SOILS

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.

TABLE OF CONTENTS.

Preface [xvii]

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]

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.

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.

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]

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:

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.

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.

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.

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.

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.

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.

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.

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.

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.

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:

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.

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.

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.

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.)

COMPOSITION OF RIVER WATERS.
PARTS PER MILLION.

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.

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.

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.

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.

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]

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]

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.

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.