content
    of ferric hydrate. We are thus left in some uncertainty as
    to the real manurial requirements of the Samoan soils, which
    doubtless represent very closely also those of Tutuila, the
    chief American island of the group.

    ANALYSES OF TROPICAL SOILS BY F. WOHLTMANN.

    Extraction with cold chlorhydric acid, sp. g. 1.15, for 48 hours.

    (A) = Tuanaimato. Virgin Forest Soil, not given.
    (B) = Le Utu Sao Vaa Cultivated Soil.
    (C) = 0—25 cm. (10 ins.)
    (D) = 75—100 cm. (30—40 ins.)
    ======================+===========================================
                          |                  SAMOAN ISLANDS.
                          |
                          |             UPOLU.       |     SAVAII.
                          |                          |
                          |   (A)    |      (B)      |  Cleared Land.
                          |          |       |       |        |
            Depth.        |          |  (C)  |  (D)  |   (C)  |  (D)
    ----------------------+----------+-------+-------+--------+---------
    Potash[152]           |  .05     |  .048 |  .022 |  .063  |  .036
                          | (.07)    |(.077) |(.043) | (.102) | (.043)
    ----------------------+----------+-------+-------+--------+---------
    Lime                  |  .07     |  .113 |  .033 |  .042  |  .023
    Magnesia              |  .37     |  .285 |  .144 |  .067  |  .074
    Ferric Oxid           |21.53     |17.600 |19.653 |15.333  |18.733
    Alumina               |12.40     | 9.621 |11.217 |18.941  |16.413
    ----------------------+----------+-------+-------+--------+---------
    Silica                |  .99     | 2.043 | 2.853 |  .366  | 1.250
    Titanic Acid          |          |       |       |        |
    ----------------------+----------+-------+-------+--------+-------
    Phosphoric Acid       |  .30     |  .179 |  .213 |29.096  |  .187
    Org. Matter and Water |18.49[153]|17.288 |12.770 |29.146  |16.332
    Nitrogen in Soil      |  .30     |  .447 |  .186 |  .697  |  .128
                          |          |       |       |        |
    Hygr. Moisture        | 6.80     |15.062 |13.242 |15.288  |12.862
    ----------------------+----------+-------+-------+--------+-------

    ======================+=================================
                          |             KAMERUN.
                          |
                          |    ISONGO.    |    MUNDAME II.
                          |       |       |       |
            Depth.        |  (C)  |  (D)  |  (C)  |  (D)
    ----------------------+-------+-------+-------+---------
    Potash[154]           |  .097 |  .104 |  .076 |  .110
                          |(.101) |(.163) |(.123) | (.168)
    ----------------------+-------+-------+-------+---------
    Lime                  |  .193 |  .154 |  .150 |  .125
    Magnesia              |  .283 | 1.415 |  .198 |  .099
    Ferric Oxid           | 7.305 | 7.497 |13.920 |11.707
    Alumina               |14.298 |14.504 | 5.223 | 5.531
    ----------------------+-------+-------+-------+---------
    Silica                |  .047 |  .108 |  .227 |  .120
    Titanic Acid          |       |       |       |
    ----------------------+-------+-------+-------+---------
    Phosphoric Acid       |  .064 |  .224 |  .131 |  .205
    Org. Matter and Water |23.335 |12.299 |10.154 | 9.549
    Nitrogen in Soil      |  .187 |  .079 |  .164 |  .103
                          |       |       |       |
    Hygr. Moisture        |20.065 |15.329 |14.811 |16.498
    ----------------------+-------+-------+-------+----------

[152] The numbers in brackets are determinations made after boiling
with acid for one hour.

[153] Soil air-dry.

[154] The numbers in brackets are determinations made after boiling
with acid for one hour.

    It is probable that for crops requiring so much potash as do
    the banana and cacao trees, potash is the first need when
    they cease to produce well on these soils.

    _Soils of Kamerun._—In the soils of Kamerun, also
    analyzed by Wohltmann, and of which two are placed alongside
    of those of Samoa, it is at once seen that the materials
    from which they have been formed are richer in both potash
    and lime than the parent rocks of the Samoan, and not quite
    so rich in iron. They are also very rich in organic matter,
    evidently down to the depth of a meter, as are those of
    Samoa. It is probably due to the high humus-content that
    these soils, washed as they have been by the second-highest
    rainfall in the world (about 35 feet annually) have not been
    as thoroughly leached as have been those of the Brahmaputra
    valley. The annual rainfall of Samoa is only from nine to
    eleven feet on the lower levels, but ranges as high as 18
    feet at higher elevations.

It is noticeable that in most of these true tropical soils the content
of magnesia is considerably above that of lime; a fact readily
intelligible from the more ready solubility of lime in carbonated
water. It is hardly doubtful that this disproportion will in many cases
explain a lack of thriftiness, which could be effectually remedied by
a simple application of lime or marl, without resorting to the more
costly fertilizers.


THE SOILS OF MADAGASCAR.

The soils of the island of Madagascar have been analyzed to the
number of about 500 by Müntz and Rousseaux, under the auspices of the
French government.[155] So large a number of analyses should give a
very full understanding of the agricultural capacity and adaptation
of so comparatively limited an area; unfortunately, we are here again
confronted by more or less imperfect data accompanying the samples
collected by government agents, and by the use of an analytical method
different from those of all other nations, and hence incommensurable
except, as in the case of Wohltmann’s method, in regard to certain
ingredients.

[155] Annales de la Science Agronomique, tome 1er, 1901, fasicules 1,
2, 3.

The French chemists use nitric instead of chlorhydric acid; cold for
phosphoric acid and lime, boiling-hot for five hours for potash;
considering the remainder as of no practical importance. Since nitric
acid is in general much less incisive than chlorhydric in its solvent
power, comparison with the analyses made by other nations becomes
difficult. As in the case of Wohltmann, magnesia, lime, and phosphoric
acid may be considered to be quite thoroughly extracted by the
treatment; while extraction of possibly available potash is doubtless
very incomplete. On the whole, however, the estimates of soil-fertility
based on percentages is very nearly the same as those assigned by
Wohltmann in the table given above. Like Wohltmann, they emphasize the
axiom that the same percentage-gauge of fertility cannot be applied in
the tropics as in the temperate zones.

_General Character of the Island._—The island of Madagascar, lying
between the 11th and 25th degrees of south latitude, is quite
mountainous in its central and eastern portion, where the coast falls
off pretty steeply into the sea, leaving only a narrow coast belt of
properly agricultural land in the lower valleys and at the mouth of
the torrential streams. The mountains rise at one point to the height
of nearly 10,000 feet. The western portion of the Island is much less
broken, has much plateau land with low intersecting ranges and streams
of moderate fall, with considerable alluvial lands near the coast.
The rocks are almost throughout gneisses and mica-schists, which, as
heretofore stated (chapter 4, p. 51), form mostly poor soils. There are
a few areas of eruptive rocks and tertiary calcareous deposits, and on
these the lands are much more thrifty. The rocks and red soils of the
central mass, however, extend seaward almost everywhere.

The rainfall is high on the east side, where the moisture of the
southeast trade winds is first condensed, the precipitation reaching
ten to twelve feet (120 to 144 inches) annually. The western portion
is relatively dry, but rains fall more or less throughout the year;
while in the eastern and central mountainous part there is a distinct
subdivision into a wet and a dry season. Here, while the rivers are
largely torrential, many large fertile valleys have been created by the
heavy denudation of the mountain slopes. This is especially the case in
the Imerina province (in which the capital, Tananarivo, is situated),
and here the valley soils are deep, and rich in humus. The western
portion is but thinly forested. The soils of most of the island are
“red” with ferric hydrate, resembling the laterite soils elsewhere; yet
the iron percentages are not usually very heavy, ranging mostly from
4 to 6, more rarely to 10% and more, of ferric oxid. Most of the red
soils are clayey, crack open in summer and become very hard in drying.

Of the 476 soils analyzed by Müntz and Rousseaux, 156 are from the
province of Imerina, 56 from the adjacent province of Betsileo,
therefore 212 from the central, mountainous part of the island. The
remainder are scattered around the coasts; the most productive being
apparently those of the northern end, Diego Suarez, which is mostly
underlaid by the eruptive rocks forming the mountain mass of Mount
Amber, from which numerous fertile valleys radiate. The valleys of the
west coast also, in the provinces of Bara, Tulear and Betsiriry, have
some very productive soils.

The subjoined table, giving fourteen analyses selected as
representative from the mass of material presented by Müntz and
Rousseaux, gives a fair general idea of the character of the soils
of the great island. It is at once apparent that lime and potash are
extremely deficient in the soils of the mountain slopes of central and
southern Madagascar, these substances having, as elsewhere in the humid
region, been leached down into the valleys; and the materials being
mostly quite clayey, these valley soils have not, as in the case of
the sandy alluvium of the Brahmaputra, themselves been again leached
of their mineral ingredients. Practically these valleys seem to form
the only profitably cultivable area of the central portion; while along
the larger river courses, such as the Mangoky, Ikopa, Mahajamba and
others, good alluvial “bottoms” and deltas form available lands. It
seems to the writer that, in view of their own expressed opinion that
tropical soils are not to be gauged on the same percentage-basis of
soil-ingredients as those of temperate regions, Müntz and Rousseaux
rather underestimate the productive value of many of these lands;
regarding which the field notes report good production, and the crops
of which are certainly not the first that they have borne in the course
of Malagassy history. It is as though their anxiety to forestall
overestimates of agricultural prospects by intending settlers, had led
them to somewhat overshoot the mark.

             ANALYSES OF MADAGASCAR SOILS BY MUNTZ AND ROUSSEAUX.
    ======================+=================================+===========
                          |             IMERINA.            | BETSILEO
    ----------------------+---------------------------------+-----------
                          |           CENTRAL MOUNTAIN REGION.
    ----------------------+----------+------------+---------+-----------
        Number of Soil.   |    4--4  |    115     |  71--4  |    272
    ----------------------+----------+------------+---------+-----------
           Locality       | Ambohitr-|  Ankazobe  | Ambohib-|Fandrandava
                          |  omby.   |   North.   |   zaka. |  Valley.
    ----------------------+----------+------------+---------+-----------
                          | Red Soil.|  Ochreous  |         |
                          |          |   Soil.    |         |
    ----------------------+----------+------------+---------+-----------
    Potash (K₂O)          |     .020 |    .006    |   .071  |    .017
    Lime (CaO.)           |     .350 |    .060    |  traces |    trace
    Magnesia (MgO.)       |     .022 |            |         |
    Ferric Oxid (Fe₂O₃)   |    9.333 |            |         |
    Phosphoric Acid (P₂O₅)|     .050 |    .032    |   .061  |    .267
    Nitrogen (N)          |     .096 |    .027    |   .030  |    .020
    ----------------------+----------+------------+---------+-----------
    Remarks               |   Small  |Deficient in|No great |    Only
                          | cultural | plant-food |cultural | moderately
                          |resource. |ingredients.|resource.|  fertile.
                          |          |May maintain|         |
                          |          |vegetation  |         |
                          |          |on account  |         |
                          |          |of humidity.|         |
    ----------------------+----------+------------+---------+-----------

    ======================+==========+=======================+========
                          | TULEAR.  |      MAINTIRANO.      |MAJUNGA.
    ----------------------+----------+-----------------------+--------
                          |                   West Coast.
    ----------------------+----------+------------+----------+--------
        Number of Soil.   |   253    |    261     | No. 267  |  343
    ----------------------+----------+------------+----------+--------
           Locality       |R. bank of| Plateau of |Village of|
                          | Sakondry |Antsoamena. |Anhozorabe|
                          | north of |            | South of |
                          |Tongobory |            |Mandroso. |
                          | Valley.  |            |          |
    ----------------------+----------+------------+----------+--------
                          |          |   Sandy    |          | Hill
                          |          |   soil.    |          |summit.
    ----------------------+----------+------------+----------+--------
    Potash (K₂O)          |    .086  |    .015    |   .014   |  .012
    Lime (CaO.)           |   4.180  |   trace    |   trace  |  trace
    Magnesia (MgO.)       |          |            |          |
    Ferric Oxid (Fe₂O₃)   |          |            |          |
    Phosphoric Acid (P₂O₅)|    .083  |    .043    |   .011   |  .017
    Nitrogen (N)          |    .075  |    .051    |   .039   |  .043
    ----------------------+----------+------------+----------+--------
    Remarks               | Potatoes,|  Manioc,   |Remarkable|Grasses,
                          |  manioc, |  maize,    |  crops,  |  tall
                          |represents|  beans,    |  manioc, | herbs.
                          |  soil of |  peanuts,  |  rice.   |
                          |  whole   |coco-trees, |          |
                          |  region. |  mangoes.  |          |
    ----------------------+----------+------------+----------+--------

    ======================+==============================
                          |       DIEGO SUAREZ.
    ----------------------+------------------------------
                          |         N. COAST.
    ----------------------+--------------+---------------
        Number of Soil.   |       3      |   331
    ----------------------+--------------+---------------
           Locality       |   Anamakia.  | Montagne
                          |              | d’Ambre.
    ----------------------+--------------+---------------
                          |   Ochr’us    | Ochr’us
                          |    earth.    |  earth.
    ----------------------+--------------+---------------
                          |              |
    Potash (K₂O)          |     .161     |   .031
    Lime (CaO.)           |     .620     |   trace
    Magnesia (MgO.)       |              |
    Ferric Oxid (Fe₂O₃)   |              |
    Phosphoric Acid (P₂O₅)|     .380     |   .124
    Nitrogen (N)          |     .124     |   .177
    ----------------------+--------------+---------------
    Remarks               |  Promises    | Crops, coffee.
                          | considerable | Good cultural
                          |  fertility.  | resources.
    ----------------------+--------------+---------------

    ======================+========================+===================
                          |        ANDEVORANTE.    |   AMBATONDRAZAKA.
    ----------------------+------------------------+-------------------
                          |                    EAST COAST.
    ----------------------+------------+-----------+----------+--------
        Number of Soil.   |    107     |     370   |    81    |  105
    ----------------------+------------+-----------+----------+--------
           Locality       |   Dist. of | Ambaniman-| Imeriman-|Sabotsy.
                          | Vatomandry.| hovinana. |   droso. |
                          |Ampitamafana|           |          |
    ----------------------+------------+-----------+----------+--------
                          |    Plain   |           |          |
                          |    valley, |   Valley. |          |Alt 1000
                          |    sandy   |           |          |   M.
                          |    soil.   |           |          |
    ----------------------+------------+-----------+----------+--------
                          |            |           |          |
    Potash (K₂O)          |    .069    |    .012   |    .490  |  .005
    Lime (CaO.)           |    .070    |    trace  |   1.200  |  .070
    Magnesia (MgO.)       |            |           |          |
    Ferric Oxid (Fe₂O₃)   |            |           |          |
    Phosphoric Acid (P₂O₅)|    .047    |    .161   |   1.566  |  .369
    Nitrogen (N)          |    .016    |    .254   |    .249  |  .164
    ----------------------+------------+-----------+----------+--------
    Remarks               |  Coffee,   |    Very   |  Amply   |Fertile
                          |  vanilla,  |   fertile | rich in  |soil fit
                          | rice, &c., |    soil.  |plant food|for all
                          |  rich red  |           |suited for|cultures.
                          | soil. Rep. |           | intensive|
                          |   whole    |           | cultures.|
                          |  valley.   |           |          |
    ----------------------+------------+-----------+----------+--------

Be that as it may, the influence of the tropical climate and rainfall
upon the composition of these soils is certainly very marked. While
gneiss is not credited with producing first-class soils, its usual
content of orthoclase feldspar should at least insure a respectable
average content of potash; but this, it will be seen, is mostly not the
case; and that of lime seems even worse, aside from the case where,
as in some regions near the coast (especially in the west and south),
calcareous formations, probably of tertiary age, have contributed to
soil-formation. At some points there seem to exist phosphate deposits,
well known elsewhere to occur in such rocks, which impart to the soils
exceptionally high percentages of phosphoric acid, even exceeding one
per cent. The phosphates of course remain practically untouched by the
leaching processes, and appear to be somewhat widely diffused; so that
the soils of Madagascar may be said to be, on the whole, well supplied
with this important plant-food.

In the central province of Imerina the valleys and lower slopes show a
fair content of both lime and potash; but in the province of Betsileo,
adjoining it on the south, nearly every one of the soils analyzed is
reported as containing only “traces” of lime, together with very small
amounts of potash in most cases. The ultimate analyses of ignited red
earths, of which an average is here given, are of interest in this
connection.

    ULTIMATE ANALYSIS OF IMERINA RED SOILS,
          IGNITED; AVERAGE OF THREE.

         Silica        55.2
         Potash          .3
         Lime          trace
         Magnesia       1.1
         Ferric oxid   10.6

It is quite obvious that only leaching-down and concentration of the
feeble resources of such material in the valleys can produce soils
worthy of permanent cultivation.

One point, however, is strikingly illustrated in several of the
analyses given in the subjoined table. We find in the original quite a
number of cases in which the field notes report considerable fertility,
while the chemists’ judgment is very unfavorable. Thus we find recorded
for the soil No. 267, taken near the village of Anjozorabe, in the
Maintirano region, “luxuriant vegetation and remarkable crops,” with
such minute proportions of potash, lime and phosphoric acid that
the authors are compelled to say that the land offers “no cultural
resources.” The same occurs in the cases of soils Nos. 370, 261,
and several others having either “good crops” or “abundant natural
vegetation.” Unless we assume that in these cases the samples were
not properly taken, we are obliged to conclude that under the local
climatic conditions, such minute amounts of plant-food are developed
with sufficient rapidity to supply good growth. This would be quite
parallel to the case of the tea soils of Assam, whose production lasted
30 years before showing exhaustion, on plant-food percentages only
slightly greater than those here noted, and determined by a much more
incisive method.

It is thus quite obvious that a different standard of interpretation
must be applied to tropical soils as compared with either the temperate
humid, or the arid regions; and that uniform methods of analysis are
needed to evolve a definite rule from the present uncertainties.


THE SOILS OF INDIA.

The soils of India have been investigated to some extent by the
geological survey of India; by Voelcker, who went there on a special
mission to investigate agricultural conditions; and since, more
especially by Leather, Bamber and Mann; and by Moreland. Leather’s
account is the most complete on the general subject and can best serve
as the basis for a review of the entire peninsula.[156]

[156] On the Composition of Indian Soils. Agr. Ledger, 1898, No. 2.

According to Dr. Leather, “the four main types of soils to be dealt
with, and which certainly occupy by far the larger of the Indian
cultivated area,” are: _The Indo-Gangetic alluvium_, covering the chief
cultivable areas of the Indo-Gangetic plain; the _black cotton soils or
regur_, occupying the main body of the plateau of the Central provinces
(the Deccan) from the Vindhya range south; the _red soils_ lying on the
metamorphic rocks of Madras; and the “_laterite_” _soils_ which are met
with in many parts of India. To these should be added the _alluvial
soils of the Brahmaputra valley_, in Assam. It is hardly to be
expected that so large an area as that of India, with its diversified
topography, and a climate ranging from about four inches of rainfall in
the northern Panjab to the world’s maximum in Assam, and southward to
typical tropical conditions, could be even thus briefly characterized.
The observers have rarely given for the several soils analyzed, special
local and climatic data, which cannot always be obtained from the
official publications; so that it is not easy to discuss them from the
points of view of aridity and humidity.

_The Indo-Gangetic Plain._—The general rain-map of India shows the
Panjab and Rajputana to be arid throughout; thence eastward the
rainfall increases to 25 and 30 inches on the Ganges; notwithstanding
which, alkali (reh) is abundant about Aligarh, Meerut and Agra. Thence
toward Calcutta there is a steady increase of rainfall until, at the
head of the Bay of Bengal, 70 inches is reached.

If under these conditions the Indo-Gangetic plain admits of any
generalizations as regards soil composition, it must be attributed in
the main to its predominantly alluvial character. It should therefore
be relatively rich in lime, magnesia and potash. So far as the first
is concerned, Leather remarks that the only rocky particles larger
than sand to be found in all this large belt of land is the nodular
limestone called kankar, formed by the deposition of calcium carbonate
within the soil, at the depth of a few feet. It occurs very generally
in India, and as stated above (chapters 9 and 19), this occurrence
of calcareous hardpan, of varying hardness, is almost universal
in the arid regions. The analysis given in the table, selected as
representative from those given by Leather, show that the general
forecast is realized in them, as soils of an arid region.

                       ANALYSES OF SOILS OF INDIA.

                                         NORTHERN INDIA.
    ============================+=================================
                                |     Indo-Gangetic Alluvium.
                                +----------------+----------------
                                |    Panjab.     |  Lower Ganges.
                                |                |
                                +--------+-------+-------+--------
                                | Sotar  |Changa | Ison, | Sibpur,
                                | Valley.|Manga. |Ganges,|Calcutta
                                | Clay   |Loamy  | Doab. |  Clay
                                | Loam.  |soils. | Sandy | soils.
                                | [157]  |       | Loams.|
    ----------------------------+--------+-------+-------+-------
    Insoluble matter            |  81.57 | 81.54 | 88.08 |  73.58
    Soluble silica              |        |       |       |
    ----------------------------+--------+-------+-------+--------
    Potash (K₂O)                |    .74 |   .54 |   .64 |
    Soda (Na₂O)                 |    .08 |   .25 |   .09 |  1.82
    ----------------------------+--------+-------+-------+--------
    Lime (CaO)                  |   1.44 |   .98 |   .47 |  1.01
    Magnesia (MgO)              |   1.97 |  1.72 |   .32 |  1.64
    Br. ox. of Manganese (Mn₃O₄)|        |   .11 |       |  7.19
    Peroxid of iron (Fe₂O₃)     |   4.32 |  5.11 |  3.10 |  7.58
    Alumina (Al₂O₃)             |   5.85 |  4.36 |  4.38 |  9.89
    Phosphoric acid (P₂O₅)      |    .23 |   .14 |   .08 |   .07
    Sulfuric acid (SO₃)         |   ?    |   .02 |   .05 |   .00
    Carbonic acid (CO₂)         |   1.13 |   .45 |   .37 |   .28
    Water and org. matter       |   2.67 |  4.78 |  2.42 |  5.93
                                +--------+-------+-------+--------
                Total           | 100.00 |100.00 |100.00 |100.00
                                |        |       |       |
    Nitrogen                    |    .02 |   .082|   .027|   .051
    ----------------------------+--------+-------+-------+--------
                                |          Brahmaputra Alluvium.
                                +---------------------------------------
                                |                Assam.[158]
                                +---------+---------+----------+--------
                                | Fezpur  |Lakhimpur|Golaghat. |Sipsagar
                                |  bank.  |   New   |Nigriling.|  Teela
                                |   Old   |Alluvium.|   New    |  land.
                                |Alluvium.|         |Alluvium. |   New
                                |         |         |          |Alluvium
    ----------------------------+---------+---------+----------+--------
    Insoluble matter            |  85.18  |   84.60 |   85.88  |  91.52
    Soluble silica              |         |         |          |
    ----------------------------+---------+---------+----------+--------
    Potash (K₂O)                |    .35  |     .24 |     .26  |    .14
    Soda (Na₂O)                 |    .30  |     .12 |     .23  |    .16
    ----------------------------+---------+---------+----------+--------
    Lime (CaO)                  |    .04  |     .11 |     .03  |    .06
    Magnesia (MgO)              |    .46  |     .33 |     .36  |    .20
    Br. ox. of Manganese (Mn₃O₄)|         |         |          |
    Peroxid of iron (Fe₂O₃)     |   2.08  |    2.78 |    2.74  |   1.52
    Alumina (Al₂O₃)             |   5.03  |    5.63 |    5.10  |   3.30
    Phosphoric acid (P₂O₅)      |    .05  |     .06 |     .06  |    .06
    Sulfuric acid (SO₃)         |    .02  |     .02 |     .02  |    .02
    Carbonic acid (CO₂)         |         |         |          |
    Water and org. matter       |   5.59  |    6.11 |    5.32  |    .18
                                +---------+---------+----------+--------
                Total           |         |         |          |
                                |         |         |          |
    Nitrogen                    |    .14  |     .20 |     .18  |    .08
    ----------------------------+---------+---------+----------+--------
                                |        Laterite Soils.
                                +-------------------------------
                                |           Bengal.
                                +----------+---------+----------
                                |Lohardaga.|  Chota  |Hazaribagh
                                |          | Nagpur. |District.
                                |          |Singhbhum|
                                |          |District.|
    ----------------------------+----------+---------+----------
    Insoluble matter            |  29.67   |  59.06  |   80.46
    Soluble silica              |          |         |
    ----------------------------+----------+---------+----------
    Potash (K₂O)                |    .10   |    .27  |     .38
    Soda (Na₂O)                 |    .04   |         |     .32
    ----------------------------+----------+---------+----------
    Lime (CaO)                  |    .38   |    .28  |    1.72
    Magnesia (MgO)              |    .21   |    .33  |     .38
    Br. ox. of Manganese (Mn₃O₄)|    .07   |    .48  |     .50
    Peroxid of iron (Fe₂O₃)     |  48.71   |  26.64  |    6.12
    Alumina (Al₂O₃)             |   8.81   |   7.27  |    7.19
    Phosphoric acid (P₂O₅)      |    .64   |    .08  |  Trace.
    Sulfuric acid (SO₃)         |          |         |  Trace.
    Carbonic acid (CO₂)         |    .06   |    .16  |     .12
    Water and org. matter       |  11.31   |   5.43  |    2.81
                                +----------+---------+----------
                Total           | 100.00   | 100.00  |
                                |          |         |
    Nitrogen                    |    .010  |    .024 |     .03
    ----------------------------+----------+---------+----------

[157] Analysis by Voelcker.

[158] Analyses by Mann.

                            SOUTHERN INDIA.
    =============================================================
                                    Madras Presidency.
    ----------------------------+--------+-----------------------
                                |Laterite|        Upland.
                                |  Soil. |      Red Soils.
                                +--------+---------+-------------
                                |Sidapet.| Madura. |Trichinopoli.
                                |        |Dindigal.| Parambalur.
                                |        |         |
                                |        |         |
    ----------------------------+--------+---------+-------------
    Insoluble matter            |  76.86 |  90.47  |    86.74
    Soluble silica              |        |         |
    ----------------------------+--------+---------+-------------
    Potash (K₂O)                |    .09 |    .24  |      .05
    Soda (Na₂O)                 |    .17 |    .12  |      .15
    ----------------------------+--------+---------+-------------
    Lime (CaO)                  |  Trace.|    .56  |      .48
    Magnesia (MgO)              |    .77 |    .70  |      .70
    Br. ox. of Manganese (Mn₃O₄)|    .19 |    .08  |      .10
    Peroxid of iron (Fe₂O₃)     |  10.09 |   3.51  |     5.70
    Alumina (Al₂O₃)             |   8.84 |   2.92  |     5.68
    Phosphoric acid (P₂O₅)      |  Trace.|    .09  |      .05
    Sulfuric acid (SO₃)         |        |         |
    Carbonic acid (CO₂)         |    .12 |    .30  |      .11
    Water and org. matter       |   2.87 |   1.01  |      .24
                                +--------+---------+-------------
                Total           | 100.00 | 100.00  |   100.00
                                |        |         |
    Nitrogen                    |    .015|    .006 |      .021
    ----------------------------+--------+----+----+-------------

    ====================================================================
                                    Madras Presidency.
    ----------------------------+-------------+-------------------------
                                |  Alluvium.  |      Regur.
                                |  Averages.  |
                                +------+------+------+----------+-------
                                |Loamy |Sandy |Regur.| Trichin- |Kistina
                                |Soils.|Soils.| Avg. |   opoli. |
                                |      |      | 18   |Parambalur|Narsa-
                                |      |      |Soils.|  Taluk.  | opet.
    ----------------------------+------+------+------+----------+-------
    Insoluble matter            |71.79 |93.09 | 68.41|   65.16  |  68.29
    Soluble silica              |      |      |      |          |
    ----------------------------+------+------+------+----------+-------
    Potash (K₂O)                |  .22 |  .04 |   .41|     .14  |   1.14
    Soda (Na₂O)                 |  .17 |  .07 |   .31|     .01  |   1.30
    ----------------------------+------+------+------+----------+-------
    Lime (CaO)                  |  .54 |  .13 |  2.90|    2.18  |   3.43
    Magnesia (MgO)              | 1.29 |  .33 |  2.27|    2.47  |   1.94
    Br. ox. of Manganese (Mn₃O₄)|  .13 |  .04 |   .17|     .25  |    .09
    Peroxid of iron (Fe₂O₃)     | 9.59 | 2.46 |  7.13|    9.27  |   6.96
    Alumina (Al₂O₃)             | 9.98 | 1.74 | 10.14|   13.76  |  10.28
    Phosphoric acid (P₂O₅)      |  .11 |  .05 |   .06|   Trace. |    .00
    Sulfuric acid (SO₃)         |      |      |      |   Trace. |  Trace.
    Carbonic acid (CO₂)         |  .09 |  .11 |  1.62|     .91  |   1.88
    Water and org. matter       | 6.09 | 1.94 |  6.58|    5.85  |   3.96
                                +------+------+------+----------+-------
                Total           |100.00|100.00|100.00|  100.00  |  99.27
                                |      |      |      |          |
    Nitrogen                    |  .037|  .015|   .03|     .024 |   .012
    ----------------------------+------+------+------+----------+-------

_The Brahmaputra Alluvium in Assam._—Aside from the immediate
alluvium of the Indus, of which no definite data are available, the
Indo-Gangetic plain represents the drainage of the _southern_ slope of
the Himalaya chain. That of most of the _northern_ slope is represented
by the Brahmaputra, which not only originates in a region of heavy
precipitation—Thibet—but continues in the same throughout its course,
and rounding the easternmost spur of the Himalaya range enters, in
southern Assam, upon the region of the maximum rainfall known. Its
alluvial deposits should therefore show the reverse characteristics of
those of the Ganges; they should, as thoroughly leached soils, be poor
in lime, magnesia and potash. We have fortunately on this subject the
excellent work done by Mr. H. H. Mann for the Indian Tea Association,
the report of which was published in 1901, and contains, besides a
large number of analyses, good descriptions of the general soil and
cultural conditions of the Assam tea districts, with suggestions for
their improvement.

The tea plantations of Assam are located almost wholly on the new and
old alluvium of the Brahmaputra river, bordered on the north by the
eastern spur of the Himalayas, on the south by the low ranges of the
Khasia hills. The soil is mostly quite sandy, the late alluvium gray in
color, the older reddish and more loamy. Of the four analyses given in
the table and fairly representing the average character of these soils,
the two first are from the north side, the latter two from the south
side of the river.

It will be noted that the prominent feature of all these soils is an
extremely low percentage of lime, the general average being about .08%
as against nearly 1.0% in the average Indo-Gangetic soils. In the
latter, potash ranges between .65 and .70%; in the Assam soils between
.25 and .35. Magnesia averages nearly 1.3 in the Indo-Gangetic, against
about .50 in the Assam tea soils. It is thus apparent that the same
general facts as regards the leaching-out of soil ingredients already
shown for eastern and western North America are strikingly verified in
northern India; but reversed as regards the points of the compass. The
preferential leaching-out of lime as compared with magnesia and potash,
is here again well exemplified. It would be interesting to have an
analysis of the Brahmaputra water to compare with that of the Ganges.
That tea should flourish for twenty to thirty years in such soils,
is a good indication of one cause at least of the total failure of
tea culture in California, where tea plants are difficult to maintain
alive, and after 25 years form rounded, scrubby bushes not over four
feet high. Similar failures of tea on calcareous soils are on record
from India. The low lime-content of the Assam soils, then, does not
necessarily imply that these soils should be limed to maintain tea
production. According to Mann, the main deficiency is in nitrogen, as
the figures imply; but whether his recommendation of green-manuring
with leguminous crops to increase the nitrogen-supply is practicable
without first supplying more lime to the Assam soils, is questionable.
Since phosphoric acid is also low, his recommendation to use freely the
basic or Thomas slag is doubtless a good one, since lime would thus
also be moderately increased.

Bamber gives a number of analyses of tea soils from low ground in
Assam, which are very rich in vegetable matter and quite acid. Like
those reported by Leather, these “bhil” soils are very poor in lime and
nitrogen, but fairly supplied with potash and phosphoric acid.

_The Regur or Black Cotton Soils of Southern India._—The
second-greatest reasonably uniform soil-area of India is that covered
by the regur, or black cotton soils, in south central India, notably
the Deccan, where these soils are said to have been cultivated without
fertilization for 2000 years and are still fairly productive.[159]
Both in their physical character, chemical composition, and cultural
characteristics, these regur soils are very similar to the “prairie
soils” of the Cotton states and especially to the “black adobe” of
California. Like the latter they are of unusual depth without change of
tint; they crack wide open during the dry season on account of their
high clay content; and the soil is thus partly inverted by the surface
soil falling into the cracks. To the latter fact Leather ascribes, in
part, the long duration of fertility in the regur lands. The regur also
contains fragments of calcareous hardpan (here called guvarayi), just
as in the Great Valley of California. The eighteen analyses of regur
given by Leather agree so nearly in their essential points that it is
admissible to average them; two other examples are however also given
in the table.

[159] That is to say, they now produce about 600 pounds, or 10 bushels
of wheat per acre, as do the Rothamstead soils after fifty years’
exhaustive cultivation. Probably both have come down to the permanent
level of production corresponding to the amount of plant-food made
currently available each year by the fallowing process in originally
very rich soils. The present product of cotton on the regur lands does
not seem to be on record; judging by the wheat product it should not be
over one hundred pounds of lint per acre.

It will be noted that while the contents of lime, magnesia and alumina
are uniformly high, the content of potash has a wide range; it rises
very high (1.14%) in the maximum, while the average is fair.

One conspicuous defect of these soils is their extremely low
content of nitrogen, in view of which their lasting productiveness
is difficult to understand; unless it be that, as in California,
their high lime-content causes a copious crop of leguminous weeds,
constantly replacing the nitrogen supply.[160] Unfortunately we have no
determinations of humus or of its nitrogen-content. Leather attributes
the black color of the regur to some mineral substance rather than
to humus; but his arguments are not quite convincing, so long as the
Grandeau test has not been made. In view of the low rainfall and the
closeness of the texture of regur, it is probable that little if any
nitrates are currently washed out of the black cotton lands.

[160] See Voelcker, Report on the Improvement of Indian Agriculture,
1892, p. 46, par. 60.

The regur soil-sheet seems to be underlaid over the greater part of its
area by a basaltic eruptive sheet (not by metamorphic rocks, as stated
by Leather), and it is not easy to conceive how such a soil stratum can
have been formed from such rocks as a sedentary formation. Elsewhere
such soils are usually rather light and porous, as is the case in the
Hawaiian and Samoan islands; and very high in iron-content. The regur
has the character of an alluvial backwater or lake deposit; but how
such a formation can have occurred on the Deccan plateau, is a question
not easily answered.

_Red Soils of the Madras Region._—Interspersed with and to seaward
of the regur lands there are in the Madras presidency considerable
bodies of “red” lands, which appear to be sedentary soils formed from
underlying dark-colored, mostly eruptive rocks. Some of these are very
rich in lime and potash, others very poor, and it seems impossible
to classify them under any definite category either from the chemical
or physical point of view, except as to their red tint. Even this
tint, however, is not always found associated with exceptionally high
contents of iron oxid, but due rather to its fine diffusion in the
soil mass. As compared with the regur, with which the “red ” areas are
interspersed, these soils contain, on the average, less lime, potash
and ferric oxid; and phosphoric acid is uniformly low. The alluvial
(brown and black) soils from the same region, exemplified in the table,
are doubtless derived partly from the regur, and their color and
composition varies accordingly.

“_Laterite Soils._”—These are defined by Wohltmann (Tropische
Agricultur, 1892) as being “the characteristic sedentary soils
(Verwitterungsböden) of the tropics, formed under the influence of
heavy precipitation, high temperatures and drought.” This definition
does not indicate their derivation from any particular rock, such as
laterite is supposed to be; but its definition puzzles even geologists,
and so, as Leather observes, the definition of laterite soils will
naturally puzzle agricultural chemists. Accordingly it is difficult to
deduce from the analyses given any definite common characters. Leather
describes those analyzed by him as red or reddish, sandy and gravelly,
the gravel or cobbles often incrusted with a dark-smooth crust of
limonite, which to the uninitiated looks as though the rock itself had
been fused and vitrified. The samples from Lohardaga and Singhbhum show
the effects of these limonite crusts upon the composition of the soils,
which resembles that of the Hawaiian soils mentioned above; but in the
latter the iron oxid is wholly pulverulent. But it is probable that, as
in the case of the latter, the high content of phosphoric acid shown in
the statement (.64 for the Lohardaga soil) is tightly locked up in the
insoluble form of ferric phosphate. Wohltmann’s definition of laterite
soils seems best represented by the “terra roxa” of Brazil, which as he
states has .02 to.08% of potash, .02 to.10% of lime, and .045 to .10%
of phosphoric acid. Humus and nitrogen are very deficient in all these
soils.

While most prominent in the coast region of Bengal, they also occur
not only near Madras (Saidapet) but also in the belt of high rainfall
on the Malabar (western) coast of the Indian peninsula.

The productiveness of the laterite soils seems throughout to be only
moderate, yet much higher than would be expected of soils of similar
composition in the temperate zones, where the rate of soil-formation is
so much slower than in the tropics.

From the analyses of “coffee soils” from Yarcand in the Sheveroy hills,
north of Madras, we learn that coffee does well with a fairly liberal
supply of lime (.30 to.44%) and phosphoric acid, but is satisfied with
a much smaller amount of potash than is found in the tea soils of Assam.

A farther systematic investigation of the soils of India, with
simultaneous accurate observations on their depth, subsoil, geological
derivation, topographical location and relations to rainfall, could
not fail to yield very important practical results. The examination of
samples collected and sent in by persons unfamiliar with the proper
mode of taking soil specimens, and the information which should
accompany them, always involves a great deal of uncertainty and waste
of labor, and indefiniteness of results.


INFLUENCE OF ARIDITY UPON CIVILIZATION.

In connection with the facts given and discussed above, as to the
relative productive capacity of lands of the humid and arid regions,
it becomes of interest to consider what influence, if any, these
differences may have had in determining the choice of the majority of
the ancient civilizations in favor of countries where nature imposes
upon the husbandman, who supplies the prime necessaries of life, the
onerous condition of artificial irrigation.[161]

[161] Verhandlungen der Deutschen Physiologischen Gesellschaft in
Berlin, December, 1892; North American Review, September, 1902.

_Preference of Ancient Civilizations for Arid Countries._—A brief
review suffices to establish the fact of such choice. Aside from Egypt,
the permanent fertility of which is ascribed to the inundations of the
Nile, we find to westward the oases of the Libyan and Sahara deserts,
the high fertility of which has become proverbial and has caused them
to be cultivated from ancient times to the present. Similarly, on both
sides of the Mediterranean Sea, we find that, instead of the humid
forest country, it was in the arid but irrigable coast countries, such
as the vegas of Valencia, Alicante, Granada, Malaga, and the even
more arid domain of which Carthage was the metropolis; and farther
east, in the Graeco-Syrian archipelago and the adjacent coasts, that
noted centers of civilization were developed and maintained. Thence
the arid belt requiring irrigation extends from Egypt and Arabia to
Palestine, Syria, Assyria, Mesopotamia and Persia, and across the Indus
through the anciently recognized regions of Indian civilization—Sindh,
the Panjab, Rajputana and the Northwestern provinces—to the Ganges,
embracing such well-known centers as Lahore, Delhi, Meerut, Agra,
etc., inhabited by much more hardy and progressive races than the
humid and highly productive tropical portions of the Indian peninsula.
Throughout the extensive and important portion of northern India,
irrigation is necessary to maintain regular production; and in default
of it, periodic famines ravage the country. Thousands of years ago,
millions upon millions of treasure were expended there upon irrigation
works, as has again been done in modern times; yet in the rainy,
forested districts we still find large areas practically tenanted by
wild beasts. In Asia Minor, as well as in Central Asia, the remains of
ancient cities once surrounded by richly productive irrigated fields,
are found where at present only the herds of nomads pasture. The
Khanates of southern Turkestan with their historic cities, illustrate
the same obstinate bias in favor of arid climates. Similarly, in the
New World, it was not in the moist and exuberantly fertile forest lands
of the Orinoco and Amazon, but on the arid western slopes of the Andes,
that the civilization of the Incas was developed. In Mexico, also, it
was the high central, arid plateau, not the bountifully productive
_tierra caliente_, over which the Aztecs chose to establish the main
centers of their empire. Even to northward, the inhabitants of the
high, dry plains of Arizona and New Mexico were, as their descendants
of the Pueblos are to-day, superior in social development to their
forest-dwelling neighbors of the Algonquin race. From time immemorial
they have practiced irrigation in connection with cultivation,
maintaining a comparatively dense population on very limited areas.

It might be thought that the desire to avoid the labor of clearing the
forest ground was the motive which guided the choice of the ancient
nations toward the cheerless-looking, treeless regions.

But if we consider the cost and labor of establishing and maintaining
irrigation ditches, it certainly seems that a stronger motive, based on
the intrinsic nature of the case, must have influenced their selection.
Neither can we with any degree of plausibility ascribe the preference
for the arid open country to the fear of enemies lurking in the forest,
since war was in early times practically the normal condition of
mankind, and was waged with little hesitation wherever booty was in
sight. It has also been asked how the ancients could have known of the
high productive capacity of arid lands; but no one who has ever seen
the springing-up of luxuriant vegetation after the periodic overflows
of the arid-region streams, or the same surrounding the springs in the
deserts, would ask that question.

_Irrigation necessitates Co-operation._—Irrigation enterprises
can be accomplished in a very limited degree only by individuals
or even families. Its permanently successful execution requires
the co-operation of at least several social groups, ultimately of
communities and states, if it is not to give rise to acrimonious
contentions or actual warfare; witness the “shotgun policy” resorted to
in the arid West in times not very remote. Irrigation, in other words,
compels co-operative social organization quite different from and far
in advance of that necessary in humid countries. And such organization
is manifestly conducive to the preservation and development of the arts
of peace, which means civilization. The most ancient systematic code of
laws known to us is that of Hammurabi, the king of arid Assyria.

_The high and permanent productiveness of arid soils induces permanence
of civil organization._—In humid countries, as is well known,
cultivation can only in exceptional cases be continued profitably
for many years without fertilization. But fertilization requires a
somewhat protracted development of agriculture to be rationally and
successfully applied in the humid regions, and the Germanic tribes,
like the North-American Indians, seem to have shifted their culture
grounds frequently in their migrations. No such need was felt by the
inhabitants of the arid regions for centuries, for the native fertility
of their soils, coupled with the fertilizing effects of irrigation
water bringing plant-food from afar, relieved them of the need of
continuous fertilization; while in the humid regions, the fertility
of the land is currently carried into the sea by the drainage waters,
through the streams and rivers, causing a chronic depletion which has
to be made up for by artificial and costly means. What with the greater
intrinsic fertility and the great depth of soil available for plant
growth, much smaller units of land will suffice for the maintenance of
a family in arid countries; a fact which is even now being illustrated
in the irrigated region of the United States, where ten acres of
irrigated land instead of 40 or 160, as in the East, form the unit.

The arid regions were, therefore, specially conducive to the
establishment of the highly complex polities and high culture, of
which the vestiges are now being unearthed in what we are in the habit
of calling “deserts;” the very sands of which usually need only the
life-giving effects of water to transform them into fruitful fields
and gardens. It is also quite natural that the wealthy and prosperous
communities so formed should in the course of time have excited the
cupidity of the “barbarous” forest-inhabiting races, and as history
records, have been over and again overwhelmed by them—a similar fate
often afterwards overtaking the conquerors in their turn, after the
Capuan ease of their existence had weakened their warlike prowess. At
the present time, the arid regions of the old world are still largely
suffering from having been overrun by the nomadic Turanians, whose
original habitat—Mongolia and Turkestan—while also arid, does not
permit of the ready realization of the advantages above outlined, on
account of the rigorous climate brought about by altitude. Mahometanism
first expelled, and has since repelled, occidental civilization from
the arid regions of the Old World, remaining to-day as an obstacle
to its progress. The peaceful aggression of railroads and telegraphs
now seems likely to gradually overcome this repulsion; and when
Constantinople and Bagdad shall be linked together by the steel bands,
the desert will lose its terrors, and Mesopotamia and Babylonia will
again become garden lands, as of old, under the abundant waters of the
Euphrates and Tigris. Until the water-supplies of the arid countries
shall have been more definitely gauged, it is impossible to foretell
to what extent food-production may be increased by their cultivation
under irrigation, after the relief from political misrule shall have
rendered such undertakings safe. But it can even now be foreseen that
with improved modern methods of cultivation, the productive area of
the world can be vastly increased by the utilization of the countries
where, as the Turcomans say, “the salt is the life of the land.”




CHAPTER XXII.

ALKALI SOILS.


_Alkali Lands and Seashore Lands._—Alkali lands proper, as already
stated, are wholly distinct in their nature and origin from the salty
lands of sea-coast marshes, past or present. The latter derive their
salts from sea-water that occasionally overflows them, or from that
which has evaporated in segregated basins or estuaries; and the salts
impregnating them are essentially “sea salts,” that is, common salt,
together with bittern (magnesium chlorid), Epsom salt (magnesium
sulfate) gypsum, etc. (see chapter 2, p. 26). Very little of what would
be useful to vegetation or desirable as a fertilizer is contained in
the salts impregnating such soils; and they are by no means always
intrinsically rich in plant-food, but vary greatly in this respect.

While seashore lands are by no means always of high fertility even when
freed from their salts, especially when very sandy, it is otherwise
when they occur near the mouths of streams or rivers, whose finest
sediments they then receive. From such lands are formed the profusely
productive Polders of Holland and northern Germany, and the equally
noted “colmates” of France and Italy. These, so soon as freed from
salt, may be considered as possessing the same advantages as “delta”
alluvial lands, and from the same causes; notably the accumulation of
the finest sediments derived from the rivers’ drainage basins.

_Origin._—Alkali lands proper bear no definite relation to the present
sea; they are mostly remote from it or from any other sea bed, so
that they have sometimes been designated as “terrestrial salt lands.”
Their existence is in the majority of cases definitely traceable to
climatic conditions alone. They are the natural result of a light
rainfall, insufficient to leach out of the land the salts that always
form in it by progressive weathering of the rock powder of which all
soils largely consist. Where the rainfall is abundant, that portion
of the salts corresponding to “sea salts” is leached out into the
bottom water, and with this passes through springs and rivulets into
the country drainage, to be finally carried to the ocean.[162] Another
portion of the salts formed by weathering, however, is partially or
wholly retained by the soil; it is that portion chiefly useful as plant
food.

[162] See Chapter 2, p. 26.

It follows that when, in consequence of insufficient rainfall, all or
most of the salts are retained in the soil, they will contain not only
the ingredients of sea-water, but also those useful to plants. In rainy
climates a large portion even of the latter is leached out and carried
away. In extremely arid climates, on the contrary, the entire mass of
the salts remains in the soils; and, being largely soluble in water,
evaporation during the dry season brings them to the surface, where
they may accumulate to such an extent as to render ordinary useful
vegetation impossible; as is seen in “alkali spots,” and sometimes in
extensive tracts of “alkali desert.” Three compounds, viz. the sulfate,
chlorid and carbonate of sodium, usually form the main mass of these
saline efflorescences. Magnesium sulfate (Epsom salt) is in many cases
a very abundant ingredient; some calcium sulfate is nearly always
present, and calcium chlorid is not infrequently found.

    In some cases the above salts are in part at least derived
    from the leaching of adjacent or subjacent geological
    deposits impregnated with them at the time of their
    formation. Such is the case in portions of Wyoming, Colorado
    and New Mexico, in the Colorado river delta, and in the
    Hungarian Plain; and it is in these cases especially that
    the chlorids of calcium and magnesium also form part of the
    saline mixture.

_Geographical Distribution of Alkali Lands._—In looking over a rainfall
map of the globe[163] we see that a very considerable portion of the
earth’s surface, forming two belts to poleward of the two tropics, has
deficient rainfall; the latter term being commonly meant to imply any
annual average less than 20 inches (500 millimeters). The arid region
thus defined includes, in North America, most of the country lying
west of the one hundredth meridian up to the Cascade Mountains, and
northward beyond the line of the United States; southward, it reaches
far into Mexico, including especially the Mexican plateau. In South
America it includes most of the Pacific Slope (Peru and Chile) south to
Araucania; and eastward of the Andes, the greater portion of the plains
of western Brazil and Argentina. In Europe only a small portion of the
Mediterranean border is included; but the entire African coast-belt
opposite, with the Saharan and Libyan deserts, Egypt and Arabia, are
included therein, as well as, south of the Equator, a considerable
portion of South Africa (Kalahari desert). In Asia, Asia Minor, Syria
(with Palestine), Mesopotamia, Persia, and northwestern India up to
the Ganges, and northward, the great plains or steppes of central Asia
eastward to Mongolia and western China, fall into the same category; as
does also a large portion of the Australian continent.

[163] See above, chapter 16, p. 294.

_Utilization of World-wide Importance._—Over these vast areas alkali
lands occur to a greater or less extent, the exceptions being the
mountain regions and adjacent lands on the side exposed to the
prevailing winds. It will therefore be seen that the problem of the
utilization of alkali lands for agriculture is not of local interest
only, but is of world-wide importance. It will also be noted that
many of the countries referred to are those in which the most ancient
civilizations have existed in the past, but which at present, with few
exceptions, are occupied by semi-civilized people only. It is doubtless
from this cause that the nature of alkali lands has until lately been
so little understood, that even their essential distinctness from the
sea-border lands has been but recently recognized in full. Moreover,
the great intrinsic fertility of these lands when freed from the
noxious salts, has been very little appreciated; their repellent aspect
causing them to be generally considered as permanently waste lands.

    _Repellent aspect._—This aspect is essentially due
    to their natural vegetation being in most cases confined
    to plants useless to man, commonly designated as “saline
    vegetation,”[164] of which but little is usually relished by
    cattle. Notable exceptions to this rule occur in North and
    South America, Australia, and Africa, where the “saltbushes”
    of the former and the “karroo” vegetation of the latter form
    valuable pasture and browsing grounds. Apart from these,
    however, all efforts to find culture plants for these lands
    generally acceptable, or at least profitable, in their
    natural condition, have not been very successful.

[164] See Chapter 23.

[Illustration: FIG. 60.—Alkali Lands in San Joaquin Valley,
California.]

    Figure 60 illustrates the usual aspect of alkali lands in
    the San Joaquin valley of California. It will be noted that
    the alkali-covered surface is only in spots, with clumps of
    vegetation between, so that cattle can find both pasture and
    browsing on a portion of such lands, even though the plants
    so growing are not usually of the most desirable kind. We
    find in all arid regions, however, considerable areas either
    wholly destitute of vegetation, or bearing only such saline
    growth as is rejected by all kinds of domestic animals.

_Effects of Alkali upon culture plants._—In land very strongly
impregnated with alkali salts, most culture plants, if their seed
germinates at all, will show a sickly growth for a short time, “spindle
up” and then die without fruiting. In soils less heavily charged the
plants may simply become dwarfed, and fruit scantily. The effect on
grown trees around which alkali has come up, is first, scanty leafage
and short growth of shoots, themselves but sparsely clothed with
leaves. This state of things is well shown in figures 61 and 62, which
represent apricot trees growing but a short distance apart, but one
coming within range of an expanding alkali spot. The characteristic
sparseness of the foliage of the “alkalied” tree as compared with the
adjacent one is well shown.

_Nature of the injury to plants from Alkali._—When we examine plants
that have been injured by alkali, we will mostly find that the visible
damage has been done near the base of the trunk, or _root crown_;
rarely at any considerable depth in the soil itself. In the case of
green herbaceous stems, the bark is found to have been turned to a
brownish tinge for half an inch or more, so as to be soft and easily
peeled off. In the case of trees, the rough bark is found to be of a
dark, almost black, tint, and the green layer underneath has, as in
the case of herbaceous stems, been turned brown to a greater or less
extent. In either case the plant has been practically “girdled,” the
effect being aggravated by the diseased sap poisoning more or less
the whole stem and roots. The plant may not die, but it will be quite
certain to become unprofitable to the grower.

[Illustration: FIG. 61.—Unaffected.]

[Illustration: FIG. 62.—Yielding to Alkali.

APRICOT TREES ON ALKALI GROUND.]

It is mainly in the case of land very heavily charged with common salt,
as in the marshes bordering the sea, or salt lakes, that injury arises
from the direct effects of the salty soil-water upon the feeding roots
themselves. In a few cases the gradual rise of salt water from below
in consequence of defective drainage, has seriously injured, and even
destroyed, old orange orchards. The natural occupancy of the ground
by certain native plants may be held to indicate that the soil is too
heavily charged with saline ingredients to permit healthy root growth
or nutrition until the excess of salts is removed. (See below, chapters
23 and 26).

The fact that in cultivated land the injury is usually found to
occur near the surface of the soil, concurrently with the well-known
fact that the maximum accumulation of salts at the surface is always
found near the end of the dry season, indicates clearly that this
accumulation is due to evaporation at the surface. The latter is
often found covered with a crust consisting of earth cemented by the
crystallized salts, and later in the season with a layer of whitish
dust resulting from the drying-out of the crust first formed. It is
this dust which becomes so annoying to the inhabitants and travelers
in alkali regions, when high winds prevail, irritating the eyes and
nostrils and parching the lips.

_Effects of Irrigation._—One of the most annoying and discouraging
features of the cultivation of lands in alkali regions is that,
although in their natural condition they may show but little alkali
on their surface, and that mostly in limited spots, these spots are
found to enlarge rapidly as irrigation is practiced. Yet since alkali
salts are the symptoms and result of insufficient rainfall, irrigation
is a necessary condition of agriculture wherever they prevail. Under
irrigation, neighboring spots will oftentimes merge together into one
large one, and at times the entire area, once highly productive and
perhaps covered with valuable plantations of trees or vines, will
become incapable of supporting useful growth. This annoying phenomenon
is popularly known as “the rise of the alkali” in the western United
States, but is equally well known in India and other irrigation regions.

The soil being impregnated with a solution of the alkali salts, and
acting like a wick, the salts naturally remain behind on the surface
as the water evaporates, the process only stopping when the moisture
in the soil is exhausted. We thus not infrequently find that after an
unusually heavy rainfall there follows a heavier accumulation of alkali
salts at the surface, while a light shower produces no perceptible
permanent effect. We are thus taught that, within certain limits, the
more water evaporates during the season the heavier will be the rise of
the alkali; provided that the water is not so abundant as to leach the
salts through the soil and subsoil into the subdrainage.

_Leaky Irrigation ditches._—Worst of all, however, is the effect
of irrigation ditches laid in sandy lands (such as are naturally
predominant in arid regions), without proper provision against seepage.
The ditch water then gradually fills up the entire substrata so far
as they are permeable, and the water-table rises from below until it
reaches nearly to the ditch level; shallowing the subsoil, drowning out
the deep roots of all vegetation, and bringing close to the surface the
entire mass of alkali salts previously diffused through many feet of
substrata.

_Surface and Substrata of Alkali Lands._—Aside from the desert proper,
in the greater portion of the alkali country “alkali spots.” _i. e._
ground covered with saline efflorescences and showing little or no
vegetation, are interspersed with larger areas apparently free from
salts and covered with the ordinary vegetation of the region. A view of
such country is given in a plate on a previous page. The alkali spots
are usually somewhat depressed below the surrounding lands, and after
rains remain covered with water for some time; the water frequently
assuming a brown or blackish tint after standing.

When a pointed steel probe is pushed down within such an alkali spot,
it will usually be found that, although the soil may appear quite
sandy, it is penetrated with some difficulty; while outside of the
spots, the probe does not encounter serious resistance until it reaches
the depth of two or three feet, when it frequently becomes impossible
to penetrate farther without the aid of a hammer. On the margin of the
spots, the transition from utter barrenness to a luxuriant vegetation
of native weeds is mostly quite sudden; as is shown in the figure, p.
425.

_Vertical Distribution of the Salts in Alkali Land._—The results of a
comparative examination of such land before and after irrigation,[165]
are shown in the annexed diagrams; in which the kind and amount of
salts is shown for every three inches of vertical depth, down to four
feet, by curves whose extension from left to right indicate the several
percentages, while the outer curved line gives the total of salts for
each of the several depths.

    Fig. 63 represents the condition of the salts in an “alkali
    spot” as found at the end of the dry season at the Tulare
    substation, California. The soil was sampled to the depth of
    two feet at intervals of three inches each. It is easy to
    see that at this time the bulk of the salts was accumulated
    within the first six inches from the surface, while lower
    down the soil contained so little that few culture plants
    would be hurt by them.

    _How Native Plants Live._—Fig. 64 represents
    similarly the state of things in a natural soil alongside
    of the alkali spot, but in which the native vegetation of
    brilliant flowers develops annually without any hindrance
    from alkali. Samples were taken from this spot in March,
    near the end of the wet, and in September, near the end of
    the dry season, and each series fully analyzed. There was
    scarcely a noticeable difference in the results obtained. It
    is seen in the figure that down to the depth of 15 inches
    there was practically no alkali found (0.035%), and it was
    within these 15 inches of soil that the native plants mostly
    had their roots and developed their annual growth. But
    from that level downward the alkali rapidly increased, and
    reached a maximum (0.529%), at about 33 inches; decreasing
    rapidly thence until, at the end of the fourth foot in
    depth, there was not more alkali than within the first foot
    from the surface. In other words, the bulk of the salts
    had accumulated at the greatest depth to which the annual
    rainfall (7 inches) ever reaches, forming there a sheet of
    tough, intractable clay-hardpan. The shallow-rooted native
    plants germinated their seeds freely on the alkali-free
    surface; their roots kept above the strongly-charged
    subsoil, and through them and the stems and foliage all the
    soil moisture was evaporated by the time the plants died.
    Thus no alkali was brought up from below by evaporation. The
    seeds shed would remain uninjured, and would again germinate
    the coming season.

[165] Hilgard and Loughridge, Bulletin No. 128, California Experiment
Station; Report California Experiment Station, 1894-95, p. 37; Bulletin
No. 30, Office of Experiment Stations; Wollny’s Forsch. Geb. Agr.
Phys., 1896.

[Illustration: FIG 63.—Diagram showing amounts and composition of
alkali salts at various depths in alkali soil, on which barley would
not grow. Taken September, 1894.

Tulare Experiment Substation, California.]

[Illustration: FIG. 64.—Diagram showing amounts and composition of
alkali salts at various depths in black alkali land, covered with
native vegetation. Taken March, 1895.

Tulare Experiment Substation, California.]

It is thus that the luxuriant vegetation of the San Joaquin plains,
dotted with occasional alkali spots, is maintained; the spots
themselves being almost always depressions in which the rain water may
gather, and where, in consequence of the increased evaporation, the
noxious salts have risen to the surface and render impossible all but
the most resistant saline growth; particularly when, in consequence of
maceration and fermentation in the soil, the formation of carbonate of
soda has caused the surface to sink and become almost water-tight.

_Upward Translocation from Irrigation._—Fig. 65 shows the corresponding
profile of the same soil after several years’ irrigation. The upward
movement of the salts is clearly seen by comparison with the previous
figure; and the surface soil has become so charged with salts that the
seeds of culture plants refuse to germinate.

Ten feet from this bare alkali ground, on which barley had refused to
grow, a crop of barley four feet high was harvested the same year,
without irrigation. Investigation proved that here the condition of
the soil was intermediate between the two preceding diagrams. The
irrigation water had dissolved the alkali of the subsoil, and the more
abundant evaporation had brought it nearer the surface; but the shading
by the barley crop and the evaporation of the moisture through its
roots and leaves had prevented the salts from reaching the surface in
such amounts as to injure the crop, although the tendency to rise was
clearly shown. By the use of gypsum, moreover, the injuriousness of the
alkali had been somewhat diminished.

The same season, grain crops were almost a failure on alkali-free
land in the same region; and in connection with this result it should
be noted as a general fact that alkali lands always retain a certain
amount of moisture perceptible to the hand during the dry season, and
that _this moisture can be utilized by crops_; so that at times when
crops fail on non-alkaline land, good ones are obtained where a slight
taint of alkali exists in the soil. Actual determinations showed that
while a sample of alkali soil containing .54% of salts absorbed 12.3%
of moisture from moist air, the same soil when leached absorbed only
2.5%—a figure corresponding to that of sandy upland loams.

[Illustration: FIG. 65.—Diagram showing amounts and composition of
alkali salts at various depths in bare alkali land, where barley would
not grow; irrigated. Taken September, 1894.

Tulare Experiment Substation, California.]

[Illustration: FIG. 66.—Distribution of Alkali Salts in Sandy Lands.]

_Alkali in Sandy Lands._—In _very_ sandy lands, and particularly when
the alkali is “white” only, the tendency to accumulation near the
surface is much less, even under irrigation. In the natural condition
the salts are in such cases often found quite evenly distributed
through soil columns of four feet, and even more. This is an additional
cause of the lesser injuriousness of “white alkali.” An illustration
of the distribution of the salts in very sandy lands, from the Tulare
substation, is given in Fig. 66. Here we see that the maximum is not
_at_, but some distance below the surface, the entire saline mass is
lower down than in the more clayey loam of the same locality, and is
more widely distributed in depth.

_Distribution of Alkali Salts in Heavy Lands._—The mode of distribution
of alkali salts in the heavier, close-grained soil of the Chino
experimental tract in southern California, is illustrated in Fig. 67.
This land is permanently moist, from a water-table ranging from five
to seven feet below the surface in ordinary years. There is therefore
no opportunity for the formation of “alkali hardpan” as in the case
of the Tulare soil; the salts always remain rather near the surface,
viz. within twelve to fifteen inches. But being in much smaller average
amounts than at Tulare (an average of about 5300 lbs. per acre), quite
a copious natural vegetation of grasses, sunflowers, and “yerba mansa”
covered the whole surface, save in a few low spots.

A similar mode of distribution of the salts is found in the still more
clayey “black adobe” lands of the Great Valley of California. The
scanty rains cannot penetrate these soils to any great depth, so that
evaporation will soon bring the salts carried by them back to within a
short distance of the surface. Their accumulation there is frequently
indicated by the entire absence of any but the most resistant alkali
weeds, even though the total of salts in the land may not be very great.

_Salton Basin._—A peculiar state of things is illustrated in the Salton
Basin, which represents what was at one time the head of the Gulf of
California, and at the lowest point of which, 268 feet below sea level,
there now lies a large deposit of rock salt. It has been cut off from
the present Gulf by the delta deposits of the Colorado river, which
now, however, overflows into the Basin at times of extreme high water.
Although appearing level to the eye, the general slope of the country
is to the lowest point of the former sea-bottom.

    The region, now in progress of settlement by means of
    irrigation water brought from the river near Yuma, was
    investigated with respect to its alkali conditions in 1900
    (Bulletin No. 140, Calif. Agric. Expt. Sta.). The annexed
    diagram 68 shows the distribution of the salts to a depth of
    21 feet. It will be noted that here also the alkali content
    becomes insignificant at 4 feet depth, but increases again
    to a second maximum at about 15 feet, below which there is a
    second decrease; below this, at 20 feet, there is a final
    very heavy increase, not only of the total salts but
    especially of common salt, which evidently represents the
    drainage toward the salt deposit. Above this level there is
    a very remarkable predominance of Glauber’s salt (sodium
    sulfate), also observable elsewhere, _e. g._ near White
    Plains, Nev., whose name is derived from the copious surface
    accumulation of the sulfate. It seems as though this must
    have been formed in some way from the common salt.

[Illustration: FIG. 67.—Amounts and composition of alkali salts at
various depths and points in the ten-acre tract at the southern
California Experiment Station; taken last week in April, 1895.]

[Illustration: FIG. 68.—Graphic illustration of distribution of salts
in Salton River section, California.]

_Horizontal Distribution of Alkali Salts in Arid Lands._—The constant
occurrence of “alkali spots” in arid lands shows at once the great
inequality of horizontal distribution of alkali impregnation. This
is as prominent in level lands as on slopes, and in extremely arid
regions it is mostly not possible to recognize even very considerable
differences without close examination. Thus in lands appearing exactly
alike on the surface, on the edge of the Salton basin in California,
on the same forty acre 1.4% (56,000 pounds per acre) was found in
the surface four feet at one point, and a hundred yards away, 12.5%
(500,000 pounds). The mapping of alkali lands is therefore somewhat
precarious unless carried into great detail. Moreover, it has been
found that the location of the salts changes from year to year,
especially in irrigated land, as might be expected. Those cultivating
alkali lands have therefore to exercise constant watchfulness, unless
the salts have been definitively eliminated by underdrainage over
a considerable area; as merely local operations may be rendered
ineffectual by the migration of the salts from neighboring tracts not
reclaimed.

_Alkali in Hill Lands._—As a rule, hill lands themselves are remarkably
free from alkali, even in the arid regions; except when water is
gathered in depressions, where strongly saline waters may be found in
Washington, Montana and elsewhere. But on level plateau lands, where
drainage is slow or imperfect, alkali appears as freely as it does in
the same regions in the stream bottoms. In the latter the leachings and
seepage of the uplands naturally causes a concentration of the salts,
and thus we find alkali salts incrusting the surface in the valleys of
the streams, as _e. g._, that of the Yellowstone, Musselshell, Judith,
Yakima and others in the north, and of Green river, Platte, Pecos, and
Rio Grande farther south; as well as in numerous valleys of central and
southern California.

_Usar Lands of India._—These lands have been investigated first by
the “Reh Commission” appointed to investigate the causes of the
deterioration of lands in the Aligarh district (south of Delhi,
between the Ganges and Jumna rivers), in 1876. The occasion of this
appointment was the appearance of “reh” (alkali salts) in a region
which had previously been free from them.[166] Subsequently, a more
elaborate investigation of the subject was made by Dr. J. W. Leather,
Agricultural Chemist to the Government of India.[167] From these
documents it appears that “usar lands” exist largely not only in the
Northwestern Provinces and Oudh, but also in the Panjab, especially
on the lands bordering the Chenab river; likewise to a slight extent
in the Bombay presidency. Leather’s investigation shows that not all
the lands designated by the natives as _usar_ contain soluble salts in
injurious amounts, some being simply lands having very hard, clayey
soils difficult to till with the imperfect methods employed. Yet the
general phenomena of the true “reh” lands are practically identical
with those of the American alkali lands, including also the calcareous
hardpan, there called _kankar_. Owing probably to the long cultivation
of the Indian lands (mostly under irrigation), the salts are there at
their maximum in the first foot, decreasing as depth increases. It is
noteworthy also that in the majority of cases the predominant salt is
carbonate of soda or black alkali, which there as in California renders
the lands impervious to water until treated with gypsum. This fact
accounts for the popular use of the same name for non-saline impervious
clay soils, and the alkali or reh lands proper.

We have an entirely analogous case in the “Szek” lands of the Hungarian
plain, some of which are simply poor refractory soils containing a
trace of soluble salts; while lower down in the valley of the Theiss
we find genuine alkali lands, both black and white, which have long
furnished carbonate of soda for local use and commerce. In this case,
however, the alkali salts seen to come largely, in some cases wholly,
from underlying saline clays whose salts in coming to the surface
suffer precisely the same transformations experienced in California and
India, in presence of calcic carbonate (see below, p. 450 ff).

[166] An abstract of the report of this commission is given in the
Report of the California Experiment Station for 1890.

[167] See Agricultural Ledger, 1897, No. 13; ibid. 1901, No. 13.

The accounts given by v. Middendorff of the nature and occurrence of
alkali lands in Turkestan (Ferghana) agree entirely with those given
above for California and India; as do also the investigations made by
other Russian observers on the saline lands of the steppes of European
Russia.


COMPOSITION AND QUANTITY OF ALKALI SALTS.

_Black and White Alkali._—Broadly speaking, the world over alkali
salts consist mainly of three chief ingredients, already mentioned,
namely, common salt, Glauber’s salt (sulfate of soda), and salsoda or
carbonate[168] of soda. The latter causes what is popularly known as
“black alkali,” from the black spots of puddles seen on the surface of
lands tainted with it, owing to the dissolution of the soil humus;[169]
while the other salts, often together with Epsom salt and bittern
(Magnesium chlorid), constitute “white alkali,” which is known to
be very much milder in its effect on plants than the black. In most
cases all three are present, and all may be considered as practically
valueless, or noxious, to plant growth.

[168] In this designation are included, in this volume, both the
normal (mono-) carbonate and the two other compounds, the bi- or
hydrocarbonate and the intermediate (so-called sesqui-) compound or
trona; all of which are commonly present simultaneously, but in utterly
indefinite relative proportions, varying from day to day and from
inch to inch of depth, inasmuch as their continued existence depends
upon the greater or less formation of carbonic acid in the soil, and
the access of air. Hence their separate quantitative determination
at any one time is of little practical interest. All naturally
occurring carbonate of soda contains, and sometimes consists of, these
“super-carbonates,” according to the greater or less exposure to air
and solar heat. They are much milder in their action on plants than the
monocarbonate, which unfortunately, in the nature of the case, always
predominates near the surface, and thus injures the root-crown.

[169] A wholly different kind of “black alkali” exists in some regions,
especially in the delta lands of the Colorado of the West and in the
Pecos and Rio Grande country in New Mexico. In these cases the dark
tint is due, not to a humic solution, but simply to moisture, which
is tenaciously retained by the chlorids of calcium and magnesium
impregnating the land, thus contrasting strongly with the gray tint of
the general dry soil.

    =========================+=================+=========================
                             |       EUROPE.   |               ASIA.
                             |-----------------+-----------------+-------
                             |      HUNGARIAN  |   ARALO-CASPIAN | ADEN.
                             |       PLAIN.    |      PLAIN.     |“HURKA”
                             |      “SZEKSO.”  |   SALINE CRUSTS.|“KARA.”
                             +---------+-------+                 |
                             |Debreczin|Kalocsa|                 |
                             |         |       |                 |
                             |         |       |                 |
    -------------------------+---------+-------+-----+-----+-----+-------
                             |         |       |     |     |     |
    Potassium Sulfate (K₂SO₄)|         |       |     |     |     |
    Sodium Sulfate (Na₂SO₄)  |     .2  |    1.6| 10.4| 18.2| 15.5|
    Sodium Carbonate (Na₂CO₃)|   48.1  |   92.5| 14.7| 12.1| 69.0|  67.2
    Sodium Chlorid  (NaCl)   |   51.7  |    4.4| 74.6| 69.7| 15.5|  32.8
    Sodium Phosphate (Na₃PO₄)|         |    1.5|     |     |     |
    Calcium Chlorid          |         |       |     |     |     |
    Magnesium Chlorid (MgCl₂)|         |       |     |     |     |
                             +---------+-------+-----+-----+-----+-------
                             |  100.0  |  100.0|100.0|100.0|100.0| 100.0
    -------------------------+---------+-------+-----+-----+-----+-------

     =========================+======================================
                              |               ASIA.
                              |--------------------------------------
                              |               INDIA
                              |               “REH.”
                              +--------------------------------------
                              |       |Gursikar,|Jellalabad,|Bayamati
                              |       |Aligarh, |Panjab,    |(Regur),
                              |       |6 feet.  |1.5 feet.  |Deccan,
                              |       |         |           |2 feet.
     -------------------------+-------+---------+-----------+--------
                              |       |         |           |
     Potassium Sulfate (K₂SO₄)|  11.1 |         |           |
     Sodium Sulfate (Na₂SO₄)  |   7.0 |    15.5 |      58.5 |   2.3
     Sodium Carbonate (Na₂CO₃)|  79.0 |    56.9 |      22.9 |
     Sodium Chlorid  (NaCl)   |   2.9 |    27.6 |      18.6 |  97.7
     Sodium Phosphate (Na₃PO₄)|       |         |           |
     Calcium Chlorid          |       |         |           |
     Magnesium Chlorid (MgCl₂)|       |         |           |
                              +-------+---------+-----------+--------
                              | 100.0 |   100.0 |     100.0 | 100.0
     -------------------------+-------+---------+-----------+--------

     =========================+======================================
                              |                  AFRICA.
                              +--------------------------+-------------
                              |             EGYPT.       |
                              |                          |
                              +---------------+----------+
                              |     Trona,    | Alkali   |   Fezzan
                              | (Commercial.) |L. Abukir.|   Trona,
                              |               |          |(Commercial.)
                              |               |          |
     -------------------------+------+--------+----------+-------------
                              |      |        |          |
     Potassium Sulfate (K₂SO₄)|      |        |     6.49 |
     Sodium Sulfate (Na₂SO₄)  | 23.6 |   38.3 |      .82 |       0.6
     Sodium Carbonate (Na₂CO₃)| 28.2 |   47.7 |     1.13 |      98.7
     Sodium Chlorid  (NaCl)   | 48.2 |   14.0 |    89.74 |        .7
     Sodium Phosphate (Na₃PO₄)|      |        |          |
     Calcium Chlorid          |      |        |     1.82 |
     Magnesium Chlorid (MgCl₂)|      |        |          |
                              +------+--------+----------+-------------
                              |100.0 |  100.0 |   100.00 |     100.0
     -------------------------+------+--------+----------+-------------

    ===========================+========================================
                               |              CALIFORNIA.
                               +------+---------+----------------+------
                               |      |         | Tulare County. |
                               |Merced|Overhiser+-------+--------+ Kern
                               |Falls.|   San   |Visalia| Expt.  |Island
                               |      | Joaquin |       |Station.|
                               |      |   Co.   |       |Tulare. |
    ---------------------------+------+---------+-------+--------+------
    Potassium Chlorid, KCl     |      |         |       |        |
    Potassium Sulfate, K₂SO₄   |      |   20.23 |   3.95|   10.13|
    Potassium Carbonate. K₂CO₃ |      |         |       |        |
    Sodium Sulfate, Na₂SO₄     |  4.67|   13.00 |       |   25.28| 88.42
    Sodium Nitrate, NaNO₃      | 12.98|         |       |   19.78|
    Sodium Carbonate, Na₂CO₃   | 75.95|   52.22 |  65.72|   32.58|  0.42
    Sodium Chlorid,  (NaCl)    |  1.46|   33.00 |   3.98|   14.75|  0.51
    Sodium Phosphate, HNa₂PO₄  |  4.94|    1.78 |   8.42|    2.25|
    Magnesium Sulfate, MgSO₄   |      |         |   1.65|        |  0.52
    Calcium Chlorid            |      |         |       |        |
    Magnesium Chlorid          |      |         |       |        |
    Ammonium Carbonate, NH₄CO₃ |      |         |       |    1.41|
    ---------------------------+------+---------+-------+--------+------
                               |100.00|  100.00 | 100.00|  100.00|100.00
    ---------------------------+------+---------+-------+--------+------

    ===========================+========================================
                               |        CALIFORNIA.
                               +-------+----------+-----------+---------
                               |       |          |           |
                               | Mojave|  Hunts,  |Westminster| Imperial
                               |Plateau|   near   |    near   |
                               |       |   San    | Santa Ana.|
                               |       |Bernardino|           |
    ---------------------------+-------+----------+-----------+---------
    Potassium Chlorid, KCl     |       |          |           |   1.15
    Potassium Sulfate, K₂SO₄   |  0.92 |     5.31 |     20.62 |
    Potassium Carbonate. K₂CO₃ |       |          |      6.59 |
    Sodium Sulfate, Na₂SO₄     | 43.34 |    66.08 |           |
    Sodium Nitrate, NaNO₃      |       |          |           |   8.21
    Sodium Carbonate, Na₂CO₃   | 15.38 |    15.85 |     62.22 |    .58
    Sodium Chlorid,  (NaCl)    | 39.34 |    11.47 |     10.57 |  31.82
    Sodium Phosphate, HNa₂PO₄  |  1.02 |          |           |
    Magnesium Sulfate, MgSO₄   |       |     0.59 |           |
    Calcium Chlorid            |       |          |           |  58.42
    Magnesium Chlorid          |       |          |           |   2.81
    Ammonium Carbonate, NH₄CO₃ |       |          |           |
    ---------------------------+-------+----------+-----------+---------
                               |100.00 |   100.00 |    100.00 | 100.00
    ---------------------------+-------+----------+-----------+---------

        =======================+====================================
                               |            WASHINGTON.
                               +--------+--------+--------+----------
                               |  Yakima|        |        | Spokane
                               |    Co. |Kittitas| Whitman|    Co.,
                               |    on  |Valley. |   Co., |Cottonwood
                               | Atahnam|        |  Lake  | Springs.
                               |  Creek.|        | Creek. |
    ---------------------------+--------+--------+--------+----------
    Potassium Chlorid, KCl     |   3.90 |   0.16 |   4.53 |   6.27
    Potassium Sulfate, K₂SO₄   |  18.44 |  15.17 |  15.90 |
    Potassium Carbonate. K₂CO₃ |        |        |        |
    Sodium Sulfate, Na₂SO₄     |        |        |        |
    Sodium Nitrate, NaNO₃      |  75.61 |  80.36 |  77.10 |  87.14
    Sodium Carbonate, Na₂CO₃   |   0.52 |   1.76 |   1.34 |   4.03
    Sodium Chlorid,  (NaCl)    |   1.53 |   2.55 |   1.13 |   2.56
    Sodium Phosphate, HNa₂PO₄  |        |        |        |
    Magnesium Sulfate, MgSO₄   |        |        |        |
    Calcium Chlorid            |        |        |        |
    Magnesium Chlorid          |        |        |        |
    Ammonium Carbonate, NH₄CO₃ |        |        |        |
    ---------------------------+--------+--------+--------+----------
                               | 100.00 | 100.00 | 100.00 | 100.00
    ---------------------------+--------+--------+--------+----------

    ========================+===========================================
                            |                  MONTANA.
    ------------------------+-------------------------------------------
                            |           UPPER MISSOURI VALLEY.
                            +-----------+-------+------+-------+--------
                            |   Upper   |Prickly| Ford | Fort  |Robert’s
                            |  Missouri | Pear  |  on  |Benton.| Creek,
                            |   Valley  |Plain. | Sun  |       | Mussel-
                            |    near   |Helena.|River.|       |  shell
                            |Centerville|       |      |       | Valley.
    ------------------------+-----------+-------+------+-------+--------
                            |           |       |      |       |
    Potassium Sulfate, K₂SO₄|     2.37  |   3.07|  1.77|   8.59|   3.07
    Sodium Sulfate, Na₂SO₄  |    56.54  |  43.38| 83.35|  47.10|  76.79
    Sodium Nitrate, NaNO₃   |     9.39  |       |      |       |
    Sodium Carbonate, Na₂CO₃|           |       |      |    .71|  13.99
    Sodium Chlorid, NaCl    |    27.47  |  14.60|  0.91|    .18|   6.15
    Magnesium Sulfate, MgSO₄|     4.23  |  38.94| 13.97|  43.42|
    Potassium Chlorid, KCl  |           |       |      |       |
                            +-----------+-------+------+-------+--------
                            |   100.00  | 100.00|100.00| 100.00| 100.00
    ------------------------+-----------+-------+------+-------+--------
    ========================+==============================
                            |           NEVADA.
                            |--------------------+---------
                            |                    |
                            |     NEAR RENO.     | Churchill
                            |                    | County.
    ------------------------+----------+---------+---------
                            |          |         |
    Potassium Sulfate, K₂SO₄|          |         |
    Sodium Sulfate, Na₂SO₄  |   52.15  |   80.30 |    0.55
    Sodium Nitrate, NaNO₃   |          |         |
    Sodium Carbonate, Na₂CO₃|   45.37  |   15.24 |   96.78
    Sodium Chlorid, NaCl    |    2.48  |    4.46 |    2.67
    Magnesium Sulfate, MgSO₄|          |         |
    Potassium Chlorid, KCl  |          |         |
                            +----------+---------+---------
                            |  100.00  |  100.00 |  100.00
    ------------------------+----------+---------+--------
    ========================+=========================================
                            |                  WYOMING.
                            |----------------------+------------------
                            |      SWEET WATER     |  LARAMIE FARM.
                            |       VALLEY.        |
                            |-------------+--------+-------+----------
                            |Independence,| Saint  |Alkali.|  Waste
                            |    Rock     | Mary’s |       |Irrigation
                            |    Lake.    |Station.|       |  Water.
    ------------------------+-------------+--------+-------+----------
                            |             |        |       |
    Potassium Sulfate, K₂SO₄|             |        |       |
    Sodium Sulfate, Na₂SO₄  |     73.17   |  88.93 |  59.29|   41.19
    Sodium Nitrate, NaNO₃   |             |        |       |
    Sodium Carbonate, Na₂CO₃|     22.98   |        |       |
    Sodium Chlorid, NaCl    |      3.85   |  11.63 |  17.01|    2.18
    Magnesium Sulfate, MgSO₄|             |        |  23.70|   43.82
    Potassium Chlorid, KCl  |             |        |       |   12.81
                            +-------------+--------+-------+----------
                            |    100.00   | 100.00 | 100.00|  100.00
    ------------------------+-------------+--------+-------+----------

    ========================+=========================================
                            |                  COLORADO.
                            +------------+---------+--------+-------
                            |    Near    |  Grand  | Rocky  | Rocky
                            |  Denver.   |Junction.| Ford.  | Ford.
    ------------------------+------------+---------+--------+-------
    Potassium Sulfate, K₂SO₄|            |         |   0.10 |  13.74
    Sodium Sulfate, Na₂SO₄  |    93.40   |   56.05 |  62.54 |  17.36
    Sodium Carbonate, Na₂CO₃|            |   18.97 |   2.08 |   5.53
    Sodium Chlorid, NaCl    |     6.60   |   24.98 |   3.86 |  11.53
    Magnesium Sulfate, MgSO₄|            |         |  31.42 |  44.43
    Magnesium Chlorid, MgCl₂|            |         |        |   5.76
    Magnesium Phosphate     |            |         |        |   1.65
                            +------------+---------+--------+-------
                            |   100.00   |  100.00 | 100.00 | 100.00
    ------------------------+------------+---------+--------+-------

    ========================+=========================================
                            |                  NEW MEXICO.
                            +-------------------------------------------
                            |                 PECOS VALLEY.
                            |------------------------+------------------
                            |      ROSWELL REGION.   | CARLSBAD REGION.
                            +-------+--------+-------+--------+---------
                            |Bremond|Michelet|Roswell|Carlsbad|Delaware
                            |       |        |       |        | River
    ------------------------+-------+--------+-------+--------+---------
    Potassium Sulfate, K₂SO₄|       |        |       |        |
    Sodium Sulfate, Na₂SO₄  | 54.61 |   2.62 | 67.46 |  35.16 |   37.11
    Sodium Carbonate, Na₂CO₃|       |        |       |        |
    Sodium Chlorid, NaCl    | 51.60 |  65.16 | 10.00 |  26.88 |   34.31
    Magnesium Sulfate, MgSO₄| 13.79 |  32.22 | 22.54 |  38.06 |   28.58
    Magnesium Chlorid, MgCl₂|       |        |       |        |
    Magnesium Phosphate     |       |        |       |        |
                            +-------+--------+-------+--------+---------
                            |100.00 | 100.00 |100.00 | 100.00 |  100.00
    ------------------------+-------+--------+-------+--------+---------

_Nutritive Salts in Alkali._—With them, however, there are almost
always associated, in varying amounts, sulfate of potash, phosphate of
soda, and nitrate of soda, representing the three elements—potassium,
phosphorus, and nitrogen—upon the presence of which in the soil in
available form, the welfare of our crops so essentially depends, and
which we aim to supply in fertilizers. The potash salt is usually
present to the extent of from 5 to 20 per cent of the total salts;
phosphate, from a fraction of one to as much as 4 percent; the nitrate
from a fraction of one to as much as 20 percent. In black alkali the
nitrate is usually low, the phosphate high; in the white, the reverse
is true. Both relations are readily intelligible from a chemical and
bacteriological point of view.

_Estimation of Total Alkali in Land._—The investigations detailed
above having shown that in California at least, outside of the axes
of valleys no practically important amount of alkali salts is usually
found at a depth exceeding four feet, it became possible to determine
approximately the amounts of salts that would have to be dealt with
when irrigation and evaporation should bring the entire amount to or
near the surface; a necessary prerequisite to the determination of
possible cultures. While, as already shown, the salts occur lower down
in very sandy lands, yet the diagram on p. 435 shows that even then, an
estimate on this basis would not be very wide of the truth. It is at
least probable that the same is measurably true of level alkali lands
elsewhere, when not underlaid by geological deposits impregnated with
salts.

The total amount of these salts ordinarily found in alkali lands (_i.
e._ in such as in the dry season show saline efflorescences on the
surface) is from about one tenth of one per cent to as much as three
per cent of the weight of the soil, taken to the depth of four feet.
The percentage of salts having been determined in samples representing
a tract, it becomes easy to calculate, approximately, the total amounts
of each salt present per acre, on the basis of the weight of the soil
per acre foot. For the soils of the arid region, such weight will
usually range from three million five hundred thousand to four million
pounds per acre-foot; the latter being the most usual figure, of which
it may be conveniently remembered, that forty thousand pounds represent
1 per cent. We are thus enabled to estimate _e. g._ the amount of
gypsum required to neutralize the carbonate of soda in the salts, or
the amounts of valuable nutritive ingredients—potash, phosphoric acid
and nitrates—present in the land in the water-soluble form.

As has been shown in the preceding discussion, the analysis at the
surface foot alone, which has frequently been alone made, gives
no definite clew whatever to the _total_ amounts of salts to be
controlled. A _full_ estimate is of special importance in enabling us
to forecast what culture plants are likely to succeed on a given tract,
by reference to the table of “tolerances” given below (chapter 23, page
467).

_Composition of Alkali Soils as a Whole._—As may be imagined, the
presence of the alkali salts finds expression in the analytical
statement of their composition, although not to the extent usually
anticipated from their superficial aspect. The table annexed gives
the composition of fourteen alkali soils, taken to the depth of one
foot, at times when there was no visible accumulation of salts on the
surface. The averages of the several ingredients determined are given
in the fifteenth column, and a comparison of its figures with those
of the general table on page 377 of chapter 20 will show some marked
characteristics. We find the average potash-content to be but little
less than twice as great as in the general average for the state of
California; in the case of lime the ratio is nearly as one to three,
in the case of magnesia nearly one to two; in that of phosphoric acid,
one to two and a half, of which in the presence of carbonate of soda
an unusually large proportion is in a readily soluble, often in the
water-soluble, condition (see preceding table).

The usual proportion of soda, of one-fourth to one-half of the
amount of potash, is changed to one-half or three-fourths; in the
case of the strongest alkali lands soda may equal or even exceed the
potash content. As the latter, however, is invariably high to very
high, it does not happen as frequently as might be supposed that the
soda content exceeds that of potash as shown by the usual method of
soil-extraction with water.

    That the _potash_ percentage should always be high in
    alkali lands, is hardly surprising when it is considered
    that the continued presence of the salts resulting from rock
    decomposition affords opportunity for the full exercise of
    the preference with which potash is known to be retained in
    soils by the formation of complex zeolitic silicates. In
    most cases the potash-percentage exceeds .75%, and rises as
    high as 2.0%; as is shown in the table.

                COMPOSITION OF ALKALI SOILS AS A WHOLE.

    (A) = White Ash, Fresno.
    (B) = Wire Grass, Visalia.
    (C) = Plains, Cross Creek, Tulare
    (D) = Salt Grass, B. V. Slough.
    =============================+========+========+========+========
                                 |        |        |        |
       COLLECTION NUMBER.        |   (A)  |   (B)  |   (C)  |   (D)
                                 |  —704— |  —585— |  —573— | —700—
    -----------------------------+--------+--------+--------+--------
    Coarse Materials > 0.55 mm   |        |14.29   | 1.50   |
    Fine Earth                   |        |85.71   |98.50   |
                                 |        |        |        |
     CHEMICAL ANALYSIS OF        |        |        |        |
           FINE EARTH.           |        |        |        |
    -----------------------------+--------+--------+--------+---------
    Insoluble matter             |85.87   |66.47   |66.08   |87.06
                                 |   88.58|   71.42|   69.46|   89.04
    Soluble silica               | 2.71   | 4.95   | 3.38   | 1.98
    -----------------------------+--------+--------+--------+--------
    Potash (K₂O)                 |  .34   | 1.22   | 1.82   |  .49
    Soda (Na₂O)                  |  .25   |  .68   |  .44   |  .35
    Lime (CaO)                   | 1.16   | 3.04   | 4.31   | 1.20
    Magnesia (MgO)               |  .50   |  .09   | 1.59   | 1.07
    Bro. ox. of Manganese (Mn₃O₄)|  .03   |  .03   |  .08   |  .03
    Peroxid of Iron (Fe₂O₃)      | 3.28   | 5.82   | 6.04   | 5.82
    Alumina (Al₂O₃)              | 3.22   | 7.14   | 8.69   |  .17
    Phosphoric acid (P₂O₅)       |  .10   |  .24   |  .74   |  .08
    Sulfuric acid (SO₃)          |  .12   |  .66   |  .26   |  .13
    Carbonic acid (CO₂)          |        | 2.55   | 2.53   |
    Water and organic matter     | 1.79   | 7.09   | 4.15   | 1.13
                                 +--------+--------+--------+--------
                 Total           |99.37   |99.97   |99.51   |99.51
                                 +--------+--------+--------+--------
    Chlorin, per cent.           |        |        |        |
    Humus                        |  .60   | 1.00   | 1.00   |  .17
      “   Ash                    |  .35   |  .84   |  .74   |  .20
                                 +--------+--------+--------+--------
      “   Nitrogen, per cent.    |        |        |        |
                    in Humus     |18.66   |14.10   |        |
                                 +--------+--------+--------+--------
      “      “    , per cent.    |        |        |        |
                    in soil      |  .11   |  .15   |        |
                                 +--------+--------+--------+--------
    Hygroscopic Moisture         | 2.22   | 8.53   | 8.74   | 2.16
      absorbed at             °C |        |        | 14°    | 15°
    -----------------------------+--------+--------+--------+--------
                                 |       Tulare Lake Alluvium,
                                 |          Tulare County.
         COLLECTION NUMBER.      |    —891—     —893—      —77—
    -----------------------------+-----------+----------+-------
    Coarse Materials > 0.55 mm   |           |          |   4.10
    Fine Earth                   |     all   |   all    |  95.90
                                 |           |          |
     CHEMICAL ANALYSIS OF        |           |          |
           FINE EARTH.           |           |          |
    -----------------------------+-----------+----------+-------
    Insoluble matter             | 54.71     | 56.92    |  67.34
                                 |      64.55|     67.15|
    Soluble silica               |  9.84     | 10.13    |
    -----------------------------+-----------+----------+-------
    Potash (K₂O)                 |  2.02     |  1.65    |   1.05
    Soda (Na₂O)                  |  2.73     |   .54    |    .84
    Lime (CaO)                   |  2.46     |  2.96    |   6.51
    Magnesia (MgO)               |  2.93     |  3.12    |   3.69
    Bro. ox. of Manganese (Mn₃O₄)|   .03     |   .04    |    .04
    Peroxid of Iron (Fe₂O₃)      |  7.46     |  6.73    |   5.05
    Alumina (Al₂O₃)              | 11.50     | 10.35    |   7.97
    Phosphoric acid (P₂O₅)       |   .11     |   .16    |    .32
    Sulfuric acid (SO₃)          |   .01     |   .01    |    .08
    Carbonic acid (CO₂)          |  1.81     |   .93    |   4.42
    Water and organic matter     |  4.34     |  5.77    |   3.71
                                 +-----------+----------+-------
                 Total           | 99.95     | 99.92    | 101.29
                                 +-----------+----------+-------
    Chlorin, per cent.           |           |          |
    Humus                        |           |   .88    |    .47
      “   Ash                    |           |  1.03    |   2.18
                                 +-----------+----------+-------
      “   Nitrogen, per cent.    |           |          |
                    in Humus     |           |          |   9.37
                                 +-----------+----------+-------
      “      “    , per cent.    |           |          |
                    in soil      |           |          |    .05
                                 +-----------+----------+-------
    Hygroscopic Moisture         | 10.50     |  8.6     |
      absorbed at             °C | 10°       |  10°     |
    -----------------------------+-----------+----------+-------

                    COMPOSITION OF ALKALI SOILS AS A WHOLE.
    ============================+==========+===============+============
                                |  Tulare  |Ten-acre tract.|Carisa Plain
        COLLECTION NUMBER       |Substation|Chino, S.B. Co.| S.L. Obispo
                                |          |               |
                                |  —1159—  |    —1284—     |   —1423—
    ----------------------------+----------+---------------+------------
    Coarse Materials > 0.55mm.  |   1.96   |   1.00        |  9.00
    Fine Earth.                 |  98.04   |  99.00        | 91.00
                                |          |               |
    CHEMICAL ANALYSIS OF        |          |               |
         FINE EARTH.            |          |               |
    ----------------------------+----------+---------------+------------
    Insoluble matter            |  72.98   |  62.62        | 57.33
                                |     73.58|      70.92    |     69.48
    Soluble silica              |   6.60   |  18.30        | 12.15
    ----------------------------+----------+---------------+------------
    Potash (K₂O)                |   1.20   |    .95        |  1.23
    Soda (Na₂O)                 |    .52   |    .50        |   .77
    Lime (CaO)                  |   1.86   |   5.07        |  4.46
    Magnesia (MgO)              |   1.81   |    .84        |  3.12
    Br. ox. of Manganese (Mn₃O₄)|    .08   |    .06        |   .01
    Peroxid of Iron (Fe₂O₃)     |   6.86   |   6.43        |  7.65
    Alumina (Al₂O₃)             |   5.66   |   4.88        |  6.16
    Phosphoric acid (P₂O₅)      |    .10   |    .21        |   .43
    Sulfuric acid (SO₃)         |    .03   |    .06        |   .06
    Carbonic acid (CO₂)         |          |   3.76        |
    Water and organic matter    |   2.54   |   1.02        |  2.63
          Total                 | 100.24   |  99.70        | 99.99
                                +----------+---------------+------------
    Chlorin, per cent           |          |    .12        |
    Humus, per cent             |    .37   |   1.99        |  1.39
      “    Ash, per cent        |    .32   |   1.13        |   .95
                                +----------+---------------+------------
      “    Nitrogen, per cent.  |          |               |
                     in Humus   |  16.75   |  10.20        | 14.36
      “       “    , per cent.  |          |               |
                     in soil    |    .06   |    .20        |   .06
                                +----------+---------------+------------
    Hygroscopic Moisture        |          |   5.81        |  8.46
      absorbed at           °C  |          |     15°       |    15°
    ----------------------------+----------+-----------+---+------------
                                |    Perris Valley,    |Sand, Coachella,
       COLLECTION NUMBER        |Jacinto river, Jacinto|  Riverside
                                | Plain, Riverside Co. |     Co.
                                |  —1758—   |  —1760—  |   —2471—
    ----------------------------+-----------+----------+----------------
    Coarse Materials > 0.55mm.  |  7.50     |  3.50    |
    Fine Earth.                 | 92.50     | 96.50    |  all
                                |           |          |
    CHEMICAL ANALYSIS OF        |           |          |
         FINE EARTH.            |           |          |
    ----------------------------+-----------+----------+----------------
    Insoluble matter            | 41.59     | 35.20    | 58.95
                                |     63.13 |     58.95|     77.54
    Soluble silica              | 21.54     | 23.75    | 11.37
    ----------------------------+-----------+----------+----------------
    Potash (K₂O)                |  1.37     |  1.16    |  1.26
    Soda (Na₂O)                 |  1.97     |   .96    |   .37
    Lime (CaO)                  |  4.23     |  8.00    |  2.71
    Magnesia (MgO)              |  3.80     |  5.69    |  2.20
    Br. ox. of Manganese (Mn₃O₄)|   .03     |   .04    |   .05
    Peroxid of Iron (Fe₂O₃)     |  9.65     |  7.33    |  6.43
    Alumina (Al₂O₃)             |  7.26     |  6.29    |  5.53
    Phosphoric acid (P₂O₅)      |   .23     |   .28    |   .21
    Sulfuric acid (SO₃)         |   .34     |   .21    |   .08
    Carbonic acid (CO₂)         |  4.19     |  6.49    |  1.05
    Water and organic matter    |  2.89     |  4.71    |  2.55
          Total                 | 99.81     | 99.84    | 99.98
                                +-----------+----------+----------------
    Chlorin, per cent           |           | Trace.   |
    Humus, per cent             |   .60     |   .91    |   .46
      “    Ash, per cent        |   .92     |  1.75    |   .42
                                +-----------+----------+----------------
      “    Nitrogen, per cent.  |           |          |
                     in Humus   |  6.66     |  7.70    |
      “       “    , per cent.  |           |          |
                     in soil    |   .04     |   .07    |
                                +-----------+----------+----------------
    Hygroscopic Moisture        |  9.43     |  8.85    |  2.64
      absorbed at           °C  |  15°      |  15°     |  15°
    ----------------------------+-----------+----------+----------------


    ============================+===============+===========
                                |Silt, Imperial,|Average of
     COLLECTION NUMBER          | S. Diego Co.  |  soils.
                                |    —2325—     |
    ----------------------------+---------------+-----------
    Coarse Materials > 0.55mm.  |               |
    Fine Earth.                 |     all       |
                                |               |
    CHEMICAL ANALYSIS OF        |               |
         FINE EARTH.            |               |
    ----------------------------+---------------+-----------
    Insoluble matter            | 62.67         | 63.08
                                |      73.60    |      72.19
    Soluble silica              | 10.93         |  9.11
    ----------------------------+---------------+-----------
    Potash (K₂O)                |   .74         |  1.17
    Soda (Na₂O)                 |   .29         |   .78
    Lime (CaO)                  |  3.75         |  3.71
    Magnesia (MgO)              |  1.68         |  2.29
    Br. ox. of Manganese (Mn₃O₄)|   .01         |   .04
    Peroxid of Iron (Fe₂O₃)     |  3.71         |  6.30
    Alumina (Al₂O₃)             |  4.26         |  6.65
    Phosphoric acid (P₂O₅)      |   .22         |   .20
    Sulfuric acid (SO₃)         |   .36         |   .17
    Carbonic acid (CO₂)         |  2.32         |  2.14
    Water and organic matter    |  8.93         |  4.19
          Total                 | 99.87         |
                                +---------------+-----------
    Chlorin, per cent           |               |
    Humus, per cent             |   .65         |   .75
      “    Ash, per cent        |   .69         |   .82
                                +---------------+-----------
      “    Nitrogen, per cent.  |               |
                     in Humus   | 10.90         |
      “       “    , per cent.  |               |
                     in soil    |   .07         |
                                +---------------+-----------
    Hygroscopic Moisture        | 2.98          | 5.63
      absorbed at           °C  | 15°           | 15°
    ----------------------------+---------------+-----------

This table exhibits also another standing characteristic of alkali
soils, which is to be anticipated from the conditions of their
formation; viz, high _lime-content_, which sometimes rises to the
extent of marliness.

In _phosphates_, also, alkali soils are almost always high; and an
unusually large proportion is found to be readily soluble.

In presence of much carbonate of soda, _nitrates_ are usually scarce
or altogether absent; while owing to the action of the alkaline
solution upon the humus, ammonia salts, or even free (or carbonated
and therefore readily dissociated and assimilated) _ammonia_ may be
present, so as to be perceptible to the senses by its odor in hot
sunshine. But in the case of “white alkali,” more especially of the
sulphate in moderate amounts, nitrification is exceedingly active and
nitrates may sometimes rise to as much as 20% of the soluble salts. As
alkali spots are usually low in the central portion and therefore more
moist than around the edges, we sometimes find ammonia salts in the
middle of a spot, while nitrates are abundant along the margin of the
same. These differences, first demonstrated by an investigation made
by Colmore,[170] illustrate some of the reactions that are essentially
concerned in the agricultural availability of alkali lands. A summary
of Colmore’s results is given in the table below.

[170] Report of the California Exp’t. St’n. for 1892-94, p. 141.

_Cross Section of an Alkali Spot._—The spot examined lies outside of
Tulare, California, substation; it being late in the season, when the
bulk of the salts is found near the surface, the samples were taken to
the depth of one foot only, at points four feet apart, from the center
out.

    AMOUNT AND COMPOSITION OF SALTS IN ALKALI SPOT FROM CENTER TO
               CIRCUMFERENCE. 4 FEET APART, 1 FT. DEPTH.
    ======================+========+========+========+========+========
                          |   1    |   2    |   3    |   4    |   5
        Mineral Salts.    | Center | Four   | Eight  | Twelve | Outer
                          |of spot.| feet.  | feet.  | feet.  | margin.
    ----------------------+--------+--------+--------+--------+--------
    Potassium sulfate     |   6.70 |   9.55 |  11.92 |  19.26 |  13.95
    Sodium sulfate        |  19.84 |  12.85 |  23.72 |  23.97 |  16.96
    Magnesium sulfate     |   3.07 |    .07 |    .95 |   2.05 |   8.29
    Sodium chlorid        |  13.80 |  23.73 |  24.12 |  24.23 |  29.69
    Sodium carbonate      |  50.72 |  50.96 |  37.55 |  35.49 |  29.94
    Sodium phosphate      |   5.57 |   2.88 |    .87 |    ?   |   1.04
    Sodium nitrate        |    .30 |    ?   |    .87 |    ?   |    .13
                          | ------ | ------ | ------ | ------ | ------
        Totals            | 100.00 | 100.00 | 100.00 | 100.00 | 100.00
                          |        |        |        |        |
    Organic matter        |  30.00 |  24.80 |  19.48 |  23.36 |  20.31
    Total soluble in soil |    .78 |    .54 |    .70 |    .37 |    .34
    Mineral salts         |    .38 |    .40 |    .54 |    .25 |    .23
    ----------------------+--------+--------+--------+--------+--------

While the table shows an obvious irregularity in some of the data
at the eight-foot point, arising doubtless from an irregularity of
surface or of texture overlooked in taking the samples, we find a very
remarkable regularity of progression in the cases of potassium sulfate,
sodium chlorid, sodium carbonate and sodium phosphate in the other
four samples. The maxima of the “black alkali” and the soluble organic
matter (humus) coincide, as does that of the phosphate; the total
mineral salts at the outer margin are only a little over half of what
is found at the center. This is natural, as owing to the deflocculating
effect of the black alkali, the center is nearly a foot lower than the
margin. The lowering of the nitrate-content at the outer margin is
obviously due to the luxuriant vegetation growing adjacent.

_Reactions between the Carbonates, Chlorids and Sulfates of Alkalies
and Earths._ That a soluble earth-salt, such as the sulfate or chlorid
of calcium, will react upon an alkaline carbonate solution so as to
form an alkali sulfate, and _e.g._ lime carbonate, is well known; the
neutralization of the sodic carbonate in the soil by means of gypsum,
above referred to, is based upon this reaction. It is not so well known
that the latter may be reversed, partly or wholly, by the presence of
carbonic acid in the solution of the soil. Although observed as early
as 1824 by Brandes, and again in 1859 by A. Müller, this reaction is
not mentioned in text-books and attracted no attention as a source of
naturally occurring alkali carbonates which in the past have formed
the basis of extensive commerce from the Orient, until in 1888, the
writer together with Weber and later with Jaffa, investigated it
quantitatively.[171] It was found that up to .75 grms. per liter, the
entire amount of sodic sulfate present in solution is transformed into
carbonate in presence of calcic carbonate, by a current of carbonic
dioxid; but the amount so transformed does not continue to increase
beyond about 4 grams per liter. A corresponding amount of calcic
sulfate is formed. In the case of potassic sulfate, the transformation
also occurs, proportionally to the molecular weight. This relation is
shown in the subjoined diagram, which also shows in the curves on the
left, the residual alkalinity left after evaporation and drying the
residue at 100° C.

[171] Proc. Am. Soc. Agr. Sci., 1888; ibid., 1890; Rep. Cal. Expt.
Sta., 1890, p. 100; Ber. Berlin, Chem. Ges., 1893; Am. Jour. Sci.,
August 1896.

[Illustration: FIG. 69.—Progressive Transformation of Alkali Sulfates
into Carbonates. (The figures along upper line represent tenths of one
per cent.)]

The corresponding reaction occurs also, of course, between sodium
chlorid and calcium carbonate, but not to the same extent, because
unlike the difficultly soluble gypsum, the reaction product is the very
soluble calcium chlorid, the presence of which in the solution limits
the reaction much sooner than when most of the decomposition product
is thrown down in the solid state. The calcium chlorid not uncommonly
found in some alkali regions is undoubtedly the product of the above
reaction.

As the saline solutions in the soil are mostly quite dilute, and
calcic carbonate is always present, it follows that whenever under the
influences which favor the oxidation of organic matter in the soil,
and the activity of the plant roots, carbonic gas is formed somewhat
copiously, alkali sulfates and chlorids present may be partially or
wholly transformed into carbonates within the soil. As a matter of
fact, it is found that this transformation occurs most readily in the
moister portions of the soil and subsoil, and _invariably so when an
alkali soil is “swamped” by excessive irrigation or rise of bottom
water_; while the reaction is again reversed whenever free access of
air reduces the carbonic dioxid below a certain point. It thus becomes
intelligible why in the diagrams showing the distribution of the salts
(this chapter pp. 431 and 432), we always find the sodic carbonate
relatively _decreasing_ as the surface is approached.

Thus, also, is explained the fact that sodium carbonate is formed more
abundantly toward the center of the root system of alkali plants, such
as the greasewood, beneath which the soil is always more abundantly
charged with “black alkali” than is the surrounding earth.

Good aeration of the soil mass, then, is essential in maintaining the
neutralization of the “black alkali” soils brought about by the use of
gypsum (land plaster).

_Inverse Ratios of Alkali Carbonates and Sulfates._—According to the
above considerations, it is not surprising that we should often find
an apparent inverse ratio between the alkali sulfates and carbonates
in soils so closely adjacent that their salts must be presumed to be
similar in composition. A striking example is shown in fig. 70, in
which this inverse ratio becomes apparent four times in succession in
one and the same soil profile. While this inference is plain on the
face of the diagram, it is not quite easy to explain in detail how
this alternation came about from the condition observed two months
previously. Most probably it was caused by corresponding alternations
of weather, in which short, warm spring showers alternated with
similarly brief periods of drying north winds; the latter causing a
reversal of the formation of sodic carbonate that had been induced by
the former.

[Illustration: FIG. 70.—Amounts and Composition of Alkali Salts at
various depths in partly reclaimed Alkali Land. Tulare Experiment
Substation, California.]

_Exceptional Conditions._—While the phenomena of alkali lands as
outlined above probably represent the vastly predominant conditions
on level lands, yet there are exceptions due to surface conformation,
and the local existence of sources of alkali salts outside of the soil
itself. Such is the case where salts ooze out of strata cropping out on
hillsides, as at some points in the San Joaquin Valley in California,
and in parts of New Mexico, Colorado and Wyoming; also where, as in
the Hungarian plain, saline clays underlie within reach of surface
evaporation.

Again, it not infrequently happens that in sloping valleys or basins,
where the central (lowest) portion receives the salts leached out of
the soils of the adjacent slopes, we find belts of greater or less
width in which the alkali impregnation may reach to the depth of
ten or twelve feet, usually within more or less definite layers of
calcareous hardpan, likewise the outcome of the leaching of the valley
slopes. Such areas, however, are usually quite limited, and are at
present scarcely reclaimable without excessive expenditure; the more
as they are often underlaid by saline bottom water. In these cases
the predominant saline ingredient is usually common salt, as might be
expected and as is exemplified in the Great Salt Lake of Utah, in the
Antelope and Perris Valleys, and in Salton basin in California; in
the Yellowstone valley near Billings, Mont.[172] in the Aralo-Caspian
desert, and at many other points.

[172] Farmer’s Bull. No. 88, U. S. Dept. Agr., 1899.

_Conclusions._—Summing up the conclusions from the foregoing facts and
considerations, we find that—

(1) The amount of soluble salts in alkali lands is usually limited;
they are not ordinarily supplied in indefinite quantities from the
bottom-water below. These salts have mostly been formed by weathering
in the soil-layer itself.

(2) The salts move up and down within the upper four or five feet
of the soil and subsoil, following the movement of the moisture;
descending in the rainy season to the limit of the annual moistening
as a maximum, and then reascending or not, according as surface
evaporation may demand. At the end of the dry season, in untilled
irrigated land, practically the entire mass of salts may be within six
or eight inches of the surface.

(3) The direct injury to vegetation[173] is caused largely within a
few inches of the surface, by the corrosion of the bark, usually near
the root crown. This corrosion is strongest when carbonate of soda
(salsoda) forms a large proportion of the salts; the soda then also
dissolves the vegetable mold and causes blackish spots in the soil,
popularly known as black alkali.

[173] For a general statement and discussion of the physiological
effects of saline solutions on plants, see chapter 26.

(4) The injury caused by carbonate of soda is aggravated by its action
in puddling the soil so as to cause it to lose its crumbly or flaky
condition, rendering it almost or quite untillable and impervious. It
also tends to form in the depths of the soil-layer a tough, impervious
hardpan, which yields neither to plow, pick, nor crowbar. Its presence
is easily ascertained by means of a pointed steel sounding-rod.

(5) While alkali lands share with other soils of the arid region the
advantage of unusually high percentages of plant-food in the insoluble
form, they also contain, alongside of the noxious salts, considerable
amounts of water-soluble plant-food. When, therefore, the action of the
noxious salts is done away with, they should be profusely and lastingly
productive; particularly as they are always naturally somewhat moist
in consequence of the attraction of moisture by the salts, and are
therefore less liable to injury from drought than the same soils when
free from alkali.




CHAPTER XXIII.

UTILIZATION AND RECLAMATION OF ALKALI LANDS.


_Alkali-Resistant Crops._—The most obvious mode of utilizing alkali
lands is to occupy them with crops not affected by the noxious salts.
Unfortunately but few such crops of general utility have as yet been
found for the stronger class of alkali lands. The question is always
one of degree, which frequently cannot be decided without an actual
determination of the amount and kind of salts to be dealt with, to
which the crops can then be adapted in accordance with the greater
or less sensitiveness of the several plants, as indicated in the
table of tolerances given farther on. But aside from this, there are
certain general measures and precautions which in any case will serve
to mitigate the effect of the alkali salts. Foremost among these, and
applicable everywhere, is the prevention of evaporation to the utmost
extent possible.

_Counteracting Evaporation._—Since evaporation of the soil-moisture
at the surface is what brings the alkali to the level where the
main injury to plants occurs, it is obvious that evaporation should
be prevented as much as possible. This is the more important, as
the saving of soil-moisture, and therefore of irrigation water, is
attainable by the same means.

Three methods for this purpose are usually practiced, viz., shading,
mulching, and the maintenance of loose tilth in the surface soil to
such depth as may be required by the climatic conditions.

As to mulching, it is already well recognized in the alkali regions
of California as an effective remedy in light cases. Fruit trees are
frequently thus protected, particularly while young, after which their
shade alone may (as in the case of low-trained orange trees) suffice
to prevent injury. The same often happens in the case of low-trained
vines, small-fruit, and vegetables. Sanding of the surface to the depth
of several inches was among the first attempts in this direction; but
the necessity of cultivation, involving the renewal of the sand each
season, renders this a costly method. Straw, leaves, and manure have
been more successfully used; but even these, unless employed for the
purpose of fertilization, involve more expense and trouble than the
simple _maintenance of very loose tilth of the surface soil throughout
the dry season_; a remedy which, of course, is equally applicable to
hoed field crops, and is in the case of some of these—_e. g._, cotton—a
necessary condition of cultural success everywhere. The wide prevalence
of “light” soils in the arid regions, from causes inherent in the
climate itself, renders this condition relatively easy of fulfilment.

_Turning-under of Surface Alkali._—Aside, however, from the mere
prevention of surface evaporation, another favorable condition is
realized by this procedure, namely the commingling of the heavily
salt-charged surface-layers with the relatively non-alkaline subsoil.
Since in the arid regions the roots of all plants retire farther
from the surface because of the deadly drought and heat of summer,
it is usually possible to cultivate deeper than could safely be done
with growing crops in humid climates. Yet even there, the maxim of
“deep preparation and shallow cultivation” is put into practice with
advantage, only changing the measurements of depth to correspond with
the altered climatic conditions. Thus while in the humid States, three
to four inches is the accepted standard of depth for summer cultivation
to preserve moisture without injury to the roots, that depth must in
the arid region frequently be doubled in order to be effective; and
will even then scarcely touch a living root in orchards and vineyards,
particularly in unmanured and unirrigated land.

A glance at fig. 63, (chap. 22, p. 431), will show the great advantage
of extra-deep preparation in commingling the alkali salts accumulated
near the surface with the lower soil-layers, diffusing the salts, say
through twelve instead of six inches of soil mass. This will in very
many cases suffice to render the growth of ordinary crops possible if,
by subsequent frequent and thorough cultivation, surface evaporation,
and with it the re-ascent of the salts to the surface, is prevented.

A striking example of the efficiency of this mode of procedure was
observed at the Tulare substation, California, where a portion of a
very bad alkali spot was trenched to the depth of two feet, throwing
the surface soil to the bottom. The spot thus treated produced
excellent wheat crops for two years—the time it took the alkali salts
to reascend to the surface.

It should therefore be kept in mind that whatever else is done toward
reclamation, _deep preparation and thorough cultivation_ must be
regarded as prime factors for the maintenance of production on alkali
lands.

_The Efficacy of Shading_, already referred to, is strikingly
illustrated in the case of some field crops which, when once
established, will thrive on fairly strong alkali soil, provided that
a good thick “stand” has once been obtained. This is notably true
of the great forage crop of the arid region, alfalfa or lucern. Its
seed is extremely sensitive to “black” alkali, and will decay in the
ground unless protected against it by the use of gypsum in sowing. But
when once a full stand has been obtained, the field may endure for
many years without a sign of injury. Here two effects combine, viz.,
the shading, and the evaporation through the deep roots and abundant
foliage, which alone prevents, in a large measure, the ascent of the
moisture and salts to the surface. The case is then precisely parallel
to that of the natural soil (see p. 432, chapter 22), except that, as
irrigation is practiced in order to stimulate production, the sheet of
alkali hardpan will be dissolved and its salts spread through the soil
more evenly. The result is that so soon as the alfalfa is taken off the
ground and the cultivation of other crops is attempted, an altogether
unexpectedly large amount of alkali comes to the surface and greatly
impedes, if it does not altogether prevent, the immediate planting of
other crops. Shallow-rooted annual crops that give but little shade,
like the cereals, while measurably impeding the rise of the salts
during their growth (see fig. 70, page 452) frequently allow of enough
rise after harvest to prevent reseeding the following season.

_“Neutralising” Black Alkali._—Since so little carbonate of soda
as one-tenth of one per cent may suffice to render some soils
uncultivable, it frequently happens that its mere transformation into
the sulfate is sufficient to remove all stress from alkali. Gypsum
(land plaster) is the cheap and effective agent to bring about this
transformation, provided water be also present. The amount required
per acre will, of course, vary with the amount of salts in the soil,
all the way from a few hundred pounds to several tons in the case of
strong alkali spots; but it is not usually necessary to add the entire
quantity at once, provided that sufficient be used to neutralize the
sodic carbonate near the surface, and enough time be allowed for the
action to take place. In very wet soil, and when much gypsum is used,
this may occur within a few days; in merely damp soils in the course of
months; but usually the effect increases for years, as the salts rise
from below.

The effect of gypsum on black-alkali land is often very striking,
even to the eye. The blackish puddles and spots disappear, because
the gypsum renders the dissolved humus insoluble and thus restores it
to the soil. The latter soon loses its hard, puddled condition and
crumbles and bulges into a loose mass, into which water now soaks
freely, bringing up the previously depressed spots to the general level
of the land. On the surface thus changed, seeds now germinate and grow
without hindrance; and as the injury from alkali occurs at or near the
surface, it is usually best to simply harrow in the plaster, leaving
the water to carry it down in solution. Soluble phosphates present
are decomposed so as to retain finely divided, but less soluble earth
phosphates in the soil.

It must not be forgotten that this beneficial change may go backward
if the land thus treated is permitted to be swamped by irrigation
water or otherwise. Under the same conditions naturally white alkali
may turn black (see above, chapter 22, p. 451). Of course, gypsum is
of no benefit whatever on soils containing no “black” alkali, but only
(“white”) Glauber’s and common salt.

_Removing the Salts from the Soil._—In case the amount of salts in
the soil should be so great that even the change worked by gypsum is
insufficient to render it available for useful crops, the only remedy
left is to remove the salts, partially or wholly, at least from the
_surface_ of the land. Three chief methods are available for this
purpose. One is to remove the salts, with more or less earth, from the
surface at the end of the dry season, either by sweeping or by means of
a horse scraper set so as to carry off a certain depth of soil. Thus
sometimes in a single season one-third or one-half of the total salts
may be got rid of, the loss of a few inches of surface soil being of
little moment in the deep soils of the arid region. Another method
affording partial relief is to flood the land for a sufficient length
of time to carry the alkali three or more feet below the surface, then
carefully preventing its re-ascent by suppressing evaporation (see this
chapter, p. 455) as much as possible. The best of all, the final and
universally efficient remedy, is to leach the alkali salt out of the
soil into the country drainage; supplementing by irrigation water what
is left undone by the deficient rainfall.

It is not practicable, as many suppose, to wash the salts off the
surface by a rush of water, as they instantly soak into the ground at
the first touch. Nor is there any certain relief from allowing the
water to stand on the land and then drawing it off; in this case also
the salts soak down ahead of the water, and the water standing on the
surface remains almost unchanged. In very pervious soils and in the
case of white alkali, the washing-out can often be accomplished without
special provision for underdrainage, by leaving the water on the land
sufficiently long. But the laying of regular underdrains greatly
accelerates the work, and renders success certain.

_Leaching-Down._—In advance of underdrainage, it is quite generally
feasible, where the land has been leveled and diked for irrigation by
surface flooding, to leach the salts out of the first three or four
feet by continued flooding, thus taking them out of reach of the crop
roots, or at all events giving the _seed_ an opportunity to escape
injury from alkali. This plan is especially effective in the case of
alfalfa, the young seedlings of which are very sensitive, while the
grown plant is rather resistant. In order to obtain this relief so
as to know what is being accomplished, the farmer should ascertain
beforehand how fast water will soak down in his ground;[174] for in
heavy clay soils, and especially in those containing black alkali,
the soakage is sometimes so slow that the upward diffusion of the
salts keeps pace with the downward soakage; in which case nothing is
accomplished by flooding, and underdrainage is the only remedy. But in
most soils of the arid region flooding from three days to a week will
remove the alkali beyond reach of the roots of ordinary crops. If
subsequently irrigation is done by means of _deep_ furrows, the alkali
salts may be either kept at a low level continuously, or if the land
be at all pervious, the alkali may ultimately be permanently leached
out into the subdrainage by farther flooding. When the alkali has not
accumulated near the surface to any great extent, irrigation by deep
furrows may, alone, afford all the relief needed.

[174] See p. 242, Chap. 13.

[Illustration: FIG. 71.—Lemon Orchard Affected by Alkali; Before Deep
Irrigation.]

In the case illustrated by figures 71 and 72, irrigation by shallow
furrows with water too strongly charged with salts had so far added to
the natural alkali-content of the land that the lemon trees were being
defoliated. Upon the advice of the California Station the deep-furrow
system was adopted, and within two years the results were as shown in
figure 72, the salts having been carried down and diluted so as to
become harmless.

_Underdrainage the Final and Universal Remedy for Alkali._—When we
underdrain an alkali soil, we adopt the very means by which the
existence of alkali lands in the humid regions is wholly prevented;
the leaching-out of the soluble salts formed in soil-weathering as
fast as they are formed. The long and abundant experience had with
underdrainage in reclaiming saline sea-coast lands, applies directly
and cogently to alkali lands. It is the universal remedy for all the
evils of alkali, and its only drawback is the first expense, and the
necessity for obtaining an outlet for the drain waters, which cannot
always be had on the owner’s land. Hence it requires co-operation or
legislation to render the great improvement of underdrainage feasible.
Such legislation is well established in the old world, and has been
enacted in several states even of the humid region. Where irrigation
is practiced as a matter of necessity, underdrainage is a correlative
necessity, both to avoid the evils of over-irrigation and to relieve
the land of noxious alkali salts.

[Illustration: FIG. 72.—The Above Orchard after Alkali was Driven Down
by Deep Irrigation, followed by Cultivation.]

The drainage law now existing in California does not go farther than
to authorize the formation of drainage districts, within which the
necessary taxes may be levied; and there is some difficulty in securing
popular action. But bitter experience will doubtless in time compel
unanimity, such as now exists, _e. g._, in Illinois, where drainage is
not nearly so urgently needed as it is in the irrigation states.

_Possible Injury to Land by Excessive Leaching._—It should not be
forgotten, however, that excessive leaching of underdrained land by
flooding is liable to injure the soil in two ways: first, by the
removal of valuable soluble plant-food; and further, by rendering the
land less retentive of moisture, such retention being favored by the
presence of small amounts of alkali salts, not sufficient to injure
crops. After the salts have been carried down to a sufficient depth to
prevent injury to annual crops, and with proper subsequent attention
to the prevention of surface evaporation, the flooding will not need
to be repeated for several years. Thus in many soils excellent crops
may be grown even in strong alkali land, pending the establishment of
permanent drainage systems.

    The importance of thoroughly washing the alkali deeply
    into the soil before the seed is planted, and keeping it
    there by proper means until the foliage of the plant shades
    the soil sufficiently to prevent the rise of moisture and
    alkali, is well illustrated in fields in the region of
    Bakersfield, Cal., where alfalfa is now growing in soils
    once heavily charged with alkali.[175] From one of these fields
    samples of soil were taken where the alkali was supposed
    to be strongest beneath the alfalfa, and also from an
    adjoining untreated alkali spot, which was said to represent
    conditions before alfalfa was planted. The results are given
    in pounds per acre in four feet depth.

    ===========================+========+==========+=========+========
                               |Sulfate.|Carbonate.| Common  |  Total
                               |        |          |  Salt.  | Alkali.
    ---------------------------+--------+----------+---------+--------
    Alkali spot before alfalfa |        |          |         |
            was planted        | 60,120 |    720   | 175,840 | 236,680
                               |        |          |         |
    Alfalfa field; alkali      |        |          |         |
                   washed down | 14,400 |          |   1,040 |  18,640
    ---------------------------+--------+----------+---------+--------

    Here the surface foot of the natural soil contained nearly
    140,000 pounds of common salt, a prohibitory amount. Similar
    experience has been had near Yuma, Arizona.

[175] Bull. 133, Cal. Expt. Sta., by R. H. Loughridge.

_Difficulty in Draining “Black” Alkali Lands._—An important exception
to the efficacy of draining, however, occurs in the case of black
alkali in most lands. In this case either the impervious hardpan or
(in the case of actual alkali spots) the impenetrability of the surface
soil itself will render even underdrains ineffective unless the salsoda
and its effects on the soil are first destroyed by the use of gypsum,
as above detailed. This is not only necessary in order to render
drainage and leaching possible, but is also advisable in order to
prevent the leaching-out of the valuable humus and soluble phosphates,
which are rendered insoluble (but not unavailable to plants) by the
action of the gypsum. Wherever black alkali is found in lands not very
sandy, the application of gypsum should precede any other efforts
toward reclamation. Trees and vines already planted may be temporarily
protected from the worst effects of the black alkali by surrounding the
trunks with gypsum or with earth abundantly mixed with it. Seeds may be
similarly protected in sowing, and young plants in planting.

_Swamping of Alkali Lands._—It should, however, be remembered that the
_swamping_ of alkali lands, whether of the white or black kind, is
fatal not only to their present productiveness, but also, on account
of the strong chemical action thus induced, greatly jeopardizes their
future usefulness. Many costly investments in orchards and vineyards
have thus been rendered unproductive, or have even become a total loss.

_Reduction of Alkali by Cropping._—Another method for diminishing the
amount of alkali in the soil is the cropping with plants that take
up considerable amounts of salts. In taking them into cultivation,
it is advisable to remove entirely from the land the salt growth
that may naturally cover it, notably the greasewoods (_Sarcobatus_,
_Allenrolfea_), with their heavy percentage of alkaline ash (12 to 20
per cent). Crop plants adapted to the same object are mentioned farther
on. Such crops should also, of course, be wholly removed from the land.

_Total Amounts of Salts Compatible with Ordinary Crops; Tolerance of
Culture Plants._—Since the amount of alkali that reaches the surface
layer is largely dependent upon the varying conditions of rainfall or
irrigation, and surface evaporation, it is difficult to foresee to
what extent that accumulation may go, unless we know the total amount
of salts present that may be called into action. This, as already
explained, can ordinarily be ascertained by the examination of one
sample representing the average of a soil column of four feet. By
calculating the figures so obtained to an acre of ground, we can at
least approximate the limits within or beyond which crops will succeed
or perish. Applying this procedure to the cases represented in the
diagrams (pp. 434, 452, chapter 22) and estimating the weight of the
soil per acre-foot at 4,000,000 pounds, we find in the land on which
barley refused to grow the figures 32,470 and 43,660 pounds of total
salts per acre, respectively corresponding to 0.203 per cent for the
first figure (the second, representing only the two surface feet, is
not strictly comparable). For the land on which barley gave a full
crop, we find for the May sample 25,550 pounds, equivalent to 0.159 per
cent for the whole soil column of four feet. It thus appears that for
barley the limits of tolerance lie between the above two figures. It
should be noted that in this case a full crop of barley was grown even
when the alkali consisted of fully one-half of the noxious carbonate
of soda; proving that it is not necessary in every case to neutralize
the entire amount of that salt by means of gypsum, which in the present
case would have required about 9½ tons of gypsum per acre—a prohibitory
expenditure.

_Relative Injuriousness of the Several Salts._—Of the three sodium
salts that usually constitute the bulk of “alkali,” only the carbonate
of soda is susceptible of being materially changed by any agent that
can practically be applied to land. So far as we know, the salt of
sodium least injurious to ordinary vegetation is the sulfate, commonly
called Glauber’s salt, which ordinarily forms the chief ingredient of
“white” alkali. Thus barley is capable of resisting about five times
more of the sulfate than of the carbonate, and quite twice as much as
of common salt. Since the maximum percentage that can be resisted by
plants varies materially with the kind of soil, it is difficult to give
exact figures save with respect to particular cases. For the sandy loam
of the Tulare substation, California, for instance, the maximum for
cereals may be approximately stated to be one-tenth of 1 per cent for
salsoda; a fourth of 1 per cent for common salt; and from forty-five
to fifty one-hundredths of one per cent of Glauber’s salt. For clay
soils the tolerance is in general markedly less, especially as regards
the salsoda; since in their case the injurious effect on the tilling
qualities of the soil, already referred to, is superadded to the
corrosive action of that salt upon the plant.

[Illustration: FIG. 73.—Alkali curve showing percentage of Alkali Salts
in field of Sugar Beets, Oxnard, Calif.]

[Illustration: FIG. 74.—Beets from corresponding positions in the above
field.]

    _Effect of Differences in Composition of Alkali Salts on
    Beets._—The marked differences which may occur as the
    result of even slight variations in the proportions of the
    several salts is well illustrated in the subjoined diagram
    of observations made by Dr. G. W. Shaw, of the Cal. Expt.
    station, upon beet fields in the neighborhood of Oxnard,
    Cal. The lands lie not far from the seashore, and saline
    water underruns them for considerable distance inland.
    The soil and subsoil are quite sandy, so that it takes
    irrigation water only about seven hours to penetrate from the
    surface to bottom water at seven feet depth. The land on
    which these observations were made are apparently level to
    the eye, though probably the alkali belts on which the sugar
    beets were “poor” are slightly depressed swales.

It will be noted that here the beets were “good” where the sulfate
(Glauber’s salt) ranged up to .8%, with .10 to .20 of common salt; but
that so soon as the latter rose above .20, the beets were poor despite
the low percentage of Glauber’s salt; then became “good” again so
soon as the common salt fell below .20%, although the Glauber’s salt
increased.


TOLERANCE OF VARIOUS CROP PLANTS.

The following table, compiled by Dr. R. H. Loughridge mainly from his
own observations,[176] gives the details of the tolerance for various
culture plants as ascertained at the several experiment substations in
California, as well as at other points in that State and in Arizona
where critical cases could be found. It is thought preferable to
investigate analytically such cases in the field, rather than to
attempt to obtain results from small-scale experiments artificially
arranged, in which sources of error arising from evaporation and other
causes are most difficult to avoid.

[176] Bulletins Nos. 128, 133 and 140, Calif. Expt. Station.

    The table is so arranged as to show the maximum tolerance
    thus far observed for each of the three single ingredients,
    as well as the maximum of total salts found compatible with
    good growth. In view of the extremely variable proportions
    between the three chief ingredients found in nature, this
    seems to be the only manner in which the observations made
    can be intelligibly presented, until perhaps a great number
    of such data shall enable us to evolve mathematical formulæ
    expressing the tolerance for the possible mixtures for
    each plant. For it is certain that the tolerance-figures
    will be quite different in presence of other salts, from
    those that would be obtained for each salt separately; or
    for the calculated mean of such separate determinations,
    proportionally pro-rated. It must also be remembered that in
    all alkali soils, lime carbonate is abundantly present, as
    is, nearly always, a greater or less amount of the sulfate
    (gypsum). As already stated, according to the investigations
    of Cameron not only these compounds, but also calcium
    chlorid, exert a protective influence against the injury to
    plant growth from compounds of sodium and potassium. The
    figures here given can therefore be regarded only as
    approximations, subject to correction by farther observation.
    They are arranged from the highest tolerances downward, for
    each of the three ingredients, as well as for the totals. The
    latter are not, of course, the sums of the figures given in
    the preceding columns, but independent data.

          HIGHEST AMOUNT OF ALKALI IN WHICH FRUIT TREES
                   WERE FOUND UNAFFECTED.[177]

    Arranged from highest to lowest. Pounds per acre in four feet depth.

    =================+===============+================+=================
        Sulfates     |   Carbonate   |     Chlorid    |  Total Alkali.
    (Glauber’s Salt).|  (Salsoda).   |  (Common Salt).|
    -----------------+---------------+----------------+-----------------
    Grapes     40,800| Grapes   7,550| Grapes    9,640| Grapes    45,700
    Olives     30,640| Oranges  3,840| Olives    6,640| Olives    40,160
    Figs       24,480| Olives   2,880| Oranges   3,360| Almonds   25,560
    Almonds    22,720| Pears    1,760| Almonds   2,400| Figs      26,400
    Oranges    18,600| Almonds  1,440| Mulberry  2,240| Oranges   21,840
    Pears      17,800| Prunes   1,360| Pears     1,360| Pears     20,920
    Apples     14,240| Figs     1,120| Apples    1,240| Apples    16,120
    Peaches     9,600| Peaches    680| Prunes    1,200| Prunes    11,800
    Prunes      9,240| Apples     640| Peaches   1,000| Peaches   11,280
    Apricots    8,640| Apricots   480| Apricots    960| Apricots  10,080
    Lemons      4,480| Lemons     480| Lemons      800| Lemons     5,760
    Mulberry    3,360| Mulberry   160| Figs        800| Mulberry   5,760
    -----------------+---------------+----------------+-----------------

[177] The several columns of figures are independent of each other; the
“total” alkali is not the summation for the three salts in the same
line.

                            OTHER TREES.
    ---------------------+-------------------+--------------------
    Kölreuteria    51,040| Kölreuteria  9,920| Or. Sycamore  20,320
    Eucal. am.     34,720| Or. Sycamore 3,200| Kölreuteria   12,640
    Or. Sycamore   19,240| Date Palm.   2,800| Eucal. am.     2,960
    Wash. Palm     13,040| Eucal. am.   2,720| Camph. Tree    1,420
    Date Palm       5,500| Wash. Palm   1,200| Wash. Palm     1,040
    Camph. Tree     5,280| Camph. Tree    320|
    ---------------------+-------------------+--------------------
    Kölreuteria    73,600
    Or. Sycamore.  42,760
    Eucal. am.     40,400
    Wash. Palm     15,200
    Date Palm       8,328
    Camph. Tree     7,020
    ---------------------

                                SMALL CULTURES.
    ---------------------+-------------------+---------------------
    Saltbush       125,640| Saltbush    18,560| Modiola       40,860
    Alfalfa, old   102,480| Barley      12,170| Saltbush      12,520
    Alfalfa, young  11,120| Bur Clover  11,300| Sorghum        9,680
    Hairy Vetch     63,720| Sorghum      9,840| Celery         9,600
    Sorghum         61,840| Radish       8,720| Onions         5,810
    Sugar Beet      52,640| Modiola      4,760| Potatoes       5,810
    Sunflower       52,640| Sugar Beet   4,000| Sunflower      5,440
    Radish          51,880| Gluten Wheat 3,000| Sugar Beet[178] 10,240
    Artichoke       38,720| Artichoke    2,760| Barley         5,100
    Carrot          24,880| Lupin        2,720| Hairy Vetch    3,160
    Gluten Wheat    20,960| Hairy Vetch  2,480| Lupin          3,040
    Wheat           15,120| Alfalfa      2,360| Carrot         2,360
    Barley          12,020| Grasses      2,300| Radish         2,240
    Goat’s Rue      10,880| Kaffir Corn  1,800| Rye            1,720
    Rye              9,800| Sweet Corn   1,800| Artichoke      1,480
    Cañaigre         9,160| Sunflower    1,760| Gluten Wheat   1,480
    Ray Grass        6,920| Wheat        1,480| Wheat          1,160
    Modiola          6,800| Carrot       1,240| Grasses        1,000
    Bur Clover       5,700| Rye            960| White Melilot    440
    Lupin            5,440| Goat’s Rue     760| Goat’s Rue       160
    White Melilot    4,920| White Melilot  480| Cañaigre          80
    Celery           4,080| Cañaigre       120|
    Saltgrass       44,000| Saltgrass  136,270| Saltgrass     70,360
    ----------------------+-------------------+---------------------
    Saltbush       156,720
    Alfalfa, old   110,320
    Alfalfa, young  13,120
    Sorghum         81,360
    Hairy Vetch     69,360
    Radish          62,840
    Sunflower       59,840
    Sugar Beet      59,840
    Modiola         52,420
    Artichoke       42,960
    Carrot          28,480
    Barley          25,520
    Gluten Wheat    24,320
    Wheat           17,280
    Bur Clover      17,000
    Celery          13,680
    Rye             12,480
    Goat’s Rue      11,800
    Lupin           11,200
    Cañaigre         9,360
    Onions          38,480
    Potatoes        38,480
    Saltgrass      381,110
    ----------------------

[178] Figures taken from Bulletin 169, Calif. Expt. Station, June,
1905.

_Comments on the Above Table._—Considering in this table, first, the
plants suitable for the stronger class of alkali lands, it may be
said generally that the search for widely acceptable kinds has not
been very successful. It is true that cattle will nibble green salt
grass (_Distichlis spicata_), but will soon leave it for any dry
feed that may be within reach. The enormous amount of salts which
it will tolerate in the soil on which it grows, and the doubtless
correspondingly large amount of those salts which it will absorb,
judging from its taste, sufficiently explain the reluctance of cattle
to feed on it to any considerable extent.

The same is true of all the fleshy plants that grow on the stronger
alkali lands, and are known under the general designation of “alkali
weeds.” When stock unaccustomed to it are forced by hunger to feed on
such vegetation to any considerable extent, disordered digestion is
apt to result; which in such ranges, however, is often counteracted by
feeding on aromatic or astringent antidotes, such as the gray sagebrush
and the more or less resinous herbage of plants of the sunflower family.

In the Great Basin region, lying between the Sierra Nevada and the
front range of the Rocky Mountains, there are, aside from the grasses,
numerous herbaceous and shrubby plants that afford valuable pasturage
for stock,[179] and some of these grow on moderately strong alkali
land; the same is true in California. It is quite possible that some of
these will be found to lend themselves to ready propagation for culture
purposes as well as they do for restocking the ranges. But thus far
none have found wider acceptance, probably because their stiff branches
and upright habit render them inconvenient to handle. It will require
more extended experience and experiment before any of these will be
definitely adopted for propagation by farmers and stockmen.

[179] See Bulletin No. 16 of the Wyoming Experiment Station; also
Bulletin Nos, 2 and 12 of the Division of Agrostology, and Farmers’
Bulletin No. 108, U. S. Department of Agriculture.


_Saltbushes, and Herbaceous Crops._

_Australian Saltbushes._—Experience in California indicates that in the
more southerly portion of the arid region, unpalatable native plants
may be largely replaced, even on the ranges, by one or more species
of the Australian saltbushes (_Atriplex spp._), long ago recommended
by Baron von Mueller of Melbourne; of which one (_A. semibaccata_)
has proved eminently adapted to the climate and soil of California
and is readily eaten by all kinds of stock. The facility with which
it is propagated, its quick development, the large amount of feed
yielded on a given area, even on the strongest alkali land ordinarily
found, and its thin, flexible stems, permitting it to be handled very
much like alfalfa, seem to commend it especially to the farmers’
consideration wherever better forage plants cannot be grown and the
climate will permit of its use. It does not, however, resist the severe
cold of the interior plateau country, and is wholly out of place in
the Pacific Coast region where summer fogs prevail. Most of the other
Australian species have an upright, shrubby habit, which adapts them
better to browsing than to pasture proper. The same is true of the
Argentine species (_A. Cachiyuyum_), which in its native pampas is
highly esteemed for that purpose, and succeeds well in California.
Of other Australian saltbushes, _A. halimoides_, _vesicaria_ and
_leptocarpa_ are the most promising; the latter is somewhat similar in
habit to the semibaccata, but is not as vigorous a grower. Since some
of the saltbushes take up nearly one fifth of their dry weight of ash
ingredients,[180] largely common salt, the complete removal from the
land of a five-ton crop of saltbush hay will take away nearly a ton of
the alkali salts per acre. This will in the course of some years be
quite sufficient to reduce materially the saline contents of the land,
and will frequently render possible the culture of ordinary crops.

[180] Analyses made at the California station show 19.37 percent of ash
in the air-dry matter of Australian saltbush. (See California Station
Bulletin No. 105; E. S. R., vol. 6, p. 718). Analyses of Russian
thistle have been reported showing over 20 per cent of ash in dry
matter. (See Minnesota Sta. Bulletin No. 34; Iowa Sta. Bull. No. 26; E.
S. R., vol. 6, pp. 552-553).

_Modiola._—Alongside of the saltbushes, the Chilean plant _Modiola
procumbens_, now generally known as modiola simply, deserves attention,
as it makes acceptable pasture where alfalfa fails to make a stand on
account of alkali. It is a trailing plant with medium-sized, roundish
foliage, and roots freely at the joints where they touch the ground.
Unlike the saltbushes it is therefore a formidable weed where it is not
wanted; but as according to California experience it resists as much as
52,000 pounds of salts per acre, even when 41,000 of these is common
salt, it is likely to be useful in many cases, particularly as an
admixture to a saltbush diet for stock, as it does not absorb as much
salt as the latter. It seems best adapted to pasturage.

As the table shows that, once grown to the age of a few years, alfalfa
will resist a percentage of alkali next to the saltbush, it will
generally be worth while, in lands otherwise adapted to alfalfa, to
prepare the land by leaching-down (see above) so as to secure a stand
of the more valuable crop.

_Native Grasses._[181]—Of all known plants that stock will eat somewhat
freely, the tussock grass (_Sporobolus airoides_, of which a figure
is given farther on), a native of the southern arid region, endures
the largest amounts of alkali; having been found growing well on land
containing the enormous amount of nearly half a million pounds of salts
per acre, although it will thrive with only 49,000 pounds in the soil.
What it will do under cultivation has never been fairly tested; but
its bare tussocks, killed by the excessive browsing of stock, testify
to its acceptableness as forage. It does not seem to absorb excessive
amounts of salts.

[181] It should be understood that the plants so referred to are
exclusively the _true_ grasses, recognized as such by every child,
and not forage plants generally; which are sometimes so designated;
not only by farmers, but by some authors who fail to appreciate the
practical importance of the distinction, which makes it necessary that
farmers should be taught to understand it.

Aside from the alkali grass proper (_Distichlis_), mentioned above,
the so-called rye grass of the Northwest (_Elymus condensatus_) is
probably, next to the tussock grass, the most resistant species among
the wild grasses. Its southern form, with several others not positively
identified, occupies largely the milder alkali lands of southern
California. This grass, though rather coarse, is regularly cut for hay
in the low grounds of Oregon and Washington.

Doubtless some of the indigenous grasses of the interior plateau
region and of the great plains east of the Rocky Mountains, such as
the buffalo and grama grasses, as well as several of the wheat grasses
(_Agropyron_) and bunch grasses (_Festuca_, _Poa_, _Stipa_, etc.) will
prove resistant to larger proportions of alkali than the meadow and
pasture grasses of the regions of summer rains.

_Cultivated Grasses._—The superficial rooting and fine fibrous roots
of the true annual grasses render them, as a whole, rather sensitive
to alkali; yet the cereals—barley, wheat, rye and oats—resist, as the
table shows, the average alkali salts to the extent of from 17,000
total salts, with not exceeding 1500 pounds of carbonate, in the case
of the more delicate varieties of wheat, to over 25,000 pounds per acre
in the case of barley, which with the gluten wheats and rye seems to
have the highest tolerance-figure. The special adaptation of gluten
wheats to arid conditions is thus emphasized. The roots of these
cereals are comparatively stout, with thick epidermis.

Among the cultivated forage grasses proper, the Australian variety of
the English ray (generally miscalled rye) grass seems most resistant.
The eastern fescues, Kentucky blue grass, and others at home in the
humid region are easily injured, as those who try to maintain lawns
on alkali-tainted lands, or by irrigation with alkali waters, know
to their sorrow. To these grasses common salt and bittern (magnesium
chlorid) seem to be particularly injurious, and they tolerate but
little “black alkali.”

On the rather close-textured soil at Chino, California, the loliums,
including the darnel (“California cheat”), and the Australian and
Italian ray (“rye”) grasses, succeed fairly on land containing as much
as 6,000 pounds of (white) salts. Most other cultivated grasses failed
conspicuously alongside of these. It must be remembered that in more
loose-textured, sandy lands than those in which these tests were made,
the above figures for tolerance would probably be increased by 30
percent or more.

_Maize_ is rather sensitive to alkali, and suffers even on slightly
alkaline land, owing doubtless to the large development of fine
white rootlets near the surface, so familiar to corn-growers. The
_Sorghums_, and especially Egyptian corn (durra) are much less
sensitive, as the table shows, and are among the first crops to be
tried on alkali lands. The related millets share this resistance more
or less, and we often see on cultivated lands in the alkali region fine
stands of barnyard grass (_Panicum crusgalli_) of which the variety (?)
_P. muticum_ is said by observers of the U. S. Dept. of Agriculture
to be specially resistant, and acceptable to stock. One of the most
successful grasses on the light alkali lands near Chino, where most
of the commonly cultivated grasses fail, was a near relative of the
barnyard grass, the _Eleusine coracana_, which produces heavy crops
of a millet-like grain much relished by poultry, and also by stock.
This grass, largely grown in Egypt, has succeeded well all over the
ground whose alkali content ranges up to 12,000 pounds per acre, but
failed where the salts reached 38,840 pounds in the surface foot.
Next to this, in point of success, were the pearl millet (_Pennisetum
typhoideum_) and teosinte, Hungarian brome grass, and Japanese millet,
on land containing about 9,000 pounds of (chiefly “white”) salts per
acre.

_Other Herbaceous Crops. Legumes._—Both the natural growth of alkali
lands and experimental tests seem to show that this entire family
(peas, beans, clovers, etc.) are among the more sensitive and least
available wherever black alkali exists; while fairly tolerant of the
white (neutral) salts. Apparently a very little salsoda suffices to
destroy the tubercle-forming organisms that are so important a medium
of nitrogen-nutrition in these plants. Excepting the melilots, alfalfa
with its hard, stout and long taproot, seems to resist best of all
these plants.

    As a general thing, taprooted plants, when once established,
    resist best, for the obvious reason that the main mass of
    their feeding roots reaches below the danger level. Another
    favoring condition, already alluded to, is heavy foliage and
    consequent shading of the ground; alfalfa happens to combine
    both of these advantages. There has been some difficulty
    in obtaining a full stand of alfalfa in the portion of the
    Chino substation tract containing from 4000 to 6000 pounds
    of (largely black) alkali salts per acre; but once obtained,
    it has done very well.

The only other plant of this family that succeeds well on this land,
and even (at Tulare) on soil considerably stronger (probably between
20,000 and 30,000 pounds) are the two melilots, _M. indica_, and
_alba_; the latter (the Bokhara clover) is a forage plant of no mean
value in moist climates, but somewhat restricted in its use in the
arid region because of the very high aroma it develops, especially
in alkali lands; so that stock will eat only limited amounts, best
when intermixed with other forage, such as the saltbushes. The yellow
melilot is highly recommended by the Arizona Experiment Station as
a green-manure plant for winter growth; but farther north it is a
summer-growing plant only, and is refused by stock. As already stated,
very few plants belonging to this family are naturally found on alkali
lands, and attempts to grow them, even where only Glauber’s salt is
present, have been but very moderately successful.

For most of the legumes the limit of full success seems to lie between
3000 and 4000 pounds to the acre. A marked exception, however, occurs
in the case of the hairy vetch, as shown in the table, where it is
credited, on the basis of repeated experiments, with a tolerance of
nearly 70,000 pounds. This amount was attained, however, in rather
sandy soils. Probably some of the Algerian vetches will likewise prove
more resistant than those which are natives of humid climates.

_Mustard Family._—As in the case of the legumes, wild plants of the
_mustard_ family are rare on alkali lands; and correspondingly, the
cultivated mustard, kale, rape, etc., fail even on land quite weak
in alkali. Their limit of tolerance seems to lie near 4,000 to 5,000
pounds per acre even of white salts. Hence turnips and radishes do not
flourish on alkali lands.

_Sunflower Family._—Several of the hardiest of the native “alkali
weeds” belong to the _sunflower_ family, and the common wild sunflowers
(_Helianthus californicus_ and _H. annuus_) are common on lands
pretty strongly alkaline. The cultivated Russian sunflower, as the
table shows, resists the effects of nearly 60,000 pounds of total
alkali, of which 52,640 pounds was sulfate (Glauber’s salt), and 5440
common salt. This, it will be seen, is a very high tolerance, so
that this sunflower, yielding such excellent poultry feed, is very
widely available. Correspondingly, the “Jerusalem artichoke,” itself
a sunflower, is among the available crops on moderately strong alkali
soils; and so, doubtless, are other members of the same relationship
not yet tested, such as the true artichoke, salsify, etc. Chicory,
belonging to the same family, yielded roots at the rate of twelve tons
per acre, on land of the Chino tract containing about 8,000 pounds of
salts per acre.

_Root Crops._—It seems to be generally true that root crops suffer in
quality, however satisfactory may be the quantity, harvested on lands
rich in salts, and especially in chlorids (common salt). It was noted
at the Tulare substation (California) that the tubers of the artichoke
were inclined to be “squashy” in the stronger alkali land, and failed
to keep well; the same was true of potatoes, which were very watery;
and also of turnips and carrots. It is a fact well known in Europe,
that potatoes manured with kainit (chlorids of potassium and sodium)
are unfit for the manufacture of starch, and are generally of inferior
quality. But this is found not to be the case when, instead of the
chlorids, the sulfate is used; hence the advice, often repeated by the
California station, that farmers desiring to use potash fertilizers
should call for the “high-grade sulfate” instead of the cheaper
kainit, which adds to the injurious salts already so commonly present
in lowland soils of the arid region. Such root crops are, however,
available for stock feed.

The common _beet_ (including the mangel-wurzel) is known to succeed
well on saline seashore lands, and it maintains its reputation on
alkali lands also. Being especially tolerant of common salt, it may
be grown where other crops fail on this account; but the roots so
grown are strongly charged with common salt, and have, as is well
known, been used for the purpose of removing excess of the same from
sea-coast-marsh lands. Such roots are wholly unfit for sugar-making.

It is quite otherwise with Glauber’s salt (sodium sulfate); and as
this is very commonly predominant in alkali lands, either before or
after the gypsum treatment, this fact is of great importance, for it
frequently permits of the successful growing of the sugar beet; as
has been abundantly proved at the Chino ranch, where land containing
as much as 60,000 pounds of salts, mostly this compound, has yielded
roots of very high grade, both as to sugar percentage and purity. But
the analyses of the Oxnard soil show that more than 10,000 pounds of
common salt will be required to render sugar beets unsatisfactory for
sugar-making.

Passing to _stem crops_, we find that _asparagus_, originally itself a
denizen of the sea-board, resists considerable amounts (not yet exactly
determined) of common salt as well as of Glauber’s salt. It is even
claimed that when grown with a dressing of common salt the asparagus is
more tender and savory. But it is quite sensitive to “black alkali,”
which must be neutralized with gypsum to render it harmless.

_Celery_ did well with 13,640 pounds, of which nearly 10,000 was common
salt. But with 30,000 pounds the plants were killed.

_Rhubarb_ was a conspicuous failure, even in the weak and mostly
“white” alkali lands of the Chino station tract.

_Textile Plants._—_Japanese hemp_, while young, seemed to have a
hard struggle with the alkali, but at the end of the season stood
eight feet high. The _ramie_ plant, also, will bear moderately strong
alkali, apparently somewhat over 12,000 pounds per acre. _Flax_ has
not been tested in cultivation; but the wide distribution of wild flax
all over the arid portions of the States of Oregon and Washington,
would seem to indicate that it is not very sensitive. Another textile
plant, the Indian mallow (_Abutilon avicennae_), was found to fail
on the Chino alkali soil. But its close relative, cotton, does not
seem to be specially sensitive, according to the experience had with
it in the Merced river bottom in California; and its culture is
extensive in Egypt, where no particular care seems to be exercised in
selecting the land for the crop. It is just possible that the saline
content of the soil has in California, as well as in the Atlantic
sea-islands, contributed to the superior length of the fiber shown in
the measurements made during the Census work of 1880.[182]

[182] Report on Cotton Culture; 10th Census of the United States, vol.
5, pp. 23 to 34.


_Tolerance of Shrubs and Trees._

_Grapevines._—The European grape, _Vitis vinifera_, is quite tolerant
of white or neutral alkali salts, and will resist even a moderate
amount of the black so long as no hardpan is allowed to form. At
the Tulare substation it was found that grapevines did well in sandy
land containing 35,230 pounds of alkali salts, of which one half was
Glauber’s salt, 9,640 pounds carbonate of soda, 7,550 pounds of common
salt, and 750 pounds nitrate of soda. They were badly distressed
where, of a total of 37,020 pounds of alkali salts, 25,620 pounds
was carbonate of soda; while where the vines had died out, there was
found a total of 73,930 pounds, with 37,280 pounds of carbonate. The
European vine, then, is considerably more resistant of alkali even in
its worst (black) form, than barley and rye, at least on sandy land;
and it seems likely that the native grapevines of the Pacific coast,
_californica_, and _arizonica_, would resist even better; a point still
under experiment.

Experience, however, has shown that vines rapidly succumb when by
excessive irrigation the bottom water is allowed to rise, increasing
the amount of alkali salts near the surface, and shallowing the soil
at their disposal. Such over-irrigation has been a fruitful cause
of injury to vineyards in the Fresno region, and would doubtless if
practiced kill most of the vines at the Tulare substation, which are
now flourishing. In such cases, sometimes the formation of hardpan is
followed by that of a concentrated alkaline solution above it, strong
enough to corrode the roots themselves, and not only killing the vines,
but rendering the land unfit for any agricultural use whatsoever. The
swamping of alkali lands, whether of the white or black kind, is not
only fatal to their present productiveness, but, on account of the
strong chemical action thus induced, greatly jeopardizes their future
usefulness. Many costly investments in orchards and vineyards have thus
been rendered unproductive, or have even become a total loss.

It should be remembered in this connection that as the roots of vines
will, when unobstructed, go to depths of fifteen and even twenty feet,
a subsequent rise of the bottom water from leaky irrigation ditches
will drown out the ends of the deep roots and thus cause the whole root
system to become diseased, inevitably resulting in unproductiveness, if
not death, of the vine.

_Citrus Trees._—Although the high figure of nearly 27,000 pounds for
the tolerance of citrus trees, as given in the table, seems to place
them rather high on the list, such high tolerance actually occurs
only in very sandy soils, and when common salt is in small proportion.
Generally speaking, the citrus tribe are rather sensitive to alkali
salts, and more especially to common salt. In fact, as to the high
tolerance-figure given in the table, observed in sandy land, the
alkali there contained only a trace of common salt. Young seedling
trees are particularly sensitive; so that it is often difficult to
obtain a stand even when, later on, the feeding roots descend beyond
the reach of injury. In the close-textured lands of Chino, young trees
hardly maintained life with more than 5,000 pounds of total salts.
Near Riverside, full-grown trees perished under the influence of
bottom water containing 0.25%, or 146 grains of salt per gallon, which
impregnated the ground; corresponding to about 9,000 pounds per acre in
four feet.

In the sandy loam lands near Corona, trees eight years old suffered
severely when by irrigation with alkali-water the alkali-content of
the land reached 11,000 pounds per acre; as illustrated in Figs. Nos.
44, and 45. At another point in the same region, two representative
trees were selected for comparison, five rows apart on land absolutely
identical; one of these retained its leaves, though suffering, the
other was completely leafless. The leaching of the alkali to the depth
of four feet gave the following results, calculated to pounds per acre:

                 Sulfates.  Carbonates.  Chlorids.  Total.

    Poor tree     4,720       1,680        2,520    8,920
    Better tree   4,120       2,360          720    7,200

Here it is apparently the excess of common salt to which the difference
is due, and this despite the higher content of carbonate of soda in the
soil bearing the better tree.

On the other hand, at the Tulare substation orange trees (sour stock)
maintain vigorous growth and good bearing in a very sandy tract which
to the depth of seven feet showed an aggregate content of 26,840 pounds
of salts (or 22,780 to four feet depth); but which is never irrigated.
(See diagram No. 66). The salts in this case consists wholly of sulfate
and carbonate of soda in the ratio of fifty-four to forty-two, implying
the presence of nearly 12,000 pounds of salsoda within reach of the
tree roots; yet in the absence of common salt, no perceptible injury or
even stress upon the trees has been noted.

According to observations made in San Diego county, Calif., lemon trees
are even more sensitive to common salt than oranges, since a total
content of 8,000 pounds per acre, about one-third of which was common
salt, seemed to render the trees wholly unprofitable.

In view of these facts, showing that common salt is the portion of
alkali by far most injurious to citrus trees, great care should be
taken in the use of irrigation waters to exclude those charged with
that compound; and also to avoid locating citrus orchards on land
already impregnated with common salt.

The _olive_ tree, as the table shows, is among the most resistant to
alkali salts, approaching the grape in this respect. This might have
been anticipated from its extended culture in the arid regions of the
old world, including Palestine and northern Africa, where alkali lands
abound. It is probable that the figure given in the table does not yet
show the extreme limit of its endurance.

California experience with the _date palm_, as the table shows, credits
it with an endurance not exceeding 8320 pounds of total salts. This is
doubtless an underestimate, for in the Sahara desert and Egypt it is
credited with being the culture which will succeed in stronger alkali
than any other cultural plant; and, according to Mr. Means of the
United States Department of Agriculture, it is sometimes irrigated with
water containing as much as 200 grains of salts per gallon. It should
be remembered, however, that these trees always grow in very sandy
lands; and in the desert regions it is often grown below the surface
of the ground, so as to render it wholly independent of the alkali
accumulations on the surface. The extreme limit of its endurance must
therefore remain in doubt until more extended experiments have made
more definite data available.


_Deciduous Orchard Trees._

Among deciduous orchard trees, strangely enough, the _almond_ stands
alongside of the fig in alkali-resistance, as indicated in the table.
The _peach_ seems to be much more sensitive, ranking near the _apricot_
and _prune_, whose tolerance is less than half as high. That the
_pear_ and _apple_, generally counted among the more northern fruits
in the humid region, should excel these stone fruits in endurance of
alkali, is rather unexpected; and the figures concerning the whole
group of these rosaceous fruits admonish us that it is unsafe to
predict, without trial, what may be the outcome of culture tests. Thus
plum trees, apparently in good condition, sometimes suddenly begin to
fail when starting to bear; the fruit appears normal on the outside
for a time, but the pit fails to form, being at times flattened out
like a piece of pasteboard; and the fruit does not mature. Yet there
is no observable injury to the base of the trunk, or to the roots. On
the other hand, pears do well even when the outside bark around the
root-crown is blackened by the action of the alkali salts. But 38,000
pounds, even of sulfate, proves too much for the pear.

The _quince_ appears to be materially more resistant than the apple or
pear. It probably ranges alongside of the fig, the soil-adaptations of
which it shares in other respects also.

The _English walnut_ resents even a slight taint of _black_ alkali;
but is fairly tolerant of “white” salts, as is shown in the peculiarly
suitable light loam soils on the lower Santa Clara river, in Ventura
county, as well as in Orange county, California.

Close figures for the limits of alkali tolerance in the case of
deciduous orchard trees cannot easily be given or determined, owing to
the difficulties inherent in the differences of root penetration in the
several soils and localities; as well as the fact already alluded to,
that in close-textured soils the tolerance is in general decidedly less
than in sandy lands. Hence the figures in the table must be taken as
more nearly representing _relative_ tolerances, rather than absolute
data to be applied in every case. As regards the stone fruits, it
should be remembered that the Myrobalan root, being at home in Asia
Minor, where alkali abounds, should when practicable be used wherever
alkali conditions exist, in preference to all but the almond, which
seems to resist well, even on its own root, but has not as wide a range
of adaptations as a grafting stock as the myrobalan. While most of the
other stone fruits at the Tulare substation were on myrabalan roots,
the stock of those in outside orchards was mostly in doubt. It is also
to be kept in mind that different varieties of the same fruit—_e.
g._, pears and apples—show a not inconsiderable variation in their
resistance.


_Timber and Shade Trees._

Of trees, forest and shade, suitable for alkali lands, some native ones
call for mention. One is the California white or valley oak (_Quercus
lobata_), which forms a dense forest of large trees on the (almost
throughout somewhat alkaline) delta lands of the Kaweah River in
California, and is found scatteringly all over the San Joaquin Valley.
Unfortunately this tree does not supply timber valuable for aught but
firewood or fence posts, being quite brittle.

The native _cottonwoods_, while somewhat retarded and dwarfed in
their growth in strong alkali, are quite tolerant of the white salts,
especially of Glauber’s salt. As they usually grow near to the water,
their tolerance for alkali salts is difficult to ascertain.

Of other trees, the _oriental plane_, or sycamore, and the _black
locust_ have proved the most resistant in the alkali lands of the San
Joaquin Valley; and the former being a very desirable shade tree, it
should be widely used throughout the regions where alkali prevails more
or less. The _ailantus_ is about equally resistant, and but for the
evil odor of its flowers, deserves strong commendation.

Of the _eucalypts_, the narrow-leaved _Eucalyptus amygdalina_ (one of
the “red gums”) and the closely related _viminalis_, seem to be least
sensitive, and in some cases have grown in alkali lands as rapidly
as anywhere. The _rostrata_, as well as the pink flowered variety of
_sideroxylon_, are now doing about as well as the _amygdalina_ at
Tulare, where at first they seemed to suffer. The common blue gum,
_globulus_, is much more sensitive.

Of the _Acacias_, the tall-growing _A. melanoxylon_ (“black acacia”)
resists pretty strong alkali, even on stiff soil; as can be seen at
Tulare and Bakersfield, California, where there are trees nearly two
feet in diameter. The beautiful _A. lophantha_ (_Albizzia_) has in
plantings made along the San Joaquin Valley railroad shown considerable
resistance, likewise; but it is quite sensitive to frost.

Of other Australian trees, one of the Australian “pines,” (_Casuarina
equisetifolia_), is doing well on fairly strong alkali land in the San
Joaquin Valley.

A remarkably alkali-resistant shrub or small tree is the pretty
_Kœlreuteria paniculata_ from China, which at Tulare is growing in
some of the strongest alkali soil of the tract. Unfortunately it is
available mainly for ornamental purposes; its wood, while small, is
very hard and makes excellent fuel.

Of trees indigenous to the Atlantic and East Central United States,
the Tulip tree, the Linden, and most other trees of the humid region,
including the English oak (_Quercus pedunculata_) become stunted
in alkali soils. The _honey locust_, being particularly adapted to
calcareous lands, does moderately well on alkali lands, but its thorns
and imperfect shade render it not very desirable. The _black locust_
and the _elms_ have on the whole done best. The eastern maples are not
successful; but the California maple (_Acer macrophyllum_) and the box
elder (_Negundo californica_) have done fairly well in the lighter
alkali lands of the San Joaquin Valley.

The _Conifers_—Pines, firs, cedars, cypress, etc., are very sensitive
to black alkali and will not endure much even of the “white” salts.
Even the native juniper of the mesas carefully adheres to the
portions—breaks and upper slopes, hilltops, etc.—which are more or less
leached by the scanty rains of these regions.


INDUCEMENTS TOWARD THE RECLAMATION OF ALKALI LANDS.

The expense involved in the reclamation of strong alkali lands
naturally gives rise to the question whether adequate advantages are
likely to be derived from such expenditure; specially when the last
resort—underdraining and leaching—has to be adopted.

Those familiar with the alkali regions are aware how often the
occurrence of alkali spots interrupts the continuity of fields and
orchards, of which they form only a small part, but enough to mar their
aspect and cultivation. Their increase and expansion under irrigation
frequently renders their reclamation the only alternative of absolute
abandonment of the investments and improvements made, and from that
point of view alone it is of no slight practical importance. Moreover,
the occurrence of vast continuous stretches of alkali lands within the
otherwise most eligibly situated valley lands of the irrigation region
forms a strong incentive towards their utilization.

[Illustration: 1st year, 2d year, 3d year, Fourth year—42 bushels.

FIG. 75.—Wheat grown on black alkali land at Tulare Substation,
California, showing improvement in successive years of reclamation
treatment.]

There is, however, a strong intrinsic reason pointing in the
same direction, namely, the almost invariably high and lasting
productiveness of these lands when once rendered available to
agriculture. This is foreshadowed by the usually heavy and luxuriant
growth of native plants around the margins and between alkali spots
(see fig. 60); _i. e._, wherever the amount of injurious salts
present is so small as not to interfere with the utilization of the
abundant store of plant-food which, under the peculiar conditions of
soil-formation in arid climates, remains in the land instead of being
washed into the ocean. Extended comparative investigations of soil
composition, as well as the experience of thousands of years in the
oldest settled countries of the world, demonstrate this fact and show
that so far from being in need of fertilization, alkali lands usually
possess extraordinary productive capacity whenever freed from the
injurious influence of the excess of useless salts left in the soil in
consequence of deficient rainfall. (See analyses, chapter 22, pp. 436,
437).

[Illustration: FIG. 76.—Grains grown on alkali land at Tulare Station,
California.]

Among many striking examples of the results of such reclamation, is
that represented in the annexed figure (75), of grain grown on strong
alkali land, before and after reclamation treatment. On the original
land even “alkali weeds” would hardly grow; while afterward a wheat
crop representing forty-two bushels per acre was grown. Additional
illustrations are shown in the second figure (76), showing crops of
wheat and barley as grown on partly reclaimed land at the Tulare
substation.

While it is certainly true that when rightly treated, alkali lands can
be rendered profusely and lastingly productive, yet close attention and
constant vigilance are needed so long as the salts remain in the soil;
and no one not determined to give such land such full attention, should
undertake to cultivate it.




PART FOURTH.

SOILS AND NATIVE VEGETATION.




CHAPTER XXIV.[183]

THE RECOGNITION OF CHARACTER OF SOILS FROM THEIR NATIVE VEGETATION;
MISSISSIPPI.


_Climatic and Soil-Conditions._—Next to climatic conditions, chief
among which are temperature and moisture, the physical and chemical
nature of the soil and subsoil is the most potent factor in determining
the natural vegetation of any region. The limitations we observe in the
adaptation of cultivated lands to certain crops, even with artificial
help, must be much more strongly pronounced when no such aid is given,
and the struggle for the survival of the fittest is continued, subject
only to seasonal variations, for thousands of years. It is obvious
that within the limits of the regional flora, _the natural vegetation
of any tract represents the best adaptation of plants to soils, in the
results of long periods of the struggle for existence between competing
species_; the survivors being those best adapted to the entire
environment.

[183] The special object of this chapter as a whole has seemed to the
writer to require a repetition of much that is already said in the
preceding chapters.

In countries uninhabited by man the chief conditions outside of the
direct influence of climate and soil that may materially affect the
results of the competition are connected with the animal creation;
and within the latter, insects are probably the most influential,
beneficially in the part they play in the fertilization of flowers,
injuriously in their role as parasites. Since in the absence of man,
the effects of fire would ordinarily be conditioned upon the occurrence
of thunderstorms, its effects would then properly come under the head
of climatic influences. But while these and some other disturbing
factors must not be forgotten in considering the relations of soils to
the natural vegetation borne by them, the common consensus of mankind
has long recognized the intimate connection existing between the two,
and has everywhere made it the basis of at least a general estimate of
the agricultural value of the land concerned.


NATURAL VEGETATION THE BASIS OF AGRICULTURAL LAND VALUES IN THE UNITED
STATES.[184]

In countries long settled, as in Europe, where the nature of the
original forest is unknown or a matter of tradition only, the
adaptations of the several kinds of land to culture plants and forest
trees has been gradually ascertained by cultural experience, and
their designations, values and uses determined accordingly. In the
United States, the character of the original forest growth is mostly
in evidence, or is definitely known by tradition, even in the older
states. West of the Alleghenies, there is as yet little difficulty in
this regard, partly because even where the original forest growth has
disappeared its character remains on record, the assessed land values
being very commonly based upon the tree growth of the wild land. In
the Southern States especially, the classification of uplands into
“pine lands” and “oak lands” is universal, and is associated with
certain limits of valuation, both by assessors and purchasers. Within
each of these two classes, however, there are well-defined gradations
of cultural value according to the kind (species) _e. g._, of pine or
oak that occupies the ground, either alone, or in intermixture with
other trees whose presence or absence is considered significant. In
the case of “bottoms” or alluvial lands, corresponding distinctions
and classifications obtain; we hear of hickory, beech, gum, and cherry
bottoms, hackberry hammocks, etc. each name being associated with
certain cultural values or peculiarities of soil, well understood by
the farming population.

[184] See above, pp. 313 to 315, chapter 18.


INVESTIGATION OF CAUSES GOVERNING THE DISTRIBUTION OF NATIVE VEGETATION.

It seems singular that such well and widely understood designations
and important distinctions should not long ago have been made the
subject of careful investigation and precise definition by agricultural
investigators. For apart from their practical importance as guides to
the purchaser of land, or settler, this correlation of land-values and
natural vegetation is of the utmost interest in offering an opportunity
for researches on the factors which determine the choice of these
several trees and the corresponding shrubby and herbaceous growths.
Moreover, the cultural results and adaptations corresponding to certain
natural growths being known from experience, a thorough knowledge
of the soils so characterized should enable us to project into new
lands, where experience is lacking, the benefits of experience already
had; even in cases where, from some cause, the natural vegetation is
different, or absent. Only very fragmentary and casual observations
in this line are on record thus far, almost the only generally
recognized chemical characterization of plant habit being that of
calciphile (lime-loving), and calcifuge (lime-repelled) ones, but with
few attempts at more than local application. Yet, to ascertain by
the physical and chemical examination of soils what are determining
factors of certain natural vegetative preferences, which are invariably
followed by certain agricultural results, should not be an unsolvable
problem, and its practical importance should justify its most active
investigation.

_Investigations in Mississippi._—In his explorations connected with
the Geological and Agricultural Survey of the State of Mississippi, as
well as, later on, in similar researches carried on in other states,
the writer was forcibly struck with the close correspondence of the
limits of geological formations with those of vegetative zones; so much
so that he was led to rely very largely on the latter as indicative of
the probable occurrence of outcrops that otherwise, in a level country,
would have passed unperceived.

These observations upon the correlations between virgin soils and their
native vegetation having originally been made by the writer, in great
detail, in the state of Mississippi, from 1855 to 1872, and that state
being from natural causes a peculiarly cogent illustration of such
correlation: it seems advisable to describe first, somewhat in detail,
the facts observed there, and subsequently to compare them with what
has been observed elsewhere by him or others.

No claim is made to an even approximately exhaustive presentation of
the whole subject, even within the United States; nor is it intended
to give complete lists of vegetation.[185] The object is to give such
facts as have been fairly well established by observation, hoping
that more thorough investigations in the same line will thereby be
stimulated.

[185] Such lists, so far as the State of Mississippi is concerned,
may be found in the writer’s Report on the Agriculture and Geology of
Mississippi, 1860. See also Plant Life of Alabama, by Charles Mohr.


VEGETATIVE BELTS IN NORTHERN MISSISSIPPI.

The diagram below is a sketch-map of the most northern part of
Mississippi, showing the narrow parallel belts of successive geological
formations or terranes running north and south, which bear the varying
zones of vegetation characteristic of each one, as indicated in the
legend beneath.

[Illustration: FIG. 77.—Sketch map of Soil Belts in Northern
Mississippi, east and west.]

        SOIL REGIONS OF NORTHERN MISSISSIPPI, SHOWING CHANGES
          FROM EAST TO WEST, AND LIME PERCENTAGES IN SOILS.

       |Lime, p.c.|    Soil Character.|       Vegetation.
       |          |                   |
    1. | .40— .60|Clay loams, clay.   |Oaks, sweet gum, tulip tree,
       |          |                   | walnut, red cedar, ash,
       |          |                   | hickories.
       |          |                   |
    2. | .05— .14|Sandy loams, sands.|Short-leaf pine, post, scarlet
       |          |                   | and black-jack oaks, black gum,
       |          |                   | chestnut.
       |          |                   |
    3. |1.00—1.40|“White lime”        |Red cedar, crab-apple, Chickasaw
       |          |prairie; clays and | plum, sturdy post and black-jack
       |          |clay loams.        | oaks, honey locust.
       |          |                   |
    4. | .30— .50 |Mellow red loams of|Oaks, hickories, walnut, tulip
       |          |“Pontotoc ridge.”  | tree, ash, cherry, umbrella
       |          |                   | tree.
       |          |                   |
    5. | .08— .18 |Heavy gray clay    |Scrubby post and black-jack oak,
       |          |soils, some gray   | short-leaf pine.
       |          |sands. “Flatwoods.”|
       |          |                   |
    6. | .15— .25|Sandy ridges and   |Post, black-jack, scarlet and
       |          |uplands, broken.   | upland willow oaks, small; some
       |          |                   | chestnut.
       |          |                   |
    7. | .25— .35 |Mellow clay loams  |Fine black, red, post, Spanish
       |          |of “Table lands.”  | and black-jack oaks, hickories,
       |          |                   | sweet gum.
       |          |                   |
    8. |2.00—5.00|Calcareous sandy    |Oaks as above, tulip tree, ash,
       |          |silt, “Bluff loess”| honey locust, linden, sassafras,
       |          |“Cane Hills.”      | umbrella tree, cane.
    ---+----------+-------------------+---------------------------------
    9. | .40—1.10 |Mississippi Bottom.|Basket, white and black oaks,
       |          |                   | ash, tulip tree'
    9a.| 1.12—    |Yazoo backland     |honey locust, pecan, shell-bark
       |          |buckshot clay.     | hickory, walnut, hackberry,
       |          |                   | cane.
    9b.| .40      |Sandy alluvium,    |Sweet gum, maple, willow oak,
       |          |“Frontland.”       | elm, hackberry.
    ---+----------+-------------------+---------------------------------
    10.| .26— .40 |Light sandy loam of|Dogwood, sweet gum, holly, ash,
       |          | “Dogwood ridge.”  | sassafras, prickly pear.
       |          |                   |
    ---+----------+-------------------+---------------------------------

    _Limestone Belt._—beginning on the east we have, first,
    a narrow belt of limestones of the carboniferous formation,
    on which there is a fine growth of various oaks, with walnut,
    hickory, sweet gum, tulip tree and red cedar, and a very
    productive soil.

    “_Pine Hills._”—Next adjoining on the west comes a
    belt of sandy, non-calcareous beds of the lower Cretaceous
    formation, about 18 miles wide. It has a hilly surface,
    and outside of the narrow valleys, the prevalent timber is
    short-leaved pine and scrubby black-jack oak, with some post
    oak and small black gum, and a few large chestnut trees.

    _“Prairie” Belt._—Westward of this belt we descend
    into a level “prairie” region, six to twelve miles wide;
    the “white lime country,” having heavy black clay soils,
    underlaid by the cretaceous “rotten limestones;” which are
    profusely productive. The sparse tree growth consists of
    stout, vigorous and dense-topped post and black-jack oaks,
    with clumps of crab-apple, Chickasaw plum thickets, and an
    occasional red cedar.

    _Pontotoc Ridge._—West of the prairie belt we ascend
    into a ridgy hill country, twelve to fourteen miles wide;
    the “Pontotoc ridge,” formed of the soft limestones and
    marls of the upper cretaceous formation, and covered with
    a deep red soil, which bears a rich growth of oaks, with
    hickory interspersed, and black walnut, umbrella and
    tulip tree even on the ridges. This is one of the finest
    agricultural regions of the State.

    _Flatwoods._—From the Pontotoc ridge and its fine
    lands and timber we descend to westward into the “Flatwoods”
    belt, three to eight miles wide; a level country underlaid
    by heavy gray non-calcareous clays of the tertiary
    formation, from which most of its soil is directly formed.
    It bears a pretty dense growth of the same species of oaks
    that characterize the prairies farther east, but the form,
    habit and size of the trees is so different that many of
    the inhabitants believe them to be different species. The
    black-jack oak looks like small, dense-topped apple trees;
    the post oak, on the contrary, has an open top of the form
    of a short-handled, spreading broom. The soil is poor and
    unthrifty, as are the few disappointed settlers, who bought
    the land on the strength of its oak-tree growth. (See page
    500).

    _Brown Loam Region. Table Lands._—Adjoining the
    Flatwoods on the west is a broad upland region, with a
    brownish-yellow soil and subsoil, extending nearly to the
    edge of the Mississippi bottom. In its eastern portion
    it is rather broken and hilly, with sandy ridge soils, a
    mixed growth of oaks and short-leaved pine, and occasional
    chestnuts; a fair farming country only. To westward the
    ridges become lower and broader, assuming a plateau
    character. The pine disappears, and black, Spanish, red and
    white oak, with much hickory, largely replaces the black
    jack and post oak; thus characterizing the fertile brown-loam
    “table-lands” that extend through western Tennessee and
    Mississippi into Louisiana, and have long been noted for
    their high production of fine upland cotton.

    _Cane Hills._—On the western border of the table-land
    region, and here forming a strip only a few miles wide along
    the edge of the Mississippi bottom, but from 70 to 450 feet
    above it, lies the remnant of what farther south constitutes
    a wide and important agricultural belt; the Bluff or Loess
    formation, locally known as “the Cane Hills.” The soil is
    largely composed of grains of sand and silt cemented by
    lime carbonate; it is therefore calcareous, and as on the
    Pontotoc ridge, described above, we find here the black
    walnut, the tulip tree, ash and others, elsewhere restricted
    to the alluvial “bottoms,” on the ridges themselves, from
    sixty to a hundred feet above the stream beds.

    _Mississippi Bottom._—At the western foot of this
    bluff there lies the great Mississippi Bottom, with its
    rich soils and varied forest growth. This also, however,
    subdivides into at least three distinct soil and vegetative
    zones, viz., the sandy “Frontlands,” which lie on the
    immediate banks of the great river and its main branches,
    and the heavy clayey “Back-land” areas, whose soils are
    partly the product of modern swamp deposits from backwaters,
    partly result from the disintegration of strongly calcareous
    clays constituting the lower part of the Bluff or Loess
    formation. A third natural subdivision is the “Dogwood
    ridge,” a narrow belt of slightly elevated land, mostly
    above ordinary overflows, which extends diagonally from the
    Mississippi river to the Yazoo bottom, and seems to be the
    continuation of “Crowleys ridge” in Arkansas. Each of these
    soil belts has its own characteristic forest growth, as
    indicated in the table below the map.

We have here along an east-and-west line of about 200 miles, eleven
markedly distinct zones of vegetation, readily recognized as such by
every farmer, and each underlaid by a distinct geological terrane. It
does seem as though a close study of these and of the soils overlying
them should lead to some definite results showing the physico-chemical
causes of these differences.

_Lime apparently a governing Factor._—The connection of some of these
changes in vegetation with the calcareous nature of the corresponding
formation has already been referred to. As regards four of the eleven
divisions, this is obvious even to the casual observer, and is well
known to the population, who speak of the “lime country” or belts
being, as a matter of common knowledge, the best land; in full accord
with what, in Kentucky and elsewhere, has passed into a popular
maxim.[186]

[186] “A lime country is a rich country.”

Taking as a guide the trees and plants which characterize the obviously
calcareous lands, our next step should be to verify, if possible, the
fact that wherever these occur naturally, lime is abundant in the
soil in comparison with those lands in which such vegetation does
not occur naturally, or perhaps even fails to flourish when planted
without special fertilization. This the writer has sought to do,
first in connection with the survey work of the state of Mississippi,
and subsequently in the wider field that has since come under his
observation.


SOIL BELTS IN SOUTHERN MISSISSIPPI.

In Mississippi, the general conclusions derived from the observations
made on the northern cross section, are corroborated many times over
in other portions of the state. Aside from the cretaceous prairie
region, there runs across the middle of the state a belt of varying
width, of calcareous tertiary beds, which also give rise to more
or less extensive tracts of “black prairie” lands, interspersed
with non-calcareous, mostly sandy ridges, the lower slopes of
which, influenced by the calcareous beds, bear an oak and hickory
growth, while the higher portions have only pine, and usually remain
uncultivated. Southward of this “central prairie” belt lies the
long-leaf-pine forest area of the state, underlaid throughout by
sandy, non-calcareous formations, with poor sandy soils, save here and
there in patches, which can be at once recognized by the replacement
of the long-leaved pine by a vigorous oak growth; as is also the case
where the pine area abuts against the calcareous “Cane Hills” on the
west. The bottom soils of this region are largely “sour,” and bear
the gallberry (_Prinos glaber_), bay galls (_Persea Carolina_), ti-ti
(_Cliftonia monophylla_), candleberry (_Myrica cerifera_), various
whortle-berries, the pitcher plants (_Sarracenia_), yellow star grass
(_Aletris_), sundews, _Xyris_, _Eriocaulon_, and other plants of
similar habits.

_Vegetative and Soil Features of the Mississippi Coast Belt._—South of
the long-leaf pine area lie the coast flats, with sour, sandy soils
underlaid by stiff clays. On these “pine meadows” of the Mississippi
coast occur some of the most striking cases of modifications of
vegetation due to physical and chemical causes.

As is well known, the long-leaved pine habitually belongs to the
dry sandy uplands of the Gulf States; the deciduous cypress, on the
other hand, is most characteristic of the swamps, where its roots
are permanently submerged in water. But on the pine meadows of the
Mississippi coast we see these two incongruous trees growing side by
side, though sadly worsted by their mutual concessions; their heights
usually ranging from 12 to 15, rarely as much as 18 feet.[187] Yet
both preserve their characteristic forms, the cypress being an exact
miniature reproduction of the usual level-topped swamp form, except
as to the “knee” feature; while the pine differs only in stature from
its giant brethren of the pine hills, from which it can be traced
down through all grades of transition. The soil on which this growth
occurs is a sour, sandy one, one and a half to three feet in depth,
underlaid by a solid, impervious gray clay, above which is usually
found several inches of coffee-colored bottom water, which drains
slowly into the sluggish water-courses, themselves carrying brownish,
sour, but very clear waters. Analysis shows the soil to be sour and
extremely poor, especially in its lime and phosphates (see chapter
19, p. 352); its herbaceous vegetation consists exclusively of very
small-seeded, “calcifuge” plants (sedges, orchids, Juncus, Hæmodoraceæ,
Xyris, Polygala, etc.). This land is wholly unproductive and affords
but indifferent pasturage, except the first season after burning-over;
probably because of the effect of the minute amount of ashes so added.
As the coast is approached, the clay subsoil has an increasing depth of
sandy soil-mass above it, and on these “sand hammocks” the long-leaved
pine gradually assumes more and more of its usual stature; the cypress
disappears, and the Cuban pine (here called pitch pine) gradually comes
in; while the sedgy vegetation diminishes and finally disappears. On
this land crops may be grown as in the long-leaf-pine uplands.

[187] R. M. Harper, who has graphically described the vegetative
features of the coastal plain of Georgia (Contr. from the Dep. of
Bot. Colum. Univ. Nos. 192, 215, 216, 1902-05; also Bull. Torr. Bot.
Club 29-32), claims the deciduous cypress of the wet pine-barrens and
ponds therein, the vegetation of which greatly resembles that of the
pine meadows of the Mississippi sea-coast, to be a distinct species,
_Taxodium imbricarium_, the leaves of which are imbricated, instead of
two-ranked and with spreading leaflets. He supports this distinction
mainly by the differences in habit from the Louisiana swamp cypress,
and the fact that the imbricated form occurs wholly on non-calcareous
land, while the other is at home in the calcareous alluvial areas.
The imbricated form has been observed and commented on before, as a
mere ecological variation, and in the writer’s opinion this is all
that can be claimed, in view of the much greater differences in the
form of other trees, notably oaks, illustrated below, caused also by
lime. There would, _à fortiori_, be reason for claiming at least three
different species of post oak and black-jack (and two of willow oak),
which differ not only in tree form but also in the form and number
of leaf lobes, and yet can be traced into one another by innumerable
transition forms. If new species are to be established on such grounds,
it is hard to see where the variations manifestly due to environment
are to come in.

But on the immediate coast, evidently under the influence of the
aboriginal “shell mounds,” the yellow sandy soil becomes blackish from
the (humus-forming) effect of the lime thus supplied; and concurrently
the coast live-oak (_Q. virens_), grape vines, the Hercules club
(_Aralia spinosa_), “l’herbe a trois quarts” (_Verbesina sp._), and
numerous leguminous plants (which are wholly absent from the pine
meadows) take possession of the land, which is very productive and has
been specially utilized in the growing of Sea Island cotton. Here the
clay stratum is 15 to 20 feet below the surface, and roots penetrate
to great depths in the pervious soil, whose great thickness makes up
for its low percentage of plant-food (see table below). This land is
distinctly limited by the extent of the shell heaps, past or present,
and shows a respectable percentage of lime.

[Illustration: FIG. 78.—Schematic profile of the Mississippi Coast
Belt, through Jackson County.]

The annexed schematic profile (fig. 78) illustrates these changes
of soil and vegetation, which furnish a striking example of the
effective modification of vegetative features by physical and chemical
soil-conditions.

It would be difficult to find a more striking exemplification of the
effect of lime carbonate, not only upon the vegetation but also upon
the physical and chemical characters of the hopelessly unproductive
soil of the sand hammocks and pine meadows; no longer brown and sour,
but jet black and neutral, modifying favorably every physical quality.
Humus likewise nowhere shows its benefits more strikingly.

_Table of Lime-Percentages._—The table below shows the average lime
percentages observed in most of the several vegetative areas mentioned
above. To meet the objection sometimes made that the vegetative changes
noted may be due to the larger amounts of phosphoric acid and potash
frequently found in calcareous lands, the percentages of the latter are
also given. Considering the origin of limestones, such a connection
is not unexpected, but it is far from constant. On the contrary, the
frequent co-occurrence of much lime and high production with small
percentages of phosphoric acid and potash leads to the conclusion,
already discussed (see chapter 19, p. 365), that in presence of
abundance of calcic carbonate, smaller percentages of phosphoric acid
may be considered adequate than when lime is deficient, on account of
greater availability. Almost the same may be said of potash; and it
is quite possible that the presence of large amounts of lime tends
to prevent the leaching-out of this base, in consequence of greater
facility for the formation of zeolites. Illustrations of this kind have
already been given (chapters 3, 22).

_Definition of “Calcareous Soils.”_—It will be noted that the very
obvious and important changes of vegetation are brought about by
comparatively slight differences in lime-content. In fact, only two
of the soils enumerated above would, according to the estimates
usually given in books on soil composition, be considered as properly
calcareous. But the decisive feature in this matter must evidently
be the _native vegetation_, which expresses the nature of the land
much more clearly and authoritatively than any arbitrary definition
or nomenclature can possibly claim to do. _A soil must be considered
as being calcareous whenever it naturally supports the vegetation
characteristic of calcareous soils._

    TABLE SHOWING NATIVE FOREST GROWTH, POPULAR ESTIMATE OF DURABILITY
    AND INITIAL PRODUCTION, AND PERCENTAGES OF LIME, PHOSPHORIC ACID
                  AND POTASH, IN MISSISSIPPI LANDS.
    ===+==========+============================+=======================
    No.|   Name.  |     Natural Vegetation.    | Production Per Acre.
       |          |                            |
    ---+----------+----------------------------+-----------------------
       |   Black  |Mainly sturdy black-jack and|400 lbs. cotton lint,
    172|  Prairie | post oak, red cedar, crab  | decreasing to 200 lbs.
       |   Soil.  | apple and honey locust.    | in 30 years.
       |          |                            |
    ---+----------+----------------------------+-----------------------
       |   Pale   |Scarlet, post and Spanish   |200 lbs. cotton first
    164|   Yellow |  oak, small.               |  year, tailing off
       |   Ridge  |                            |  to 75 to 100 lbs.
       |   Loam.  |                            |  by 5th year.
    ---+----------+----------------------------+-----------------------
       | Pontotoc |Oak, hickory, walnut, sweet |350-400 lbs. cotton
    226|  Ridge   |and black gum, honey locust.| lint, 300 lbs. after
       |  Soil.   |                            | 20 years.
    ---+----------+----------------------------+-----------------------
       |   Heavy  |Post and black-jack oak,    |20 bushels of corn
    230| Flatwood | and some short-leaved pine.| first year--then
       |   Soil.  |                            | nubbins only.
    ---+----------+----------------------------+-----------------------
       |Black-Jack|Black-jack oak, pine,       |Unproductive.
    345|   Ridge  |  huckleberry.              |
       |   Soil.  |                            |
    ---+----------+----------------------------+-----------------------
       |  Oak and |Red and post oak, pig-nut   |200 lbs. cotton lint
       |   Pine   | hickory, short-leaved pine.| at first; 100 lbs.
    142|  Upland  |                            | 6-8 years.
       |   Loam.  |                            |
    ---+----------+----------------------------+-----------------------
       |   Brown  |Black, post, Spanish and    |400 lbs. cotton--250
    219|Table-land| black-jack oak, hickory,   | lbs. after 20 years.
       |   soil.  | sweet and black gum.       |
    ---+----------+----------------------------+-----------------------
       | Bluff or |Cane, black and white oak,  |400 lbs. cotton,
    237|  Loess   | tulip tree, linden,        | decreasing to 250 lbs.
       |  Soil.   | sassafras.                 | after 30 years.
    ---+----------+----------------------------+-----------------------
       |“Buckshot”|Hickory, ash, sweet gum,    |800 lbs. cotton lint,
    390|   Soil.  | pecan, cow oak, honey      | reduced to 500 lbs.
       |          | locust, cane, crab-apple,  | in 30 years
       |          | plum.                      | cultivation.
    ---+----------+----------------------------+-----------------------
       |   Pine   |Long-leaved pine, with      |20-30 bushels corn for
    206|   Hill   |  scarlet and post oak,     | one year--then
       |   Soil.  |  scattered small and       | nubbins. Usually
       |          |  occasional pig-nut        | good sweet potatoes
       |          |  hickory.                  | for 2-3 years.
    ---+----------+----------------------------+-----------------------
       |   Pine   |Dwarf long-leaved pine,     |Unproductive. Bears
    215|   Meadow | cypress, sedges, orchids,  | small-seeded “sour”
       |   Soil.  | Hæmodoracæ, etc.           | vegetation.
    ---+----------+----------------------------+-----------------------
       |   Shell  |Live and water oak, cedar,  |300 lbs. sea-island
     88|  Hammock | magnolia, holly, dogwood,  |cotton for 15 years.
       |   Soil.  | sweet gum, hickory,        |
       |          | sassafras, grape, Hercules |
       |          | club, etc.                 |
    ---+----------+----------------------------+-----------------------

    ===+==========+===============+=======+=====+=====+======+=======
    No.|   Name.  |    Physical   |  K₂O. | CaO.| MgO.| P₂O₅.| Humus.
       |          |   Character.  |       |     |     |      |
    ---+----------+---------------+-------+-----+-----+------+-------
       |   Black  |Heavy adhesive |       |     |     |      |
    172|  Prairie | dark-colored  |  .333 |1.367| .363| .104 | 1.25
       |   Soil.  | clay.         |       |     |     |      |
       |          |               |       |     |     |      |
    ---+----------+---------------+-------+-----+-----+------+-------
       |   Pale   | Pale yellow   |       |     |     |      |★
    164|   Yellow |  sandy loam.  |  .093 | .069| .126| .033 |  .50
       |   Ridge  |               |       |     |     |      |
       |   Loam.  |               |       |     |     |      |
    ---+----------+---------------+-------+-----+-----+------+-------
       | Pontotoc |   “Mulatto”   |       |     |     |      |
    226|  Ridge   |  medium loam. |  .374 | .281| .234| .082 | 1.00
       |  Soil.   |               |       |     |     |      |
    ---+----------+---------------+-------+-----+-----+------+-------
       |   Heavy  |   Heavy gray  |  .753 | .178| .831| .052 |  .305
    230| Flatwood |     clay.     |       |     |     |      |
       |   Soil.  |               |       |     |     |      |
    ---+----------+---------------+-------+-----+-----+------+-------
       |Black-Jack|   Very sandy  |       |     |     |      |★
    345|   Ridge  |     loam.     |  .073 | .142| .100|Trace.|  .20
       |   Soil.  |               |       |     |     |      |
    ---+----------+---------------+-------+-----+-----+------+-------
       |  Oak and |     Medium    |       |     |     |      |
       |   Pine   |      loam.    |       |     |     |      |★
    142|  Upland  |               |  .236 | .092| .196| .091 |  .50
       |   Loam.  |               |       |     |     |      |
    ---+----------+---------------+-------+-----+-----+------+-------
       |   Brown  |               |       |     |     |      |
    219|Table-land|   Clay loam.  |  .630 | .270| .450| .210 |  .79
       |   soil.  |               |       |     |     |      |
    ---+----------+---------------+-------+-----+-----+------+-------
       | Bluff or |   Sandy loam. |  .511 |5.921|3.278| .143 |  .72
    237|  Loess   |               |       |     |     |      |
       |  Soil.   |               |       |     |     |      |
    ---+----------+---------------+-------+-----+-----+------+-------
       |“Buckshot”|    Heavy      |       |     |     |      |★
    390|   Soil.  | calcareous    | 1.104 |1.349|1.665| .304 | 1.0
       |          |  clay soil.   |       |     |     |      |
    ---+----------+---------------+-------+-----+-----+------+-------
       |   Pine   | Light sandy   |       |     |     |      |
    206|   Hill   |  loam; mere   |  .259 | .129| .180| .030 |  .35
       |   Soil.  |  sand at      |       |     |     |      |
       |          |  3 feet.      |       |     |     |      |
    ---+----------+---------------+-------+-----+-----+------+-------
       |   Pine   | Very sandy    |       |     |     |      |
    215|   Meadow | loam, gray    |  .061 | .023| .069| .021 |
       |   Soil.  | clay subsoil. |       |     |     |      |
    ---+----------+---------------+-------+-----+-----+------+-------
       |   Shell  |  Very sandy   |       |     |     |      |
     88|  Hammock |  loam, deep.  |  .080 | .115| .065| .107 |  .75
       |   Soil.  |               |       |     |     |      |
    ---+----------+---------------+-------+-----+-----+------+-------
        Starred values (★) are approximate.


DIFFERENCES IN THE FORM AND DEVELOPMENT OF TREES.[188]

It will be noted that in the above table, as well as in the discussion
preceding it, identical species of trees are ascribed to vegetative
areas of widely different productive capacity. Perhaps the most
striking example is that the cretaceous prairies and the adjoining
flatwoods belt, standing respectively highest and lowest in the scale
of productiveness, are yet bearing specifically identical tree-growth,
to-wit, the post oak (_Quercus minor_) and the black-jack oak (_Q.
marylandica_). While to the field botanist[189] there can be no
question as to the absolute specific identity of the two trees as
growing on the respective areas, yet the mode of development of both
is so different in the two cases, that, as before remarked they are
popularly supposed to be different “kinds.”

[188] It is a matter of regret to the writer that owing to the long
distance intervening and the difficulty of securing competent and
sympathetic observers for such work, it has not been possible for him
to secure photographs of the tree-forms here discussed. At the time his
own observations were made, photography was practically unavailable as
yet, and the figures given are therefore based upon sketches made at
the time, and partly upon recollection. They represent types rather
than definite individuals, which were however described when fresh in
mind, in the Report on the Agriculture and Geology of Mississippi,
1860, pages 254 et seq.

[189] It has been already, and doubtless will be again and
increasingly, attempted to make distinct “species” of these widely
different forms of trees. But this is simply begging the question.
Mere external diagnostic marks will not avail here; it would have to
be shown that the seed of these different forms do not produce the
other forms under changed conditions. Until this has been done, the
number-less transition forms which he that runs may observe in the
field, throw upon the species-makers the onus of proof of differences
of specific value—if it be possible to define such value.

_Forms of the Post Oak._—The post oak of the prairie lands is a tree
50 to 70 feet high, with a stout, excurrent, rather conical trunk,
often somewhat curved to one side above, and densely clothed from
within 12 or 15 feet of the ground with comparatively short, sturdy
branches set squarely to the trunk, much crooked (geniculate), often
reflexed downward; altogether forming a dense head, beneath whose
thick foliage, a bird or squirrel is quite secure from the hunter’s
aim.—In the flatwoods, on the contrary, the post oak has a thin, rather
short trunk, divided up at 15 or 20 feet height into long, rod-like
branches, spreading broom-fashion, and scantily clothed with short
twigs bearing tufts of leaves; thus forming an open head, in which no
creature can hide effectually. On the brown-loam table-lands, again,
the post oak has a straight, rather slender, excurrent trunk with long
and more or less crooked limbs projecting at a large angle, sometimes
even drooping, and freely divided up into lateral, leafy branches; the
trees attain from 40 to 55 feet in height. Again, on the high sandy
ridges which are interspersed in the eastern portion of the brown loam
area, we find, generally associated with a similarly depauperated form
of the black-jack oak, and with the Upland Willow oak (_Q. cinerea_), a
form of the post oak intermediate between that of the Flatwoods and the
Table lands; twelve to fifteen feet high, with thin trunk, “sprangling”
long, crooked branches, clothed with sparse tufts of leaves. These four
strikingly distinct types are shown schematically, in their extreme
development, in the subjoined figures.

It is hardly necessary to say that between these extreme forms there
are many degrees of transition, corresponding to the transitions
between the several soil-classes respectively represented by them; or
they may be developed into depauperated types. Thus, for example, the
forms of the post and black-jack oak found on the sandy ridges of the
yellow loam region, hardly need experience in the observer to interpret
them as characterizing a wretchedly poor soil.

_Forms of the Black-jack Oak._—Not less striking are the
characteristics of the forms of the _black-jack oak_ as developed upon
these several kinds of land. The black-jack of the prairies is a low
tree with a dense rounded head, often somewhat flattened above, and a
low, thick-set trunk divided up into square-set branches, so densely
clad with foliage that no light penetrates into the interior, and birds
can safely hide and nest within it. The height rarely exceeds 35 feet,
the head being 20 to 30 feet across.

The Flatwoods form, on the contrary, rarely exceeds 15 feet in height,
with a very rough bark and a small, rather dense, rounded top, giving
the whole the appearance of a small apple tree. Practically the same
form is seen on poor, clay ridges of “hog-wallow” land.

[Illustration: Loam Upland.]

[Illustration: Sandy Ridges.]

[Illustration: Flatwoods.]

[Illustration: Black Prairie.

FIG. 79.—Extreme Forms of Post Oak (_Quercus minor Marsh., obtusiloba_
Mich.).]

On the brown-loam lands the black-jack, like the post oak, has a
rather slender, often somewhat crooked, but excurrent trunk 35 to 50
feet high, with more or less crooked limbs of moderate length, well
provided with leafy branches, but forming altogether a rather open
crown. A depauperated form of this type occurs on the sandy ridges of
the yellow-loam region and is 12 to 15 feet high, with slender, crooked
branches, clothed with scanty foliage; as shown in Figure No. 80,
alongside of the other typical forms.

[Illustration: Loam Upland.]

[Illustration: Flatwoods.]

[Illustration: Sandy Ridges.]

[Illustration: Black Prairie.

FIG. 80.—Extreme Forms of Black-Jack Oak (_Q. marylandica Mursh, nigra
Wangh_).]

In all these variations of the tree forms, there is also a concomitant
variation in the forms and other characters of the leaves. Thus in
the compact forms of the black-jack oak, the trilobate leaf is almost
completely obliterated, the leaf being simply rounded-cuneate, somewhat
auriculate at base. In the sparse-branched upland forms the leaves are
deeply three-lobed, and the ferruginous tomentum of the lower surface
is much less pronounced. The lobation of the post oak also varies
considerably both in the numbers of lobes and in their obtuseness.
Similar differences prevail in the case of the black and Spanish oaks;
thus in the latter, the long terminal, falcate lobe is always most
pronounced on “rich” soils, while on poor ones the trilobate leaf
predominates.

Of course all these forms may be found bearing acorns, so that they
undoubtedly represent adult trees.

_Characteristic Forms of other Oaks._—Similar general features are
repeated in the case of the other species of oaks, and also more
or less in other kinds of trees; though mostly less pronouncedly
than with the two species above described. Among the more striking
are the two forms of the willow oak (_Q. phellos_), which on low,
undrained ground assumes the low, rounded, “apple-tree” form, while on
well-drained uplands of good fertility it is a beautiful, slender tree
producing almost the effect of the acacia type; it is then a sign of
first-class land. The scarlet oak rather reverses these types; on good,
“brown-loam” upland it is of rounded form, not very tall, with sturdy,
rough-barked trunk; while on poor hillside lands its tall, smooth,
white trunk stands out as a conspicuous admonition to the land-seeker
to beware of a poor purchase. The black and Spanish oaks also indicate,
by tall thin trunks, a deterioration of the land as compared with the
lower and more sturdy growth on areas relatively richer in lime.

_Sturdy Growth on Calcareous Lands._—One feature invariably repeated,
not only in Mississippi but throughout the United States, is that _in
many strongly calcareous soils the growth of all trees, as well as of
shrubs and of many herbaceous plants, is of a more sturdy and thick-set
habit_ than that of the same species grown on thin, sandy, or generally
on non-calcareous land. This effect is quite as apparent in the arid
region of the Pacific Coast as in the Atlantic States, on the prairies
of the Middle West, and of the Gulf Coast. The experienced farmer
recognizes this habit of the tree-growth as a sign of good land, and
the reverse, viz., trees of lank, tall and thin growth, as evidence to
the contrary, from the Atlantic to the Pacific.

_Cotton Plant._—The cotton plant affords very striking evidences of
this influence of lime. On the bottom lands of a creek in Rankin
county, Mississippi, the writer found a “patch” of cotton with
luxuriant stalks reaching above the head of a man on horseback, but
almost devoid of “squares” or blooms. The soil was very dark and
rich-looking, but was derived from a non-calcareous tertiary terrane
surrounding the heads of the stream. A few rods below, the latter
crosses the line of a calcareous terrane, from which copious marly
debris have been washed down on the bottom soil. Here the cotton was
just half as high as above, and thickly covered with squares, blooms,
and bolls.

Another similar example was noted on the Chickasawhay river, in Wayne
county, Miss. Where that stream flows through the non-calcareous,
lignitiferous area of the tertiary formation, its bottom lands bear
cotton crops of medium productiveness only, the stalks being of the
usual height of about three feet, and only fairly bolled. But a short
distance below the point where the soft marls of the marine tertiary
are cut into by the stream, the cotton plants on the bottom lands
are from 18 to 20 inches high only, closely branched, and literally
thronged with cotton bolls, so that the fields appear a solid mass
of white. The only objection urged against this land is that to pick
such cotton “breaks the backs” of the pickers. The tree growth of the
bottom, of course shows a corresponding change.

_Lime Favors Fruiting._—In connection with the obvious changes of
form and stature caused by the presence of an abundant supply of lime
carbonate in soils, there is another that has been long noted in
cultivation, but is no less striking in the native vegetation. The
abundant fruiting of oaks on such lands as compared with the same
species on non-calcareous soils is a matter of common note in the
Mississippi Valley states; and the same is true of other trees, and
of herbaceous plants as well. The fruit on the lime soils is often
smaller, unless much humus is present; but the statement made in
Europe that cultivated fruits, and especially grapes, are sweeter on
calcareous lands, is abundantly verified in the native fruits of the
Mississippi Valley states as well; where the various wild berries,
haws, plums, etc., are well known to the younger part of the population
to be much sweeter and higher-flavored in certain (calcareous)
localities than in others, besides being usually more abundant.

This is entirely in accord with the well-known fact that the
application of lime checks the excessive wood and leaf growth resulting
from excess of nitrogen as well as moisture; while on the other hand,
the injurious effects of overdressing with lime or marl are known to
be repressed by the use of stable manure, or by green-manuring. The
repression of excessive wood growth by lime would seem to offer a
simple explanation of the compact habit of growth on calcareous lands;
and the extraordinary sweetness of fruits grown in the arid region as
compared with the same in the humid, is fully in accord with the high
lime-content of the arid lands.

_Stunted Growth._—In practice it will be found in most cases that a
stunted native growth is due not so much to lack of plant-food in the
soil, as to unfavorable physical conditions. Among these, _shallowness,
and extreme heaviness_ of the soil are the most common causes. The
“scab lands,” underlaid by impervious rock at a depth too slight for
culture plants, as in many plateaus of the Pacific Northwest, and in
rocky or mountainous regions generally, are cases in point. Strata of
impervious clay often produce the same result; but in this case, should
such clay be intrinsically capable of supporting plant growth, the
land can often be made available for orchard purposes by blasting with
dynamite (see chapter 10, p. 181).

The post oak (and black-jack) flats of the Mississippi Valley states
are familiar examples of land whose dwarfed tree growth causes it
to be avoided by settlers; similarly, a dwarfed growth of red elm
(_Ulmus rubra_), hackberry and ash indicates in the flood plain of
the Red river of Louisiana a heavy “waxy” red clay, or “gumbo” land,
scarcely available for agricultural purposes.[190] The gray or white
“cray-fishy” bottom and bench lands of the Southwestern States, so
poor in lime, phosphates and humus as to be worthless under existing
conditions, are characterized by an easily recognized scrubby growth of
Water and Willow Oaks (_Q. nigra_, or _aquatica_, and _phellos_), with
low, rounded tops; while the same trees, when well developed, indicate
highly productive lands.

[190] Rep. of Geological Reconnaissance of Louisiana; New Orleans,
1873, p. 27.

_Physical_ vs. _Chemical Causes of Vegetative Features_.—The extent
to which the modifications of form alluded to above are referable to
chemical and physical causes respectively, can be approached by the
discussion of the presence or absence of certain trees from soils
of extreme physical character, but otherwise normally constituted.
As has been shown above, the black-jack and post oaks belong, as
species, equally to the heaviest and lightest soils within the state of
Mississippi; to the black and yellow “prairie” soils, as well as to the
sandy ridges of the yellow-loam region; showing for these two species
as such, an independence of physical conditions and an extraordinary
adaptability, found in few other trees. They are frequently found
either alone or associated with only a few other species of local
adaptation, such as, in the prairie lands, the crab-apple, wild plum,
and the juniper or red cedar. On the soils of intermediate or loam
character, on the contrary, they are always associated with other oaks
as well as with hickory, and in that association attain what may be
considered their normal type or form.

From the fact that the dense, rounded top is formed by the black-jack
oak both on the rich prairie lands and on the poor soils of the
Flatwoods, it would seem that that form is the outcome of a physical
cause, viz., the extreme “heavy-clay” character of both kinds of land;
and we may note that exactly the reverse effect is observed in the
form growing on the poor sandy ridges, as shown in fig’s 79 and 80.
Yet it will also be noted that in the case of the post oak, the poor,
heavy-clay soil of the Flatwoods produces an open, broom-shaped top,
while the form assumed on the sandy ridges is substantially the same
for both species. Care must therefore be exercised in drawing general
conclusions as to the effects produced by either physical or chemical
causes, _alone_, upon tree forms.

_Lowland Tree Growth._—The variations occurring in the valleys or
alluvial bottoms are less obvious to superficial observation, yet
equally important and cogent to the close observer. In the properly
alluvial lands, one dominant condition, that of _adequate moisture_
supply, is almost always fulfilled, irrespective of soil quality. In
addition to this, as stated in chapter 2 (see page 24), practically all
the alluvial lands of the humid region may be considered as being of
a more or less _calcareous_ character, as compared with the adjacent
uplands. These two important conditions dominate in a great measure the
minor ones of variation in soil-texture. Yet where, as is largely the
case in the southern part of the State of Mississippi, the amount of
calcic carbonate is insufficient to overcome the sourness of the soil,
the vegetative contrasts become extremely striking and characteristic,
as explained above.

_Contrast Between “First” and “Second” Bottoms._—A very striking phase
of transition between the alluvial bottoms and the uplands proper in
the Cotton States are the second bottoms or hammocks of the streams,
whose soil and tree-growth in most cases differ markedly from those
of the first bottom; and these being usually closely adjacent, often
afford a very striking contrast to the latter. From some antecedent
geological cause not fully understood, these hammocks, usually elevated
from 4 to 10 feet above the present flood plain, have almost throughout
soils of a fine sandy, pulverulent or silty nature, frequently in
strong contrast to heavy clay soils in the first bottom.

They seem, moreover, to have been at some time subject to prolonged
maceration under water, resulting in the reduction of the ferric oxid,
and its accumulation in the lower portion of the deposit in the form of
bog-ore spots or “black gravel.” Since such a process always results
in the abstraction of phosphoric acid from the general mass of the
soil, to be accumulated in the bog ore in an inert condition,[191]
these hammock soils, usually whitish or gray in color, are almost
throughout poor in phosphates as well as in lime; the latter having
been definitively leached out. The resulting vegetation, as may be
imagined, is widely different from that of the bottom proper, as well
as, frequently, from that of the adjacent uplands; and though level and
fair to see, these hammocks are usually unthrifty and last but a short
time under exhaustive cultivation. Accordingly, their forest growth
is prevalently that of the poorer class of uplands, viz., small-sized
post and black-jack oaks, and in the low ground depauperated water oak,
or less commonly willow oak, of the low, stunted type indicative of a
soil of inferior productiveness. The luxuriant growth of the present
alluvial bottom is often seen within a few feet of the unthrifty
vegetation of these hammocks. It is usually only in the limestone
regions, and in the lower course of the larger streams, that the
hammocks or second bottoms are found to be of good fertility.

[191] See Chapter 2, p. 24.

_The Tree Growth of the First Bottoms. The Cypress._—Among the
trees occupying the low ground of the first bottom in the southern
Mississippi states, the deciduous cypress (_Taxodium distichum_)
deserves special mention as an example of extreme variation in form.
In sloughs and swampy tracts, as is well known, the cypress grows with
roots submerged throughout the season, excepting only the excrescences
known as “knees,” which project above the water, probably performing
some function in connection with the aeration of the root, which is
essential to the root functions in all plants. The trunk rising from
the water is supported by numerous projecting buttresses for from
8 to 15 feet above the water; higher up it becomes cylindrical for
a height of from 40 to 70 feet, then divides up into a few widely
spreading, thick, almost conical branches, whose twigs and foliage form
an almost level surface to the head. This level-topped forest growth
characterizes at once the submerged areas of the river and coast swamps.

But the cypress is by no means confined to the swamps and sloughs;
it is also found occupying the better class of hammock lands, 12 or
15 feet above water level. In this case, however, the tree assumes a
shape and growth so wholly different from that described above as to
lead to a popular assertion of a difference of species. As a matter
of fact, however, the cones of these upland cypresses, when dropping
into the water below them, reproduce exactly the common swamp form.
The extraordinary difference in the aspect of the tree under these
different conditions is best seen in the subjoined diagram, showing
the upland cypress to assume the form of the tall willow oak, with
which it is sometimes locally associated.

[Illustration: Swamp.]

[Illustration: Upland.

FIG. 81.—Forms of Deciduous Cypress on overflowed and on bench-land.]

The trees from which the annexed sketch is taken grew within thirty
feet of each other, on Yellow creek, a small tributary of the Tennessee
river, in Tishomingo county, Miss. The soil stratum is underlaid by a
shaly limestone, and bears lime vegetation.

The fact that the deciduous cypress grows without difficulty on the
moister class of lowlands in California, 12 or 15 feet above bottom
water, is of interest in this connection. It then assumes the upland
form shown in the figure above, although not growing quite as tall. The
calcareous nature of these soils is probably an important factor in
this apparently incongruous adaptation of a subtropical swamp tree to
arid conditions. In its swamp form the cypress usually grows in rather
shallow, heavy clay soil, into the dense subsoil of which the roots
penetrate but little.

_Other Lowland Trees._—The lowland hickories, like their brethren on
the highlands, seem on the whole to prefer the lighter or loamy bottom
soils to those of a heavier character. This is especially true of the
Pecan. The latter, as well as the shell-bark hickory, is especially
indicative of the highest class of bottom soils. The black walnut,
while apparently also best suited in loamy soils, is also more or
less found on heavy bottom lands, provided they are sufficiently
calcareous; and the same is measurably true of the tulip or white-wood
tree. The most frequent occupants of heavy bottom lands, however, are
the black gum and sweet gum, so that “gum swamps” are usually found
to be of that character.[192] But in the prairie region, where the
bottom soils are very calcareous and heavy, as well as in corresponding
soils of the “buckshot” lands of the great Mississippi Bottom, the
chestnut-white (cow- or basket-) oak sometimes occupies such ground
almost exclusively. Among the accompanying trees are especially the
honey locust, the crab-apple, mulberry and sweet gum, as well as ash.

[192] Hence perhaps the vernacular name “gumbo” for heavy, adhesive
clay soils in the north central states; which may also, however, be
derived from a comparison with the “gummy” pods of the cultivated okra
or gumbo plant.

_General Forecasts of Soil Quality in Forest Lands._—While the oaks and
pines mentioned as forming the bulk of the timber constitute in the
cotton states the _primà facié_ evidence, as it were, of the general
character of the land, there are numerous other trees and plants which
serve the discriminating land-seeker as a guide for the quality of
the soils in different localities. While everywhere, well-developed
black, red, Spanish and white oaks are considered as signs of a high
quality of land, the tall, thin scarlet, the upland willow, and the
barrens scrub oak are considered as indications detracting materially
from the producing value wherever they prevail. The various hickories
are throughout considered as indicating good land when mixed with
the oaks, or by themselves; while the presence of walnut, linden and
tulip tree will usually raise the estimate of uplands to the highest
class. On the other hand, the occurrence of small black-gum trees and
short-leaved pine, with low huckleberry, among the oaks of whatever
kind, accompanied as they usually are by the disappearance of the
black, white and Spanish oaks, will materially depress the land-values.

The appearance of well-formed oaks, as well as of hickory, is therefore
at once welcomed as an evidence of soil improvement, while that of
low huckleberry bushes and small black gums indicates the reverse. An
increase in the thickness and retentiveness of the soil stratum is
also usually indicated by the occurrence of short-leaved pine in the
long-leaf-pine areas.

The black, red, white, and Spanish oaks belong altogether to soils
of medium physical constitution, only their _size_ upon such lands
depending upon the relative richness in plant-food; but without such
changes producing any notable variation in their _form_. Clearly
then, these species are intolerant of extreme physical conditions,
and are practically restricted to soils of “loamy” character and easy
cultivation.




CHAPTER XXV.

RECOGNITION OF THE CHARACTER OF SOILS FROM THEIR NATIVE VEGETATION.
UNITED STATES AT LARGE. EUROPE.


The application of the above data outside of Mississippi can mostly be
verified only in a fragmentary way from such data as are casually given
in the reports of State Surveys, as well as from such observations as
the writer has been able to make personally elsewhere. In the latter
category the most copious refer to the states of Alabama, Louisiana and
Illinois.

ALABAMA.—The observations of Prof. Eugene A. Smith, and those of
Dr. Chas. Mohr, are especially valuable and cogent as to the close
correspondence of the soil and vegetative phenomena with those
observed in Mississippi.[193] They are faithfully reproduced on the
corresponding geological areas, including also the Flatwoods. Northwest
of Mobile, on the Mississippi line, the long-leaf-pine forest is
interspersed with more or less continuous areas bearing a fine oak
growth, with hickories and other trees indicating a calcareous soil.
This feature is most extensively developed in Alabama in what is known
as the “lime-sink region,” on the borders of which the vegetative
transition in passing from the non-calcareous sandy pine land, can be
observed in the most striking manner and with frequent alternations.
Northward of the long-leaf-pine belt, the tertiary and cretaceous
areas show in Alabama the same features as in Mississippi, viz.,
black calcareous prairies alternating with ridge lands, among which
in the cretaceous area the Pontotoc ridge is represented by a series
of isolated knobs, popularly known as Chunnenugga ridge, closely
resembling the former in its soils and vegetative character.

[193] See Plant Life of Alabama, by Charles Mohr, Vol. VI. Contr. U. S.
Nat. Herb., U. S. Dep’t Agr.; Alabama Ed. of Same, Ala. Geol. Survey,
1901.

In northern Alabama, according to Dr. Smith, on the various stages
of the Carboniferous formation, ranging from a sandy or conglomerate
character to that of limestones of various degrees of purity, soils
contrast strikingly with each other, agreeing closely with those seen
in the neighboring part of Mississippi. Here, moreover, the contrast
between the natural vegetative character as well as cultural value
of the lands derived from the magnesian limestones (the “barrens”)
contrasts strikingly with those originating in the purer limestones, on
which the blue grass is at home.

LOUISIANA.—As to Louisiana, whose geological formations correspond
closely to those of Mississippi, it may be said in general that the
vegetative phenomena coincide completely with those observed in
Mississippi. The “white-lime country” of northeastern Mississippi
is represented in Louisiana only by patches occurring here and
there on a line laid from Lake Bistineau to the coast at Petite
Anse Island. But the chief characteristics of the calcareous area,
among them especially that of the occurrence of red cedar and clumps
of crab-apple, persistently reappear. The “Central Prairie Region”
of Louisiana is quite narrow, but on it there reappear precisely
the same characteristics described in connection with that area in
Mississippi. In the long-leaf-pine region of Louisiana there occur,
as in Mississippi, some isolated patches of a calcareous character,
the largest of which is on the Bayou Anacoco in Vernon Parish, near
the western border of the State. As we emerge from the sandy lands of
the long-leaf pine area to that underlaid by the calcareous formation,
we find, first, a change to oak and short-leaved pine, then the oak
forest alone; finally, on a level black prairie of considerable
extent, the post and black-jack oak in their thick-set form, clumps
of crab-apple, red haw and honey locust, here and there a red cedar;
exactly as has already been described in connection with the prairie
lands of Mississippi. To southward of the long-leaf-pine area lies a
broad belt of level, generally treeless, sandy prairie, in part dotted
with groves of timber, but otherwise with nearly the same peculiar,
small-seeded herbaceous vegetation observed in the corresponding
portion of Mississippi. But in Louisiana there intervenes between these
gray sour lands and the shell hammocks of the immediate sea-coast
with their groves of live-oak, a belt of black calcareous prairie,
increasing in width and clayeyness towards the West, and acquiring
considerable extension in the corresponding portion of Texas. On these
prairies we again find the calciphile vegetation, including the honey
locust, clumps of crab-apple and red haw, etc., but not usually any oak
growth, except (near the sea-coast) the live-oak. In the hilly country
of northern Louisiana there is reproduced substantially the vegetative
character of the “short-leaf pine and oak” uplands of Mississippi (see
map on p. 490, chapter 24), save in that, owing to the occasional
outcropping of the calcareous materials of the Tertiary, small
prairies with black soil are spotted about here and there. Bordering
the Mississippi Bottom there are a series of oak-upland ridges with
a brown loam soil corresponding to the fertile area in northwestern
Mississippi, with small patches of the “Cane hills” loess soils,
bearing a corresponding tree growth.[194]

[194] See “Final Report of a Geological Reconnoisssance of Louisiana,”
published by the New Orleans Academy of Science in 1871.

In _Western Tennessee_ the vegetative zones so distinctly shown in the
adjacent portion of Mississippi are not so strikingly outlined, but
so far as they do exist, the phenomena observed accord exactly with
those heretofore described. The same holds true of _Western Kentucky_,
as is well set forth and graphically described in the reports of the
geological surveys of that state by Dr. David Dale Owen, and later by
Dr. R. H. Loughridge.

_North Central States._—North of the Ohio River the materials of the
geological formations are not nearly as much varied as they are south
of the same; consequently the vegetative features are also much more
uniform. It must be remembered that from the Alleghenies nearly to
the Mississippi, the states of Ohio, Southern Michigan, Indiana and
Illinois are largely covered by drift deposits overlying the older
formations, except that along the Ohio and Mississippi rivers lies the
calcareous loam of the Loess or Bluff formation.

Within the states mentioned, however, not only are the older
underlying formations very generally calcareous, but calcareous sand
and gravel form a large proportion of the drift deposits, which in
most cases overlie the rocks. Hence we find from the Alleghenies to
the Mississippi a predominance of the oak forests which characterizes
calcareous soils, as in the better class of uplands in Mississippi
and Tennessee; interrupted only here and there by sandy belts or
ridges bearing inferior growth, among which, again, the black-jack and
post oaks, with short-leaved pine, are conspicuous. But in a large
portion of Illinois, as well as in Western Indiana, the oak forest
is interrupted by more or less continuous belts, and sometimes by a
wide expanse, of black prairie, generally treeless or bearing only
clumps of crab-apple and haw, and underlaid more or less directly by
the carboniferous limestones, whose disintegration has materially
contributed to the black prairie soils; which are noted for their high
and long-continued productiveness. The lower ground is characterized,
besides clumps of crab-apple and red haw, by the frequent occurrence of
the honey locust, the lead plant (_Amorpha fruticosa_), the button-bush
(_Cephalanthus occidentalis_), and among herbs by the polar plant
(_Silphium laciniatum_), the prairie burdock (_S. terebinthinaceum_),
the swamp rose-mallow (_Hibiscus moscheutos_), the sneezewort
(_Helenium autumnale_), the wild indigo (_Baptisia tinctoria_ and
_leucophcæa_).

The black-jack and post oak are not nearly as frequently found on
the prairies of Illinois as on those of Mississippi and Alabama; but
where they occur they assume a similar habit, including the occurrence
of the dwarfed, apple-tree-shaped form on the low ridges with heavy
yellow clay soil, that sometimes intersect the prairies. The post oak,
moreover, in a form quite similar to that described as occurring on
the Flatwoods of Mississippi, forms the timber of the “post oak flats”
occasionally found between the low ridges bordering the streams, or
along the edges of the prairies. The herbaceous vegetation of these
post oak flats distinctly characterizes them as being poor in lime. In
the loamy uplands, where the calcareous ingredient is more abundant,
the open-headed form of the black-jack and post oak are also found,
interspersed with a luxuriant growth of black, red and white oak, with
more or less of hickory, which here assume a magnificent development,
much superior to that seen south of the Ohio. These yellow-loam uplands
correspond very closely in their soil-composition and agricultural
character to the brown-loam area of Mississippi and Tennessee, which
lies inland from the Loess belt. Where these uplands approach the
prairie or the outcrops of a limestone formation, there is usually
added to the oak growth the linden, the wild cherry and the ash; the
latter two also usually appear in the bottoms of the streams and on the
slopes adjacent, together with the walnut and butternut, and in the
lowest ground the sycamore.

The tree growth of the Loess belt bordering the Ohio and Mississippi,
so far as climatic differences permit, agrees almost precisely with
that described in the corresponding portions of Mississippi and
Tennessee. The change from the oak and hickory growth covering the
yellow-loam uplands toward the more calcareous area is evidenced by
the appearance of large sturdy trees of sassafras, together with
the linden and sugar maple. Descending from the “bluff” toward the
rich bottom-prairie with its black, heavy soil, we at once encounter
the familiar indices of the more highly calcareous land, viz., the
honey locust, clumps of crab-apple and red haw, with hackberry,
Kentucky coffee tree and mulberry on the lower ground. In late summer
and during autumn, a tall growth of the iron weed (_Vernonia_),
several _Eupatoriums_ (_E. perfoliatum_ and _purpureum_, the white
and the purple boneset) and of the blue-spiked _Verbena_ are very
characteristic, as are also several species of _Cassia_ (Carolina
coffee, etc.,) and the swamp rose-mallow.

_Upland and Lowland Vegetation in the Arid and Humid Regions._—In
the humid countries there is commonly a marked difference between
the vegetation of the uplands and lowlands, arising not merely from
the difference in the moisture supply, but evidently of a specific
nature. When we discuss the characteristic plants in detail, it becomes
obvious that it is _lime_ vegetation that, in most cases, forms the
characteristic differences between upland and lowland forest growth; a
natural consequence of the leaching-down of the lime from the higher
land to the lower levels. By way of counter-proof we find that when
the uplands themselves are of a calcareous nature, a part at least
of the lowland flora ascends into them. As prominent examples may
be mentioned the Tulip tree (_Liriodendron_), black walnut, ash,
Kentucky coffee tree, Hercules’ club, etc., which are lowland trees
over the greater part of their area of occurrence; but in the loess
or Cane hills bordering the Mississippi and its larger tributaries,
as well as in the limestone regions of the southwestern and western
states, are conspicuous in the uplands as well. The tall southern
cane (_Arundinaria macrosperma_), usually considered a plant of the
low river bottoms, originally covered the loess or “Cane hills” of
the lower Mississippi, with their highly calcareous soils. The same
is true of many other trees and shrubs characterizing limy lands. Of
course there are some whose habitat is dependent upon the concurrent
presence of _both_ lime and moisture, such as the sycamore, cottonwood,
hackberry, pawpaw, etc., which are naturally found only in stream
bottoms or on low hammocks.

In the arid region, on the contrary, the main difference in upland
and lowland vegetation is (outside of mountain influences) entirely
referable to moisture-conditions; the proof being that so soon as the
uplands are irrigated the lowland flora takes possession. Both uplands
and lowlands being abundantly calcareous, there then is no cause for
any material differences. This substantial uniformity of upland and
lowland plant growth is particularly striking in the comparatively
restricted floras of Eastern Oregon and Washington, and in Montana,
where the more luxuriant growth of the valleys is almost the only
contrast seen when their vegetation is compared with that of the
uplands adjacent.

_Forms of Deciduous Trees in the Arid Regions._—Since, as shown above,
the soils of the arid regions are almost throughout calcareous, we
should expect that the forms of the native trees would in general
conform to the rule given above. As regards the deciduous trees this
is very generally true: We rarely see on the Pacific slope, south of
Oregon, anything to compare with the tall oaks of the Atlantic forests.
The native oaks are as a rule of low, spreading growth, with stout,
short trunks; and as they rarely form dense forests, the timbered areas
have an orchard-like appearance, characteristic of the landscapes of
the arid region, from the Mezquit Plains of Texas to Eastern Oregon
and Washington. Only where a very abundant supply of moisture prevails
do we find occasional exceptions. The trees of the humid region when
transplanted to California have a perverse tendency to branch low, so
that only the most persistent trimming-up will induce them to form
trunks at all like those found in their native climes. In some cases
no amount of trimming will result in the formation of anything more
than bushes.

It may be objected that the arid climate as such, and not the
calcareous nature of the soil, is the cause of this tendency. It is
unquestionable that this low-branching habit is a distinct advantage to
the plants, whose trunks would otherwise be frequently scorched by the
hot summer sun; as happens when Eastern settlers try to grow “standard”
fruit trees, with the result that a “sore,” or sunburnt streak is
formed on the southwest side of the exposed trunk. All orchard trees
should therefore be pruned “vase-shape” in arid climates, partly for
this, partly for other reasons. But this cannot explain the fact that
seedlings from eastern acorns act precisely as do acclimated trees; so
that it is not a case of the survival of the fittest to endure arid
conditions.

_Tall Growths of Conifers._ Moreover, while the rule holds good with
almost all deciduous trees, it is not applicable to the Conifers; which
in the case of the Sequoias (redwoods and “big-trees”), sugar pine and
others, exemplify some of the tallest growths known in the world. The
Eastern Cedar or Juniper grows tall only on sufficiently calcareous
soils, and in the Mississippi Valley states at least, wherever it
occurs is an unfailing indication of calcareous lands. The extended
occurrence of the spruce on the Allegheny Ranges, where limestone
formations prevail so largely, seems to indicate a similar preference
for calcareous lands. And this is certainly true of the black locust,
which reaches its extreme southern range in the cretaceous hills of
Northeastern Mississippi, showing the stout, stocky form it also
assumes when planted in the calcareous black-prairie lands of Illinois.

_Herbaceous Plants as Soil Indicators._ While herbaceous plants are not
as generally considered by land-seekers in judging of soil fertility
and character, it goes without saying that very many are quite as
characteristic as the tree vegetation, especially when deep-rooting, so
as not to indicate merely the character of a few inches of surface soil.

In the Middle West of the United States especially, a large number
of the Compositæ serve as marks of high productive capacity. This is
particularly true of the larger species of the sunflower tribe, among
which _Helianthus grosse-serratus_ and _doronicoides_ are perhaps the
most generally notable; while farther west, beginning with Kansas, the
“Sunflower State,” and its northern neighbor, _H. annuus_, whether
native or introduced, becomes conspicuous also. The _Silphiums_
(compass sunflowers) have nearly the same significance, _S. laciniatum_
and _perfoliatum_ being prominent on the prairies of Illinois and
Indiana; but in land under cultivation they are mostly replaced by a
luxuriant growth of the Ragweed, _Ambrosia trifida_. Various species
of _Bidens_ (beggar ticks), notably the _B. aristata_ and _cernua_,
accompany the true sunflowers in the lower grounds of these regions,
as do also _Heliopsis laevis_, _Coreopsis triperis_ and _Rudbeckia_
(_Obeliscaria_) _pinnata_. _Rudbeckia hirta_ and _purpurea_, though
also occurring on rich soils, are not characteristic of them. The
larger species of golden rods (_Solidago_), notably _S. canadensis_,
_rigida_ and _speciosa_ (not ordinarily distinguished by farmers) share
substantially the distribution of the large sunflowers mentioned above.
Of the Asters, only _A. novæ-angliæ_ serves as a reliable guide to
high-class lands in the Middle West,[195] but a very copious growth of
asters and solidago of various species is always a welcome indication
of land quality, and indicates soils of good lime content, if not
absolutely calcareous.

[195] In view of its specific designation and the reputed poverty of
New England soils, this is rather unexpected.

_Leguminous Plants._—It is generally understood that most leguminous
plants, and among them especially the clovers, indicate rich, or
rather, calcareous lands. The very large proportion of lime contained
in the ash of legumes at once creates this presumption, which is
fully confirmed by experience so far as our ordinary culture plants
of that relationship are concerned. The favoring effect of lime on
the development of bacteria, so essential to the full development of
cultivated legumes, has already been referred to. The favoring effect
of gypsum sown even in small amounts with clover and other legumes, may
probably be referable to the known action of that salt in promoting
nitrification, which in the first stages of leguminous growth is so
highly favorable to a vigorous and early start of the crop, and to
a more copious production of the nitrogen-assimilating nodules. The
quick change noted in meadows and pastures of languishing production
so soon as moderately limed, by the appearance of clover among the
herbage, at once reminds us that the Rhizobia do not flourish in acid
lands. The great prevalence of leguminous plants of all kinds in the
arid region—clovers (not fewer than twenty-three species in California
alone), Lupins, Astragalus and related genera, at once remind us of the
universal prevalence of calcareous soils in these regions, as shown
above. _Mutatis mutandis_, we find precisely the same general facts in
the arid regions of the other continents.

Nevertheless, it must be kept in mind that not all plants of the
leguminous order are positively “calciphile.” Within the United States,
it is especially the genera _Desmodium_ (_Meibomia_) and _Lespedeza_,
which are very numerously represented in the long-leaf pine region
of Mississippi, where the soils are so poor in lime. Whether under
these conditions these plants develop the rhizobian nodules, has not,
so far as the writer is aware, been definitely observed. Certain it
is that quite a number of these plants occur on both calcareous and
non-calcareous soils, and on the latter assume a much more vigorous
development than in the pine woods. But it is evident that they, with
a few others (_e. g._ _Galactia mollis_, _Cassia chamæcrista_ and
_nictitans_) are more or less _indifferent_ to the lime-content of
soils, and cannot therefore be relied upon in judging the quality of
lands. In Mississippi and northern Alabama, the _Tephrosia virginica_
(“devil’s shoestring”), associated with chestnut and short-leaved
pine, is characteristic of the poorest non-calcareous lands, and bears
seeds but very scantily. It disappears so soon as calcareous lands are
approached, together with the chestnut tree.


EUROPEAN OBSERVATIONS AND VIEWS ON PLANT DISTRIBUTION AND ITS
CONTROLLING CAUSES.

The writer has thus far presented and discussed mainly his own
observations made in the United States, without reference to the
previous and contemporaneous work on the same subject in Europe. There
arose certain discrepancies which could not well be explained without a
previous full consideration of American conditions.

As is well known, for nearly twenty years the accepted theory in
Europe was that of Thurman,[196] which attributes the distribution of
the native floras entirely to physical conditions; thus anticipating by
more than half a century the corresponding hypothesis lately brought
forward by the U. S. Bureau of Soils. Thurmann classes plants simply
as hydrophile and xerophile, thus differing from most of our modern
ecologists merely in omitting the transition phase of “mesophytes,”
which now serves as a convenient pigeon-hole for an indefinite variety
of plants.

[196] Essai de Phytostatique appliquée à la chaine du Jura et aux
contrées voisines. 2 vols. 8vo. Berne, 1849.

While gradually many were led by their observations to doubt
the correctness of Thurmann’s exclusive physical theory, Fliche
and Grandeau[197] were apparently the first to impair by their
investigations the confidence in the accepted view. They investigated
exhaustively the conditions under which the maritime pine and the
chestnut tree, both antagonistic to lime, would flourish, and proved
that the presence of any considerable amount of lime in the land would
cause them to languish or die, although the physical conditions so far
as ascertainable were exactly alike. It is interesting to note what
were the lime-percentages which caused these differences; viz., for the
“non-calcareous” soil and subsoil, respectively, .35 and .20%; for the
calcareous land, 3.25 and 24.04%, the latter evidently being decidedly
“marly.” The composition of the ash of these trees is very instructive,
and is therefore given in full. Alongside of the ash of the maritime
pine on the two soils is given that of the Corsican pine, a lime-loving
tree.

    COMPOSITION OF PINE ASHES ON CALCAREOUS AND NON-CALCAREOUS LANDS.
    -----------------+-------------------------------+--------------
                     |        MARITIME PINE,         | CORSICAN PINE,
                     |        PINUS PINASTER.        |PINUS LARICIO.
    -----------------+-----------------+-------------+--------------
                     |On non-calcareous|On calcareous|On calcareous
                     |      soil.      |    soil.    |    soil.
                     |                 |             |
    Potash           |      16.04      |     4.95    |     13.56
    Soda             |       1.91      |     2.52    |      2.24
    Lime             |      40.20      |    56.15    |     49.13
    Magnesia         |      20.09      |    18.80    |     13.49
    Ferric Oxid      |       3.83      |     2.07    |      3.29
    Silica           |       9.18      |     6.42    |      7.14
    Phosphoric acid  |       9.00      |     9.14    |     11.33
    -----------------+-----------------+-------------+--------------
              Total  |     100.25      |   100.04    |    100.18
    -----------------+-----------------+-------------+--------------
      Ash per cent.  |       1.32      |     1.54    |      2.45
    -----------------+-----------------+-------------+--------------

[197] Annales de Chimie et de Physique, 4me série, tome 29; ibid. 5me
série, Tome 2. Also, ibid, tome 18, 1879.

    It is very interesting to note in these analyses the inverse
    ratio in the absorption of potash and lime by the maritime
    pine, which seems to be unable to defend itself against
    excessive absorption of lime and thus experiences a dearth
    of potash which naturally interferes with the formation of
    starch and chlorophyl; hence probably induces the chlorosis
    so well known to occur on excessively calcareous soils. The
    lime-loving Corsican pine takes up a larger total amount of
    ash and more phosphoric acid, and nearly three times as much
    potash, but considerably less lime than did the maritime
    pine on the same calcareous soil.

    The corresponding analyses made by Fliche and Grandeau,
    of the leaves and wood of chestnut grown on the same two
    kinds of soils, gave in general the same results; and
    they add that the smaller content of iron absorbed by the
    calcifuge trees when grown on calcareous soil point also
    to a deleterious influence upon the normal formation of
    chlorophyl.

Following Fliche and Grandeau, Bonnier[198] made corroborative tests by
sowing seeds of the same plants, both calciphile and calcifuge, upon
the two kinds of soils, and noting the differences in their mode of
growth and internal structure.

[198] Bull. de la Société Botanique de France, tome 26, 1879.


_Calciphile, Calcifuge and Silicophile plants._

The subject has been somewhat exhaustively discussed by Contejean[199]
who enumerates and has classified under the three general heads of
calciphile, calcifuge and indifferent, over 1700 species of European
plants. Unfortunately he had but few soil analyses at his disposal,
and was inclined to consider as non-calcareous, most soils that gave
no effervescence with acids. But notwithstanding this disadvantage so
far as his contention of the efficacy of chemical soil-composition,
and especially of lime is concerned, he disproves very effectually
the physical theory of Thurmann, by numerous examples from France
and elsewhere in Europe; and also disposes very definitely of the
claim that there is a special class of “silicophile” plants. He
concludes that silica (and sand) is merely a neutral and inert medium
which offers refuge to the plants “expelled” by lime; and that clay
similarly exerts no chemical but only a purely physical action.
That potash, phosphoric acid and nitrogen, while most essential
as plant-foods, exert otherwise little if any effect on general
plant distribution. He alludes similarly to magnesia; and his final
conclusion is that “chemical are in general more potent than physical
influences,” and that the most widely active influences are carbonate
of lime and chlorid of sodium. He does not, of course, deny the potent
influence of moisture upon plant distribution.

[199] Geographie botanique. Influence du terrain sur la vegetation.
Baillère et Fils, Paris, 1881, 143 pp.

Since these publications were made, many observers have investigated
the subject, and the broad distinction between lime-loving
or calciphile and lime-repelled or calcifuge plants has been
very generally recognized and discussed: but the cause of this
discrimination by plants is still more or less the subject of
controversy. Some still claim that the calcifuge plants (such as the
chestnut, the huckleberries and whortle-berries, the heather and many
other Ericaceæ, most sedges, etc.) are repelled by calcareous lands
because they need a large supply of silica, which they suppose cannot
well be assimilated in presence of much lime; hence they also designate
the calcifuge plants as “silicophile”; while others attribute the
preference of calciphile plants to the physical effects produced upon
the soil by lime, as outlined above (chapter 20, page 379).

The contention that the presence of much lime in soils renders silica
insoluble and hence unassimilable by plants, is at once negatived
by the fact that waters exceptionally rich in silica, partly simply
dissolved by carbonic acid, partly in the form of water-soluble
alkali-silicates, are very abundantly found in the arid region. This
is especially the case in California, where moreover a number of
species of very rough-surfaced horsetail rushes and grasses prove the
ready absorption of silica when wanted, even in strongly calcareous
soils. But the question is whether the supposed class of silicophile
plants is a reality or merely a theoretical fiction, based upon the
habit of speaking of “siliceous” soils as a class apart from other and
especially heavier or clay soils. As a matter of fact, the siliceous
soils usually so called are simply those poor in clay and lime—in other
words, “light” lands, the outcome of the weathering of quartzose rocks
into sandy soils, which in the humid region are always poor in lime
because thoroughly leached. In the arid region, on the contrary, sandy
lands are quite commonly just as calcareous as the heavier soils, and
show no “silicophile” flora.

According to the writer’s observations and views, it being obvious that
some plants are practically indifferent to the presence or absence
of lime in the soil except in so far as it influences favorably
the physical conditions, _moisture_ must always stand first as the
condition of maximum crop production, and as a _conditio sine qua non_
of the best development of plants on all kinds of soils; its best
measure being a matter of special adaptation to each species. But
this being understood, he agrees with Contejean as to the commanding
influence of lime in determining the adaptation of soils to plants,
both cultivated and wild. At the same time, it is obvious that the
absence of the opportunity to observe _really_ native vegetation,
adapted to the soils through ages, has created for European observers
difficulties which are readily solved where original native floras are
available.

Schimper[200] says pointedly that observations prove that the
differences between the location of plants on calcareous and siliceous
soils are not constant, but vary from province to province; that _e.
g._, the list of indifferent (bodensteter) plants for the Alps do
not hold good in the Dauphiné, still less between the Carpathians
and Skandinavia. According to Wahlenberg the following species are
calciphile in the Carpathians, and according to Christ indifferent
in Switzerland: _Dryas octopetala_, _Saxifraga oppositifolia_, most
of the leguminous species, _Gentiana nivalis_, _G. tenella_, _G.
verna_, _Erica carnea_, _Chamæorchis alpina_, _Carex capillaris_.
_Geum reptans_ is reported by Bonnier to be exclusively calciphile on
Mont Blanc, exclusively silicophile in the Dauphiné; indifferent in
Switzerland. A great number of similar contradictions are reported by
others as well, and the entire subject thus becomes rather vague; so
that Schimper and others suggest that climatic conditions may in part
be responsible for these discrepancies.

[200] Pflanzengeographie, p. 111 & ff.

In all, or nearly all these cases, it is tacitly assumed that the
underlying geological formation has essentially been the source of
the soil, and that its character is determined accordingly. But this
assumption is wholly arbitrary unless confirmed by actual direct
examination. A soil-formation overlying limestone on the slopes of a
range may be wholly derived from non-calcareous formations lying at a
higher elevation, or may have been leached of its original lime-content
by abundant rains. The feldspars constituting rocks designated as
granite, may or may not be partially or wholly of the soda-lime instead
of the potash series; the mica may or may not be partially replaced by
hornblende, in which cases the soil would be calcareous to the extent
of determining the character of the flora as calcifuge or calciphile,
without its being at all evident in the physical character of the soil,
which would still be “granitic” or “siliceous.” Such observations in
order to be critically decisive, clearly require that the observer
should be, not merely a systematic botanist, nor a mere geologist or
chemist, but all these combined. There is good reason to believe that
most or all of these supposed contradictions would disappear before a
critical physical and chemical examination of both the soils and the
rocks from which they are supposed to have been derived. Contejean
himself, in placing so many of his long catalogue of plants into the
doubtful groups, suggests many cases in which the above considerations
may explain the apparent discrepancies.

_What is a calcareous soil?_ The definition adopted for this volume
has been given in a previous chapter (chapter 19, page 367); viz,
that _a soil must be considered calcareous so soon as it naturally
supports a calciphile flora_—the “lime vegetation” so often referred
to above and named in detail. Upon this basis it has been seen that
some (sandy) soils containing only a little over one-tenth of one per
cent of lime show all the characters and advantages of calcareous
soils; while in the case of heavy clay soils, as has been shown, the
lime-percentage must rise to over one-half per cent to produce native
lime growth. While in the United States observations of the contrasts
between calciphile and calcifuge floras are easily made in the field,
and the facts must attract the attention of any fairly qualified
observer, in Europe they would have to be made the subject of special
cultural investigation based upon soil analysis; a procedure not yet
fully accredited abroad, any more than in the United States. In a
general way it has however been recognized by Maercker, as shown at the
end of the preceding chapter. How far this estimate was based upon
American precedents, can now be only conjectured. Certain it is that
the European definition of calcareous soils remains to the present
day a wholly different one from that stated above; and from this have
arisen the greater part of the doubts and differences of opinions among
European botanists as to the classification of plants in relation to
calcareous soils. Two per cent of lime (equivalent to nearly double
the amount of carbonate) is the prevailing European postulate for a
calcareous soil. Some go so far as to postulate effervescence with
acids, requiring about 5% of the carbonate.

_Predominance of Calcareous Formations in Europe._—It is not generally
recognized even among geologists how abnormally predominant are
_limestone_ formations in Europe. In all works on European agriculture
we find the “lime sand” mentioned as a normal ingredient of soils,
specially provided for (or against) in the operations of soil
examination. Its presence is the rule, its absence the exception.
Soils as poor in lime as are those of the long-leaf and short-leaf
pine regions of the United States, are there very exceptional and
(like the “Haideböden” of northern Germany) have long remained almost
uncultivated. Calcareous soils being the rule in the regions of intense
culture, the ideas of both agriculturists and agricultural chemists
have in Europe, in the main, been based upon them as normal soils; so
that instead of comparing calcareous, and non-calcareous soils properly
speaking—_i. e._, such as would not bear native lime-vegetation—the
majority of comparisons has actually been made between soils which, in
the American sense, were all or chiefly within the calcareous class. It
is characteristic of this state of things that the injuriousness of an
_excess_ of lime is among the foremost themes of European (especially
French and English) agricultural writers, as against the beneficent
effects prominently assigned to lime in America. No such popular
saying as that “a lime country is a rich country” exists in Europe;
on the contrary, we constantly hear, and see in books, the mention of
“poor chalk lands,” and in France especially the deleterious effects
of excess of lime upon crops is the theme of remark. Excess of lime
in their marly lands has been the despair of French vintners, and
Viala was specially sent to America to find some vine to serve as
a grafting stock which would resist the tendency to chlorosis which
renders many of the American phylloxera-resistant vines useless to the
viticulturists of France. Viala did not find such grapevines until he
reached the cretaceous (chalk) area of Texas, where the native vines
had long ago adapted themselves to marly soils; and these vines have
solved the problem for French viticulture.

And England, France, Belgium and most of western Europe _are_ rich
countries, largely owing to their abundant limestone formations; and
it may be questioned whether, had this been otherwise, Europe would so
long have remained the center of civilization; for starving populations
are not a good substratum for high mental culture and progress. It
may equally be asked whether the invariably calcareous character of
arid soils, as heretofore shown, has not, together with their general
high quality, been largely a determining factor in the location and
persistence of so many ancient civilizations in arid lands; as outlined
in chapter 21, page 417. In this connection, the proper distinction
between calcareous and non-calcareous soils passes from the domain of
natural science to that of the history of human civilization.




CHAPTER XXVI.

THE VEGETATION OF SALINE AND ALKALI LANDS.


_Marine Saline Lands._—While the saline alluvial lands of the sea-coast
differ both in their mode of origin and in their nature from the alkali
soils or “terrestrial saline lands,” as they have been called in
Europe, their vegetation has in many respects a common character. Not
only is there much similarity, sometimes even identity, in the kinds
of plants inhabiting these lands, but their saline ingredients induce
certain changes of form and structure in plants not properly “saline”
but more or less tolerant of soluble salts, by which the saline or
alkali character of the lands may be recognized.

Just as in the case of lime we must distinguish between the plants
definitely repelled by a large amount of this substance in the soil
(calcifuge), while others prefer the soils in which lime is abundant
(calciphile), and still others appear to be indifferent to its presence
and are governed in their habitat by the physical conditions presented:
so in the case of saline lands the salts may attract or repel certain
plants. The latter class is much the largest; while there is also a
number of plants which are more or less indifferent to the presence
of salts, provided these be not in very great excess. Such plants
constitute the next-largest class; while those attracted by salts, and
whose welfare is conditioned upon their presence, are comparatively few
in number, and still fewer among them are of economic importance. Hence
the soluble salts have largely a negative importance for agriculture;
the question usually being how to utilize the land until the
undesirable surplus of salts can be got rid of, partially or wholly, as
the case may be; the former usually in seashore lands, the latter in
the alkali lands proper; in which a small remnant, not sufficient to
injure crop plants, is usually desirable (see chap. 23, p. 462).

_General Character of Saline Vegetation._—Those familiar with seashore
marshes cannot fail to note the fleshiness and succulence of the
characteristic plants. This “incrassation” belongs not only to the
saline flora proper, but is acquired to a greater or less degree when
plants not ordinarily at home on saline ground are transferred to it
artificially, or by saline overflows; while at the same time the leaves
usually become smaller, and the growth more compact. Correspondingly,
when saline plants are transferred to non-saline ground, the leaves
generally become thinner and larger, and the growth more slender. The
well-known “Russian thistle” is a case in point, as is also its close
relative, the soda saltwort (_Salsola soda_); although the latter does
not often venture as far from the saline lands as does the former
(_Salsola kali tragus_), which now seems to have become a world-wide
weed, with only a shade of preference for alkali lands.

_Structural and Functional Differences Caused by Saline Solutions._—It
has been definitely shown by the investigations of Schimper, Brick,
Hoffmann, Lesage, Rosenberg and others, that the peculiarities or
changes of structure brought about by saline solutions are essentially
those pertaining to xerophile (drought-enduring) vegetation; which in
general tend to the diminution of evaporation from the plant surfaces.
It may be said, roughly speaking, that the absorption of water by the
roots begins to diminish so soon as the concentration of the saline
solution approaches or exceeds one-half of one per cent; while when
it rises as high as three per cent, water-absorption by the roots
ceases even in the wettest soils, and the plant suffers from drought
quite as much as from any directly injurious effects of the salts.
Different plants of course differ in the measure of concentration which
brings about these phenomena, which vary also with the character of
the soluble salts. It is stated that injurious or useless salts like
common salt act at lower concentrations than _e. g._, saltpeter, which
is useful. The difference in external structure are: diminution of the
size of leaves, assumption of cylindrical or spinous forms, sinking-in
of the breathing pores below the outer surface, dense hairy covering,
resinous exudations, etc. Internally we find that xerophile plants have
developed on their upper or outer leaf-surfaces instead of one, several
layers of “palisade” (long and erect, closely-packed) cells, through
which transpiration is extremely slow, as is also the transmission
of heat. When salt-tolerant plants are grown on saline soils, their
palisade cells are relatively lengthened.

Coincident with these external means for the retardation of
evaporation, the leaves of xerophiles are frequently supplied
with special water-storage cells, which supply moisture for the
physiological processes when the root supply falls short. The cactus
tribe and similar-looking plants are examples of the latter provision,
which causes even animals suffering from thirst to resort to them,
although they eschew the saline vegetation.

_Absorption of the Salts._—The true halophytes or exclusive salt
plants, which refuse to grow on lands not containing a large
proportions of salt, often absorb so much salt that on drying it blooms
out on their surface; they usually have, even when green, a distinctly
salty taste, and their ash is rich in chlorids, specially of sodium.
Such is the case of the samphire, common in saline marshes everywhere.
The total ash is usually very high, often varying with the salinity of
the water or soil in which they have grown. Thus the salt-content of
the ash of samphire may vary by several per cent. In other cases, as in
that of one of the Australian saltbushes investigated at the California
station, neither the ash content nor the composition of the ash varies
materially whether the plant be grown on strong alkali land, or on
uplands whose total saline content does not exceed (in four feet depth)
.015% or 2500 pounds per acre.

The following table gives the composition of the ash of this saltbush
alongside of that of two other prominent alkali-plants of the same
relationship, occurring, one in the San Joaquin valley of California,
in strongly saline lands, the other in the Great Basin region of the
interior, on lands strongly impregnated with carbonate of soda. All
these, it will be seen, take up very large amounts of sodium salts,
notably the chlorid; the Australian plant most so, the “greasewood” of
the Great Basin least so; a large proportion of the alkali salts being
evidently, in the latter case, contained in the form of organic salts,
which in the ash become carbonates.


              ANALYSES OF ASHES OF SALINE AND ALKALI PLANTS.

    (A) = Australian Saltbush, Atriplex semibaccata.†
    (B) = Bushy Samphire,  Allenrolfea occidentalis.†
    (C) = Greasewood, Sarcobatus vermiculatus.†
    (D) = Saltgrass, Distichlis spicata.‡
    (E) = Tussock grass, Sporobolus airodies.‡
    (F) = Prickly Pear, Opuntia macrocentra.‡
    ========================+======+======+======+======+======+======
                            | (A)  | (B)  |  (C) |  (D) | (E)  |  (F)
    ------------------------+------+------+------+------+------+------
    Ash, air-dried plant, %.| 19.37| 12.03| 13.81| 11.61|  7.99| 24.18
    ------------------------+------+------+------+------+------+------
    Potash (K₂O)            | 11.42| 18.53| 30.11|  3.30|  5.78|  1.61
    Soda (Na₂O)             | 35.39| 39.45| 32.58|  2.38|  5.15|  2.76
    Lime (CaO)              |  5.75|  1.36|  8.70|  5.25|  8.05| 65.66
    Magnesia (MgO)          |  3.23|  1.09|  1.09|  2.95|  4.15| 26.70
    ------------------------+------+------+------+------+------+------
    Br. ox. of Manganese    |      |      |      |      |      |
               (Mn₃O₄)      |   .22|      |      |   .16|   .25|
    ------------------------+------+------+------+------+------+------
    Peroxid of Iron (Fe₂O₃) |  3.33|  7.06| not  |  2.22|  2.39|  1.19
    Alumina (Al₂O₃)         |      |      |det’d.|      |      |
    ------------------------+------+------+------+------+------+------
    Silica                  | 16.24| 11.81|  4.00| 78.73| 66.79|   .81
    Phosphoric acid (P₂O₅)  |  2.80|  3.51|  5.60|   .83|  1.25|   .47
    Sulfuric acid (SO₃)     |  2.64|  4.93|  5.90|  3.20|  4.52|   .64
    Chlorin, percent        | 24.33| 15.30| 11.00|  1.40|  2.13|   .21
    ------------------------+------+------+------+------+------+------
          Totals            |105.35|103.04| 99.79|100.31|100.46|100.05
      Less excess, O: Cl    |  5.35|  3.25|  2.50|   .31|   .46|   .05
    ------------------------+------+------+------+------+------+------
          True totals       |100.00| 99.79| 97.29|100.00|100.00|100.00
    ------------------------+------+------+------+------+------+------

    (G) = Shad scale, Atriplex canescens.‡
    (H) = Alfalfa Hay. (Cal.)†
    (I) = Timothy Hay.

                                     OF FORAGE CROPS.
    ============================+========+========+=======
                                |  (G)   |   (H)  |  (I)
    ----------------------------+--------+--------+-------
      Ash, air-dried plant, %.  |   4.23 |   9.85 |  6.15
    ----------------------------+--------+-------+-------
    Potash (K₂O)                |  25.17 |  43.72 | 28.80
    Soda (Na₂O)                 |   6.23 |   4.48 |  2.70
    Lime (CaO)                  |  25.97 |  20.51 |  9.83
    Magnesia (MgO)              |  16.63 |   2.56 |  3.60
    Br. ox. of Manganese (Mn₃O₄)|    .51 |        |
    ----------------------------+--------+--------+-------
    Peroxid of Iron (Fe₂O₃)     |   5.89 |   2.95 |
    Alumina (Al₂O₃)             |        |        |
    ----------------------------+--------+--------+-------
    Silica                      |  11.94 |   5.87 | 35.00
    Phosphoric acid (P₂O₅)      |   3.11 |   5.00 | 10.80
    Sulfuric acid (SO₃)         |   4.93 |   6.92 |  3.90
    Chlorin, percent            |   2.07 |  10.25 |  5.00
    ----------------------------+--------+--------+-------
          Totals                | 100.45 | 102.26 | 99.70
      Less excess, O: Cl        |    .45 |  22.6  |  1.13
    ----------------------------+--------+--------+-------
          True totals           | 100.00 | 100.00 | 98.57
    ----------------------------+--------+--------+-------
      † Jaffa, Cal. St’n. Rept. 1894-95, p. 169.
      ‡ Goss, New Mex. St’n. Bull. No. 44; recalculated.

It will be noted that the saltbush hay contains nearly one-fifth of
its (air-dry) weight of ash, of which nearly 40% is common salt. It
therefore has a distinctly salty taste, and is always moist to the
touch, containing-ordinarily over 15% of moisture. It is therefore much
liked by stock when fed intermixed with other hay, and thus supplies
all the salt needed by cattle. The greasewood is much less liked by
stock, and bushy samphire is wholly rejected by them. Comparing with
these fleshy plants the ash of the two grasses, the first a world-wide
“salt grass,” the other a common grass of the American arid region,
we note that not only do they contain much less soluble ash than the
saltbushes, but especially much smaller amounts of sodium salts;
proving that even when growing in company with the saltbushes on
strongly impregnated land, they can repel from absorption these to them
useless or injurious salts. But in the case of the “shad scale,” also a
“saltbush” of the Great Basin, the ash-content is remarkably low—only
about one-fifth of that of its Australian relative—and it differs
widely from the latter in having but a very low proportion of soda,
and a very high one of lime and potash, approaching in these respects
to our usual forage crops; and being also fairly rich in nitrogen, it
forms acceptable browsing when other pasture plants are scarce. It
therefore does not exert the laxative action produced by the exclusive
feeding on the more saline herbages.

The exceptionally high ash-content of the cactus or prickly pear, also
given in the table, arises, it will be noted, not from the soluble
salts but from the absorption of extraordinarily high proportions of
lime and magnesia. Owing probably to the latter substance, and also the
oxalate form in which lime is usually found in the cactus tribe, this
plant when used as forage is also somewhat laxative.

Altogether, this table offers remarkable examples of wide differences
in the kind and amount of ash ingredients absorbed by plants growing
upon similar soils and under identical climatic conditions; indicating
a selective power which no merely physical theory of soil-action in
plant growth can explain.

_Injury to Plants from the Various Salts._—The early observers,
especially Contejean, were predisposed from their observations of lime
on vegetation to ascribe the action of salt upon marine vegetation
to the sodium component. But the wide differences in the effects of
different sodium compounds, notably of common salt and Glaubers salt,
led some to the conclusion that the acidic ingredients are the chief
determining factors. Moreover, it was soon found that a single salt is
more injurious than a mixture of several, such as sea water. This also
led to the inference that the varying degree of dissociation of these
salts essentially influences the effects.

    Kearney and Cameron have investigated these relations,[201]
    and have by artificial cultures in solutions of varying
    concentration and composition studied the behavior of plant
    roots and the limits of their endurance. They found for the
    several salts occurring in alkali soils, taken separately,
    the following figures, in 100,000 parts of water:

       Magnesium sulfate          7
          “      chlorid         12
       Sodium carbonate          26
          “    sulfate           53
          “    chlorid          116
          “    bicarbonate      167
       Calcium chlorid        1,377

[201] Report No. 71, U. S. Dep’t. of Agriculture, 1902.

    It will be noted that in many respects the results given in
    this table stand in marked contrast to the facts observed
    in alkali lands everywhere; and therefore while interesting
    physiologically, are not directly applicable to practice.
    Magnesium sulfate, which according to this table is the most
    injurious of all, is a common ingredient of alkali lands
    from Wyoming to New Mexico, as also is sodium sulfate; yet
    there, as well as in the Musselshell valley in Montana, and
    at many other points, it shows no specially deleterious
    action either upon native or cultivated plants, and in
    Europe as well as in New England the mineral kieserite is
    freely used as a fertilizer at many points. That sodium
    sulfate should be twice as harmful as sodium chlorid or
    common salt, and half as harmful as the carbonate or black
    alkali, is again wholly contrary to actual experience,
    which as shown elsewhere in this chapter, indicates that
    the majority of plants will tolerate between three and four
    times as much of sodium sulfate as of common salt; while the
    ratio of tolerance as against the carbonate seems sometimes
    to rise as high as ten to one.

    It is clearly evident, however, that it is the metallic or
    basic ingredient that in the main determines the toxicity of
    these salts. The universal presence of lime in some form in
    all alkali lands doubtless explains the discrepancies
    mentioned, since lime is especially potent in counteracting
    the injurious effects; thus throwing additional light upon
    the importance of the lime-content of alkali soils proper,
    and also upon the causes of the narrow limitations of
    the littoral (marine saline) flora; inasmuch as, unlike
    alkali soils, marine alluvial lands are by no means
    always calcareous. Cameron goes so far as to attribute
    the favorable effects of gypsum upon black alkali not so
    much to the conversion of the latter into neutral sulfate,
    as to the effect of gypsum solution in counteracting the
    saline effects. This interpretation, however, seems rather
    far-fetched, since there can be no question about the double
    decomposition of gypsum with carbonate of soda; or the
    intense injuriousness of carbonate of soda in the actual
    corrosion of vegetable tissues. The corresponding protective
    influence of various salts, more especially of those of
    lime, against the injurious effects of pure common salt on
    marine animals, has already been mentioned (chapter 20, page
    380), and later investigations by Osterhout on marine algæ,
    show the same relation to hold true for them also.

_Reclamation of Marine Saline Lands for Culture._—The reclamation of
sea-coast lands and marshes for agricultural use is based in general
upon the same methods as those already outlined for alkali lands in
chapter 20; except that in this case no chemical neutralization is
possible, since common salt cannot be changed by any practically
feasible means. It must be removed by leaching, and this, in the humid
countries in which such reclamations have chiefly been made, is usually
done by the agency of rains, aided by ditching. The “polder” lands thus
reclaimed along the shores of the North Sea, from Belgium to Prussia,
are especially esteemed for their productiveness, doubtless owing to
the alluvium of the numerous rivers tributary to that sea, which is
distributed along its shores and in the numerous inlets and bays. The
tides are of course excluded by dikes provided with gates opening
outward, so as to permit of the outflow of rain- or irrigation-water
used for leaching purposes.

Out of reach of stream alluvium no exceptional fertility is to be
expected of seashore lands, which then commonly assume the form of sand
dunes or bars, incapable of nourishing any cultural vegetation. Of the
latter, the groups listed below as tolerant of alkali salts, may also
be considered with reference to reclaimed seashore lands; the first
cereal to succeed being usually barley, the first root crop, beets.
Asparagus is also available while salt is being leached out.


THE VEGETATION OF ALKALI LANDS.

The general character of alkali-land vegetation is not unlike that of
saline seashore lands; some species of plants are common to both, but
the alkali lands harbor a much greater variety of plants, owing to
the differences in climates and soils as well as to the nature of the
impregnating salts. Moreover, owing to the very causes which underlie
the presence of these salts, viz, aridity, the xerophile or dry-land
character of the alkali-land flora is much more pronounced than that of
the saline seashore vegetation. In view of the very complex conditions,
the discussion of the alkali-flora is of necessity much more complex
than that of the marine group; and the data for its full elucidation
with respect to the nature of the soils and salts are as yet very
incomplete.


RECLAIMABLE AND IRRECLAIMABLE ALKALI LANDS AS DISTINGUISHED BY THEIR
NATURAL VEGETATION.

While, as shown above (chapter 20), the adaptation or non-adaptation
of particular alkali lands to certain cultures may be determined by
sampling the soil and subjecting the leachings to chemical analysis, it
is obviously desirable that some other means, if possible available to
the farmer himself, should be found to determine the reclaimability and
adaptation of such lands for general or special cultures.

In alkali lands, as in others, the natural plant-growth affords
such means, both as regards the quality and quantity of the saline
ingredients. The most superficial observation shows that certain plants
indicate extremely strong alkali lands where they occupy the ground
alone; others indicate pre-eminently the presence of common salt;
the presence or absence of still others form definite or probable
indications of reclaimability or non-reclaimability. Many such
characteristic plants are well known to and readily recognized by the
farmers of the alkali districts. “Alkali weeds” are commonly spoken
of almost everywhere; but the meaning of this term—_i. e._, the kind
of plant designated thereby—varies materially from place to place,
according to climate as well as the quality of the soil. It is obvious
that if these characteristic plants were definitely observed, described
and named, while also ascertaining the amount and kind of alkali they
indicate as existing in the land, lists could be formed for the several
regions, which would indicate, in a manner intelligible to the farmer
himself, the kind and degree of impregnation with which he would have
to deal in the reclamation work; thus enabling him to go to work on the
basis of his own judgment, without previous chemical examination.

A study of the lands of California having this purpose in view, was
undertaken in the years 1898 and 1899 by the California Station; but
lack of funds prevented its prosecution beyond the ascertainment
of those plants the abundant occurrence of which prove the land
to be irreclaimable without the use of the universal remedy, viz,
underdrainage, which on the large scale is usually beyond the means
of the land-seeker. The botanical field work and collection of soil
samples was carried out by Mr. Jos. Burtt Davy; the chemical work,
as heretofore, being done by Dr. R. H. Loughridge. The results here
reported are therefore essentially their joint work. It is hoped that
in the future, a more comprehensive study and close comparison of
the native vegetation with the chemical determination of the quality
and kind of alkali corresponding to certain plants, or groups of
plants, naturally occurring on the land, may enable us to come to a
sufficiently close estimate of the nature and capabilities of the
latter from the native vegetation alone, or with the aid of test plants
purposely grown, for the farmers’ purposes.

_Plants Indicating Irreclaimable Lands._—The plants herein-after
mentioned and figured are, then, to be understood as indicating,
_whenever they occupy the ground as an abundant and luxuriant growth_,
that such land is irreclaimable for ordinary crops, unless underdrained
for the purpose of washing out surplus salts. The occurrence merely of
scattered, more or less stunted individuals of these plants, while a
sure indication of the _presence_ of alkali salts, does not necessarily
show that the land is irreclaimable.

The plants which may best serve as such indicators in California are
the following:

    Tussock-grass (_Sporobolus airoides_ Torr.), Fig. 82.

    Bushy Samphire (_Allenrolfea occidentalis_ [Wats.] Ktze.),
    Fig. 83.

    Dwarf Samphire (_Salicornia subterminalis_ Parish, and
    other species), Fig. 84.

    Saltwort (_Suaeda torreyana_ Wats., and _S. suffrutescens_,
    Wats.), Fig. 85.

    Greasewood (_Sarcobatus vermiculatus_ [Hook.] Torr.),
    Fig. 86.

    Alkali-heath (_Frankenia grandifolia campestris_ Gray),
    Fig. 87.

    Cressa (_Cressa truxillensis_ Choisy), Fig. 88, perhaps
    identical with _C. cretica_ auct.

    Saltgrass (_Distichlis spicata_), Fig. 89.

TUSSOCK-GRASS (_Sporobolus airoides_, Torr.); Fig. 82.

(“Bunch grass” of New Mexico).

The three sets of Tussock-grass soil which have been analyzed show
that the total amount of all salts present is in no case less than
49,000 pounds per acre, to a depth of four feet; and that it sometimes
reaches the extraordinarily high figure of 499,000 pounds. Of these
amounts the neutral salts (Glauber’s salt and common salt) are usually
in the heaviest proportion (Glauber’s salt, 19,600 to 323,000 pounds
per acre; common salt, 3,500 to 172,800); the corrosive salsoda varying
from 3,000 to 44,000 pounds.—Tussock-grass apparently cannot persist
in ground which is periodically flooded. It is of special importance
because it is an acceptable forage for stock.

Tussock-grass is a prevalent alkali-indicator in the hot, arid portions
of the interior, from the upper San Joaquin Valley, the Mojave desert,
and southward; also through southern Nevada and Utah as far east as
Kansas and Nebraska. In the San Joaquin Valley it has not been found
farther north than the Tulare plains, although east of Reno it occurs
near Reno. Coville observes that in the Death Valley region “it is
confined principally to altitudes below 1,000 meters” (3,280 feet).
Hillman, however, reports it from near Reno, Nevada, at an altitude
which cannot be much less than 4,500 feet.

[Illustration: FIG. 82.—Tussock-Grass—_Sporobolus airoides_ Torr.]

The tussocks formed by this grass, which are unfortunately not shown in
the figure, sometimes appear as veritable little grass trees, and when
denuded by the browsing of cattle seem like trunks 18 and 20 inches
high. It is therefore very easily recognized; but it should be noted
that in view of the extraordinary range of its tolerance, shown above,
its scattered occurrence does not necessarily indicate irreclaimable
land.

BUSHY SAMPHIRE. (_Allenrolfea occidentalis_ [Wats.] G. Ktze.) FIG. 83..

This plant is locally called greasewood, but as this name is much more
commonly used for _Sarcobatus vermiculatus_, it seems best to call
Allenrolfea “bushy samphire,” as it closely resembles the true samphire
(_Salicornia_).

Bushy Samphire usually grows in low sinks, in clay soil which in
winter is excessively wet, and in summer becomes a “dry bog.” Wherever
the plant grows luxuriantly the salt content is invariably high, the
total salts varying from 327,000 pounds per acre, to a depth of three
feet, to 494,520 pounds in four feet. The salts consist mainly of
Glauber’s and common salts (a maximum of about 275,000 pounds each);
salsoda varies from 2,360 to 4,800 pounds per acre. The percentage
of common salt and total salts is higher than for any other plant
investigated, and the content of Glauber’s salt is also excessive.
The areas over which this plant grows must therefore be considered
among the most hopeless of alkali lands, for although its salts are
“white,” submergence during winter precludes the growth of Australian
saltbushes. Full underdrainage alone could reclaim the soil-areas it
occupies. Bushy Samphire is common on low-lying alkali lands in the
upper San Joaquin Valley, California, and extends northward along the
eastern slopes of the Coast Range to Suisun Bay. It is also abundant in
the Death Valley region, apparently overlapping the southward range of
the _Sarcobatus_, the greasewood properly so-called.

DWARF SAMPHIRE (_Salicornia subterminalis_, Parish, and other species
of the interior); Fig. 84.

[Illustration: FIG. 83.—Bushy Samphire—_Allenrolfea occidentalis_ (S.
Wats.) G. Ktze.]

[Illustration: FIG. 84.—Salicornia subterminalis. Alkali samphire.

A. Much-branched form.

B. Slender form.

C. Flower with the perianth removed showing the simple pistil and the
two stamens.

D. Portion of flowering spike, showing two joints. The flowers are
impressed in the joints in opposite clusters of three. In each cluster
the middle flower stands slightly above the two laterals as shown in
the lower joint.]

The three or four species of Dwarf Samphire which grow in the interior
valleys of the State are not usually very abundant, save locally.
Wherever the species do occur, however, they may be considered as
indicating excessively saline soils. Dwarf Samphire soil has shown
a total salt content of 441,880 pounds per acre in a depth of four
feet. The neutral Glauber’s salt amounts to 314,000 pounds, almost
as much as in Tussock-grass soil; common salt up to 125,640 pounds
while the salsoda varies from 2,200 to 12,000. We may consider the
plant as indicative of almost the highest percentage of common salt,
Glauber’s salt and total salts. Like the preceding species it indicates
land strongly charged with salts, more especially common salt, and
susceptible to cultivation only after reclamation by underdrainage.

_Salicornia subterminalis_, _S. herbacea_ (L.), _S. mucronata_, and
another species, all occurring inland, differ materially in habit and
botanical characters from the one so conspicuous in submerged salt
marshes along the seashore; but all alike indicate strongly saline
soils, reclaimable only by thorough drainage.

SALTWORT (_Suaeda torreyana_, Wats., _S. suffrutescens_, Wats., and
perhaps one other species); Fig. 85.

Samples of saltwort soil from Bakersfield and Byron Springs,
California, taken to a depth of one foot and three feet respectively,
show that this plant grows luxuriantly in a soil containing 130,000
pounds of total salts per acre in the first foot, and with 10,480
pounds of the noxious salsoda, and 39,760 pounds of common salt in
three feet; while only a sparse growth is found on soils containing
only 3,700 pounds of salts in three feet. It thus appears to indicate
a lower percentage of salsoda than does Greasewood, but a higher
percentage than Bushy Samphire. Further investigation is necessary
to determine the exact relation of the different salts to the growth
of the plant, and as to whether carbonates occur in large quantity;
but enough data have been gathered to show that a luxuriant growth of
Suaeda torreyana indicates a soil reclaimable only by thorough-drainage.

Suaeda torreyana occurs on low alkali lands throughout the State of
California, from San Bernardino to Honey Lake, in the desert sinks,
and in the Great Valley, in appropriate locations. Sometimes it is
replaced by _S. suffrutescens_ and perhaps other species, but all the
saltworts appear to grow in similar habitats, and it is probable that
the soil-conditions are practically the same for all these species.
They indicate land too heavily impregnated for the growth of ordinary
crops, but which will perhaps allow the Australian saltbush to succeed.

[Illustration: FIG. 85.—Saltwort—_Suaeda Torreyana_, Wats.]

GREASEWOOD (_Sarcobatus vermiculatus_) [Hook. Torr.]; FIG. 86.

This, the true _Greasewood_ of the desert region east of the Sierra
Nevada, and not either of the plants known under that name in the San
Joaquin Valley and in Southern California, invariably indicates a
heavy impregnation of the land with black alkali or carbonate of soda.
Since, as before stated, black alkali is most likely to occur in low
ground, we frequently find the true greasewood forming bright green
patches in the swales, and on the benches of periodic streams, as well
as on the borders of alkali ponds or lakes. Stock unaccustomed to it
will frequently go to these patches on a run, only to turn away badly
disappointed after taking a few bites, the plant being both bitter and
salty.

[Illustration: FIG. 86.—Greasewood (proper)—_Sarcobatus vermiculatus_
(Hook) Torr.]

    A. Appearance of a branch when not in blossom.
    B. Spiny-branchlet from the same.
    C. Branchlet bearing cones of male flowers.
    D. Cone of male flowers, enlarged.
    E. Branch bearing fruits.
    F. Cluster of fruits, enlarged.
    G. Vertical section through a fruit, showing the seed with its
         curved embryo, (enlarged).

Where a luxuriant growth of this plant is found, the soil may contain
from 38,000 to 117,000 pounds of total salts per acre, of which
sometimes nearly half is carbonate of soda; the content of common salt
is usually low, and Glauber’s salt or sulfate of soda, sometimes with
considerable proportion of epsom salt, forms a variable proportion of
the total.

Greasewood is distinctly a plant of the Great Basin, only reaching
California in the adjacent counties of Lassen, Alpine, Mono, and
northern Inyo. It is very abundant on the lower levels of Honey Lake
valley, Cal.

The Sarcobatus is chiefly found on silty or sandy soils of good
native fertility (see page 445, chapter 22), so that when its excess
of salsoda is neutralized by means of gypsum, the land becomes very
productive. Unfortunately the cost of the amount of gypsum required
to render such soils adapted to the tolerance of most culture plants
is often prohibitive; but where the correction of only small spots is
called for, the “white alkali” resulting from the gypsum treatment
would be tolerated by many culture plants.

ALKALI-HEATH (_Frankenia grandifolia campestris_ Gray); Fig. 87.

[Illustration: FIG. 87.—Alkali-Heath—_Frankenia grandifolia campestris_
A. Gray.]

Alkali-heath is perhaps the most widely distributed of any of the
California alkali plants. Its perennial, deep-rooting habit of growth,
and flexible, somewhat wiry rootstock, which enables it to persist
even in cultivated ground, render it a valuable plant as an alkali
indicator. The salt-content where Alkali-heath grows luxuriantly is
invariably high, ranging from 64,000 to 282,000 pounds per acre;
salsoda varies from 680 to 19,590 pounds; common salt ranges from 5,000
to 10,000 pounds. Such soils would not be benefited by the application
of gypsum, as the salts are already largely in the neutral state. Of
useful plants only Saltbushes and Tussock-grass are likely to flourish
in such lands, when not too wet.

While Alkali-heath is thus one of the most alkali-tolerant plants, it
is at the same time capable of growth with a minimum of salts (total
salts 3,700 pounds, salsoda 680 pounds). Where only a sparse growth of
this plant occurs, therefore, the land should not be condemned until a
chemical examination of the soil has been made.

Alkali-heath is found on soils of very varying physical texture
and degrees of moisture; while on soils of uniform texture and
moisture-conditions, but differing in chemical composition, it varies
with the varying salt-content.

It has been found that Australian saltbush (_Atriplex semibaccata_) can
be successfully grown on the “goose-lands,” of the Sacramento Valley,
on soil producing a medium crop of Alkali-heath; it remains to be
shown whether it will do equally well on soils producing a dense and
luxuriant growth of the same.

Alkali-heath is widely distributed throughout the interior valleys of
California; a closely related form grows in the salt-marshes of the
sea-coast.

CRESSA (_Cressa cretica truxillensis_ Choisy); Fig. 88.

Cressa soils show a low percentage of the noxious salsoda, but
comparatively heavy total salts (161,000 to 282,000 pounds per acre.)
Common salt varies from 5,760 to 20,840 pounds per acre in four feet.
The maximum is lower than in the case of Alkali-heath, but Cressa
seems to be much more closely restricted to strong alkali than does
the former species. Cressa appears to be as widely distributed through
the interior valleys of California as Alkali-heath. The Cressa is a
cosmopolitan plant, occurring, as its name indicates, on the Ionian
Islands, as well as in North Africa, Syria, and other arid countries of
the world.

SALTGRASS, _Distichlis spicata_.—This grass is of world-wide
distribution, and always indicates a sensible content of soluble salts,
without apparently any special preference for either of the three most
commonly occurring ones. Its maximum tolerance, as will be seen by
the preceding table, is very high, yet at the same time it will grow
luxuriantly on lands containing so little that other saline plants
like the samphires, saltwort or greasewood will refuse to grow. On
the shores of Honey Lake, California, it may often be seen incrusted
with the salts of the water concentrated by a long season of drought,
yet maintaining life, though somewhat stunted. On lands lightly
impregnated, stock will often eat it quite freely, so that it has been
mistaken for Bermuda grass, to which its habit and foliage bears some
resemblance. But Bermuda grass, while not as sensitive to alkali as
most forage grasses, will probably not bear much over 12,000 pounds per
acre.

[Illustration: FIG. 88.—Cressa—_Cressa cretica truxillensis_, Choisy.]

The mere _presence_ of the salt grass cannot therefore be taken as a
definite indication of anything more than that there is an unusual
amount of salts in the soil; whether or not there is more than will be
tolerated by the ordinary culture-plants, must be judged either from
the accompanying plants, or by experiment or analysis.

[Illustration: FIG. 89.—Salt-Grass—_Distichlis spicata_ (L.) Greede.]

_Relative Tolerance of the Different Species._—The following table
shows in systematic order the tolerance of the several plants discussed
above, for the different salts, so far as the data available permit.
The column marked _optimum_ shows under what proportions of salts the
plants grew in about equal luxuriance, therefore under, apparently,
the most favorable conditions. Both above and below the proportions
mentioned in that column, the luxuriance (size) and (usually) the
abundance of the plants was less; showing that while excessive amounts
of salts depressed their welfare, yet they also suffered when the
proportions dropped below a certain point. Whether this was partly or
wholly the result of competition with other plants, is an unsettled
question.

      TABLE SHOWING MAXIMUM, OPTIMUM, AND MINIMUM OF SALTS
         TOLERATED BY EACH OF THE SEVERAL ALKALI PLANTS.
    -------------------------------+-----------------------------
                                   |  Pounds Per Acre in feet.
                                   +-----------+--------+--------
                                   |  Optimum. |Maximum.|Minimum.
    -------------------------------+-----------+--------+--------
            _Total Salts._         |           |        |
    Bushy Samphire                 |   494,320 | 494,520| 135,060
    Dwarf Samphires                |   441,880 | 441,880| 441,880
    Alkali-heath                   |{  281,960}| 499,040|   3,720
                                   |{   64,300}|        |
    Cressa                         |   281,960 | 281,960| 161,160
    Saltworts                      |   130,000 | 153,020|   3,720
    Greasewood                     |    58,560 |  58,560|   2,400
    Tussock-grass                  |    49,000 | 499,040|  49,000
                                   |           |        |
        _Carbonate_ (Salsoda).     |           |        |
    Tussock-grass                  |    23,000 |  44,460|   3,040
    Alkali-heath              [202]| {  19,590}|  19,590|     680
                                   | {     680}|        |
    Greasewood                     |    18,720 |  18,720|   1,280
    Dwarf Samphires                |    12,120 |  12,120|   2,200
    Saltworts                      |    10,480 |  12,120|   1,120
    Cressa                         |     5,440 |   5,440|     680
    Bushy Samphire                 |     4,800 |   4,800|   1,500
                                   |           |        |
       _Chloride_ (Common Salt).   |           |        |
    Bushy Samphire                 |   212,080 | 275,160|  56,800
    Dwarf Samphires                |   125,640 | 125,640| 125,640
    Saltworts                      |    39,760 |  52,900|   1,040
    Cressa                         |    20,840 |  20,840|   5,760
    Alkali-heath                   |{   10,180}| 212,080|   1,040
                                   |{    5,760}|        |
    Tussock-grass                  |     6,200 | 172,800|   3,530
    Greasewood                     |     3,680 |   3,680|     160
                                   |           |        |
     _Sulphates_ (Glauber’s salt). |           |        |
    Dwarf Samphires                |   314,040 | 314,040| 314,040
    Bushy Samphire                 |   277,640 | 277,640|  50,080
    Cressa                         |   275,520 | 275,520| 134,880
    Alkali-heath                   |{  275,520}| 323,200|   1,560
                                   |{   34,530}|        |
    Saltworts                      |    44,160 | 104,040|   1,560
    Greasewood                     |    36,160 |  36,160|     960
    Tussock Grass                  |    19,640 | 323,200|  19,640
    -------------------------------+-----------+--------+--------

[202] This plant grows with equal luxuriance in soils containing only
680 pounds of carbonates.




APPENDICES.




APPENDIX A.

DIRECTIONS FOR TAKING SOIL SAMPLES. ISSUED BY THE CALIFORNIA EXPERIMENT
STATION.


In taking soil specimens for examination by the Agricultural Experiment
Station, the following directions should be carefully observed; always
bearing in mind that the examination, and especially the analysis, of
a soil is a long and tedious operation, which cannot be indefinitely
repeated.

_First._—Do not take samples at random from any points on the land, but
consider what are the two or three chief varieties of soil which, _with
their intermixtures_, make up the cultivable area, and carefully sample
these, each separately; then, if necessary, sample your particular
soil, noting its relation to these typical ones.

_Second._—As a rule, and whenever possible, take specimens from spots
that have not been cultivated, nor are otherwise likely to have been
changed from their original condition of “virgin soils”—_e.g._, not
from ground frequently trodden over, such as roadsides, cattle-paths,
or small pastures, squirrel holes, stumps, or even the foot of trees,
or spots that have been washed by rains or streams, so as to have
experienced a notable change, and not be a fair representative of their
kind.

_Third._—Observe and record carefully the normal vegetation, trees,
herbs, grass, etc., of the average virgin land; avoid spots showing
unusual growth, whether in kind or in quality, as such are likely to
have received some animal manure, or other outside addition.

_Fourth._—Always take specimens from more than one spot judged to
be a fair representative of the soil intended to be examined, as an
additional guarantee of a fair average, and mix thoroughly the earth
_taken from the same depths_.

_Fifth._—After selecting a proper spot, pull up the plants growing
on it, and sweep off the surface with a broom or brush to remove
half-decayed vegetable matter not forming part of the soil as yet. Dig
or bore a vertical hole, like a post-hole, and note at what depth a
change of tint occurs. In the humid region, or in humid lowlands of
the arid, this will usually happen at from six to nine inches from the
surface, and a sample taken _to_ that depth will constitute the “soil.”

In California and the arid region generally, very commonly no change
of tint occurs within the first foot, sometimes not for several feet;
hence, especially in sandy lands, the “soil” sample will usually be
taken to that depth, so as to represent the _average of the first foot_
from the surface down.

_Samples taken merely from the surface, or from the bottom of a hole,
have no definite meaning, and will not be examined or reported upon._

Place the “soil” sample upon a cloth (jute bagging should not be used
for the purpose, as its fibres, dust, etc., become intermixed with the
soil) or paper, break it up, mix thoroughly, and put _at least a quart_
of it in a sack or package properly labeled, for examination.

This specimen will, ordinarily, constitute the “soil.” Should the
change of color occur at a less depth than six inches, the fact should
be noted, but the specimen taken to that depth nevertheless, since it
is the least to which rational culture can be supposed to reach.

In the same way take a sample of each foot separately to a depth of at
least three feet; preferably four or five, especially in the case of
alkali soils, or suspected hardpan.

_Sixth._—Whatever lies beneath the line of change, or below the minimum
depth of six inches, will constitute the “subsoil.” But should the
change of color occur at a greater depth than twelve inches, the “soil”
specimen should nevertheless be taken to the depth of twelve inches
only, which is the limit of ordinary tillage; then another specimen
from that depth down to the line of change, and then the “subsoil”
specimens beneath that line.

The depth down to which the last should be taken will depend on
circumstances. It is always necessary to know what constitutes the
foundation of a soil, down to the depth of three feet _at least_, since
the question of drainage, resistance to drought, root-penetration,
etc., will depend essentially upon the nature of the substratum. In
the arid region, where roots frequently penetrate to depths of ten
or twelve feet or even more, it is frequently necessary to at least
_probe_ the land to that depth or deeper. The specimens should be taken
in other respects precisely like that of the surface soil, _each to
represent the average of not more than twelve inches_. Those of the
materials lying below the third foot from the surface may sometimes be
taken at some ditch or other easily accessible point, and if possible
should not be broken up like the other specimens.

If there is _hardpan or heavy clay_ present, an unbroken lump of it
should be sent, for much depends on its character.

_Seventh._—When in the case of cultivated lands, it is desired to
ascertain the cause of differences in the behavior or success of a
crop on different portions of the same field or soil area, do not send
only the soil which bears unsatisfactory growth, but also the one
bearing normal, good growth, for comparison. In all such cases, try to
ascertain by your own observations whether or not the fault is simply
in the subsoil or substrata; in which case a sample of surface soil
sent for examination would be of little use. In such examinations the
soil probe will be of great service, and save much digging or boring.

_Eighth._—Specimens of alkali or salty soils should preferably be taken
towards the end of the dry season, when the surface layers will contain
the largest amount of salts. A special sample of the first six inches
should in that case be taken separately by means of a post-hole auger,
and then, in a different spot close by, a hole four feet deep should
be bored, and _the earth from the entire four-foot column_ intimately
mixed before the usual quart sample is taken. Samples of the plants
growing on the land should in all cases be included in the package, as
they indicate very closely the agricultural character of the land.

_All samples taken while the land is wet should be air-dried before
sending; in the case of alkali soils this is absolutely essential._

_Ninth._—All peculiarities of the soil and subsoil, their behavior
under tillage and cultivation in various crops, in wet and dry seasons,
their location, position, “lay,” every circumstance, in fact, that
can throw any light on their agricultural qualities or peculiarities,
should be carefully noted, and _the notes sent by mail_. _Without
such notes, specimens cannot ordinarily be considered as justifying
the amount of labor involved in their examination._ Any fault found
with the behavior of the land in cultivation or crop-bearing should
be specially mentioned and described. The conditions governing
crop-production are so complex that even with the fullest information
and the most careful work, cases are found in which as yet the best
experts will be at fault.




APPENDIX B.

SUMMARY DIRECTIONS FOR SOIL—EXAMINATION IN THE FIELD OR ON THE FARM.


While the general principles upon which the cultural value and
adaptations of lands should be judged, have been given in the text
of this volume, it seems advisable to summarize their practical
application to land examination here, for convenient reference.

The directions given in Appendix A for the sampling of soils having
been carried out, the samples so taken may be subjected to farther
examination by any intelligent farmer to good purpose, and often with
great saving of time and expense.

Spread the samples from the several depths in regular order upon a
table or bench, and note the differences in color and texture apparent
to the eye or touch, and whether they will or will not crush readily
between the fingers, wet and dry. Whatever the fingers can do, can
similarly be done by the harrow, cultivator, clod crusher or roller.

The tilling qualities of the surface soil and immediate subsoil are the
first and most important matter to be ascertained; including especially
their behavior to water. Place some air-dried lumps in a shallow dish
with a little water; observe whether they take up the water quickly
or slowly, and whether in so doing the lumps fall to pieces or retain
their form. Slow penetration, and maintenance of form, will at once
indicate a soil somewhat refractory and difficult to till; while if
the water is taken up easily and the lump falls to pieces, the land
is easily cultivated and will absorb the rainfall and irrigation
water readily. The darkening of the tint on wetting will also give an
approximate idea of its humus-content.

Then take a wetted lump and work it between the fingers and on the palm
of the hand, until its “stickiness” or adhesiveness ceases to increase.
This “hand test” is of first importance and in skillful hands will
largely supersede the need of elaborate mechanical analysis. It will
at once enable the operator to classify the soil as a light or heavy
loam, clay loam or clay soil; it will show directly what will be the
result of plowing the land when wet, the liability to the formation of
a plowsole, and whether a single or a double team will generally be
needed to cultivate it properly. Also whether stock can be allowed to
pasture the land soon after rain. Comparison with the known land of
neighbors will also thus become easy, and in a measure the crops best
adapted to the physical qualities of the soil, subsoil and substrata,
taking into account their respective depths, will at once be at least
approximately determined. The presence of coarse and fine sand in
greater or less amounts will also be thus readily ascertained, allowing
estimates of the percolative properties; the latter can, of course,
be more practically tested in the field, in the manner described in
chapter 13, page 242.

A more definite estimate of the amount and kind of sand present in the
soil materials can be obtained by washing the _kneaded_ sample into
a tumbler, and allowing a thin stream of water to flow into it from
a faucet while gently stirring the turbid water. The clay, together
with the finest silts, will thus be carried off over the rim of the
glass, and sand of any desired degree of fineness, according to the
strength of the stream of water used, will be left behind. The kind and
amount of these sandy materials can then be estimated, or definitely
ascertained by weighing or measuring.

This will, generally speaking, be as far as the uninstructed farmer
can readily go; but these simple operations will give him an insight
into the nature of his soil and subsoil that will enable him to avoid a
great many costly mistakes.


RECOGNITION AND MEANING OF THE SEVERAL SOIL MINERALS.[203]

Those somewhat familiar with scientific methods and operations, and
supplied with pocket lens or microscope, can profitably go much
farther towards the definite ascertainment of the permanent cultural
value of the land, by the study of the minerals of which the sand is
composed, and which as a rule represent those from which the entire
soil has been formed; therefore indicate in a general manner its
chemical composition. Such examinations are specially feasible and
important when soils are not far removed from their parent rocks, as
in most of the arid region, and in the states north of the Ohio. In
the Southwestern states, in the coastal plain of the Gulf of Mexico,
the original soil minerals have usually been too far decomposed to
admit of definite identification. Sand is there as a rule made up of
quartz grains of many varieties, with only an occasional tourmaline and
pyroxene.

[203] For more details see chapters 3 and 4.

Among the prominent soil minerals, quartz is almost always recognizable
by its glassy luster and the irregular fracture—absence of definite
planes or facets of cleavage, causing the grains to be abraded or
rounded nearly alike in all directions. The _feldspars_, on the
contrary, always show a tendency to cleave into fragments having
definite, obviously oblique angles, which are perceptible even when
the grains are somewhat worn; because of the difference in the ease
with which wear takes place in the several directions. _Potash
feldspar_, moreover, which is the most important to be recognized
because it indicates a relatively large supply of potash in the
natural soil, is but rarely glassy in luster, but mostly dull white,
or reddish-white.—The _lime_ and _lime-soda_ feldspars rarely show as
definite forms, because of their tendency to form complex crystalline
aggregates (twins): and their definite recognition requires somewhat
complex (polarizing) appliances in connection with the microscope. In
such cases, however, the accompanying minerals (hornblende, pyroxene,
mica and others) often afford valuable indications, because of their
known association with soda-lime feldspars in certain rocks.

An abundant occurrence of _hornblende_ fragments, characterized by
their flat, tabular form, and bottle-green or black tint, indicate,
almost always a fairly good supply of lime in the soil, but leaves
the potash-supply in doubt. _Pyroxene_ (distinguished by its smooth,
polished surface from the angularly-weathering, usually rusty
hornblende fragments) rarely occurs with potash feldspar; and soils
strongly charged with it are mostly poor in potash.

_Mica_ occurs in so many rocks and is of so little consequence as
a soil-ingredient from the chemical point of view, because of its
difficult decomposition, that its presence can mostly only serve to
corroborate or contradict conclusions as to the derivation of a soil
from some particular rock or region. But mica serves a good purpose in
improving the tilling qualities of soils. Its thin scales must not be
mistaken for the tabular fragments of hornblende.

_Calcite_ in its several forms is mostly easily recognized both by
its form under the microscope, and by the effervescence its granules
show when touched with an acid. This effervescence can generally be
observed on touching the wetted soil with chlorhydric acid, so soon as
the content exceeds two per cent; but something depends upon the size
of the grains, as when these are very small, the giving-off of gas is
less readily noted. To facilitate it, the wetted soil may be warmed
before touching it with the acid. The recognition of the presence of
lime carbonate in soils is so important as to justify considerable
trouble in rendering it definite. When the aid of a chemist cannot be
commanded, fairly definite conclusions may be drawn from the character
of the native vegetation; regarding which, detailed information may
be found in Parts III and IV of this volume. But where, as in the
arid region, this criterion is not available, since the controlling
factor there is the moisture supply, a presumption may be gained by the
application of a slip of moistened red litmus paper to the _wetted_
soil. Should the red paper be turned blue within one or two minutes
it would indicate the presence of carbonate of soda (“black alkali”)
as well as of lime carbonates: but if blued only after twenty minutes
or more, it would indicate the presence of the carbonates of lime and
magnesia. If not changed at all, the conclusion would be that either
lime carbonate is in very small supply, or that the soil is in an acid
condition. (See chapter 8, p. 122).

_Saline and Alkali Soils._—The presence of an unusual or injurious
amount of _soluble salts_, as in the case of sea-coast and alkali
soils, is commonly easily ascertained in the field; where, if the
surface soil is at all seriously contaminated with soluble salts of any
kind, these may be seen on the surface during a dry season, forming a
whitish efflorescence, which in most cases is definitely crystalline.
In doubtful cases a tablespoonful of the surface soil may be leached
with water, and the first ten or fifteen drops caught in a clean,
bright silver spoon and evaporated. Or the soil may be stirred up with
about twice its bulk of water and the mixture be allowed to clear by
settling, then evaporating. A slight whitish film will almost always
remain in the spoon; but if the amount be somewhat considerable, the
presence of soluble salts is very readily recognized by pouring a
few drops of clear water on one side of the spot, and then allowing
it to flow gently over the spot to another place, where it is again
slowly evaporated. Any considerable amount of salts present will be
shown both in the diminution of the original spot, and in the soluble
residue accumulated where the water was last evaporated. Should _common
salt_ be present to any considerable extent, the residue in the silver
spoon will, if the last drops be allowed to evaporate slowly, show
square or cubical crystals to the naked eye, and certainly to a common
pocket lens. The residue may also be tested with red litmus paper for
carbonate of soda, which would quickly turn it blue.

More detailed examination requires chemical reagents and experience,
but the above tests should be sufficient to prevent the mistaking of
mere white spots, whose humus has been destroyed by fermentation caused
by bad drainage, with true alkali caused by excess of soluble salts; a
mistake not uncommon in both the arid and humid regions.




APPENDIX C.

SHORT APPROXIMATE METHODS OF SOIL EXAMINATION USED AT THE CALIFORNIA
EXPERIMENT STATION.


BY R. H. LOUGHRIDGE.

The California Experiment Station has for many years given the farmers
of the State the privilege of having their soils examined to ascertain
any physical defects, deficiency in plant-food, or the presence of
alkali salts. They have quite generally taken advantage of this, and
the number of samples of soil sent in each year has been very large.

A complete analysis of a soil-sample requires fully 15 days; hence the
necessity of adopting some quick methods for the determination of the
main elements of fertility, viz., humus, lime, potash, and phosphoric
acid, that would at the same time give results sufficiently accurate
for practical purposes. Similarly for alkali salts in the soil; the
leaching-out and analysis of which often occupies more than a week.

The following methods have been adopted, which shorten the time of
examination for the plant-food of a soil to about one hour, except for
potash, which requires a much longer time. For alkali salts the time is
reduced to two days, and less if a pressure filter be used.

_Humus._—The Grandeau method of ammonia extraction requires the removal
of the lime and magnesia with weak hydrochloric acid, washing out of
the acid and then digestion with weak ammonia; all of which, with a
soil rich in humus, may require many days, though a number of samples
may be put through at the same time.

The method adopted to determine adequacy or inadequacy of the humus
(for this is all that is intended in this examination) is completed in
less than half an hour. It is based on the color of the humus-extract
and avoids the necessity of removal of the lime from the soil.

The soil is pulverized in a mortar with a rubber pestle, and passed
through a half-millimeter sieve. Seven grams of the fine earth is
placed in a test tube with 15 or 20 cc. of a ten per cent solution of
caustic potash and boiled for ten or fifteen seconds, then allowed to
settle. The humus is dissolved and the density of the color of the
solution is an indication of adequacy or inadequacy. A dense black,
non-translucent solution shows the presence of at least one per cent
of humus in the soil; a deep brown translucent color indicates about
one-half of one per cent; while a light brown color clearly shows a
deficiency in the soil, and a need of a good green-manure crop.

_Lime._—Two grams of fine earth is treated with a little hydrochloric
acid, boiled for a few seconds, and ammonia is added to precipitate
the iron and alumina; the whole, with the soil-residue, is quickly
thrown on a filter to separate the mass from the lime solution, and
washed. After adding ammonium chlorid the lime is precipitated with
oxalate of ammonia, and its adequacy for soil-fertility judged of by
the turbidity of the solution, or the bulk of the precipitate. Or the
latter may be filtered off, dried and weighed. We thus obtain a measure
of the carbonate and humate of lime present, by comparing it with the
precipitate obtained from a soil whose percentage of lime has been
correctly ascertained.

_Potash._—The determination of potash in the soils requires more time
than either of the other ingredients, and is more rarely made by us.
Our knowledge of the soils of the State of California obtained through
many analyses, gives us a clue to those localities where potash would
probably be deficient, as well as to those whose soils are generally
extremely rich in potash; the percentages reaching usually from .5 to
as much as 1.5 per cent and more.

For the determination, two grams of the fine earth is digested in
hydrochloric acid over a steam bath for two days, the insoluble residue
filtered off, the filtrate evaporated to dryness to render the silica
insoluble, again filtered and the iron; alumina and lime removed by
precipitation with ammonia and oxalate of ammonia and filtration. The
filtrate is then evaporated to dryness, the ammonia salts destroyed
with _aqua regia_ or driven off by heat, and the alkalies changed to
chlorids. Any residue is then filtered off and platin-chlorid added to
precipitate the potash, which is separated and determined in the usual
way, either by reduction of the platinum by ignition, or by measurement
in a Plattner’s potash tube.

_Phosphoric Acid._—The determination of phosphoric acid is based on
the volume of the phospho-molybdate precipitate in a tube made like a
Plattner’s potash tube, but having a wider interior diameter for the
smaller portion (not greater than 3 millimeters), and a length of 50
mm. With this diameter, one mm. in height of the precipitate obtained
by our short method indicates one one-hundredth of one per cent of
phosphoric acid in the soil. The unit of measure must be obtained
for each tube, unless of uniform diameter, and is ascertained by
taking a soil whose phosphoric-acid percentage has been determined
gravimetrically and giving it the following quick treatment; which
must, of course, be closely followed in each soil to be examined.

Two grams of the fine earth is ignited in a platinum dish to destroy
the organic matter, transferred to a test-tube containing 5 cc. of
nitric acid and made to boil for only a couple of seconds, thus
preventing the solution of silicates to any material extent. It is not
allowed to stand, but a little water is immediately added and it is
quickly thrown on a small filter and washed with a little water. The
phosphoric acid is then precipitated with molybdic acid at the proper
temperature; allowing it to settle, the liquid is drawn off and the
precipitate transferred to the measuring-tube. It settles into the
small part in a short time if the latter is not too narrow, and is then
measured with a millimeter scale. This represents the percentage as
found in the soil by the gravimetric method, and serves as a guide for
other examinations, whose agreement with gravimetric determinations is
generally quite close, and quite sufficient for practical purposes.
The rapidity with which the solution is made and separated from the
soil is a matter of special importance for comparative results, or
determination of percentages; for if the acid solution be allowed to
stand for some time before filtration from the soil, silica passes into
solution also, and the volume of the molybdate precipitate is increased
by it; thus vitiating the results and adding to the time required for
the method. By this short method the practically important phosphoric
acid in the soil may be approximately determined within half an hour.


SHORT METHOD FOR ALKALI SALTS.

The old method of obtaining solutions of the salts by leaching the
soil on a filter until all of the alkali had been washed out has been
replaced by the following short one. 50 or 100 grams of the well-mixed
soil is placed in a bottle containing 200 cc. of water, shaken up
occasionally during 12 hours and allowed to settle. The solution may
then be passed through a common filter (or preferably a pressure
filter) and an aliquot part (usually 50 cc.) of the filtrate evaporated
to dryness in a platinum basin and ignited at a temperature just below
redness to destroy any organic matter that may be present. The basin
and contents are weighed and the soluble salts are dissolved in a very
little water and separated by filtration through a small filter into a
50 cc. cylinder and the alkali carbonates and chlorids determined by
titration, being calculated as sodium compounds.

The material remaining on the filter and in the basin, consisting of
insoluble earth, carbonates and calcium sulfate, is gently ignited
in the basin and weighed; the difference between this and the first
weight gives approximately the _total soluble salts_, which should
substantially correspond to the titrations made.

The sulfates are determined by differences between these and the total
alkalies. The solution may contain some sulfate of magnesia, or calcium
and magnesium chlorids, and these are determined gravimetrically.

Nitrates, which may have been destroyed in the first ignition, are
determined in the original solution by the picric method. Any magnesia
rendered insoluble by the ignition may usually be accounted for as
chlorid, unless much nitrate is present which is rarely the case in
carbonated alkali. If much nitric acid was found, it should be first
assigned to magnesia.




INDEX.


                    A.
                                                PAGE
    Absorption and movements of water in soils, 221
      of solids from solutions, 267
      of gases by soils, 272, 275
    Acacias, tolerance of alkali, 480
    Accessory minerals, 50
    Acid, strength used in soil analysis, 341
    Acidic and basic eruptive and metamorphic rocks, 49
    Acidity, neutrality, alkalinity of soils, 322
    Acids of different strengths, analysis with; table, 326, 341
    Action of plants in soil formation, mechanical and chemical, 19
    Aeration and reduction as influencing nitrification, 147
      effects of insufficient, in soils, 280
      excessive, injury in arid regions, 280
    Aerobic and anerobic bacteria, 144
    Air, functions in soils, 279
     “ of soils, composition of, 280
    Air-space in soils; figure, 108
    Alabama, vegetation and soil-characters, 511
    Alaska current, effects on California climate, 296
    Alinit, 149
    Alkali carbonates and sulfates, inverse ratio, 451, 452
      carbonates, effects on clay, 62
      effects on culture plants; figure, 426, 427
    Alkali-heath, range, tolerance of alkali, figure, 544, 545
    Alkali lands, crops for strong, 468
      effects of irrigation on, 428
      efficacy of shading, 457
      exceptionally productive when reclaimed, 483
      fertilization not needed in, 483
      formation from leachings of slopes, 453
      geographical distribution of, 423
      high and lasting production when reclaimed, 482
      inducements toward reclamation of, 481
      in the San Joaquin valley, Cal., figure, 425
      possible injury to, from excessive leaching, 462
    Alkali lands, summary of conclusions, 453
      surface and substrata of, 429
      utilization and reclamation of, 455
        of world-wide importance, 424
      vegetation of, 534
    Alkali-resistant crops, 455
    Alkali salts, black and white, 441
      composition of, 439
      composition of; general table, 442, 443
      distribution in heavy lands, 436, 437
      effects on beet crop, 465
      horizontal distribution of, 439
      in hill lands, 439
      in sandy lands, 433, 435
      in Salton Basin, distribution of, 4, 36, 438
      leaching-down of, 459
      nature of, 423
      plant food in, 441, 444
      reactions between, 449, 450
      reduction by cropping, 463
      relative injuriousness of, 464
      removal from the soil, 458
      removal by deep-furrow irrigation; figure, 460, 461
      tolerance of various crop plants; table, 466, 467
      total in lands; estimation of, 444
      underdrainage the universal remedy for, 460
      upward translocation from irrigation, 433
      vertical distribution in soils, 429, 431, 432, 434
    Alkali soils and seashore lands, 422
      calcareous character of, 28
      composition of, as a whole, 445, 446, 447
      how native plants live in, 430
      origin of, 422
      repellent aspect, cause of, 424
      retention of silica in, 392
    Alkali spots, white, 286
    Alkali, rise of, 428
      turning under of surface, 456
      weeds as cattle food, 468
        study of, by Loughridge and Davy, 535
    Aluminic hydrate, in soils of California and Mississippi;
      table  101, 390
    Alluvial soils, 12
    Ammonia-forming bacteria, 149
    Ammonia gas, absorption of, by soils; figure, 274, 275
    Ammonic carbonate, effects on glass, 18
    Ancient civilizations, preference for arid countries, 417
      rare in humid countries, 418
    Apatite, 63
    Arid and humid climates, rock-weathering in, 47
    Arid and humid regions, criteria of soils of, 371
      contrast between soils of, 28
      soils of, 111, 371
    Arid and humid soils, general comparison; table, 375, 377
    Arid belts, subtropic, 298, 299
      utilization of, 299
    Arid conditions, local, in tropical countries, 401
    Aridity, influence upon civilization, 417
    Arid region, bunch grasses on soils of, 111
      standing hay in, 300
      upland soils of; table, 373, 374
    Arid soils, productiveness induces permanent civil organization, 412
    Arroyo Grande and Yazoo buckshot soils, 345
    Asparagus, resistant to salts, 475
    Atmosphere, composition of; table, 16
    Azotobacter, 156
      Lipman on, 156

                    B.
    Bacteria, active in soil-formation, 20
      aerobic and anaerobic, 144
      denitrifying, 148
      food and functions of, 145
      in soils, numbers of, 142
      micro-organisms of soils, 142
      multiplication of, 144
      nitrifying, 146
    Bacterial life, effect on soil, condition, 149
      relation of carbonic dioxid to, 281
    Bacteroids, 151
      Mork figures, 152, 153
    Basaltic rocks, 49
    Basalts, red soils from, 52
    Basic slag, 64
    Basin irrigation, advantages of, 243
    Bauxite in soils, 101, 390
    Beet, sugar, effects of salts on, 474
      tolerant of common salt, 474
    Bhil soils, 414
    Bicarbonate of soda, 78
    Black-alkali lands, difficulty in draining, 462
      neutralizing of, 457
      waters, use of, 250
      why so called, 78
    Black earth of Russia, humus in; table, 130
    Black sand, 45
    Black prairie soils, 53
    Black soils and lands, 283
    Blizzards in continental America, 298
    Blown-out lands, 9
    Blue tint in clays and subsoils, 45
    Bodengare, 149, 150, 281
    Bog ore, formation of, in subsoils, 46, 66
    Bones, composition of, 64
    Bone meal, efficacy of, 65
    Borax, borate of soda, 79
    Bottom water, 227
      rise of, from irrigation, 227, 230
    Bottoms, first and second, contrast between, 506, 507, 509
    Bottoms, first, tree growth of, 507, 509
    Brahmaputra alluvium, Assam, 413
    Brown iron ore, 44
    “Buckshot” soils of Yazoo bottom, 116
    “Bunch grasses” as alkali-resistants, 471
    Burning-out of humus, effects of, 118
    Burrowing animals, work in soil-formation, 160

                    C.
    Calcareous clay, crumbling on drying, 116
      formations, predominance in Europe, 525
      soils, definition of, 367, 496, 524
        solubility of alumina and silica in, 389
      subsoils, and hardpans, 162
    Calciphile, calcifuge and silicophile plants, 521
    Calcite, calcareous spar, 39
      recognition of, 39
    Caliche, in Chile, Nevada, and California, 66, 67
    Capillarity, 189
    Capillary water, reserve of, 229
      rise of, 202 to 207
    Carbonated water, action on feldspar, 32
      action on silicates, 18
      universal solvent, 17
    Carbonate of soda, 77
      injury to soils and plants, 78
    Carbonates, chlorids and sulfates of earths and alkalies,
      reactions between, 449, 450
    Carbonic acid, 17
      secreted by roots, 20
    Carbonic dioxid, absorption of, by soils, 274
      heavily absorbed by ferric and aluminic hydrates, 278
      occurrence, formation, 17
      relation to fungous activity, 281
    Cascade range, climatic divide in N. W. America, 297
    Caves in limestone regions, 41
    Celery, moderate tolerance of alkali, 475
    Centrifugal elutriator, Yoder’s, 92
    Cereals, alkali-resistance, barley, gluten wheats, 471
    Channels, cutting-out by gravel, 6
    Charcoal, absorption of gases by, 276, 277
    Chemical absorption by soils, 270
      action of roots, 20
      analysis of soils (in general), 323
      character of soil, recognition of, 322
      decomposition, causes intensifying, 21
      processes of soil formation, 16
    Chile saltpeter, 66
    Chernozem, 130
      analyses of, table, 364
    Chestnut, American, a calcifuge tree, 491, 519
    Chlorin, largest ingredient of sea water, 27
    Chlorite, 36
    Chlorosis of vines in marly lands, 526
    Churn elutriator, Hilgard’s; figure, 91
    Circling of hill lands, 220
    Citrus fruits, injury to, from alkali, 478
      lemons most sensitive, 478
      sensitiveness to common salt, 477
    Classification of rocks, 47
      of soils, 10
    Clay as a soil ingredient, 83
      colloidal, 59
      functions of, in soils, 59
        maintains crumb structure, 110
    Clays, claystones, clay shales, 48
      colors of, 58
      formation, flocculation and deposition of, 33
      maturing of, 60
      plasticity and adhesiveness, influence of fine powders on, 85
        influence of ferric hydrate on, 85
    Clay-sandstones, soils from, 57
    Clays, separation of, by subsidence, by centrifuge, 89
      varieties, enumeration of, and characters, 57, 58
        fusibility of, 58
    Claystones, soils from, 59
    Cleavage of rocks, 3
    Cleopatra’s needle, 2
    Climate, 287
    Climatic and seasonal conditions, 21
    Climates, continental, coast and insular, 297
    Coffee soils, calcareous, 417
    Colloidal clay, amount in soils. Table, 84
      analysis of, by Loughridge, 385
      effects of alkali carbonates upon, 62
      investigation by Schloesing, 59
      properties of, 61
      separation of, by boiling and kneading, 61
    Colloid humates, 133
    Colluvial soils, 12
    Colors of soils, advantages of, 283
    Common salt, injuriousness in soils, 76
      recognition of, 76
      removal from soils, 76
    Conglomerates, 48
    Conifers, tall growth of, in arid regions, 517
    Contraction of soils in wetting and drying, 114
    Co-operation, favored by need of irrigation, 419
    Corsican and maritime pine, ash analyses, 520
    Cotton, compact growth and heavy boiling on calcareous soils, 503
    Cracking of clay soils in drying, 113
    Cressa; range, tolerance of alkali, figure, 545, 546
    Creep, 12
    Crops, alkali-resistant, 455
    Crumbling of calcareous clays on drying, 116
    Crumb-structure of soils; figure, 110
    Crusting of soils, effects of, 111, 117, 221
    Cultivated soils, analysis of, 325
      investigation of, 316
    Cultural experience the final test, 324
    Cutting-out of channels by water-borne gravel, 6
    Cypress, different forms of; figures, 507, 508

                    D.
    Date palm, resistance to alkali, 478
    Decomposition, chemical, of rocks, 16
    Decolorizing action, of soils, charcoal, 267
    Deep-rooting of native plants in arid region, 174
    Deforestation, effects of, 219
    Deltas, formation of, 7
    Denitrifying bacteria, 148
    Deserts, effects of winds in, 8
    Desert sands, only lack water to become productive, 420
    Dew, formation of, 307
      rarely adds moisture to soils, 308
      within the soil, 308
    Differentiation of soil and subsoil, causes of, 121
    Distance between furrows and ditches, 241
    Dolomite, 42
    Drainage, rights-of-way for, 461
    Drainage waters, use for irrigation, 250
    Drain waters, analyses of, table, 22
      leaching effects of, 271
    Drouth, resistance to, in arid soils, 167
    Dust soils, nature of, 104
      slow penetration of water in, 105
    Dust storms, 9
    Dynamite, used for shattering dense substrata, 181

                    E.
    Earth’s crust, known thickness, xxiv
    Earthworms, action of in soil-formation, 158
    Ecological studies, 314
    Egypt, obelisks of, 2
    Elements constituting earth’s crust, table of, xxiv
      important to agriculture, list of, xxiv
    Elutriator, Hilgard’s, figure, 91
    Epsomite, epsom salt, in soil, 78
    Eremacausis, 129
    Erosion in arid regions, 219
      in Mississippi table lands; figures, 218
      lowering of land by, 15
      of rocks by sand; figure, 10
    Eruptive rocks, basic and acidic, 49
      rocks, soils from, 52
    Eucalyptus, tolerance of alkali, 480
    European observations on plant distribution, 519
      standards of plant-food adequacy—Maercker’s table, 369
    Europe, predominance of calcareous formations in, 525
    Evaporation and crop yields, calculated, 193
      and crop yields, observed (Fortier), 194
      and plant growth, 193
      counteracting, in alkali lands, 455
      dependence on air temperature; Fortier’s experiments, table, 255
      from reservoirs and ditches, 257
      from water surfaces, 254
        wet and moist soils, 254
      in different climates, 192, 256
      in different localities, California, 255
      restrained by loose surface layer, 255
      through roots and leaves, amount of, 262, 263
    Expansion by oxidation, 18

                    F.
    Farmyard or stable manure, 72
    Feldspars, weathering of, 31
      products of, 32
    Ferghana, alkali lands in, 441
    Ferric hydrate, effects of, 100
      functions of, in soils, 285
      high absorptive power of, 277
      in Hawaiian soils; table, 356
      more diffused in humid than in arid soils, 392
    Ferric phosphate, unavailability of, 356
    Ferroso-ferric hydrate and oxid, 18, 45
    Ferrous oxid, 18
    Ferruginous lands, injury from swamping of, 233
    Fertilizers, mineral, 63
      waste of, by leaching, 269
    Flocculation and floccules, 91
    Flocculated structure; cements maintaining, 110, 111
    Flood-plains of rivers, 14, 15
    Fool’s gold, 75
    Force exerted by roots, 19
    Forecasts, general, of soil quality in forest lands, 507
      of soil values, popular, 313
    Forest trees, forms of, 499 to 502
      of Atlantic states on alkali lands, 481
    Form and development of trees, differences in, 498
    Forms of leaves, variation in, 502
      black-jack oak, 499, 501
      post oak, 499, 500
      trees, deciduous, in arid region, 516
      willow, scarlet, black and Spanish oaks, 502
    Freezing water, effects of, 3
    Frost, effect of soils, 118
    Fruiting, favored by lime in soils, 503
    Fungi and molds, action of, 123
      functions in humus-formation, 157

                    G.
    Gases, absorption of, by soils, 272, 275
      partial pressure of, 276
    Germination of seeds, 309
    Glacier flour, fineness and fertility of, 5
      physical analysis of, 5
    Glaciers, grinding and abrasion by; figure, 3
    Glauber’s salt, 77
    Glauconite, in calcareous sandstones, 56
    Gneiss soils, 51
    Gobi desert, migration of lakes, 9
    Going-back of orchards, 182
    Grain-sizes, effect on percolation; table, 224
      influence on soil texture, 100
    Grandeau method of humus estimation, 132
    Granite soils, potash and phosphoric acid in, 50
    Granitic rocks, weathering of, 47
      sand, formation in arid climates, 2
    Grano-diorite soils, of Sierra Nevada, 51
    Granular sediments, influence upon tilling qualities, 102
    Grape-vine, alkali, tolerance of, 475
    Grasses, cultivated, sensitive to alkali, 471
    Greasewood, range, tolerance of alkali; figure, 542, 543
    Greenstones, soils from, 51
    Ground water, depth most favorable to crops, 228
      variation of surface of, 228
    Gulf-stream, 295
    Gypsum or selenite, formation from sea-water evaporation, 42
      how recognized, 42

                    H.
    Halite, 76
    Hardpans, causes, formation and cements of, 185
    Hardpan, physical, analysis of, 103
      plowsole, 241
    Hawaiian Islands, humid and arid sides of, 297
      soils, analyses of, 356
    Hay bacillus; figure, 149, 150
    Heat and cold, effects on rocks, 1
      of high and low intensity, 304
      reflection and dispersion from soil surface, 304
      relations to soils and plant growth, 301
      trapping of suns, 288
    Heaviest clay soils, physical analysis of, 115
    Heaving-out of grain, 119
    Hematite, 44
    Herbaceous plants as soil indicators, 517
    Hog-wallows, 114
    Hornblende and pyroxene, 33
      weathering of, 33
    Horsetail rushes, secretion of silica by, 31
    Humates and ulmates, 133
      cementing effects of, 111
    Humid and arid climates, rock-weathering in, 47
    Humid region, upland soils of; table, 372, 374
    Humification in soils, 20
      normal conditions of, 129
      tests; Snyder, tables, 140
    Humin substances, formation of, 123
    Humus, amidic constitution of, 125
      and coal, amounts of, from vegetable substance, 128
      amount in soils, 133
      ash of, from Minnesota soils, analysis, 134
      decrease of nitrogen-content with depth, 135
      determination in soils, 132
      distribution in the surface soil, 157
      functions in soils, 21
      in arid and humid regions, 138
      in black earth of Russia, 130
      in Minnesota soils, 131
      in North Dakota soils, 133
      in the surface soil, 120
      losses from cultivation and fallow, 131
      nitrogen of, 124, 135
      percentage in soils, and nitrogen-content of,
        tables, 135, 136, 137
      porosity of, 124
      progressive changes in soils, 126
      relation to bacterial content, 144
      scanty in arid soils, but rich in nitrogen, 397
      substances, physical and chemical nature of, 124
      variation of, with original materials, 139
      volume weight of, table, 125
      versus adipocere, 140
    Hydraulic elutriation, 90
    Hydromica, 35
    Hydrous silicates in soils of arid region, 388

                    I.
    Ice-flowers on soils, 119
    Immediate plant-food requirements, ascertainment of, 333
      productiveness, chemical tests of, 337
      productiveness vs. permanent value of soils, 318, 327
    India, climatic contrasts, 401
    Indian soils, table of analyses, 410 412
      types of soils, 411
    Indo-Gangetic plain; calcareous hardpan, kankar, 411
    Injury from excessive runoff, prevention of, 220
      to plants from the various salts, 531
      to soils and plants from carbonate of soda, 78
    Insects, work in soil-formation, 160
    Insoluble residue of soils; less in arid than humid, 384
    Insufficient rainfall, leaves sea salts in soils, 28
    Insular climate, of Britain, western Europe, 298
    Introduction, xxiii
    Injury from swamping, permanent, 232
    Iron carbonate solution, how formed, 44
      coloring clays, 58
      minerals, 44
      pyrite, how recognized, 75
    Irrigation, basin, advantages and disadvantages, 244
      by check flooding, 237
        flooding, 237
        furrows, 238
        lateral seepage, 242, 241
        shallow, deep and wide furrows; diagram, 239
        surface sprinkling, 237
        underground pipes, 245
      ditches, leaky, effects on alkali lands, 429
      excessive surface rooting caused by, 245
      methods of, 236
      necessitates co-operation, 419
    Irrigation water, abundant use of saline, 249
      duty of, 251
      economy in use of, 243
      effects of saline, figures, 247
      heavy losses in using, 252
      limits of salinity, 246, 248
      loss by evaporation, 252
      loss by percolation; diagram, 253
      quality of, 246
      saline, how to use, 249
      testing penetration of, 242
      temperature of, 244
    Irrigation, winter, advantages of, 236
    Isinglass, 43

                    J.
    Janesville loam, chemical analysis of, 331
    Japan current, 296
    Jasper and hornstone pebbles, weathering of, 30

                    K.
    Kainit, composition of, 71
    Kaolinite and clay; kaolin, 32
      assumes plasticity on trituration with water, 60
      crystalline form of, 22, 59
      lacks plasticity, 60
    Kaolinization, results in zeolite-formation, 395
      slow in arid regions, 87, 386

                    L.
    Landholding, units of, smaller in arid than in humid region, 420
    Landlocked lakes, water of, 27
    Land plaster as a fertilizer; effects on soils, 43
    Landslides, 12
    Laterite soils, Wohltmann’s definition, 416
      Terra roxa of Brazil, 416
    Leaching of the land, 22
    Legumes, bacteria of, 150
      mostly sensitive to alkali, 472
    Leguminous plants, mostly calciphile; exceptions, 518
    Leucite, potash content, 32
    Lichens, action on rock-surfaces, 19, 20
    Lignites and coal, how formed, 127
    Lime a dominant factor in productiveness, 353
    Lime carbonate in sea water, 26, 27
      removed from earth’s surface, 41
      summary of effects in soils, 379
    Lime-content, effects of high, in soils, 365
      effects on availability of phosophates, table, 366
    “Lime-country is a rich country”, 365
    Lime, excess of, in arid soils, 378
    Lime feldspars, leave lime carbonate in soils, 32
      in alkali lands protects plants from salts, 532
      lands, failure of tea on, 414
    Lime-loving trees, 490 to 492, 497
    Lime, most abundantly leached out, 24
      percentages, what are adequate, 367
        in coast-belt soils, 496, 497
        in heavy clay soils; table, 368
    Lime renders lower amounts of plant-food adequate, 354
    Limestone countries, 53
        Rotten, 54
        soils, excluded from comparison of arid and humid soils, 376
        residual, 53
    Limestones, impure, as soil-formers, 40, 53
      residual soils of, how formed, 40
      soft, or marls, 40
      slow disintegration of pure, 63
    Limit of acid action on soils, investigation of, by Loughridge, 340
    Limonite, 44
    Loamy and sandy soils, show little shrinkage, 117
    Loose surface layer, illustration of effect, figure, 258, 260
      prevention of evaporation by, 257
    Loss of humus in summer mulch, 132
    Louisiana, vegetation and soil-characters, 512
    Lowland tree-growth, 506
    Lysimeter, 227

                    M.
    Madagascar, character and soils of, 405, 406
      climate and rocks of, 406
      methods used by Müntz and Rousseaux, 406
      potash and lime leached into valleys, 407
      red soils, 407, 409
      table of soil analyses, 408
    Madras, red soils of, 415
    Magnesia, effects of excess over lime, 382
      exceeds lime in tropical soils, 405
      high in arid soils, 381
      leached out next to lime, 24
      proper proportions to lime, 383
    Magnesian limestones as soil-formers, 42
    Magnesian soils largely poor, 36
    Magnetite, 45
    Maize and sorghums, alkali-resistance, 471
    Maize roots in humid and arid region, 175, 176
    Manganese, more in humid than arid soils, 383
      stimulant effects on crops, 383
    Marble and limestones, formation of, 39
    Marls, gypseous and calcareous, 43
    Marly substrata, 186
    Marine saline lands, 527
      first crops for, 533
      reclamation for culture, 534
    Matière noire; active nitrification of, 132, 360
    Mechanical analysis of soils, 88
    Melilots, white and yellow, alkali resistance, 473
    Mesas of arid region, 14
    Mesopotamia, rehabilitation of, 421
    Metamorphic rocks, 46
    Methods of irrigation, 236
      soil analysis, 325
    Mica as a soil ingredient, 35
      weathers slowly, 35
      mistaken for gold and silver, 35
    Mica-schist soils, 51
    Micro-organisms of soils, 142
    Mineral fertilizers, 63
      ingredients of soils, minor, 63
    Minerals injurious to agriculture, 73
      major soil-forming, and rock-forming, list of, 29
      tints of, 18
      unessential or injurious to soils, 75
    Mirabilite, 77
    Mississippi, changes in vegetation from east to west in northern, 490
      investigations in, by writer, 489
      northern, vegetative belts in; map, 490
        vegetative belts, descriptions of, 490, 491, 492
    Mississippi river, sediment carried by, 7
    Mississippi, southern, central prairie, long-leaf-pine belts, 493
      coast-belt; pine meadows; profile, 495
        live-oak or shell hammocks, 495
    Mississippi valley, climate of, 298
    Mississippi water, annual variations in, 25
      generalized composition of, 25
    Modiola, of Chile, 469
    Moisture hygroscopic, table, 196
        influence of temperature and air-saturation, 197
        method of determining, 197, 198
        Mitscherlich’s objections, 199
        utility to plant growth, 199
      available to growing plants, 211
      distribution in soil, as affected by vegetation, 264
      evaporated from forests, 265
        Eucalyptus, 265
      in Russian forests and steppes, 265
      requirements of crops in the arid region, 212
        Loughridge’s tables of same, 214
      supplied by tap roots, 229
      useful to crops retained by alkali lands, 433
      wasted by weeds, 264
    Moraines, in North Central states, 5
    Mosses, follow lichens in rock decomposition, 20
    Moulds and fungi, action of, 123
    Mountain chains, arid climate under lee of, 294
      effects of, on rainfall, 293
    Muddy waters, 251
    Muir glacier, analysis of mud, 5
    Mulches, loss of humus, 132
    Mulching with straw, sand, 266
    Mustard family, sensitive to alkali, 473
    Myrobalan root, use for grafting in alkali lands, 479

                    N.
    Native vegetation, basis of land values for farmers, 488
      causes governing its distribution not an unsolvable problem, 489
      result of struggle for existence, 487
    Native grasses for alkali lands, 470
    Native growth, cogency of conclusions based on, 314
    New Mexico, soils from; analysis, 378
    Nile water, Letheby’s analyses of, 25
    Nitrate deposits, origin of, 67
      of soda, 66
    Nitrates, waste of, by leaching, 24, 68
    Nitrification, active in matière noire, 360
    Nitrification and denitrification, 145
      in alkali lands, 68
      in soil of “ten-acre tract.”, 359
      intensity in arid climates, 68
      list of substances favoring, 147
      of organic matter in soils, experiments, 358
      not active in unhumified matter, 359, 360
    Nitrifying Bacteria, 146
    Nitrobacterium; conditions of activity, 146
    Nitrogen-absorbing bacteria, 156
    Nitrogen, absorbed more abundantly than oxygen, 278
      accumulation of, in humus, 124
      adequacy in humus, lowest limit of, 363
        in soils, 357
      availability of, in soils; ascertainable, 363
      content of humus, 135
      deficiency, pot test, figure, 362
      determination of, in soils, 357
      hungry soils; table, 361
      percentages in humus, what are adequate, 360
      supply of plants, views on, 150
    Nitrosomonas, figure, 246
    Nodules of legumes, 151
    North Central States, herbaceous vegetation on
      calcareous soils, 514, 515
      lowland growth in uplands, when, 515
      vegetation and soil-character, 513
    Nutritive salts in alkali, 441

                    O.
    Ocean currents, Gulf stream and Japan stream, 295, 296
    Olive, resistance to alkali, 478
    Organic and organized constituents of soils, 120
    Organisms influencing soil-conditions, 142
    Oxalic acid, secretion by lichens, 19
    Oxidation, expansion by, 18
    Oxids constituting earth’s crust; table, 31
    Oxygen, action in weathering rocks, 18
      proportion of, in earth’s crust, 30

                    P.
    Pamperos, 9
    Peat bogs, 122
    Peaty soil, shrinkage, 117
    Percolation in natural soils: diagram, 223, 225, 226
      rate of, as influenced by grain-sizes, 224
    Permanent value of land vs. Immediate productiveness, 340
    Physical and chemical causes of vegetative features, 505
      conditions of plant growth, 319
    Physical analyses, correlation with popular names, 96
        results of, 94
        table. Mississippi and California soils, 98
      analysis of soils, 88
      constituents of soils, 10
    Physico-chemical investigation of soils, 313
    Physiological soil analysis, 333
    Phosphate fertilizers, importance of, 65
    Phosphate fertilization, in arid region, 393
      in California, 393
    Phosphoric acid, limits of adequacy in soils, 355
      minute amounts leached from soils, 24
      no constant difference between arid and humid soils, 393
      rendered inert by ferric hydrate, 355
    Phosphorites, low-grade, of Nevada, Russia, 63, 64
    Plane tree, oriental, resistant to alkali, 480
    Plant-adaptation “varying from province to province”, 523
    Plant associations, plant formations, 315
    Plant distribution, Thurman’s physical theory of, 519
    Plant-development under different temperatures, 309
    Plant-food, accumulation in finest parts of soils, 87
      high percentages mean high land value, 346
      ingredients, condition of, in soils, 319
      in virgin soils, lowest limit of, table, 352
      limits of adequacy, 353
      minute amounts may produce large crops, 410
      percentages, what are high, 346
      percentages, low, 346
      water-soluble, reserve, unavailable, 320
    Plant-growth on arid subsoils, 166
    Plants, deep-rooting in arid region, 174
      indicating irreclaimable alkali lands, 535, 536
    Plant root action, cannot be imitated in laboratory, 324
    Plasticity, absence of, in fine powders, 60
      of clay, causes of, 60
        lost by burning, 60
    Plot tests, difficulties and uncertainties of, 334
      plan of, figure, 335
    Plowsole, how formed, by shallow irrigation, 186, 241
    Poor chalk lands, 525
    Pore-space, 108
    Porosity of humus, 124
    Port Hudson bluff, lignite in, figure, 128
      recession of, 116
    Potash, abundant in arid soils, 395
      and soda in arid and humid region, 394
    Potashes, production of detrimental to agriculture, 69
    Potash feldspar, supplies potash to soils, 32
      fertilization first in humid, last in arid region, 396
      from sea water, 69
      limits of adequacy in soils, 354
      minerals, orthoclase feldspar, 68
      preferential retention of, in soils, 272
      Salts, Stassfurt, 69
      slightly leached out, 24
      sulfate, high-grade, 71
    Pot-culture tests, 336
    Powders, absorption of various gases by, table, 277
    Prairie soils, black, 53
    Preparation of soils for physical analysis, 89
    Productive capacity and duration, forecast of, 346
    Progress of humification and formation of coal, table, 126, 127
    Pulverulent soils of arid regions, 87
    Purifying action of soils, 269
    Putrefactive processes, relation to carbonic gas and
       anaerobic bacteria, 282
    Putty soils, 103
    Pyroxene, augite, 33, 34

                    Q.
    Qualifications required for soil study, 524
    Quality of irrigation water, 246
    Quince, resistance to alkali, 479
    Quartz and allied rocks, 29
      sand most prominent ingredient of soils, 30
      veins, formation of, 31

                    R.
    Rain belts, temperate and tropical, 295
    Rainfall, amount of, 215
      distribution in California and Montana, 290
        in the United States, 215
        most important, 290
        on the globe, figure, 294
      influence on soil formation, 22
      insufficient, forms alkali soils, 28
        leaves lime behind, 28
      natural disposition of, 216
    Rains, beating, 221
      cold and warm, 302
    Reclaimable and irreclaimable alkali lands, 534
    Red foothill soils of California, 34
    Red or rust-colored soils, 34
      advantages of, 284
    Regur soils, Deccan, India, 414
      formation of, 415
      “guvarayi” hardpan, 415
      present production, 414
    Reh of India, 440
    Reserve plant-food in soils, 320
      of zeolites, carbonates, phosphates, 321
    Residual soils, 11, 13, 22
    Rhizobia, adaptation to symbiosis, 154
      inoculation of soils with, 154
      increase of crops by inoculation with, 155
      of legumes, 150
      mode of infection, 154
      varieties of forms, 154
    Rhubarb, sensitive to alkali, 475
    Rhyolites, soils from, 53
    River bars, formation of, 7
    Rivers, amount of dissolved matters carried by, 24
      flood-plains of, 13, 14
      sediment carried by, 24
      waters, analyses of, table, discussion, 23, 24
      white and green, 4
    Rock crystal, 29
    Rocks as soil-formers, 47
      chemical decomposition of, 16
      cleavage of, 3
      definition of, xxiii
      disintegration of, under extremes of temperature, 2
      effects of heat and cold on, 1
      erosion of by sand, 10
      forming minerals, 29
      fragments, rounding of, by flowing water, 6
    Rock powder, 85
    Rock-weathering in arid and humid climates, 47
    Rohhumus, 122
    Rolling of soils, of influence of, on heat, 305
    Root action, limitation of, 351
    Root bacillus, figure, 149, 150
    Root crops, effects of alkali upon, 474
    Root development in the arid and humid regions, 169 to 176
    Rooting, deep, from proper irrigation, 243, 245
    Roots, chemical action of, 21
      force exerted by, 19
      secrete carbonic acid, 20
    Root system in the humid region; figure, 168
    Rotten Limestone, soils from, analyses, 54
    Runoff of rain water, 216
    Russia, black earth of; roots and humus in, 130, 363
    Rye grass, giant, of Northwest; uses, 470

    S.
    Saline and alkali lands, vegetation of, 527
      plants, analyses of ashes of, 530
        selective power of, 531
    Saline and xerophile vegetation, similarity of, 528
    Saline contents of waters, variations of, 250
      solutions, structural and functional differences caused by, 528
      vegetation, general character of, 527
    Saltbushes, Australian, growth and use in California, 469
      of Great Basin, probable usefulness, 468
    Saltgrass; range, tolerance of alkali, figure, 546, 547
    Salton Basin, profile of salts in, 438
    Salts, absorption of, by saline plants, 529
    Saltwort: range, tolerance of alkali, figure, 540, 542
    Samoa and Kamerun soils, analyses by Wohltmann; method, 402
      table of, 404
    Samphire, Bushy and Dwarf; range, alkali-tolerance,
      figure, 538, 539, 540
    Sand blasts, effects on cobbles, 10
      coarse, effect of, on clays, 105
      erosion of rocks by; figure, 10
      hammocks, of Gulf coast, 56
    Sands of arid and humid regions, differences in, 86, 386
      table of analyses, 387
    Sand, silt and dust, 85
    Sandstones, 48
      argillaceous, 57
      calcareous, formation of, 56
      rich soils from, 56
      dolomitic, often form poor soils, 56
      ferruginous, poor soils from, 56
      siliceous, poor soils from, 55
      varieties of, 55
      zeolitic, soils from, 57
    Sandstone soils, lightness of, 55
      soils, poor, of humid region, 55
    Sand storms, 9
    Sandy lands, of arid regions, highly productive, 386
    Sandy soils, 30
    Schöne’s elutriator, figure, 90
    Shrinkage, extent of, in drying soils; figure, 113, 114
    Schübler on calcareous soils, 115
    Sea water, average composition of, table, 26
      chief ingredients useless to plants, 28
      minor constituents of, 27
      sources of salts in, 26
    Sedentary soils, 11, 13, 40
    Sedimentary rocks, 47, 48
    Sediment deposited by Mississippi in Gulf, 7
    Sediments, exhibition of, from physical analysis, 95, 96
      number of, in physical analysis, 93
      table of diameters and hydraulic values, 94
    Seeds, germination of, 309
    Semi-humid and semi-arid region, 377, 397
    Serpentine, 36
    Sieves, use in physical analysis of soils, 88
    Silica, absorption and secretion by plants, 31
      and alumina, soluble; quantitative relations, 385
      solubility in water, 31
      soluble, retained in alkali soils, 391
    Silicate minerals, 31
    Silicates of soda and potash, soluble, 31
    Silicon, abundance of, in rocks, xxxi
    Silicophile plants, a fiction, 522
    Sinkholes, 43
    Soapstone, 36
    Soda in arid and humid regions, 394
    Soda, nitrate of, 66
    Sodium salts, leached out by drains and rivers, 24
    Soil analysis, change of views regarding, 317
      discrepant methods used in, 402
      practical utility of, 318
    Soil and subsoil, causes and processes of differentiation, 120
      ill-defined, 120
    Soil bacteria, numbers of, 141
    Soil character, recognition from native vegetation, 487, 511
    Soil-dilution experiments, 347
      figures, 348, 349, 350
      table of, 350
    Soil-examination, short approximate methods for,
      used at California station, 560
      summary directions for, in field or farm, 556
    Soil-formation influenced by rainfall, 22
      physical processes of, 1
    Soil-forming processes accelerated by high temperatures, 398
    Soil-grains, number of, 99
        surface of, 99
        determination by air-flow, 99
        by “Benetzungswärme”, 99
      investigation, historical review of, 313
      moisture, regulation and conservation of, 234
      phosphates, solubility in water; Schloesing fils, 332
      probe, mode of using, 177
      profiles in arid and humid region, 165
    Soil, samples, directions for taking, by Calif. Station, 553
      sedentary or residual, 11, 13, 40
      study, qualifications needed for, 524
      surveys, early, of Kentucky, Arkansas and Mississippi, 316
      temperature, annual range near surface in arctic
        and tropical regions, 303
        change with depth; table, 303
        influence of evaporation on, 307
        influence of soil material, 306
        influence of surface conditions, 303
        influence of vegetation and mulch, 305
      tests by crop analysis, Godlewski, Vanderyst, 337, 338
        by extraction with organic acids; Dyer, Maxwell, 339
      water, different conditions of, 195
    Soils, acid-soluble and water-soluble portions most important, 324
      alluvial, 12, 13
      ancient, in geological formations, xxix
      calcareous, definition of, 367, 496, 524
      classification of, figure, 11
      colluvial, 12, 24
      definition of, xxix
      derived from various rocks, 49
      effects of crusting on, 221
      indefinite action of dilute acids on, 326
      interpretation of analyses; Wohltmann, 403
      physico-chemical investigation of, 313
      (see Table of Contents)
    Solar radiation, influence of, 302
    Solubility, continuous, of soils in water, 328
      King’s table, rich and poor soils, 330
      Schultze’s table, rich soil, 328
      Ulbricht’s table, poor soil, 329
    Solvent action of water upon soils, 327
      power of water, 17
    Solubility, increased with nitrogen-content, 141
    Sour grasses, 123
      humus, antiseptic properties of, 122
      soils, 122
    Souring of soils, by cultivation, 123
    Stable or farmyard manure, 72
      composition of, table, 73
      green-manuring only substitute, 74
      method of using, in humid region, 74
      physical effects of, 73
      use of, in the arid region, 74
    Stalactites and stalagmites, 41
    Stassfurt Salts, discovery of, 69
      importance to agriculture, 70
      origin of, 70
    Stassfurt Salts, nature of, 71
    Stonecrops, succeed mosses, 20
    Stone fruits, resistance to alkali, 478
    Stratified rocks, derived from crystalline, 29
    Stunted growth, caused by shallow or very heavy soils, 504
    Sturdy growth on calcareous lands, 502, 503
    Subsidence method, 89
    Subsoils, arid region, 163
      and deep plowing, 164
      calcareous, 162
      rawness of, in humid climates, 163
    Substrata in arid region, importance of, 173
      faulty, with figures, 177 to 180
      impervious, injury from; figure, 181
      leachy, 182
      marly, 186
    Subterranean rivers, 41
    Sulfate of potash, high-grade, 71
      of soda, dust from, 77
        injuriousness to plants, 77
        occurrence in arid regions, 77
    Sulfates, reduction of, 232
    Sulfuric acid in arid and humid regions, 394
    Summer mulch, loss of humus in, 132
    Sunflower family, resistance to alkali, 473
    Sun’s heat, penetration into the soil, 302
    Surface crusts, formation of, 111, 117
      physical analyses of, 118
    Surface, hydrostatic and ground waters, 215
    Surface waters, chemical effects of percolation, 161
      physical effects of percolation, 161
    Swamping of alkali lands, consequences of, 451, 463
      irrigated lands, results of, 231
    Symbiosis, adaptation to, of Rhizobia, 154
    Szek of Hungarian plain, 440

                    T.
    Tabashir, 31
    Talc and serpentine, 36
    Tap-roots, moisture supplied by, 229
    Tea, failure on calcareous lands, 414

    Temperature, annual mean of, 289
      conditions, ascertainment and presentation of, 288
      extremes, on high mountains and plateaus, 288
      of stellar space, 288
      seasonal, monthly and daily means, 289, 291
    Temporary vs. permanent productive capacity of soils, 340
    Tennessee and Kentucky, vegetation and soil-character, 513
    Terraces, river and lake, 14
    Testing penetration of irrigation water, 242
    Textile plants, tolerance of alkali, 475
    Thomas or basic slag, 64
    Thurman’s physical theory of plant distribution, 519
    Tillage; effects of; figure, 109, 110
      how maintained in nature, 111
    Titanium in soils, xxxi
    Time of acid-digestion, different; table, 342
    Tolerance of alkali by culture plants, 463
      alkali plants; table, 548, 549
    Topography, influence of, on climate, 293
    Trachytes, 53
    Trona, Urao, 77
    Tropical soils, 398
      are highly leached, 400
      often highly colored with iron, 400
      do not need early fertilization, 399
      humus in; abundant, but low in nitrogen, 399
        few determinations made, 399
        possible calculation of, 399
      investigations of, 401
      laterites, not always rich in iron, 400
        mostly have low plant-food percentages, 400
        resemble the “nimble penny”, 400
    Tubercles of legumes, figures, 151, 154
    Tufa, calcareous, 41
    Tussock grass, food value of, 470
      range, alkali-tolerance, figure, 536, 537

                    U.
    Ulmin substances, 122
    Underdrainage, advantages of, 235
    Underdrains, effects of, 234
    Unhumified organic matter does not nitrify, 148
    Unhumified vegetable matter, utility of, 135, 360
    United States, good field for comparative soil study, 524
    Upland and lowland growth in arid and humid regions, 515
    Usar lands of India, character of, 440
      not all alkali lands, 440

                    V.
    Vegetative belts, lime a governing factor of, 492
    Virgin lands, advantages of soil study in, 318
    Virgin soils, analysis by extraction with strong acids, 340
    Vivianite, 65
    Volatile part of plants, xxxii
    Volcanic ash, form soils rapidly; soils from, 20, 52
    Volcanic glass, 53
    Volume of soils, changes on wetting and drying, 122
    Volume-weight of soils, 107

                    W.
    Walnut, black, a lime-loving tree, 490 to 497
      tolerant of white alkali, 479
    Washing-away and gullying of land, 217
    Water, capillary, 201
        ascent in soil columns, figure, 202, 205
          uniform sediments, figure, 204, 207
        held at different heights in soil column, table, 208
        expansion and contraction in absorbing, 208, 209
        maximum and minimum of water-holding power, termination of;
        figure, 202, 207
        movements in moist soils, 210
      carbonated, solvent power, 17
      carrying power, 14
      controlling factor of soil temperature, 301
      density of, 190
      effects of flowing, 5
    Water extraction of soils, practical conclusions from, 332
    Water, hard, 41
      hygroscopic, 196
      of lan-locked lakes, 27
      loss of, by irrigation in shallow furrows, 240
      physical factors of, 188
      regulation of temperature by, 191
      relations to heat, 189
      requirements of growing plants, 192
        plants in arid regions, 195
      sidewise penetration of, in soils, 241
      of soils, chapters on, 188, 215, 234
      solvent action upon soils, 327
      solvent power, 17, 191
    Water-soluble plant-food, 321
      specific heat of, 190, 191
      table, 227
      vaporization of, 191
    Watery soil extracts, from European soils; tables, 327, 329
      American soils, King, 329, 330
    Wave action on shores, figure, 7
    Weathering, by oxygen, carbonic acid, water, 16, 17
      “  in humid and arid regions, 2, 86
    Weight of soils, per acre-foot, 107
    White soils, nature of, in humid regions, 285
      in arid regions, 286
    Wind deposits, 105
    Winds, action of, in forming soils, 8
      cyclones, and anti-cyclones, 293
      effects of, in deserts, 8
      heat the cause of, 291
      land and sea breeze, 291
      trade, and monsoons, 291, 292
    Winter irrigation, 236
    Wire-basket tests, of Bureau of Soils, 337

                    X.
    Xerophile vegetation, similarity to saline, 529

                    Y.
    Yazoo bottom, soils of, 116
    Yazoo “buckshot” and Arroyo Grande soils, 345

                    Z.
    Zeolites, decomposition by acids, 36, 38, 39
      exchange of bases in analcite and leucite, 37
      formation of, 37
      importance in soils, 38
      rocks cemented by, 38
    Zeolitic sandstones, 57




AUTHORS REFERRED TO.

    [NOTE.—In cases where no special credit is given
    in this volume for investigations made or data given from
    the Southwestern States and the Pacific Coast, these should
    be understood as work done, mostly under the writers
    direction, or by himself and assistants, in connections
    with the geological surveys of Mississippi and Louisiana,
    as well as the Tenth Census of the United States, by Drs.
    Eugene A. Smith and R. H. Loughridge; the chemical work for
    the Pacific Northwest, under the auspices of the Northern
    Transcontinental Survey, by M. E. Jaffa and Geo. E. Colby;
    that in California, at the Experiment Station, by the
    latter two, Dr. R. H. Loughridge, and temporary assistants.
    It would be impossible to segregate, without excessive
    prolixity, the credit to be assigned to each of these
    participants.]

                A.
    Adametz, L., 142, 281.
    Agassiz, L., 4.
    Aso, K., 383.

               B.
    Bamber, —, 401, 410, 414.
    Batholomew, J. G. 294.
    Beyerinck, M. W., 151, 156.
    Blumtritt, E., 276.
    Bonnier, G., 521.
    Böttcher, O., 393.
    Boussingault, J. B., 151, 276, 313.
    Brick, —, 528.
    Brock, see Morck, D.
    Burri, R., 148.
    Butler, O., 151.

               C.
    Cameron, F. K., 380, 466, 532, 533.
    Clarke, F. W., XXIV, 23.
    Colby, G. E., note above.
    Colmore, C. A., 448.
    Contejean, Ch., 521, 523, 531.
    Coville, F. V., 536.
    Crochetelle, J., 146, 147.

               D.
    Darton, N. H., 10.
    Darwin, Ch., 158.
    Davy, J. B., 535.
    Deherain, P. P., 146, 147.
    Detmer, W., 127.
    Djemil, —, 159.
    Duclaux, P. E., 144.
    Duggar, J. F., 155.
    Dumont, J., 146, 147.
    Dyer, B., 339, 357.

               E.
    Ebermayer, E., 279, 305.
    Eckart, C. F., 212.
    Eichorn, —, 327.
    Ermann, G. A., 303.

               F.
    Fawcett, W., 355.
    Fischer, Hugo, 156.
    Fliche, P., 520, 521.
    Forbes, R. H., 219.
    Fortier, S., 194, 254.
    Fraenkel, L., 142.
    Frank, A., 151.
    Fuelles, P., 281.
    Furry, F. E., 73.
    Furuta, T., 383.

               G.
    Geikie, J., 14.
    Gerlach, —, 156.
    Gilbert, G. K., 2.
    Gilbert, J. H., 151, 192.
    Godlewski, E., 337, 393
    Goss, A., 376, 530.
    Grandeau, L., 132, 133, 139, 357, 520, 521.

               H.
    Haberlandt, F., 310.
    Hall, A. D., 210, 227.
    Hare, R. F., 378.
    Harper, R. M., 494.
    Hartwell, B. L., 123.
    Headden, H. P., 18.
    Hedin, Sven, 9.
    Hellriegel, F., 130, 151, 192.
    Henrici, 200.
    Hillman, F. H., 536.
    Hiltner, L., 154.
    Hoffmann, R., 528.
    Hohl, J., 143.
    Hunt, T. S., 23.

               J.
    Jaffa, M. E., 135, 381, 450, 530.
    Johnson, S. W., 60, 380.

               K.
    Katayama, T., 383.
    Kearney, T. H., 532.
    Kedzie, R. C., 343, 375.
    Kellner, O., 393.
    King, F. H., 99, 108, 109, 168, 192, 193, 210, 211, 212, 224,
                     228, 236, 305, 325, 328, 332.
    Kinsley, J. S., 143.
    Knop, W., 197.
    Koch, R., 156, 281.
    Kossovitch, P., 363.
    Kosticheff, P., 130, 157.
    Kröber, 156.
    Krocker, F., 22.
    Kuntze, O., 67, 68.

               L.
    Ladd, E. F., 131, 133, 134, 141.
    Langley, S. P., 288.
    Lawes, J., 151, 192.
    Lea, E. C., 387.
    Leather, J. W., 401, 410, 411, 412, 414 to 417, 440.
    Lemberg, J., 272
    Lesage, M., 528.
    Letheby, H., 23.
    Liebig, J. von, 150, 313.
    Liebscher, G., 354.
    Lipman, J. G., 156.
    Loeb, J., 380.
    Loew, O., 23, 42, 382, 383.
    Loughridge, R. H., 87, 207, 213, 214, 240, 259, 340 to 342,
                           385, 430, 462, 466, 513, 535, 560.

               M.
    Maercker, M., 65, 369.
    Mann, H. H., 401, 410, 413.
    Manson, M., 294.
    Maxwell, W., 339.
    May, D. W., 42, 380.
    Mayer, A., 199, 207, 209.
    Mayo, N. S., 143.
    Mazurenko, D. P., 87.
    Means, T. H., 248, 478.
    Merrill, G. P., 2, 13, 167.
    Middendorff, V., 441.
    Mitscherlich, E. A., 99, 199.
    Miquel, P., 142, 281, 359.
    Mohr, Chas., 489, 511.
    Moore, G. T., 154.
    Morck, D., 154.
    Müller, A., 449.
    Müller, P. E., 122, 184.
    Müntz, A., 142, 355, 370, 401, 402, 406 to 410.
    Murray, John, 23, 24.
    Myers, H. C., 6, 144.

               N.
    Naegeli, C. v., 129.
    Nagaoka, M., 65, 393.
    Nobbe, F., 154.

               O.
    Osterhout, W. J. V., 533.
    Ototzky, L., 265.
    Owen, D. D., 316, 317, 343, 513.

               P.
    Peter, A. M., 175.
    Peter, R., 316,317, 343.
    Pichard, P., 147.
    Porter, J. L., 23, 24.
    Pumpelly, R., 110.

               R.
    Rafter, G. W., 217.
    [204]Ramann, E.
    Reade, T. M., 41.
    Regnault, V., 26.
    Reichert, E., 276.
    Richthofen, F. von, 110.
    Risler, E., 354.
    Rosenberg, S., 528.
    Rousseaux, E., 355, 370, 401, 402, 406 to 410.
    Rudzinski, D., 87.
    Russell, I. C., 24.

               S.
    Saussure, H. E. de, 150.
    Schimper, A. F. W., 523, 528.
    Schloesing, Th., 59, 111, 354.
    Schloesing, Th., fils, 332, 393.
    Schmidt, C., 23.
    Schöne, H. E., 90.
    Schübler, J. J., 116, 197, 313.
    Schultze, H., 328, 329.
    Seton, E. T., 159, 160.
    Shaler, N. S., 12.
    Shaw, G. W., 465.
    Smith, E. A., 511.
    Snyder, H., 131, 133, 134, 139.
    Stenhouse, —, 276.
    Stockbridge, H. E., 307, 308.
    Stone, C. H. H., 25.
    Stubenrauch, A. V., 222.
    Stutzer, A., 149.

               T.
    Thurmann, J., 519, 520.
    Tolman, L. M., 387.
    Tourney, J. W., 216.
    Traphagen, F. W., 23.
    Tuxen, C. F. A., 184.

               U.
    Udden, J. A., 106.
    Ulbricht, R., 328.

               V.
    Vanderyst, H., 338.
    Ville, G., 151.
    Voelcker, J. A., 22, 410.
    Vogel, J. H., 156.

               W.
    Wagner, P., 65.
    Ward, M., 144.
    Warington, R., 108, 146.
    Washington, H. S., xxiv.
    Way, J. T., 22, 73.
    Weber, A. H., 450.
    Wheeler, H. J., 123.
    Whitney, M., 94, 195, 207, 316, 321, 330, 332, 337.
    Wilfarth, H., 151.
    Williams, W. E., 60, 100.
    Winogradsky, S., 146, 156.
    Wohltmann, F., 355, 370, 401, 402 to 405, 406, 416.
    Wolff, E., 22, 73.
    Wollny, E., 110, 113, 125, 147, 159, 195, 264, 279,
                281, 284, 305, 306.
    Wüllner, —, 198.
    Wunder, G., 327.

               Y.
    Yoder, P. A., 92.

               Z.
    Zöller, P. H., 22.

[204] This writer’s valuable “Boden Runde” (1905) unfortunately came to
hand too late to be considered in this volume.

Printed in the United States of America.





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