Transcriber’s Notes:
Underscores “_” before and after a word or phrase indicate _italics_
in the original text.
Small capitals have been converted to SOLID capitals.
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SOILS
[Illustration: ·The MM Co·]
THEIR FORMATION, PROPERTIES, COMPOSITION,
AND RELATIONS TO CLIMATE AND PLANT GROWTH
IN THE HUMID AND ARID REGIONS
BY
E. W. HILGARD, PH.D., LL.D.,
PROFESSOR OF AGRICULTURE IN THE
UNIVERSITY OF CALIFORNIA, AND DIRECTOR
OF THE CALIFORNIA AGRICULTURAL EXPERIMENT STATION
New York
THE MACMILLAN COMPANY
LONDON: MACMILLAN & CO., LTD.
1921
COPYRIGHT, 1906,
BY THE MACMILLAN COMPANY.
Set up and electrotyped. Published July, 1906.
Norwood Press:
Berwick & Smith Co., Norwood, Mass., U.S.A.
SUMMARY OF CHAPTERS.
1. ORIGIN AND FORMATION OF SOILS.
Introduction.
Chapter I. Physical Processes of Soil Formation.
“ II. Chemical Processes of Soil Formation.
“ III. Chief Soil-forming Minerals.
“ IV. The Various Rocks as Soil-Formers.
“ V. Minor Mineral Ingredients of Soils. Mineral
Fertilizers. Minerals Injurious to Agriculture.
2. PHYSICS OF SOILS.
Chapter VI. Physical Composition of Soils.
“ VII. Density, Pore Space, and Volume-Weight of Soils.
“ VIII. Soil and Subsoil; Causes and Processes of
Differentiation; Humus.
“ IX. Soil and Subsoil; Organisms Influencing
Soil-Conditions. Bacteria.
“ X. Soil and Subsoil in their Relations to Vegetation.
“ XI. Water of Soils; Hygroscopic and Capillary Moisture.
“ XII. Water of Soils; Surface, Hydrostatic, and Ground
water; Percolation.
“ XIII. Water of Soils; Conservation and Regulation of
Soil Moisture. Irrigation.
“ XIV. Absorption by Soils of Solids from Solutions.
Absorption of Gases. The Air of Soils.
“ XV. Colors of Soils.
“ XVI. Climate.
“ XVII. Relations of Soils and Plant-Growth to Heat.
3. CHEMISTRY OF SOILS.
Chapter XVIII. Physico-Chemical Investigation of Soils in
Relation to Crop Production.
“ XIX. Analysis of Virgin Soils by Extraction with
Strong Acids, and its Interpretation.
“ XX. Soils of Arid and Humid Regions.
“ XXI. Soils of Arid and Humid Regions continued.
“ XXII. Alkali Soils, their Nature and Composition.
“ XXIII. Utilization and Reclamation of Alkali Lands.
4. SOILS AND NATIVE VEGETATION.
Chapter XXIV. Recognition of the Character of Soils from their
Native Vegetation. Mississippi.
“ XXV. Recognition of the Character of Soils from their
Native Vegetation. United States at large, Europe.
“ XXVI. Vegetation of Saline and Alkali Lands.
TABLE OF CONTENTS.
PREFACE xiii
INTRODUCTION, xxiii.—Definition of Soils, xxiii.—Elements
Constituting the Earth’s Crust, xxiii.—Average Quantitative
Composition of the Earth’s Crust, xxiii.—Clarke’s Table,
xxiv.—Oxids Constitute Earth’s Crust, xxiv.—Elements
Important to Agriculture; Table, xxv.—The Volatile Part of
Plants, xxvi.
CHAPTER I.
AGENCIES OF SOIL FORMATION, 1.—1. _Physical Agencies_,
1.—Effects of Heat and Cold on Rocks, 1.—Unequal
Expansion of Crystals, 2.—Cleavage of Rocks, 3.—Effects
of Freezing Water, 3.—Glaciers; Figure, 3.—Glacier Flour
and Mud, 4.—“Green” and “White” Rivers, 4.—Moraines,
5.—Action of Flowing Water, 5.—Enormous Result of
Corrasion and Denudation, 6.—Effects of Winds, 8.—Dunes,
8.—Sand and Dust Storms in Deserts, Continental
Plateaus and Plains, 8.—Loess of China, 9.—Migration
of Gobi Lakes, 9.—_Classification of Soils_, 10.—Their
Physical Constituents, 10.—Sedentary or Residual Soils,
11.—Colluvial Soils, 12.—Alluvial Soils. Diagram,
12.—Character of these Soil Classes, 13.—Richness of
Flood-plain and Delta Lands, 14.—Lowering of the Land
Surface by Soil Formation, 15.
CHAPTER II.
CHEMICAL PROCESSES OF SOIL FORMATION, 16.—2. _Chemical
Disintegrations or Decomposition_, 16.—Ingredients of
the Atmosphere, 16.—Effects of Water; of Carbonic Acid,
17.—Carbonated water a universal solvent, 17.—Ammonic
carbonate, effect on silicates, 18.—Action of oxygen;
on ferrous compounds, 18.—_Action of Plants and their
Remnants_, 19.—_A._ Mechanical; Force of Root Penetration,
19.—_B._ Chemical; Action of Root Secretions, 19.—Bacterial
Action, 20.—Humification, 20.—_Causes Influencing Chemical
Action and Decomposition_, 21.—Heat and Moisture,
21.—Influence of Rainfall on Soil-Formation, 22.—Leaching
of the Land, 22.—Residual Soils, 22.—Drain Waters; River
Waters. Tables of Solid Contents, 22.—Amount of Dissolved
Matters Carried into the Sea; Amount of Sediment, 24.—Sea
Water, Composition of; Waters of Landlocked Lakes,
25.—Results of Insufficient Rainfall; Alkali Lands, 28.
CHAPTER III.
ROCK-AND SOIL-FORMING MINERALS, 29.—Quartz, quartzite,
jasper, hornstone, flint, 29.—Solubility of silica in
water; absorption by plants, 30.—Silicate Minerals,
31.—Feldspars, their Kaolinization, 31.—Formation of Clays,
33.—Hornblende or Amphibole, Pyroxene or Augite, 33.—Their
Weathering and its Products, 33.—Mica, Muscovite and
Biotite, 35.—Hydromica, Chlorite, 35.—Talc and Serpentine;
“Soapstone”, 36. The Zeolites; Exchange of Bases in
Solutions, 36.—Importance in Soils, in Rocks, 38.—Calcite,
Marble, Limestones; their Origin, 39.—Impure Limestones
as Soil-Formers, 40.—Caves, Sinkholes, Stalactites, Tufa,
41.—Dolomite; Magnesian Limestones as Soil-Formers,
42.—Selenite, Gypsum, Land Plaster; Agricultural Uses,
42.—Iron Spar, Limonite, Hematite, Magnetite, 44.—Reduction
of Ferric Hydrate in Ill-drained Soils, 45.
CHAPTER IV.
THE VARIOUS ROCKS AS SOIL-FORMERS, 47.—General Classification,
47.—Sedimentary, Metamorphic, Eruptive, 47.—Sedimentary
Rocks; Limestones, Sandstones, Clays, Claystones, Shales,
47.—Metamorphic Rocks: Formed from Sedimentary, 48.—Igneous
or Eruptive Rocks, Basic and Acidic, 49.—_Generalities
Regarding Soils Derived from Various Rocks_, 49.—Variations
in Rocks themselves. Accessory Minerals, 50.—Granites;
not always True to Name; Sierra Granites, 51.—Gneiss.
Mica-schist, 51.—Diorites, 51.—Diabases, 51.—Eruptive
Rocks; Glassy ones Weather Slowly; Basaltic Oxidize
Rapidly, 52.—Red Soils of Hawaii, Pacific Northwest,
52.—Trachyte Soils; Light-colored, rich in Potash.
Rhyolites generally make Poor Soils, 53.—_Sedimentary
Rocks_, 53.—Limestones, 53.—“A Limestone Country is a
Rich Country,” 53.—Residual Limestone Soils; from “Rotten
Limestone” of Mississippi; Table, 54.—Shrinkage of Surface,
55.—Sandstone Soils, 55.—Vary According to Cement, and
Nature of Sand, 55.—Calcareous, Dolomitic, Ferruginous,
Zeolitic, 56.—Clay-sandstones, Claystones, 57.—_Natural
Clays_, 57.—Great Variety, Enumeration and Definition,
58.—Colors of Clays, 58.—_Colloidal Clay_, Nature and
Properties, 59.—Plasticity; Kaolinite Non-plastic,
59.—Causes of Plasticity, 60.—Separation of Colloidal Clay,
its Properties, 61.—Effects of Alkali Carbonates on Clay,
62.
CHAPTER V.
THE MINOR MINERAL INGREDIENTS OF SOILS; MINERAL
FERTILIZERS, 63.—Minerals Injurious to Agriculture,
63.—_Minerals used as Fertilizers_, 63.—Apatite;
Phosphorites of the U. S., Antilles, Africa, Europe,
63.—Phosphatic Iron Ores, “Thomas Slag,” 64.—Animal
Bones; Composition and Agricultural Use, 64.—Vivianite,
Dufrenite, 65.—Chile Saltpeter, 66. Occurrence in Nevada,
California, 66.—Origin of Nitrate Deposits, 67.—Intensity
of Nitrification in Arid Climates, 68.—Potash Minerals,
68.—Feldspars not Available, 68.—Depletion of Lands by
Manufacture of Potashes, 69.—Discovery of Stassfurt
Salts, 69.—Origin of these Deposits, 70.—Nature of the
Salts, 71.—Kainit, 71.—Potash Salts in Alkali Soils,
72.—Farmyard or Stable Manure; Chemical Composition,
Table, 72.—Efficacy largely due to Physical Effects
in Soils, 73.—Green-manuring a Substitute for Stable
Manure, 74.—Application of Stable Manure in Humid and
Arid Climates, 74.—_Minerals Unessential or Injurious to
Soils_, 75.—Iron Pyrite, Sulphur Balls, 75.—Occurrence
and Recognition. Remedies 75.—Halite or Common Salt,
76.—Recognition of Common Salt, 76.—Mirabilite or Glauber’s
Salt; in Alkali Lands; not very Injurious, 77.—Trona
or Urao; Carbonate of Soda, “Black Alkali,” 77.—Injury
Caused in Soils, 78.—Epsomite or Epsom Salt, 78.—Borax,
79.—Soluble Salts in Irrigation Waters, 79.
CHAPTER VI.
PHYSICAL COMPOSITION OF SOILS, 83.—Clay as a Soil Ingredient,
83.—Amounts of Colloidal Clay in Soils, 84.—Influence of
Fine Powders on Plasticity, 85.—Rock Powder; Sand, Silt
and Dust, 86.—_Weathering in Humid and Arid Regions_,
86.—Sands of the Humid Regions, 86.—Sands of Arid Regions
not Sterile, 86.—_Physical Analysis of Soils_, 88.—Use of
Sieves. Limits, 88.—Use of Water for Separating Finest
Grain-Sizes, 89.—Elimination of Clay by Subsidence and
Centrifugal Method, Hydraulic Elutriation, 90.—Schöne’s
Instrument, 90.—Churn Elutriator with Cylindrical Tube,
91.—Figures of Same, 91.—Yoder’s Centrifugal Elutriator,
92.—Number of Grain-sizes Desirable, 93.—Results of such
Analyses, 93.—Physical Composition Corresponding to
Popular Designations of Soil-Quality. Table, 96.—Number
of soil-grains per Gram, 99.—Surface Offered by various
Grain-sizes, 99.—Influence of the several Grain-sizes
on Soil Texture, 100.—Ferric Hydrate, its Effects on
Clay, 100.—Other Substances, 101.—Aluminic Hydrate,
101.—Influence of Granular Sediments upon the Tilling
Qualities of Soils, 102.—“Physical” Hardpan, 103.—Putty
Soils, 103.—Dust Soils of Washington; Table, Physical
Analyses of Fine Earth, 104.—Slow Penetration of Water,
105.—Effects of Coarse Sand, 105.
CHAPTER VII.
DENSITY, PORE-SPACE AND VOLUME-WEIGHT OF SOILS,
107.—Density of Soil Minerals, 107.—No Great Variation,
107.—Volume-weight most Important, 107.—Weight per
Acre-foot, 107.—Air-space in Dry Natural Soils. Figure,
108.—May be Filled with Water, 108.—Effects of Tillage.
Figures, 109.—Crumb or Flocculated Structure; Cements,
109.—How Nature Tills, 111.—Soils of the Arid Regions;
do not Crust, 112.—Changes of Soil-Volume in Wetting and
Drying, 112.—Extent of Shrinkage, 113.—Expansion and
Contraction of Heavy Clay Soils. Figure, 113.—Contraction
of Alkali Soils on Wetting, 114.—“Hog Wallows,”
114.—Physical Analyses of such Soils. Table, 115.—Crumbling
of Calcareous Clay Soils on Drying, 116.—Yazoo Bottom, Port
Hudson Bluff, 116.—Loamy and Sandy Soils, 117.—Formation of
Surface Crusts, Physical Analyses, 117.—Effects of Frost on
the Soil; Heaving; Ice-flowers, 118.
CHAPTER VIII.
SOIL AND SUBSOIL; CAUSES AND PROCESSES OF DIFFERENTIATION
HUMUS, 120.—Soil and Subsoil ill-defined, 120.—_The
Organic and Organized Constituents of Soils_, 120.—Humus
in the Surface Soil, 120.—Soil and Subsoil; Causes
of their Differentiation, 121.—Ulmin Substances or
Sour Humus, 122.—Sour Soils, 122.—Cultivation Induces
Acidity, 123.—Humin Substances, 123.—Porosity of Humus,
124.—Physical and Chemical Nature of the Humus Substances.
Table, 124.—Chemical Nature, 125.—Progressive Changes and
Effect on Soils, 126.—The Phases of Humification, Wood to
Anthracite; Table, 127.—Amounts of Humus and Coal formed
from Vegetable Matter, 128.—Figure, From Port Hudson Bluff,
128.—Conditions of Normal Humification, 129.—Eremacausis in
the Arid Regions, 129.—Black Earth of Russia; Kosticheff’s
Table, 130.—Losses of Humus from Cultivation and Fallowing,
131.—Estimation of Humus in Soils; Unreliability of
Combustion Methods, 132.—Grandeau Method, “Matière Noire,”
132.—Amounts of Humus in Soils, 133.—Humates and Ulmates,
134.—Mineral Ingredients in the Humus, 134.—Functions
of the Unhumified Organic Matter, 135.—The Nitrogen
Content of Humus, 135.—Table for Arid and Humid Soils,
136.—Decrease of Nitrogen Content in Humus with Depth,
138.—Table, Russian River Soils, 139.—Influence of the
Original Material upon the Composition of Humus, 139.—Table
of Snyder, 139.—Effect of Humus in rendering Mineral Plant
Food Available, 140.
CHAPTER IX.
SOIL AND SUBSOIL (_continued_), 142.—ORGANISMS INFLUENCING
SOIL-CONDITIONS. BACTERIA, 142.—_Micro-organisms of the
Soil._ Bacteria, Moulds, Ferments, 142.—Numbers at Various
Depths, given by Early Observers, 142.—Investigations of
Hohl; Mayo and Kinsley. Tables, 143.—Multiplication of the
Bacteria, 144.—Aerobic and Anaerobic Bacteria, 144.—Food
Materials required, 145.—Functions of the Bacteria,
145.—Nitrifying Bacteria. Figures, 146.—Conditions of
their Activity. Table, 146.—Effects of Aeration and
Reduction, 147.—Unhumified Organic Matter does not Nitrify,
148.—Unhumified Vegetable Matter, Functions in Soils,
148.—Denitrifying Bacteria. Figures, 148.—Ammonia-forming
Bacteria. Figures, 149.—Alinit, 149.—Effects of Bacterial
Life on Physical Soil Conditions, 149.—Root-bacteria, or
Rhizobia of Legumes, 150.—Figures of Root Excrescences
and Corresponding Bacteroids, 152.—Varieties of Forms,
154.—Mode of Infection, 154.—Cultural Results, 155.—Table
Showing Increased Production by Soil Inoculation,
155.—Other Nitrogen-absorbing Bacteria, 156.—Distribution
of Humus in the Surface Soil, 157.—Fungi, Moulds and Algae,
157.—_Animal Agencies_—Earthworms, Insects, Burrowing
Quadrupeds, 158.
CHAPTER X.
SOIL AND SUBSOIL IN THEIR RELATIONS TO VEGETATION,
161.—Physical Effects of the Percolation of Surface
Waters, 161.—Chemical Effects; Calcareous Subsoils and
Hardpans, 161.—“Rawness” of Subsoils in Humid Climates,
162.—Subsoils in the Arid Region, 163.—Deep Plowing and
Subsoiling in the Arid Region; examples of Plant growth on
Subsoils, 164.—Resistance to Drought, 167.—Root System in
the Humid Region, 168.—Figures of the Root System of an
Eastern (Wisconsin) Fruit Tree, 168.—_Comparison of Root
Development in the Arid and Humid Regions_, 169.—Prune
on Peach Root, 169.—Adaptation of Humid Species to Arid
Conditions, 169.—Grapes, 170.—Kentucky and California
Maize, 175, 176.—Hops, 172.—Deep-Rooting in the Arid
Region, 174.—Goose Foot and Figwort, 174.—_Importance
of Proper Substrata in the Arid Region_, 173.—Injury
from Impervious Substrata. Figure, 177.—Faulty Lands of
California. Figure, 178.—Shattering of Dense Substrata
by Dynamite, 181.—Leachy Substrata, 182.—“Going-back”
of Orchards, 182.—Hardpan, Formation and Varieties,
183.—Nature of the Hardpan Cements, 184.—Bog Ore,
Moorbedpan and Ortstein; Calcareous and Alkali Hardpan,
184.—The Causes of Hardpan, 185.—“Plowsole,” 186.—Marly
Substrata, 186.
CHAPTER XI.
THE WATER OF SOILS. HYGROSCOPIC AND CAPILLARY MOISTURE,
188.—General Properties, 188.—Physical Factors
of Water compared with other Substances. Table,
188.—Capillarity or Surface Tension, 189.—Heat Relations,
190.—Density, 190.—Specific Heat and its Effects,
190. Ice, 191.—Vaporization, 191.—Solvent Power,
191.—Water-requirements of Growing Plants, 192.—Evaporation
from Plants in Different Climates, 192.—Relations between
Evaporation and Plant Growth. Table, 193.—Fortier’s
Experiments. Figure, 194.—Different Conditions of
Soil Water, 196.—_Hygroscopic Water in Soils; Table_,
196.—Influence of Temperature and Air-Saturation,
197.—Utility of Hygroscopic Water to Plant Growth,
199.—Mayer’s Experiments, 200.—Summary, 200.—_Capillary
Water_, 201.—Ascent of Water in Soil-Columns. Table,
202.—Ascent in Uniform Sediments. Figure, 204.—Maximum and
Minimum Water-holding Power, 207.—Capillary Water held at
different Heights in a Soil Column. Table, 208.—Capillary
Action in Moist Soils, 210.—Proportion of Soil Moisture
Available to Plants, 211.—Moisture Requirements of Crops in
the Arid Region, 211.—Tables of Observations in California,
214.
CHAPTER XII.
SURFACE, HYDROSTATIC AND GROUND WATER. PERCOLATION,
215.—Amount of Rainfall, 215.—Natural Disposition of Rain
Water, 216.—The Surface Runoff, 216.—Washing-away and
Gullying in the Cotton States, 217.—Injury in the Arid
Regions, 219.—Deforestation, 219.—Prevention of Injury to
Cultivated Lands from Excessive Runoff, 220.— _Absorption
and Movements of Water in Soils_, 221.—Determination of
Rate of Percolation. Diagram, 221.—Summary, 224.—Influence
of Variety of Grain-sizes, 224.—Table of King’s
Experiments, 224.—Percolation in Natural Soils. Figure,
225.—Ground or Bottom Water, 227.—Lysimeters, Surface of
Ground Water; Variations, 227.—Depth of Ground Water most
Favorable to Crops, 228.—Moisture Supplied by Tap Roots,
229.—Reserve of Capillary Water, 229.—Injurious Rise of
Bottom Water from Irrigation, 230.—Consequences of the
Swamping of Irrigated Lands; Prevention, 231.—Permanent
Injury to certain Lands, 231.—Reduction of Sulfates,
232.—Ferruginous or Red Lands, 233.
CHAPTER XIII.
WATER OF SOILS; THE REGULATION AND CONSERVATION OF SOIL,
MOISTURE; IRRIGATION, 234.—Loosening of the Surface,
234.—Effects of Underdrains; Rain on Clay Soils,
235.—Winter Irrigation, 236.—_Methods of Irrigation_,
236.—Surface Sprinkling, 237.—Flooding, 237.—Check
Flooding. Furrow Irrigation, 237, 238.—Figure Showing
Penetration, 239.—Figure Showing Faulty Irrigation
in Sandy Lands, 239.—Distance between Furrows and
Ditches, 241.—Irrigation by Lateral Seepage, 242.—Basin
Irrigation of Trees and Vines; Advantages and Objections,
243.—Irrigation from Underground Pipes, 245.—_Quality of
Irrigation Waters_, 246.—Saline Waters; Figures of Effects
on Orange Trees, 246.—Limits of Salinity, 246.—Mode of
Using Saline Irrigation Waters; Apparent Paradox, 249.—Use
of Drainage Waters for Irrigation, 250.—“Black Alkali”
Waters, 250.—Variations in the Salinity of Deep and Shallow
Wells, 250.—Muddy Waters, 251.—_The Duty of Irrigation
Waters_, 251.—Causes of Losses, 252.—Loss by Percolation.
Figure, 252.—_Evaporation_, 253.—Tables Showing same
at California Stations, 255.—Evaporation in Different
Climates; Table, 255.—Evaporation from Reservoirs and
Ditches, 257.—Prevention of Evaporation; Protective Surface
Layer, 257.—Illustrations of Effects of Tillage; Table,
258.—Evaporation through Roots and Leaves, 262.—Weeds
waste Moisture, 264.—Distribution of Moisture in Soils
as Affected by Vegetation, 264.—Forests and Steppes,
265.—Eucalyptus for Drying Wet Lands, 265.—Mulching;
Effects on Temperature and Moisture, 266.
CHAPTER XIV.
ABSORPTION BY SOILS OF SOLIDS FROM SOLUTIONS. ABSORPTION
OF GASES, AIR OF SOILS, 267.—_Absorption of Solids_,
267.—Desalination, 267.—Decolorization, 267.—Complexity
of Soil-Action, Physical and Chemical, 268.—“Purifying”
Action of Soils on Gases and Liquids, 269.—Waste of
Fertilizers, 269.—Variation of Absorptive Power,
270.—Generalities Regarding Chemical Action and Exchange,
270.—Drain Waters, 271.—Distinctions not Absolute,
272.—_Absorption or Condensation of Gases by the Soil_,
272.—Proof of Presence of Carbonic and Ammonia Gases in
Soils, 273.—Absorption of Gases by Dry Soils. Figure,
274.—Composition of Gases Absorbed by Various Bodies from
the Air. Table, 275.—Discussion of Table, 277.—_The Air of
Soils_, 279.—Empty Space in Dry Soils, 279.—Functions of
Air in Soils, 279.—Insufficient and Excessive Aeration,
280.—Composition of the Free Air of Soils, 280.—Carbonic
Dioxid vs. Oxygen, 281.—Relation to Bacterial and Fungous
Activity, 281.—Putrefactive Processes, 282.
CHAPTER XV.
COLORS OF SOILS, 283.—Black Soils, 283.—“Red” Soils,
284.—Origin of Red Tints, 285.—White Soils,
285.—Differences in Arid and Humid Regions, 286.—White
Alkali Spots, 286.
CHAPTER XVI.
CLIMATE, 287.—Heat and Moisture Control Climates, 287.—Climatic
Conditions, 287.—_Ascertainment and Presentation of
Temperature Conditions_, 288.—Annual Mean not a Good
Criterion, 289.—Extremes of Temperature are most
Important, 289.—Seasonal and Monthly Means, 289.—Daily
Variations, 290.—_The Rainfall_, 290.—Annual Rainfall
not a Good Criterion, 290.—Distribution most Important,
290.—_Winds_, 291.—Heat the Cause of Winds, 291.—Trade
Winds, 291.—Cyclones, 292.— Influence of the Topography
on Winds; Rains to Windward of Mountains, Arid Climates
to Leeward, 293.—General Distribution of Rainfall on the
Globe. Figure, 294.—Ocean Currents, 295.—The Gulf Stream,
295.—The Japan Stream, 296.—Contrast of Climates of N. W.
America, 297.—Continental, Coast and Insular Climates,
297.—Subtropic. Arid Belts, 298.—Utilization of the Arid
Belts, 299.
CHAPTER XVII.
RELATIONS OF SOILS AND PLANT GROWTH TO HEAT, 301.—Temperature
of Soils, 301.—Water Exerts Controlling Influence,
301.—Cold and Warm Rains, 302.—Solar Radiation,
302.—Penetration of the Sun’s Heat into the Soil,
302.—Change of Temperature with Depth, 303.—Surface
Conditions that Influence Soil Temperature, 303.—Heat of
High and Low Intensity, 304.—Reflection vs. Dispersion
of Heat, 304.—Influence of Vegetation, and of Mulches,
305.—Influence of the Nature of the Soil-Material,
306.—Influence of Evaporation, 307.—Formation of
Dew, 307.—Dew rarely adds Moisture, 308.—Dew within
the Soil, 308.—Plant Development under Different
Temperature-Conditions, 309.—Germination of Seeds; Optimum
Temperature for each Kind, 309.—Artificial Heating of
Soils; by Steam Pipes or Water, 310.
CHAPTER XVIII.
PHYSICO-CHEMICAL INVESTIGATION OF SOILS IN RELATION TO CROP
PRODUCTION, 313.—_Historical Review of Soil Investigation_,
313.—Popular Forecasts of Soil Values, 313.—Cogency of
Conclusions Based upon Native Growth, 314.—Ecological
Studies, 315.—Early Soil Surveys of Kentucky, Arkansas
and Mississippi, 316.—Investigation of Cultivated Soils,
316.—Change of Views, 317.—Advantages for Soil Study
offered by Virgin Lands, 318.—Practical Utility of Soil
Analysis; Permanent Value _vs._ Immediate Productiveness,
319.—_Physical and Chemical Conditions of Plant Growth_,
319.—Condition of Plant-food Ingredients, in the Soil,
319.—Water-soluble, Reserve, and Insoluble Part,
320.—Hydrous or “Zeolitic” Silicates, 321.—Recognition of
the Prominent Chemical Character of Soils, 322.—Acidity,
Neutrality and Alkalinity, 322.—Chemical Analysis,
323.—Water-Soluble and Acid-Soluble Portions most
Important, 324.—We cannot Imitate Plant-root Action, 324
Cultural Experience the Final Test, 324.—Analysis of
Cultivated Soils, 325.—Methods of Analysis, 325.—_The
Solvent Action of Water upon Soils_, 327.—Extraction of
Soils with Pure Water, 327.—Continuous Solubility of
Soil Ingredients. Tables, 328.—King’s Results. Table,
329.—Composition and Analysis of Janesville Loam,
331.—Solubility of Soil Phosphates in Water, 332.—Practical
Conclusions from Water Extraction, 332.—_Ascertainment
of the Immediate Plant-food Requirements of Cultivated
Soils by Physiological Tests_, 333.—Plot Tests; their
uncertainties. Diagram, 334.—Crop Analysis as a Test
of Soil Character, 337.—Chemical Tests of immediately
Available Plant Food; Dyer’s Method, 338.
CHAPTER XIX.
ANALYSIS OF VIRGIN SOILS BY EXTRACTION WITH STRONG ACIDS AND
ITS INTERPRETATION, 340.—Loughridge’s Investigation on
Strength of Acid and Time of Digestion, 340.—_Writer’s
Method_, 342.—Virgin Soils with High Plant-food Percentages
are always Productive. Table, 343.—Discussion of Table,
343.—Low Plant-food Percentages not always Indication of
Sterility, 346.—What are “Adequate” Percentages of Potash,
Lime, Phosphoric Acid and Nitrogen, 347.—Soil-Dilution
Experiments, 347.—Table of Compositions, 350.—Figures of
Plants and their Root-Development, 351.—Limitation of Root
Action, 351.—Lowest Limits of Plant-food Percentages and
Productiveness Found in Virgin Soils, 353.— _Limits of
Adequacy of the Several Plant-food Percentages in Virgin
Soils_, 353.—Lime a Dominant Factor in Interpretation,
353.—Potash, 354.—Phosphoric Acid, 355.—Action of Lime and
Ferric Oxid, 355.—Table of Hawaiian Ferruginous Soils,
356.—Unavailability of Ferric Phosphate, 356.—Nitrogen,
357.—Nitrification of the Organic Matter of the Soil,
358.—Analysis of Soil from the Ten-Acre Tract at Chino,
Cal., 358.—Experiments and Results; Matière Noire the
Only Guide, 360.—What are Adequate Nitrogen Percentages
in the Humus? 360.—Table of Humus and Nitrogen-Content
of Californian and Hawaiian Soils, 361.—Confirmatory
Experiment. Figure, 362.—Data for Nitrogen-Adequacy. Table,
363.—_Influence of Lime upon Soil Fertility_, 365.—“A
Lime Country is a Rich Country,” 365.—Effects of High
Lime-Content in Soils, 365.—Table of Soils showing Low
Phosphoric Acid with High and Low Lime-Content, 366.—What
are Adequate Lime-percentages? Differ for Light and Heavy
Soils, 367.—Table Showing Need of High Lime Percentages
in Heavy Clay Soils, 368.—European Standards for Land
Estimates, 369.—Maercker’s Table, 369.
CHAPTER XX.
SOILS OF THE ARID AND HUMID REGIONS, 371.—Composition of
Good Medium Soils; Table, 371.—Criteria of Lands of
the Two Regions, 371.—Tables of Soil-Composition in
Both Regions, 372.—Soils of the Humid Region governed
by Time, 374.—Soils of the Arid Region Governed by
Moisture, 374.—Lime and Magnesia Uniformly High in Arid
Soils, Despite Scarcity of Limestone Formations; Potash
also High, 374.—_General Comparison of the Soils of the
Arid and Temperate Humid Regions_, 375.—Basis of Same,
376.—New Mexico and Analysis of Soil, 376.—General Table,
377.—Discussion of the Table, 378.—_Lime_; Summary of
Physical and Chemical Effects of Lime Carbonate in Soils,
378.— Discussion of Summary, 379.—_Magnesia_: Its role
in Plant Nutrition, 381.—_Manganese_: Its Stimulant
Action, 383.—The “_Insoluble Residue_” or Silicates,
384.—_Soluble Silica and Alumina_, 384.—Analysis of Clay
from Soil, 385.—Difference in Sand of Arid and Humid
Regions. Table, 386.—Soluble Silica or Hydrous Silicates
more Abundant in Arid than in Humid Soils, 388.—_Aluminic
Hydrate._ Table, 389.—Retention of Soluble Silica in Alkali
Soils, 391.—_Ferric Hydrate_, 392.—_Phosphoric Acid_,
392.—Sulfuric Acid, 394.—_Potash_ and _Soda_, Retained
more in Arid Soils, 394.—Arid Soils Rich in Potash,
395.—_Humus_, Low in Arid Soils, but Rich in Nitrogen,
396.—The Transition Region, 397.
CHAPTER XXI.
SOILS OF ARID AND HUMID REGIONS CONTINUED, 398.—_Soils of the
Tropics_, 398.—Humus in Tropical Soils, 399.—Investigations
of Tropical Soils, 401.—_Soils of Samoa and Kamerun_,
402.—Soils of the Samoan Islands, 403.—Soils of Kamerun,
404.—_Soils of Madagascar_, 405.—_Soils of India_, 410.—The
Indo-Gangetic Plain, 411.—The Brahmaputra Alluvium in
Assam, 413.—Black Soils of Deccan, 414.—Red Soils of the
Madras Region, 415.—Laterite Soils, 416.—_Influence of
Aridity upon Civilization_, 417.—Preference of Ancient
Civilizations for Arid Countries, 417.—Irrigation
Necessitates Co-operation, 419.—High and Permanent
Productiveness of Arid Soils Induces Permanence of Civil
Organization, 419.
CHAPTER XXII.
ALKALI SOILS, THEIR NATURE AND COMPOSITION, 422.—Alkali
Lands _vs._ Seashore Lands, 422.—Origin, 422.—Deficient
Rainfall, 423.—Predominant Salts, 423.—Geographical
Distribution, 424.—Their Utilization of World-wide
Importance, 424.—Repellent Aspect, Plate, 424.—Effects
of Alkali upon Culture Plants. Figures of Apricot Trees,
426.—Nature of the Injury, External and Internal,
426.—Effects of Irrigation, 428.—Leaky Irrigation Ditches,
429.—Surface and Substrata of Alkali Lands, 429.—Vertical
Distribution of the Salts in Alkali Soils, 429.—How
Native Plants Live, 430.—Figures of various Phases of
Reclamation, 431.—Upward Translocation from Irrigation,
433.—Distribution of Alkali in Sandy Lands, 433.—In Heavier
Lands, 436.—Salton Basin or Colorado Delta, 436.—Diagram of
Alkali Distribution in Same, 438.—Horizontal Distribution
of Alkali Salts in Arid Lands, 439.—Alkali in Hill
Lands, 439.—Usar Lands of India, 440.—“Szek” Lands of
Hungary, 440.—Alkali Lands of Turkestan, 441.—_Composition
and Quantity of Salts Present_, 441.—Nutritive Salts,
441.—Black and White Alkali. Tables, 442.—Estimation of
Total Alkali in Land, 444.—_Composition of Alkali Soils
as a whole._ Tables, 445.—Presence of much Carbonate
of Soda, 448.—Cross Section of an Alkali Spot. Table,
448.—_Reactions between the Carbonates and Sulfates of
Earths and Alkalies._ Figure of Curve, 449.—Inverse
Ratios of Alkali Sulfates and Carbonates. Diagrams,
451.—Exceptional Conditions, 453.—Summary of Conclusions,
453.
CHAPTER XXIII.
UTILIZATION AND RECLAMATION OF ALKALI LAND,
455.—Alkali-resistant Crops, 455.—Counteracting
Evaporation, 455.—Turning-under of Surface Alkali,
456.—Shading, 457.—Neutralizing Black Alkali,
457.—Removing the Salts from the Soils, 458.—Scraping
off, 458.—Leaching-Down. Figure, 459.—Underdrainage, the
Final and Universal Remedy for Alkali, 460.—Possible
Injury to Land by Excessive Leaching, 462.—Difficulty in
Draining “Black” Alkali Land, 462.—Swamping of Alkali
Land, 463.—Removal of Alkali Salts by Certain Crops,
463.—Tolerance of Alkali by Culture Plants, 463.—Relative
Injuriousness of the several Salts. Effects on Sugar Beets,
464.—Table of Tolerances; Comments on same, 467.—Saltbushes
and Native Grasses. Australian Saltbushes, 469.—Modiola;
Native and Cultivated Grasses, 469.—Other Herbaceous Crops,
472.—Legumes, 472.—Mustard Family, 473.—Sunflower Family,
473.—Root Crops, 474.—Stem Crops, 475.—Textile Plants,
475.—Shrubs and Trees, 475.—Vine, Olive, Date, Citrus
Trees. Deciduous Orchard Trees. Timber and Shade Trees,
475.—Inducements toward the Reclamation of Alkali Lands,
481.—Wheat on Reclaimed Land at Tulare; Figure, 482.—Need
of Constant Vigilance, 484.
CHAPTER XXIV.
THE RECOGNITION OF SOIL CHARACTER FROM THE NATIVE
VEGETATION; MISSISSIPPI, 487.—Climatic and Soil Conditions,
487.—Natural Vegetation the Basis of Land Values in the
United States, 488.—Investigation of Causes Governing
Distribution of Native Vegetation, 488.—Investigations
in Mississippi, 489.—Vegetative Belts in Northern
Mississippi, 490.—Sketch Map of Same, with Tabulation of
Lime Content and Native Vegetation, 490.—Lime Apparently a
Governing Factor, 492.—Soil Belts in Southern Mississippi,
493.—Vegetative and Soil Features of Coast Belts. Diagram,
495.—Table of Plant-Food percentages and Native Growth,
496.—Definition of Calcareous Soils, 496.—_Differences in
the Form and Development of Trees_, 498.—Forms of the Post
Oak. Figures, 498.—Forms of the Black Jack Oak. Figures,
500.—Characteristic Forms of other Oaks, 502.—Sturdy Growth
on Calcareous Lands, 502.—Growth of Cotton, 503.—Lime
Favors Fruiting, and compact Growth, 504.—Physical vs.
Chemical Causes of Vegetative Features, 505.—Lowland Tree
Growth, 506.—Contrast between “First” and “Second” Bottoms,
506.—Tree Growth of the First Bottoms. The Cypress,
507.—Figures of Swamp and Upland Cypress, 508.—Other
Lowland Trees, 509.—General Forecasts of Soil Quality in
Forest Lands, 509.
CHAPTER XXV.
RECOGNITION OF THE CHARACTER OF SOILS FROM THEIR NATIVE
VEGETATION. UNITED STATES AT LARGE, EUROPE, 511.—Forest
Growths outside of Mississippi; Alabama, Louisiana, Western
Tennessee, and Western Kentucky, 511.—North Central States
East of the Mississippi River, 513.—Upland and Lowland
Vegetation in the Arid and Humid Region, 515.—Forms of
Deciduous Trees in the Arid Region, 516.—Tall Growth of
Conifers, 517.—Herbaceous Plants as Soil Indicators,
517.—Leguminous Plants Usually Indicate Rich or Calcareous
Lands, 518.—European Observations and Views on Plant
Distribution and its Controlling Causes, 519.—Composition
of Pine Ashes on Calcareous and Non-calcareous Lands.
Table, 520.—Calciphile, Calcifuge, and Indifferent Plants,
521.—Silicophile _vs._ Calciphile Flora, 523.—What is a
Calcareous Soil? 524.—Predominance of Calcareous Formations
in Europe, 525.
CHAPTER XXVI.
THE VEGETATION OF SALINE AND ALKALI LANDS, 527.—_Marine Saline
Lands_, 527.—General Character of Saline Vegetation,
527.—Structural and Functional Differences Caused by Saline
Solutions, 528.—Absorption of the Salts. Table, 529.—Injury
from the Various Salts, 531.—Reclamation of Marine Saline
Lands for Culture, 533.—_The Vegetation of Alkali Lands_,
534.—_Reclaimable_ and _Irreclaimable Alkali Lands_ as
Distinguished by their Natural Vegetation, 534.—Plants
Indicating Irreclaimable Lands, 535.—Tussock Grass; Bushy
Samphire; Dwarf Samphire; Saltwort; Greasewood; Alkali
Heath; Cressa; Salt Grass, 536.—Relative Tolerances of the
different Species; Table, 549.
APPENDICES.
A.—Directions for taking Soil Samples, issued by the
California Experiment Station, 553.
B.—Summary Directions for Soil-Examination in the Field
or Farm, 556.
C.—Short Approximate Methods of Chemical Soil-Examination
Used at the California Experiment Station, 560.
General Index, 565.
Index of Authors referred to, 591.
PREFACE.
This volume was originally designed to serve as a text and reference
book for the students attending the writer’s course on soils, given
annually at the University of California, who complained of their
inability to find in any connected treatise a large portion of
the subject matter brought before them. As all these students had
preliminary training in physics, chemistry and botany, no introductory
chapters on these general subjects were necessary or contemplated; the
more so as good elementary treatises embracing the needful preparation
are now numerous.
As time progressed, however, outside demands for a book embodying the
writer’s soil studies in the humid and arid regions, especially the
latter, became so numerous and pressing that the scope of the work has
gradually been much enlarged to conform to these demands; and this,
rather than completeness of detail, when such detail can be found well
given elsewhere, has been the guide in the necessary condensation of
the whole. To give the entire subject matter full elucidation, would
require several more volumes.
It may not be unnecessary to explain at the outset why and how this
treatise deviates in many respects from previous publications on the
same general topic. From boyhood up it has fallen to the writer’s lot
to be almost continuously in more or less direct contact with the
conditions and requirements of newly settled regions, as well as with
those hardly yet invaded even by the pioneer farmer; where the question
of cultural adaptation was yet undetermined or wholly in the dark.
Being during his active life constantly called upon in his official
capacity to give information and advice to pioneer farmers or intending
settlers in regard to the merits and adaptations of virgin soils, the
writer’s attention was naturally and forcibly directed toward soil
investigation as a possible means of determining, beforehand, the
general prospects and special features of agriculture in regions where
actual experience was either non-existent or very brief and partial.
In the pursuit of these studies he has been favored by exceptional
opportunities, extending over a varied climatic area reaching on the
south from the Gulf of Mexico to the Ohio, across to the Pacific coast,
and to British Columbia on the north. That a systematic investigation
of soils over so large an area, covering both humid and arid regions,
should lead to some unexpected and novel results, is but natural; and
it is the discussion of these results in connection with those obtained
elsewhere, and with some of the prevailing views based thereon, that
must serve as the justification for the present addition to an already
well-stocked branch of literature.
From the very beginning of the scientific study of agriculture, the
investigation of soils with a view to the _à priori_ determination
of their adaptation, permanent value, and best means of cultural
improvement, has formed the subject of continuous effort. It is not
easy to imagine a subject of higher direct importance to the physical
welfare of mankind, whose very existence depends on the yearly returns
drawn by cultural labor from the soil.
It is certainly remarkable that after all this long-continued effort,
even the fundamental principles, and still more the methods by which
the object in view is to be attained, are still so far in dispute that
a unification of opinion in this respect is not yet in view; and a
return to pure empiricism is from time to time brought forward to cut
the Gordian knot.
While this state of things is primarily due to the intrinsic complexity
and difficulty of the subject itself, it has unquestionably been
materially aggravated by accidental, partly historic conditions.
Foremost among these is the fact that until within recent times, soil
studies have borne almost entirely on lands long cultivated and in most
cases fertilized: thus changing them from their natural condition to a
more or less artificial one, which obscures the natural relations of
each soil to vegetation.
The importance of these relations is obvious, both from the theoretical
and from the practical standpoint. From the former, it is clear that
the native vegetation represents, within the climatic limits of the
regional flora, the result of a secular process of adaptation of plants
to climates and soils, by natural selection and the survival of the
fittest. The natural floras and sylvas are thus the expression of
secular, or rather, millennial experience, which if rightly interpreted
must convey to the cultivator of the soil the same information that
otherwise he must acquire by long and costly personal experience.
The general correctness of this axiom is almost self-evident; it is
explicitly recognized in the universal practice of settlers in new
regions, of selecting lands in accordance with the character of the
forest growth thereon; it is even legally recognized by the valuation
of lands upon the same basis, for purposes of assessment, as is
practiced in a number of States.
The accuracy with which experienced farmers judge of the quality of
timbered lands by their forest growth, has justly excited the wonder
and envy of agricultural investigators, whose researches, based upon
incomplete theoretical assumptions, failed to convey to them any such
practical insight. It was doubtless this state of the case that led a
distinguished writer on agriculture to remark, nearly half a century
ago, that he “would rather trust an old farmer for his judgment of land
than the best chemist alive.”[1]
[1] “The Soil Analyses of the Geological Surveys of Kentucky and
Arkansas.” S. W. Johnson in _Am. Jour. Sci._, Sept. 1861.
It is certainly true that mere physico-chemical analyses, unassisted
by other data, will frequently lead to a wholly erroneous estimate of
a soil’s agricultural value, when applied to cultivated lands. But
the matter assumes a very different aspect when, with the natural
vegetation and the corresponding cultural experience as guides, we seek
for the factors upon which the observed natural selection of plants
depends, by the physical and chemical examination of the respective
soils. It is further obvious that, these factors being once known, we
shall be justified in applying them to those cases in which the guiding
mark of native vegetation is absent, as the result of causes that have
not materially altered the natural condition of the soil.
It is probable that, had agricultural science been first developed
in regions where the external conditions permitted the carrying-out
of such a course of investigation, instead of in the abnormally
temperate, even and humid climate of middle Europe, with its
long-cropped, worn fields, and very predominantly calcareous soils,
the present condition of this science might differ not immaterially
from that actually existing. As a matter of fact, it has attained
its present state under very disadvantageous external conditions,
which frequently necessitated a recourse to highly complex and
laborious methods and artificial appliances, for the establishment and
maintenance of the conditions which elsewhere might have been found
abundantly realized in nature; thus permitting, by the multiplication
of observations over extended and widely varied areas, the elimination
and control of accidental errors of experiment and observation.
Just as in historical geology the subdivisions of formations observed
and accepted in Europe formed for many years a procrustean bed upon
which the facts observed elsewhere had to be stretched, so in the
domain of soil physics and chemistry, and even in vegetable physiology,
the observations made in the really exceptional climates and soils
of middle Western Europe, have often erroneously been construed as
constituting a general basis for unalterable deductions.
The rapid extension of civilization and the carrying of minute
scientific research into other regions, now rendered possible by the
improved means of communication, has shown the one-sidedness of some of
the views prevailing heretofore, inasmuch as they are really applicable
only to accidental and rather exceptional conditions.
It is therefore one object of this volume to present and discuss
summarily the facts of physical and chemical soil constitution and
functions with reference to the additional light afforded on the wider
basis, embracing both the humid and the arid regions; of which the
latter has, as such, received but scant and desultory attention thus
far, to the detriment of both the work of the agricultural experiment
stations and of agricultural practice. The book therefore includes the
discussion both of the methods and results of direct physical, chemical
and botanical soil investigation, as well as the subject matter
relating to the origin, formation, classification and physical as well
as chemical nature of soil, usually included in works on scientific
agriculture.
In the presentation of these subjects, it has been the writer’s aim
to reach both the students in his own classes and in the agricultural
colleges generally, as well as the fast increasing class of farmers of
both regions who are willing and even anxious to avail themselves of
the results and principles of scientific investigation, without “shying
off” from the new or unfamiliar words necessary to embody new ideas. It
would seem to be time that the latter class, and more especially those
constituting farmers’ clubs, should learn to understand and appreciate
both the terms and methods of scientific reasoning, which are likely to
form, increasingly, the subjects of instruction in the public schools.
But in order to segregate to some extent the generally intelligible
matter from that which requires more scientific preparation than can
now be generally expected, it has been thought best to use in the text
two kinds of type; the larger one embodying the matter presumed to be
interesting and intelligible to the general reader, while the smaller
type carries the illustrative detail and discussion which will be
sought chiefly by the student.
As regards the chemical nomenclature used in this volume, the writer
has not thought it advisable to follow the example set by some late
authors in substituting for the well-known names of the bases and
acids, those of the elements, and still less, those of the intangible
ions. Any one who has taught classes in agricultural chemistry will
have experienced the difficulty and loss of time unnecessarily incurred
in the incessantly recurring transposition of terms, and complication
of formulæ, serving no useful purpose save that of academic
consistency. It is of at least doubtful utility to present to the
farmer, _e. g._, the inflammable and dangerous elements phosphorus and
potassium as prime factors in the success of his crops, and of healthy
nutrition.
Inasmuch as all the elements are presented to and contained in the
plant in compounds only, and these compounds are themselves, in the
dilute solutions used by plants, known to be largely dissociated into
their basic and acid groups, it seems to be most natural to present
them under the corresponding, even if not absolutely theoretically
correct names of acids and bases, to which the farmer and the trade
have been accustomed for half a century. Upon these considerations the
long-used designations of potash, soda, lime, phosphoric, sulfuric,
nitric and other acids and bases have been retained in this volume,
adding the chemical formula where, as in analytical statements, a doubt
as to their meaning might arise. Assuredly, the diffusion of scientific
knowledge should not be needlessly hindered by the adoption of a
pedantic mode of presentation.
The great breadth of the subject of this volume has rendered
inadvisable any extended bibliography, such as it has of late become
customary to add to works of this kind. References have therefore been
restricted to publications specially discussed, and to such as are not
widely known on account of limited circulation.
The author’s warmest acknowledgments are due to Professor R. H.
Loughridge, of the University of California, for efficient and
sympathetic assistance, both in the revision of the manuscript, and
active personal help in the preparation of the illustrations. Without
his coöperation the preparation and publication of the volume would
have been much longer delayed.
Acknowledgments are also due for helpful suggestions and criticism
to Professors L. H. Bailey, of Cornell University, F. H. King, of
Wisconsin, and Jacques Loeb of the University of California.
E. W. HILGARD.
BERKELEY, CALIFORNIA,
_November 15, 1905_.
INTRODUCTION.
_Definition of Soils._—In the most general meaning of the term, a soil
is the more or less loose and friable material in which, by means of
their roots, plants may or do find a foothold and nourishment, as well
as other conditions of growth. Soils form the uppermost layer of the
earth’s crust; but the term does not indicate any such definite average
texture as is sometimes implied by its popular use to designate certain
loose, loamy materials found in older geological formations. We do find
in these, not unfrequently, layers that in the past have served to
support vegetation, as evidenced by remains of plants found therein.
But as a rule, such ancient soils are much compacted and otherwise
changed, and would not now be capable of performing the office of
plant nutrition without previous, long-continued exposure to the same
agencies by which all soils were originally formed from pre-existing
rocks. Within the latter category must be included, in scientific
parlance, not only the hard rocks known as such in daily life, but also
such soft materials as clay, sand, marls, etc., which often compose,
partially or wholly, the bodies of wide-spread geological formations.
_Elements Constituting the Earth’s Crust._—More than seventy elementary
substances have been found within the portion of the earth accessible
to man; most of these are present only in very minute proportions; of
those occurring in relatively considerable quantities, a list showing
their approximate proportions is given below.
_Average quantitative composition of the Earth’s Crust._—The total
thickness of the outer shell of the earth, thus far known to us, does
not exceed about 95,000 feet, as observed in the accessible rock
deposits. Estimates of the proportions in which the more abundant
elements contribute to the composition of these constituent rocks, have
repeatedly been made. The latest and most widely accepted of these, by
F. W. Clarke, of the U. S. Geological Survey, is given herewith. It
includes the constituents of the sea and atmosphere as well; these two
constitute about 7 per cent of the whole, 93 per cent being solid rocks.
RELATIVE ABUNDANCE OF THE ELEMENTS TO A DEPTH OF TEN KILOMETERS.
SOLID CRUST OCEAN MEAN,
(93 PER CENT). (7 PER CENT). INCLUDING AIR.
Oxygen 47.29 85.79 49.98
Silicon 27.21 25.30
Aluminum 7.81 7.26
Iron 5.46 5.08
Calcium 3.77 0.05 3.51
Magnesium 2.68 0.14 2.50
Sodium 2.36 1.14 2.28
Potassium 2.40 0.04 2.23
Hydrogen 0.21 10.67 0.94
Titanium 0.33 0.30
Carbon 0.22 0.002 0.21
Chlorin 0.01 2.07 0.15
Phosphorus 0.10 0.09
Manganese 0.08 0.07
Sulphur 0.03 0.09 0.04
Barium 0.03 0.03
Nitrogen 0.02
Fluorin 0.02 0.02
Chromium 0.01 0.01
It will be noted that one-half of the total consists of oxygen, and
that nearly 86% (or 47.29% of the 49.98%) of this amount is contained
in the solid rocks; nearly 2.50% of the remainder in sea and other
water; and .41% in the atmosphere, in the free condition, in which it
serves for the respiration of animals and plants, and for the various
processes of slow and rapid combustion, or “oxidation.” This relatively
small proportion of the whole, is, nevertheless, the most directly
important for the maintenance of organic life.
_Oxids Constitute Earth’s Crust._—The vast predominance of oxygen in
the above list suggests at once that most of the other elements must
exist in combination with it, _i. e._, as “oxids.” H. S. Washington[2]
has lately revised the estimates heretofore made, on the basis of a
very large number of analyses made by him and others, of rocks within
the United States, and gives the following table; alongside of which
is placed a revised estimate by Clarke, which also includes rocks from
abroad; both being given in terms of oxids of the several elements.
[2] U. S. Geol. Survey, Professional Paper No. 14, p. 108.
WASHINGTON. CLARKE.
Silica SiO₂ 57.78 59.89
Alumina Al₂O₃ 15.67 15.45
Peroxid of Iron Fe₂O₃ 3.31 2.64
Protoxid of Iron FeO 3.84 3.53
Magnesia MgO 3.81 4.37
Lime CaO 5.18 4.91
Soda Na₂O 3.88 3.56
Potash K₂O 3.13 2.81
Water, basic H₂O⁺ 1.42 1.52
Water, acid H₂O⁻ .36 .40
Ferric Sulphid FeS₂ 1.03 .60
Phosphoric acid P₂O₅ .37 .22
Manganese Protoxid MnO .22 .10
The salient point which at once attracts attention in these tables is
the great predominance of the oxid of silicon—silica, silicic acid,
quartz, etc.,—over all other substances. While quartz occurs alone
in enormous masses, as will be shown later, probably the greater
proportion is found in combination with other oxids, notably those of
aluminum, calcium, iron, magnesium, and the alkali metals potassium and
sodium. Chlorin and fluorin, however, do not occur as oxids.[3]
[3] A trifling amount of chlorin is found oxidized in the form of
sodium perchlorate, in the nitre deposits of Chile.
_The Chemical Elements Important to Agriculture._—Of the numerous
elements known to chemists, only eighteen require mention in connection
with either soil formation or plant growth; and of these only thirteen
or fourteen participate in normal plant growth. They are the following:
METALLIC ELEMENTS. NON-METALLIC ELEMENTS.
Potassium Carbon
Sodium Hydrogen
Calcium Oxygen
Magnesium Nitrogen
Iron Phosphorus
Manganese Sulphur
Aluminum Chlorin
Titanium Fluorin
Iodin
Silicon.
Of this list, titanium, though a very constant ingredient of soils in
the form of titanic dioxid, is not known as performing any important
function in soils, and is not, so far as known at present, ever taken
up by plants. Aluminum, in the form of its compounds with oxygen
and silicon, is a very prominent and physically very important soil
ingredient, but does not, apparently, perform any direct function in
plant nutrition, and is absent from their ash, except in the case of
some of the lower plants (horsetails and ferns).
Iodin appears to be normally present in all seaweeds, and occurs in
traces in some land plants. Fluorin is a normal ingredient of animal
bones, and its presence in plant ashes is often easily shown. The
remaining fourteen, however, are always present in plants; carbon,
hydrogen, oxygen and nitrogen forming the volatile or combustible part,
while the rest occur in the ashes.
It is true that other elements, or rather their compounds, are
sometimes found in plants, being taken up by them from solutions
existing in the soil. Thus the alkalies caesium and rubidium, also
barium, strontium, zinc, copper, boron and some others, may be absorbed
when present in soluble form. But they are neither necessary nor
beneficial to plant economy, and when in considerable amounts are
harmful. Thus fifteen elements, ommiting iodin and titanium, alone
require discussion.
_The Volatile Part of Plants_, as already stated, consists of carbon,
hydrogen, oxygen and nitrogen. Of these, carbon is obtained by the
plant exclusively from the carbonic (dioxid) gas of the air; hydrogen
and oxygen, from the soil in the form of water; nitrogen, directly
from the soil but indirectly also from the air, through the agency of
certain bacteria. The _ash ingredients_ of course are all derived from
the soil through the roots, and must all be present in the latter in
an available form, to a sufficient extent to supply the demands of
vegetation.
_The Agencies of Soil Formation._—With respect to their mode of
formation, soils may be defined as the residual product of the physical
disintegration and chemical decomposition of rocks; with, ordinarily,
a small proportion of the remnants of organic life. The agencies
producing these changes are those classed under the general term
“atmospheric” or “meteorological;” they include therefore the action
of _temperature_—heat and cold—that of _water_, and that of _air and
its ingredients_. In popular parlance, it includes the processes of
_weathering_; nearly the same processes are involved in the “fallowing”
of soils.
PART I.
THE ORIGIN AND FORMATION OF SOILS.
CHAPTER I.
THE PHYSICAL PROCESSES OF SOIL FORMATION.
Since the physical and mechanical effects of the agencies mentioned
above usually precede, in time, the chemical changes, which are
materially facilitated by the previous pulverization of the rocks, the
former should be first considered.
_Effects of heat and cold on rocks._—Most rocks are aggregates of
several simple minerals; a few only (limestone, quartzite and a few
others) expand or contract alike in all their parts. Of the minerals
composing the compound rocks, scarcely any expand to exactly the same
extent under the influence of the sun’s heat, especially when their
colors differ; nor, in the great majority of cases, does one and the
same mineral expand alike in all three directions. It follows that at
each change of temperature there is a tendency to the formation of
minute fissures between adjacent crystals or masses of different simple
minerals; and especially in the case of large crystals of certain
kinds, this action alone will gradually result in the disruption of the
rock surface, so that individual crystals may be detached with little
difficulty. In any case, the cracks so formed are gradually widened
by a frequent repetition of the changes of temperature, coupled with
access of air, water, dust, and the rootlets of plants; all of which
brings about a gradually increasing rate of surface crumbling. This is
especially conspicuous at the higher elevations of mountains, where the
temperature changes are very great and abrupt; and also in the clear
atmosphere of deserts, where owing to the extent and suddenness of
temperature-changes between day and night, caused by the free radiation
of heat into the clear sky, even homogeneous pebbles are known to be
almost explosively disrupted in the mornings and evenings of clear
days.
Such effects may often be strikingly observed on small surfaces of
compound crystalline rocks, such as granite, exposed on glaciers, where
the daily changes of temperature are often extreme, viz., from below
the freezing point to as much as 130 degrees Fahr. (54.4 degrees C). In
such cases one may sometimes scoop off the disintegrated rock by the
handful, while yet the mineral surfaces are almost perfectly fresh.
On a larger scale, the disruption and scaling off of huge slabs of
granite, and rocks of similar structure, may be observed in southern
California on the southwestern side of rock exposures, where slabs
from a few inches to ten and more feet in length and eight or ten
inches thick, have slid off, perhaps still leaning against the parent
rock, which has been rounded off by a succession of such events into
the domelike form so characteristic of granite mountains. Merrill[4]
reports similar exfoliations to occur especially on the peninsula of
California, on Stone Mountain in Georgia, and elsewhere.
[4] See Rocks, Rock-weathering, and Soils, page 246; also paper on
Domes and Dome Structure, by G. K. Gilbert, in Bulletins of the Geol.
Society Am., Vol. 15, pp. 29-36.
A striking exemplification of the effects of frequent and rapid changes
of temperature on rocks, and of humid and dry climates as well, is seen
in the case of the great monoliths of Egypt, one of which now stands in
the Central Park, New York. In the quarries of Syene in Upper Egypt,
where most of these monoliths were obtained, the rough blocks that
were in progress of quarrying when the work was abandoned, quite two
thousand years ago, still show an almost perfectly fresh surface; and
the same is true of the finished obelisks in Lower Egypt, where both
the changes of temperature and the rainfall are somewhat greater. It
is a matter of public note that one of “Cleopatra’s Needles” which was
set up in Central Park nearly thirty years ago, but originally erected
at Heliopolis on the Nile, is in great danger of destruction from the
influence of a totally different climate, in which both the temperature
changes and the rainfall are much more frequent and severe than in
Egypt. The large crystals of feldspar and quartz which compose the
(syenite) rock material have had fine fissures formed between them by
often-repeated expansion and contraction; which when filled with water
and subsequently changed to ice, the latter’s expansion in freezing
(see below) has still farther enlarged them and caused a scaling-off,
which threatens to obliterate the hieroglyphic inscriptions. Thus
temperature-changes and a rain followed by freezing may in a few days
produce a greater effect than a thousand years of Egyptian climate.
_Cleavage of rocks._—Many kinds of rocks have definite directions
of ready cleavage. The most common and obvious cases of this kind
are schists, slates and shales, cleaving readily into plates or
irregular flat or lens-shaped fragments. Such structure greatly favors
disintegration, especially when the layers are on edge at steep
angles. But there are other apparently structureless, massive rocks,
particularly basalts and other eruptive rocks related to them, as well
as many sandstones and claystones, that have a strong tendency to
cleave into more or less definite forms when struck; such as columns
or prisms, square, six-sided or diamond-shaped blocks, etc. Similar
forms are naturally produced in them under the influence of changes
of temperature; by the formation of minute cracks at first, then
enlargement of these by the several agencies already mentioned.
_Effects of freezing water._—The irresistible force exerted by the
expansion of water in freezing, amounting to about 9 per cent of its
bulk, is a powerful factor in widening and deepening fissures and
cracks of rocks; not uncommonly, whole masses of rock are rent into
fragments by this agency, which is one of the most common causes of
“rock falls” on the brink of precipices. By the freezing process cracks
and crevices are enlarged, and the surfaces exposed to weathering are
still farther increased; and the rock fragments or soil particles are
loosened and rendered more liable to be removed from the original site,
whether by gravity, wind or water.
_Glaciers._—Ice in the form of the glaciers that descend from mountain
chains (see figure 1), and of the moving ice sheets that have covered
large portions of North America and Europe in past ages and now cover
Greenland and the South Polar continent, exerts a most potent action
in abrading and grinding even the hardest rocks; not so much by the
direct friction of the moving ice itself, as by the cutting, scoring,
grinding and crushing action which the stones imbedded in the ice,
or carried and shoved by it, exert upon the rocky channels in which
the ice stream moves, as well as upon each other. The product of this
grinding process is largely very fine (hence “glacier flour”), so that
it remains suspended in the water of the glacier-streams until their
velocity is permanently checked when reaching a plain or lake. This
suspended stone-flour imparts to the glacier streams their distinctive
character of “white rivers,” as contradistinguished from the clear,
dark “green rivers” that have their origin outside of glaciated areas.
This difference can be readily observed in traveling along any of
the glacier-bearing mountain chains of the world, and is frequently
expressed in the names of the streams.
[Illustration: FIG. 1.—Zermatt Glacier (Agassiz).]
The physical analysis of mud from the foot of Muir glacier,[5] Alaska,
at its sea front, made by Professor Loughridge, shows the prevalent
fineness of the materials brought down by the glacier waters.
[5] Collected by Dr. W. E. Ritter of the University of California.
PHYSICAL COMPOSITION OF GLACIER MUD.
============+====================+===============
MATERIAL. | DIAMETER. | PER CENT.
------------+--------------------+---------------
Clay | ? | 16.57 } 70.31
Fine silt | .0023 -- .016 mm. | 53.74 }
Fine silt | .016 to .025 mm. | 4.38
Medium silt | .025 to .036 mm. | 7.06
Coarse silt | .036 to .047 mm. | 5.91
Coarse silt | .047 to .072 mm. | 3.76
Fine sand | .072 to .12 mm. | 1.14
Medium sand | .12 to .16 mm. | 1.56
| | -----
Total | | 94.12
------------+--------------------+---------------
It will be noted that over 70 per cent of this mud consists of
extremely fine, wholly impalpable materials; but little of which is
true clay.
The fineness of the glacier flour renders it peculiarly suitable for
the rapid conversion into soil, and such soils are usually excellent
and remarkably durable. The great and lasting fertility of the soils
of southern Sweden is traced directly to this mode of origin, and
doubtless the great American ice sheet of glacier times is similarly
concerned in the high quality of the soil of our “north central”
states, from the Ohio to the Great Lakes and the Missouri.
The accumulations of rocks and debris of all sizes in the “moraines”
or detrital deposits of glaciers and ice-sheets form another class of
glacier-made lands which cover extensive and important agricultural
areas (drift areas), both in the old and new worlds. Such lands are
undulating or slightly hilly, and the soil usually contains imbedded
in it stones of a great variety of kinds and sizes, partly angular,
partly rounded and polished by friction. Of course the frequent and
violent changes of temperature occurring on the surface of a glacier,
aid materially in reducing the rocks carried by it to the condition in
which we find the material of the moraines; which commonly form lateral
or cross ridges in valleys formerly occupied by glaciers.
_Action of flowing water._—The action of flowing water is doubtless at
this time the most potent mechanical agency of soil formation. From the
sculpturing of the original simple forms in which geological agencies
left the earth’s surface into the complex ones of modern mountain
chains, to the formation of valleys, plains, and basins out of the
materials so carried away, its effects are prodigious. The torrents
and streams in carrying silt, sand, gravel and bowlders, according to
velocity and volume, do not merely displace these materials; the rock
fragments of all sizes not only score and abrade the bed of the rill
or stream, but by their mutual attrition produce more or less of fine
powder similar to that formed by glacier action; usually more mixed in
its ingredients than the former, because derived from a wider range of
drainage surface. In the glacier stream itself, it is easy to trace the
gradual transition from the sharp stone fragments lying in the water as
it issues from the terminal ice cave at the lower end of the glacier,
to the rounded shingle found a few miles below.
[Illustration: FIG. 2.—Erosion of Hawaiian Hills, near Honolulu. (Phot.
by H. C. Myers.)]
On slopes where water flows only during rain or the melting of
snow, the same erosive effects may be seen as between the heads of
ravines and their outlets. (See figure 2,) It is there too that
the surprisingly rapid cutting-out of channels by the aid of water
charged with rock fragments or gravel, can readily be observed, and
the enormous power of water erosion convincingly shown. In the United
States the stupendous gorges of the Columbia and Colorado rivers,
the former cut to a depth of over 2000 feet into hard basalt rock,
the latter to over 5,000 feet, partly into softer materials, partly
into granite, are perhaps the most striking examples of this power;
the manifestations of which can, however, be as convincingly seen in
thousands of minor rivers and streams.
All the materials so carried off from the higher slopes are finally
deposited on a lower level; whether only a short distance away on a
lower slope (colluvial soils), or farther away in the flood plain of
streams, rivers, or lakes (alluvial soils). Other things being equal,
the finest materials are of course, carried farthest, and often into
the sea; in which, however, they cannot long remain suspended, but are
quickly thrown down, forming river bars, flood plains, and deltas. The
fineness of the material of delta soils, like that of those made from
glacier flour, insures them the same advantage, viz. great fertility
and durability.
It is calculated that the Mississippi River carries into the Gulf of
Mexico annually some 7469 millions of cubic feet of earthy deposits,
which would fill one square mile of surface to the height of 268 feet,
or would cover that number of square miles to the depth of one foot.
[Illustration: FIG. 3.—Cliffs and caves on sea-beach at La Jolla,
Calif. showing effects of Wave action.]
_Wave-Action._—The powerful effects of the beating of waves upon abrupt
shores of seas or lakes are in evidence all over the world, and these
effects are so characteristic that they can be recognized even where
no sea or lake exists at present. Gravel and sand are carried in the
surf and serve as grinding materials, wearing even the hardest rocks
into grooves, rills, channels and caves, defining sharply the varying
degrees of hardness or tough resistance in different parts of rocky
cliffs; frequently undermining them and causing extensive rock falls.
The latter then serve for a time to break the violence of the waves’
onset, and may even cause permanent shore deposits to be formed under
their lee.
Such deposits are very generally formed on gently sloping beaches,
and as the water gradually recedes, sometimes by elevation of the
ground, beach lines or beach-terraces are left, which indicate the
successive levels of the lake or sea. Such old beach lines or terraces
and level-surfaced “buttes” in the Great Basin country, and “bench
lands” elsewhere, show in their structure the characteristic lines of
wave-deposition.
_Effects of Winds._—The action of winds in transporting soil particles
(dust and sand) is familiar; and the accumulations that may be formed
under the influence of regular, continuous winds are sufficiently
obvious on lee shores having sandy beaches, inland of which the
formation of sand dunes at times assumes a threatening magnitude. Where
winds are irregular, frequently reversing their direction, of course
the local effects will be less obvious, and the transportation of
material actually occurring will often not be noticed. Yet there can be
no doubt of the importance of wind action in soil formation, and there
are cases in which no other agency can explain the facts observed over
widely extended areas. This is especially true with regard to the soil
masses of the high plains or plateaus of the dry continental interiors,
where not only the regularity of the prevailing winds, but also the
structure (or absence of structure) and pulverulent character of the
soil itself, renders this the only rational mode of accounting for its
presence where we find it.
The effects that may be exerted by regular winds are well
illustrated in the plains and deserts of Africa as well as
those of central Asia. Here we find a distinct subdivision
of the desert (rainless) areas into the _stony_, from
which the wind has swept all but the bedrock and gravel
and where scarcely any natural growth, and certainly no
cultivation is possible in the almost total absence of
soil. The next subdivision is the _sandy_ desert,
to leeward of the stony area, where the winds are less
violent and regular, and where, therefore, the sand has
been dropped and is waited back and forth by “sand
storms,” the surface being covered with moving sand dunes.
Still farther to leeward we find the region in which the
finer portions of the desert surface has been deposited;
here we have “_dust_ storms” so long as the land is
not irrigated: but the application of water renders the
soil abundantly fruitful. Such is the case of the Oases
and fertile border-lands of the Sahara and Libyan deserts.
In the cultivated portions of the Mojave and Colorado deserts in
California, plowing of the land during a dry time is not uncommonly
followed by a bodily removal of the loosened soil to neighboring
fields, sometimes leaving a gravel surface behind. Such “blown-out
lands” exist naturally at numerous points in the Colorado desert.
Sven Hedin (Central Asia and Tibet, Vol. II.) shows that from the
effects of the violent storms that prevail in the Gobi or Takla Makan
desert, Lop-nor lake, the sink of the Tarim river, has in the course
of time shifted its bed as much as fifty miles in consequence of the
excavation of the southern part of the desert by the wind; while the
sand so blown out, together with the deposits from the rivers, now
tends to fill up the present (southern) lake, which is gradually
returning northward toward its original site, now a desert, but around
which formerly a dense population existed.
The great plains of North America, the pampas of South America, the
plateaus of Mongolia and especially the fertile loess region of
northwestern China, are also cases in point. The dense dust storms of
these regions are familiar and unpleasant phenomena, which are often
observed even by vessels at sea off the east coast of South America,
where the dust-laden “pamperos” at times compel them to proceed with
the same precautions as in a fog; and the same is true of the northeast
winds blowing off the Sahara desert on the west coast of Africa.
The effects of windstorms carrying sand in the _erosion of rocks_ are
very obvious and striking in many parts of the world; nowhere probably
as much so as on the great plains of western North America, where the
geological composition of the “bad lands” is frequently impressed
upon the rock surfaces very prominently. The strikingly grotesque
forms are frequently brought out in this way, especially in the case
of “mushroom” rocks, where a hard stratum has remained as a covering
while softer layers underneath have been worn away. The illustration
annexed shows such a case on the plains of Wyoming as figured in the
Report of the U. S. Geological Survey, on the Central Great Plains,
by N. H. Darton. Striking examples of the same effects are seen on
the shores of Lake Michigan in the Grand Traverse region, where the
rocky cliffs are visibly worn away and carved under the influence of
the regular “sand-blasts” of northwest winds. On a smaller scale the
effects of these sand-blasts may be noted in the cobble-deserts, where
we frequently find the cobbles worn away on the windward side in a very
characteristic manner; the lee side remaining rounded and smooth, while
the structure of the rock is strongly outlined on the windward side.
[Illustration: FIG. 4.—“Mushroom rocks,” produced by Wind action,
Wyoming. (Darton.)]
CLASSIFICATION OF SOILS.
_The physical Constituents_ of soils are thus, in the most general
terms, first, _rock powder_ (“sand”) more or less changed by
weathering; second, _clay_, as one of the chief results of the
weathering process of silicate minerals; and thirdly, _humus_, the
dark-colored remnant of vegetable decay. According to the obvious
predominance of one or the other of these primary ingredients, soils
are popularly, in the most general sense, classed as “heavy” and
“light”; the former term corresponding as a rule to those in which
clay forms a prominent ingredient, while sandy and humous or “mold”
soils usually fall under the latter designation, because of their easy
tillage. For practical purposes these subdivisions are both convenient
and important, and they form the ordinary basis of land classification.
Beyond these, the degree of fineness of the rock debris, and their
physical and chemical constitution, determine distinctions such as
gravelly, sandy, silty, loamy, calcareous, siliceous, magnesian,
ferruginous, and others of less general application, though locally
often of considerable importance.
For the purposes of discussion and definition, however, another basis
of classification is needed, which essentially concerns both the origin
and the adaptations of lands.
[Illustration: FIG. 5.—Diagram illustrating the genetic relation of
different soil classes to each other.]
1. _Sedentary Soils._—When soils have been formed without removal
from the site of the original rock, by simple weathering, they are
designated as _sedentary_, or _residual_ soils, or “soils in place.”
In the case of these, the original rock underlies the soil or subsoil
at a greater or less depth, according to the intensity and duration
of the weathering process, and is usually more or less softened and
decomposed at the surface where it meets the soil layer. The latter of
course bears some of the distinctive characters of the parent rock, and
its composition and adaptations may, in a measure, be directly inferred
from that origin. Such soils usually contain, especially in their lower
portions, some angular fragments of the parent rock. In some cases
sedentary soils may have been partially derived from rocks that have
been removed from above the present country rock by erosion, and in
that case fragments of such vanished rock may also be present.
Sedentary soils are most commonly found on rock plateaus and on slopes
or plains underlaid by rock strata of but slight inclination, where the
velocity of the “runoff” rainfall is not sufficient to dislodge the
rock debris. Extended areas of such soils exist in the granitic areas
of the southern Alleghenies, in the “black prairies” of the Cotton
States, and on the “basaltic” plateaus of the Pacific Northwest.
2. _Colluvial Soils._—When the soil mass formed by weathering has been
removed from the original site to such a degree as to cause it to
intermingle with the materials of other rocks or layers, as is usually
the case on hillsides, and in undulating uplands generally, as the
result of rolling or sliding down, washing of rains, sweeping of wind,
etc., the mixed soil, which will usually be found to contain angular
fragments of various rocks, and is destitute of any definite structure,
is designated as a _colluvial_[6] one. Colluvial soil masses are
frequently subject to disturbance from landslides, which are usually
the result of water penetrating underneath, between the soil mass and
the underlying rock, or sometimes simply of complete saturation of the
former with water. Aside from such catastrophic action, they commonly
have a slow downward movement in mass (creep), which ordinarily becomes
perceptible only in the course of years; most quickly where there are
heavy frosts in winter, which act both by direct expansion, and by the
state of extreme looseness in which the soil mass is left on thawing.
Colluvial soils form a large portion of rolling and hilly uplands, and
are of very varying degrees of productiveness.
[6] The term “overplaced,” used for such soils in late memoirs of
the U. S. Geological Survey, is at least superfluous, in view of the
perfectly understood term already in general use, and does not seem to
commend itself for adoption by any special or superior fitness; nor
does the suggestion of Shaler (The Origin and Nature of Soils, 12th
Rep. U. S. Geol. Survey) to include the colluvial soils within the
alluvial class, commend itself either from a theoretical or practical
point of view, since but few useful generalizations can apply to both
classes.
3. _Alluvial Soils._—When soils are the result of deposition by
streams, the material having been gathered along the course of the
stream from various sources and carried to a distance before being
deposited, the soil is designated as _alluvial_. These are the soils
of the valleys, flood-plains, and sea- and lake-borders, past and
present. Being of mixed origin, their general character may vary
from one extreme to the other, both as regards physical and chemical
composition. Since, moreover, they represent the finer portions of
the soils of the regions drained by the water-courses, alluvial
soils are as a rule of a fine texture; and as representing the most
advanced decomposition products of the parent rocks, they are usually
preëminently fertile. This is proverbially true of the flood-plains of
rivers, and still more of their deltas—the bodies of lands formed near
their outlets into seas or lakes.
_Character of these soil-classes._—_Sedentary_ soils are as a matter of
course, other things being equal, dependent entirely on the parent rock
for their specific character; and taking into consideration the various
rocks (usually one or few) from which they may have been derived,
nearly the same is true of _colluvial_ soils, except that a portion of
the clay and finest pulverulent matters may in their case be carried
down on the lower slopes and into the valleys and streams, by the
hillside rills.
According to the calculation of Merrill (_Rocks,
Rock-weathering, and Soils_, p. 188) granite when
transformed into soil _without loss_ would increase
in weight by 88%; more than doubling its bulk. More
usually, the leaching process _diminishes_ their
volume as compared with the parent rock.
_Alluvial_ soils are also of course to a certain extent dependent upon
the character of the rocks and surface deposits occurring within the
drainage area of the depositing stream. As a rule their composition
is much more generalized; but their character as to the relative
proportions of sand and clay is essentially dependent upon the velocity
of the water current. Thus in the upper portions of valleys, where the
slope is relatively steep and the velocity therefore high, a large
proportion of cobbles and gravel is often present in the deposits,
sometimes to the extent of rendering cultivation impracticable, or at
least unprofitable. As the slope and velocity decrease, first coarse
and then fine sand will be the prominent component of the deposited
soil; while still lower down, in the region of slack water, the
finest sand or silt, together with clay, will predominate. According
to Hopkins,[7] flowing water will, at a velocity of three inches per
second, _carry_ in suspension only fine clay (and silt); at eight
inches it will carry sand as large as linseed. At one and one-third
inches, it will _move_ pebbles one inch in diameter; and at a velocity
of two inches per second, pebbles of egg size are moved along the
stream bed. Since the velocity of streams subject to freshets will
vary greatly from time to time, deposits of very different grain
will in such cases be found alternating with one another in the soil
stratum of the flood plain. In fact, this alternation and the more or
less stratified structure resulting therefrom, is the distinguishing
mark of alluvial soils as such. It is true that this peculiarity is
also sometimes found in the case of lands now lying far above the
flood-plains of present rivers; but this is due to the elevation of
the land or the depression of the river channels at a former period,
prior to which such lands (commonly known as river terraces, benches
or second bottoms) were formed. The same is true of lake terraces
(“mesas”), which cover enormous areas in some parts of the world,
more particularly in western North America. It must nevertheless be
remembered that such alluvial terrace or bench soils differ in some
respects from the modern alluvials, on account of their long exposure
to atmospheric action alone; one result of which is that they are
usually much poorer in humus, and therefore of lighter tints, than
the more modern soils of alluvial origin. Other differences will be
adverted to hereafter.
[7] Geikie, “Text-book of Geology,” 3d ed.
As a matter of course the above distinctions, especially between
colluvial and alluvial soils, cannot be rigorously maintained in all
cases. There are transitions from one class to the other, so that it
is sometimes optional with the observer to which of the two classes a
particular soil may be considered as belonging. On the lower slopes of
the hills bordering alluvial valleys the colluvial slope-soil may often
be found alternating with the alluvial deposits, or bodily washed away
to be redeposited as alluvium at a greater or less distance.
One characteristic of the flood-plain lands of all the larger rivers,
and more or less of all streams subject to periodic overflows, is
that the land immediately adjoining the banks is both higher and more
sandy than are the lands farther back from the stream. The cause of
this phenomenon is that as lateral overflow diminishes the velocity of
that flow, its coarser portions are deposited near the river banks,
while the finer particles are carried farther away, until finally only
the finest—clay-substance—reach the lagoons or lakes filled with the
overflow or backwater, and are there in the course of time deposited
as heavy clay “swamp” soils. The same occurs where rivers empty into
lakes or the sea; and these slack-water or delta lands are, as a rule,
the most productive on the river’s course. The continued productiveness
of alluvial soils is moreover in many cases assured by the deposition,
during overflows, of fresh soil-material brought down from the head
waters of the streams. The Nile, and the Colorado river of the West,
illustrate this point.
_Lowering of the land-surface by soil formation._—It is evident that
the soil-forming agencies must in the course of time materially affect
both the surface conformation and the absolute level of the land. The
sharp pinnacles and crests of rock are abraded into the rounded forms
now characterizing our uplands and lower ranges of hills and mountains;
and it is estimated that, _e. g._, the general level of the drainage
basin of the Mississippi river is lowered about one foot in 7,000
years, the material being carried into the lowlands and the sea.