The Nature
and Properties
of
NINTH EDITION
The Nature
and Properties of
-- ' . . . , •• ~,-... - • - . - ~ ... ,·."IItoS,,·~· ... ~ .... __ ~_, ... ~~~. -~-. . ~. ':. ,." ¥:_:'.,- . . . . ',. ... , ~. .
Copyright @ 1984. Macmillan Publishing Company. a division of Macmillan. Inc. Printed in the United States of America
All rights reserved. No part of this book may be reproduced or transmitted in any form or by any means. electronic or mechanical. including photocopying. recording. or any information storage and retrieval system. without permission in writing from the Publisher.
Earlier editions by T. Lyttleton Lyon and Harry O. Buckman copyright 1922. 1929. 1937. 1943. and 1952 by Macmillan Publishing Co .• Inc. Earlier editions by Harry O. Buckman and Nyle C. Brady copyright @ 1960 and 1969 by Macmillan Publishing Co .. Inc. Copyright renewed 1950 by Bertha C. Lyon and Harry O. Buckman. 1957 and 1965 by Harry o. Buckman. 1961 by Rita S. Buckman. Earlier edition by Nyle C. Brady copyright @ 1974 by Macmillan Publishing Co .. Inc.
Macmillan Publishing Company
866 Third Avenue. New York. New York 10022 Collier Macmillan Canada. Inc.
Library of Congress Cataloging in Publication Data Brady. Nyle C.
The nature and properties of soils. Includes bibliographies and index. 1. Soil science. I. Title.
S591.B79 1964 631.4 83-19545 ISBN 0-02-313340-6 (Hardcover Edition) ISBN 0-02-946030-1 (International Edition) Printing: 5678 Year: 789012
ISBN
0-02-313340-6Copyright @ 1984. Macmillan Publishing Company. a division of Macmillan. Inc. Printed in the United States of America
All rights reserved. No part of this book may be reproduced or transmitted in any form or by any means. electronic or mechanical. including photocopying. recording. or any information storage and retrieval system. without permission in writing from the Publisher.
Earlier editions by T. Lyttleton Lyon and Harry O. Buckman copyright 1922. 1929. 1937. 1943. and 1952 by Macmillan Publishing Co .• Inc. Earlier editions by Harry O. Buckman and Nyle C. Brady copyright @ 1960 and 1969 by Macmillan Publishing Co .. Inc. Copyright renewed 1950 by Bertha C. Lyon and Harry O. Buckman. 1957 and 1965 by Harry o. Buckman. 1961 by Rita S. Buckman. Earlier edition by Nyle C. Brady copyright @ 1974 by Macmillan Publishing Co .. Inc.
Macmillan Publishing Company
866 Third Avenue. New York. New York 10022 Collier Macmillan Canada. Inc.
Library of Congress Cataloging in Publication Data Brady. Nyle C.
The nature and properties of soils. Includes bibliographies and index. 1. Soil science. I. Title.
S591.B79 1964 631.4 83-19545 ISBN 0-02-313340-6 (Hardcover Edition) ISBN 0-02-946030-1 (International Edition) Printing: 5678 Year: 789012
PREFACE
Unprecedented population increases have occurred since World War II. mostly in those countries with low agricultural productivity and low per capita in-comes. Population growth coupled with rising energy costs and worldwide inflation has taxed the ability of farmers to meet human food requirements. In fact, in some areas. such as sub-Saharan Africa. the per capita food produc-tion has actually declined in the past decade. In other heavily populated regions. including South Asia. per capita food production has remained reasonably steady, but at levels so low as to dictate widespread nutritional deficiencies and constant hunger for tens of millions of people. Meeting human food require-ments continues to be a major worldwide challenge. not only for the developing nations but for all of us.Soils playa critical role in meeting human food needs. Their conservation and wise use are essential today and will be even more critical in the future as population pressures increase. These pressures are already forcing the use of lands for food production that good judgment says should be kept in forests and rangelands. This change in land use encourages accelerated erosion and runoff losses that reduce soil productivity and simultaneously force sediment-burdened floodwaters on downstream populations. Political leaders and devel-opment analysts alike agree that at no time in the history of humankind has the welfare of people been so dependent on the wise use and management of soils.
Effective conservation and management of soils require an understanding of these natural bodies and of the processes going on within them. These processes. which are vital to the production of plants. also influence the many other uses made of soils. The primary purpose of this text is to help the reader to gain a better understanding of the nature and properties of soils and to learn ways of making soils more useful to humankind. I hope that it will be as useful to the student preparing to work in tropical areas as in the United States or Europe.
This ninth edition recognizes the role soils must play in enabling the world to feed itself. But most of the changes from the eighth edition are dictated by the numerous recent advances in soil science. For example. specific
vi
Prefacesis is placed on new and innovative "conservation tillage" systems. which have expanded so rapidly in the United States in recent years. These systems maintain soil cover and thereby reduce soil erosion and water runoff. Attention is given to the effects of these systems on soil properties as well as on soil productivity in a variety of agro-climatic zones. including those in the tropics. New knowledge is presented of nitrogen transformations both in well-drained upland soils and in soils of wetland paddy fields where much of the food for developing countries is produced. Our improved understanding of the nature of soil colloids. including the soil clays in tropical areas. is reflected in several chapters. Likewise. recently acquired knowledge of the charge prop-erties of different soils and of the nature of soil acidity is included.
The text has been rather extensively reorganized. The introductory chapter has been expanded to include general information on the soil as a source of nutrients for plants. Chapters 2-8 cover the basic physical. chemical. and biolog-ical properties of soils. There follow three chapters (9-11) dealing with the essential plant nutrients and three (12-14) concerned with soil formation. classi-fication. and use. Chapter 13 on soil classification has been revised to conform with recent changes in Soil Taxonomy.
Chapters 14-19 deal with the practical management of soils. first through improved water conservation and management and then through use of lime. fertilizer. and organic supplements. The treatment of pollution in Chapter 20 has been updated to include information on the dangers of domestic and indus-trial wastes containing excessive quantities of heavy metals. Finally. Chapter 21 focuses on the role of soils in meeting human food needs. All chapters have been revised to incorporate new knowledge of soil processes.
The format of the text has been improved to make it more attractive and readable. A photograph at the beginning of each chapter gives the reader a general idea of the subject to be covered. The charts and graphs have been redrawn. and many new illustrations. both drawings and photographs. have been added to better explain the text material. Ray Weil and Wybe Kroontje have written a Study Guide that will greatly aid the student in mastering the concepts involved in soil processes.
I am indebted to many soil and crop scientists for advice and counsel in preparing this text and for permission to reproduce their graphs and charts. I also appreciate the helpful suggestions of Macmillan's reviewers: Wybe Kroontje. Virginia Polytechnic Institute and State University; Murray H. Mil-ford. Texas A&M University; George Van Scoyoc and William McFee. Purdue University; R. L. Thomas. University of Guelph; and Ray Weil, University of Maryland. I am especially grateful to my wife. Martha. for her encouragement and for her help in all stages in the preparation and checking of the figures and manuscript. Dr. Joyce Torio was also most helpful in finding backup mate-rial. including photographs. and in typing. checking. and proofreading the figures and parts of the manuscript.
CONTENTS
/
/
1
JThe Soil in Perspective
1
L;
/
What Is Soil?
1.1 3
1.2 Evolution of Modem Concepts of Soil 4
1.3 The Approach-Edaphological Versus Pedological 7
1.4 A Field View of Soil 8
1.5 The Soil Profile 9
1.6 Topsoil and Subsoil 10
1.7 Mineral (Inorganic) and Organic Soils 12
1.8 General Definition of Mineral Soils 12
1.9 Four Major Components of Soils 13
1.10 Mineral (Inorganic) Constituents in Soils 14
1.11 Soil Organic Matter 14
1.12 Soil Water-A Dynamic Solution 16
1.13 Soil Air-Also a Changeable Constituent 17
1.14 The Soil-A Tremendous Biological Laboratory 17
1.15 Clay and Humus-The Seat of Soil Activity 18
1.16 Soils as a Habitat for Plants 19
1.17 The Essential Elements 20
1.18 Essential Macronutrient Content of Soils 21
1.19 Forms of Macronutrients in Soils 23
1.20 Macronutrient Cations 24
1.21 Macronutrient Anions 26
1.22 Soil Solution 28
1.23 Soil and Plant Relations 30
1.24 Summary 33
References 33
..
(
2
-
Some Important Physical Properties of
\
Mineral Soils
35
''---2.1 Soil Texture (Size Distribution of Soil Particles) 36
2.2 Physical Nature of Soil Separates 38
2.3 Mineralogical and Chemical Compositions of Soil
Separates 39
2.4 Soil Textural Classes 42
2.5 Determination of Soil Class 43
2.6 Structure of Mineral Soils 46
2.7 Particle Density of Mineral Soils 48
2.8 Bulk Density of Mineral Soils 49
2.9 Pore Space of Mineral Soils 52
2.10 Aggregation and Its Promotion in Arable Soils 57
2.11 Structural Management of Soils 60
2.12 Soil Consistence 63
2.13 Tilth and Tillage 65
2.14 Conservation Tillage 67
References 70
/'~ \
3
Soil Water: Characteristics a
n
d Behavior
73
3.1 Structure and Related Properties of Water 74
3.2 Capillary Fundamentals and Soil Water 76
3.3 Soil Water Energy Concepts 79
3.4 Soil Moisture Content Versus Tension 85
3.5 Measuring Soil Moisture 86
3.6 Types of Soil Water Movement 90
3.7 Saturated Flow Through Soils 90
3.8 Unsaturated Flow in Soils 93
3.9 Water Movement in Stratified Soils 94
3.10 Water Vapor Movement in Soils 96
3.11 Retention of Soil Moisture in the Field 97
3.12 Conventional Soil Moisture Classification Schemes 100
3.13 Factors Affecting Amount and Use of Available Soil
Moisture 102
3.14 How Plants Are Supplied with Water-Capillarity and
Root Extension 104
3.15 Soil Water amd Soil Microorganisms 107
3.16 Conclusion 108
References 108
4
Soil Air and Soil Temperature
109
4.1 Soil Aeration Defined 110
4.2 Soil Aeration Problems in the Field 111
4.3 Means of Characterizing Soil Aeration 113
4.4 Factors Affecting Soil Aeration 118
4.5 Effects of Soil Aeration on Biological Activities 121
4.6 Other Effects of Soil Aeration 124
4.7 Aeration in Relation to Soil and Crop Management 126
4.8 Soil Temperature 127
4.9 Absorption and Loss of Solar Energy 130
4.10 Specific Heat of Soils 132
4.11 Heat of Vaporization 133
4.12 Movement of Heat in Soils 134
4.13 Soil Temperature Data 134
4.14 Soil Temperature Control 138
References 139
@)
Soil Colloids: Their Nature and Practical
Significance
141
5.1 General Constitution of Layer Silicate Clays 142
5.2 Mineral Colloids Other Than Layer Silicates 143
5.3 Organic Soil Colloids-Humus 147
5.4 Adsorbed Cations 148
5.5 Fundamentals of Layer Silicate Clay Structure 150
5.6 Mineralogical Organization of Silicate Clays i53
5.7 Source of the Negative Charge on Silicate Clays 160
5.8 Chemical Composition of Silicate Clays 162
5.9 Genesis of Silicate Clays 164
5.10 Geographic Distribution of Clays 168
5.11 The Effect of pH on Surface Charges of Soil Colloids 169
5.12 Cation Exchange 173
5.13 Cation Exchange Capacity 175
5.14 Percentage Base Saturation of Soils 179
5.15 Cation Exchange and the Availability of Nutrients 181
5.16 Anion Exchange 182
5.17 Physical Properties of Colloids 183
5.18 Conclusion 187
References 187
~
Soil Reaction: Acidity and Alkalinity
189
6.1 Source of Hydrogen and Hydroxyl Ions 190 6.2 Colloidal Control of Soil Reaction 1966.3 Major Changes in Soil pH 198
6.4 Minor Fluctuations in Soil pH 200 6.5 Hydrogen Ion Variability in the Soil Solution 200 6.6 Active Versus Exchange Acidity 201
6.7 Buffering of Soils 202
6.8 Buffer Capacity of Soils and Related Phases 203
6.9 Importance of Buffering 205
6.10 Soil-Reaction Correlations 206
6.11 Response of Higher Plants to Soil Reaction 209
6.12 Determination of Soil pH 211
6.13 Soil Acidity Problems 213
6.14 Methods of Intensifying Soil Acidity 214
6.15 Reaction of Soils of Arid Regions 215
6.16 Reaction of Saline and Sodic Soils 215
6.17 Growth of Plants on Halomorphic Soils 218
6.18 Tolerance of Higher Plants to Halomorphic Soils 219
6.19 Management of Saline and Sodic Soils 220
6.20 Conclusion 222
References 222
7
Organisms of the Soil
223
7.1 Organisms in Action 224
7.2 Organism Numbers. Biomass. and Metabolic Activity 226
7.3 Earthworms 229
7.4 Termites 232
7.5 Soil Microanimals 233
7.6 Roots of Higher Plants 236
7.7 Soil Algae 238
7.8 Soil Fungi 239
7.9 Soil Actinomycetes 242
7.10 Soil Bacteria 244
7.11 Conditions Affecting the Growth of Soil Bacteria 245
7.12 Injurious Effects of Soil Organisms on Higher Plants 245
7.13 Competition Among Soil Microorganisms 247
7.14 Effects of Agricultural Practice on Soil Organisms 248 7.15 Activities of Soil Organisms Beneficial to Higher Plants 250
References 251
8
Organic Matter of Mineral Soils
2538.1 Sources of Soil Organic Ma tter 254
8.2 Composition of Plant Residues 254
8.3 Decomposition of Organic Compounds 256
8.4 Energy of Soil Organic Matter 260
8.5 Simple Decomposition Products 260
8.6 The Carbon Cycle 261
8.7 Simple Products Carrying Nitrogen 263
8.8 Simple Products Carrying Sulfur 263
8.9 Mineralization of Organic Phosphorus 264
8.10 Humus-Genesis and Definition 264 8.11 Humus-Nature and Characteristics 266 8.12 Direct Influence of Organic Compounds on Higher Plants 268 8.13 Influence of Soil Organic Matter on Soil Properties 268 --'~
8.14 Carbon/Nitrogen Ratio 269
8.15 Significance of the Carbon/Nitrogen Ratio 269 8.16 Amount of Organic Matter and Nitrogen in Soils 272 8.17 Factors Affecting Soil Organic Matter and Nitrogen 274 8.18 Regulation of Soil Organic Matter 278
References 281
9
Nitrogen and Sulfur Economy of Soils
2839.1 Influence of Nitrogen on Plant Development 284
9.2 Forms of Soil Nitrogen 285
9.3 The Nitrogen Cycle 285
9.4 Immobilization and Mineralization 287
9.5 Utilization of Ammonium Compounds 288
9.6 Ammonia Fixation 289
9.7 Nitrification 290
9.8 Soil Conditions Affecting Nitrification 291
9.9 Fate of Nitrate Nitrogen 293
9.10 Denitrifica tion 294
9.11 Nitrification Inhibitors 298
9.12 Ammonia Volatilization 299
9.13 Biological Nitrogen Fixation 300 9.14 Symbiotic Fixation with Legumes 303 9.15 Fate of Nitrogen Fixed by Legume Bacteria 305 9.16 Do Legumes Always Increase Soil Nitrogen? 306 9.17 Symbiotic Fixation with Nodule-Forming Nonlegumes 307 9.18 Symbiotic Nitrogen Fixation Without Nodules 308 9.19 Nonsymbiotic Nitrogen Fixation 309
.
Contents Xl9.20 Addition of Nitrogen to Soil in Precipitation 310
9.21 Reactions of Nitrogen Fertilizers 311
9.22 Practical Management of Soil Nitrogen 312
9.23 Importance of Sulfur 313
9.24 Natural Sources of Sulfur 315
9.25 The Sulfur Cycle 319
9.26 Behavior of Sulfur Compounds in Soils 320
9.27 Sulfur Oxidation and Reduction 321
9.28 Sulfate Retention and Exchange 323
9.29 Sulfur and Soil Fertility Maintenance 323
References 324
10
Supply and Ava
il
ability of Phosphorus and
Potassium
327
10.1 Importance of Phosphorus 328
10.2 The Phosphorus Cycle 329
10.3 The Phosphorus Problem 330
10.4 Phosphorus Compounds in Soils 331
10.5 Factors That Control the Availability of Inorganic Soil
Phosphorus 334
10.6 pH and Phosphate Ions 335
10.7 Inorganic Phosphorus Availability in Acid Soils 336 10.8 Inorganic Phosphorus Availability at High pH Values 340 10.9 pH for Maximum Inorganic Phosphorus Availability 341
10.10 Availability and Surface Area of Phosphates 341
10.11 Phosphorus-Fixing Power of Soils 343
10.12 Influence of Soil Organisms and Organic Matter on the
Availability of Inorganic Phosphorus 343
10.13 Availability of Organic Phosphorus 344
10.14 Intensity and Quantity Factors 345
10.15 Practical Control of Phosphorus Availability 348 10.16 Potassium-The Third "Fertilizer" Element 349
10.17 Effects of Potassium on Plant Growth 349
10.18 The Potassium Cycle 350
10.19 The Potassium Problem 350
10.20 Forms and Availability of Potassium in Soils 353 10.21 Factors Affecting Potassium Fixation in Soils 356
10.22 Intensity and Quantity Factors 358
10.23 Practical Implications in Respect to Potassium 360
References 361
11
Micronutrient Elements
363
11.1 Deficiency Versus Toxicity 365
11.2 Role of the Micronutrients 365
11.3 Source of Micronutrients 368
11.4 General Conditions Conducive to Micronutrient Deficiency 370
11.5 Factors Influencing the Availability of the Micronutrient
Cations 371
11.6 Chelates 374
11.7 Factors Influencing the Availability of the Micronutrient
Anions 376
11.8 Need for Nutrient Balance 378
11.9 Soil Management and Micronutrient Needs 379
References 383
12
Origin, Nature, and Classification of Parent
Materials
385
12.1 Classification and Properties of Rocks 386
12.2 W ea thering-A General Case 387
11.3 Mechanical Forces of Weathering 390
12.4 Chemical Processes of Weathering 391
12.5 Factors Affecting Weathering of Minerals 394
12.6 Weathering in Action-Genesis of Parent Materials 396
12.7 Geological Classification of Parent Materials 397
12.8 Residual Parent Material 398
12.9 Colluvial Debris 399
12.10 Alluvial Stream Deposits 399
12.11 Marine Sediments 403
12.12 The Pleistocene Ice Age 404
12.13 Glacial Till and Associated Deposits 408
12.14 Glacial Outwash and Lacustrin Sediments 409
12.15 Glacial Eolian Deposits 410
12.16 Agricultural Significance of Glaciation 412
12.17 Conclusion 413
References 413
13
Soil Formation, Classification, and Survey
415
13.1 Factors Influencing Soil Formation 416
13.2 Weathering and Soil Development Processes 421
13.3 The Soil Profile 423
...
Contents Xlll13.4 Concept of Individual Soils 427
13.5 Soil Classification in the United States 429
13.6 New Comprehensive Classification Systems-Soil
Taxonomy 430
13.7 Soil Orders 436
13.8 Soil Suborders. Great Groups. and Subgroups 454
13.9 Soil Families and Series 457
13.10 Soil Phases. Associations. and Catenas 460
13.11 Other Classification Systems 463
13.12 Soil Survey and Its Utilization 465
13.13 Land Capability Classification 466
13.14 Soil Interpretations for Nonfarm Uses 472
References 473
14
Organic Soils (Histosols): Their Nature,
Properties, and Utilization
475
14.1 Genesis of Organic Deposits 476
14.2 Area and Distribution of Peat Accumulations 478
14.3 Peat Parent Materials 479
14.4 Uses of Peat 481
14.5 Classification of Organic Soils 481
14.6 Physical Characteristics of Field Peat Soils 484
14.7 The Colloidal Nature of Organic Soil 486
14.8 Chemical Composition of Organic Soils 488
14.9 Bog Lime-Its Importance 490
14.10 Factors That Determine the Value of Peat and Muck Soils 491
14.11 Preparation of Peat for Cropping 491
14.12 Management of Peat Soils 492
14.13 Organic Versus Mineral Soils 494
References 494
15
Losses of Soil Moisture and Their
Regulation
495
15.1 Interception of Rainwater by Plants 496
15.2 The Soil-Plant-Atmosphere Continuum 496
15.3 Evapotranspiration 500
15.4 Magnitude of Evaporation Losses 502
15.5 Efficiency of Water Use 504
15.6 Evaporation Control: Mulches and Cultivation 506
15.7 Vaporization Control in Humid Regions-Minimum Tillage 510
15.8 Vaporization Control in Semiarid and Subhumid Regions 511
.
15.9 Evaporation Control of Irrigated Lands 512
15.10 Types of Liquid Losses of Soil Water 515
15.11 Percolation and Leaching-Methods of Study 515
15.12 Percolation Losses of Water 516
15.13 Leaching Losses of Nutrients 519
15.14 Land Drainage 522
15.15 Surface Field Drains 523
15.16 Subsurface or Underground Drains 525
15.17 Benefits of Land Drainage 529
References 530
16
Soil Erosion and Its Control
533
16.1 Significance of Runoff and Soil Erosion 534
16.2 Accelerated Erosion-Mechanics 537
16.3 Types of Water Erosion 538
16.4 Factors Affecting Accelerated Erosion-Universal Soil-Loss
Equation 538
16.5 Rainfall and Runoff Factor (R) 539
16.6 Soil Erodibility Factor (K) 541
16.7 Topographic Factor (L5) 542
16.8 Cover and Management Factor (C) 544
16.9 Support Practice Factor (P) 546
16.10 Calculating Expected Soil Losses 548
16.11 Sheet and Rill Erosion Control 549
16.12 Conservation Tillage Practices 549
16.13 Gully Erosion 557
16.14 Wind Erosion-Its Importance and Control 558
16.15 Soil Loss Tolerance 566
16.16 Conservation Treatment in the United States 566
16.17 Summary of Soil Moisture Regulation 568
References 568
L
ime and Its Soil
-
Plant Relationships
571
17.1 Liming Materials 572
17.2 Chemical Guarantee of Liming Materials 574
17.3 Fineness Guarantee of Limestone 576
17.4 Changes of Lime Added to the Soil 578
17.5 Loss of Lime from Arable Soils 579
17.6 Effects of Lime on the Soil 580
17.7 Crop Response to Liming 582
17.8 Overliming 582
17.9 Shall Lime Be Applied? 583
17.10 Form of Lime to Apply 583
17.11 Amounts of Lime to Apply 585
17.12 Methods of Applying Lime 586
17.13 Lime and Soil Fertility Management 588
References 588
18
Fertilizers and Fertilizer Management
589
18.1 The Fertilizer Elements 590
18.2 Three Groups of Fertilizer Materials 590
18.3 Nitrogen Carriers-Two Groups 591
18.4 Inorganic Nitrogen Carriers 592
18.5 Phosphatic Fertilizer Materials 597
18.6 Potassium Fertilizer Materials 602
18.7 Sulfur in Fertilizers 603
18.8 Micronutrients 604
18.9 Organic Sources of Nutrients 605
18.10 Mixed Fertilizers 605
18.11 Effect of Mixed Fertilizers on Soil pH 609
18.12 The Fertilizer Guarantee 610
18.13 Fertilizer Inspection and Control 611
18.14 Fertilizer Economy 612
18.15 Movement of Fertilizer Salts in the Soil 613
18.16 Methods of Applying Solid Fertilizers 614
18.17 Application of Liquid Fertilizers 616
18.18 Factors Influencing the Kind and Amount of Fertilizers
to Apply 617
18.19 Kind and Economic Value of Fertilized Crop 617
18.20 Chemical Condition of the Soil-Total Versus Partial
Analysis 619
18.21 Quick Tests for Available Soil Nutrients 620
18.22 Broader Aspects of Fertilizer Practice 623
References 625
19
Recycling Nutrients Through Animal Manures and
Other Organic Wastes
627
19.1 Farm Manure Significance 628
19.2 Quantity of Manure Produced 630
19.3 Chemical Composition 632
19.4 Storage. Treatment. and Management of Animal Manures 635
.
19.5 Utilization of Animal Manures 638
19.6 Long-Term Effects of Manures 639
19.7 Urban and Industrial Wastes 639
19.8 Sewage Effiuent and Sludge 641
19.9 Composts 64il
19.10 Integrated Recycling of Wastes 649
References 651
20
Soils and Chemical Pollution
653
20.1 Chemical Pes ticides-Background 654
20.2 Kinds of Pesticides 656
20.3 Behavior of Pesticides in Soils 657
20.4 Effects of Pesticides on Soil Organisms 663
20.5 Contamination with Toxic Inorganic Compounds 665
20.6 Potential Hazards of Chemicals in Sewage Sludge 667
20.7 Behavior of Inorganic Contaminants in Soils 671
20.8 Prevention and Elimination of Inorganic Chemical
Contamination 674
20.9 Organic Wastes 675
20.10 Use of Organic Wastes for Crop Production 677
20.11 Soils as Organic Waste Disposal Sites 679
20.12 Soil Salinity 680
20.13 Acid Rain 682
20.14 Radionuclides in Soil 684
20.15 Three Conclusions 685
References 686
21
Soils and the World's Food Supply
689
21.1 Expansion of World Population 690
21.2 Factors Influencing World Food Supplies 690
21.3 The World's Land Resources 691
21.4 Potential of Broad Soil Groups 694
21.5 Problems and Opportunities in the Tropics 696
21.6 Requisites for the Future 703
References 706
Glossary of Soil Science Terms
709
Index
737
[Preceding page] Soils are natural three-dimensional bodies used for many purposes,
the most important of which is to produce food and fiber for humankind. [Courtesy USDA Soil Conservation Service.]
People are dependent on soils-and to a certain extent good soils are dependent upon people and the use they make of them. Soils are the natural bodies on which plants grow. Society enjoys and uses these plants because of their beauty and because of their ability to supply fiber and food for humans and for animals. Standards of living are often determined by the quality of soils and the kinds and quality of plants and animals grown on them.
But soils have more meaning for humankind than as a habitat for growing crops. They underlie the foundations of houses and factories and determine whether these foundations are adequate. They are used as beds for roads and highways and have a definite influence on the length of life of these struc-tures. In rural areas soils are often used to absorb domestic wastes through septic sewage systems. They are being used more and more as recipients of other wastes from municipal, industrial, and animal sources. The deposition of undesirable silt in municipal reservoirs makes the protection of soils in upstream watersheds as important to city dwellers as to their counterparts on the farm or in the forest. Obviously, soils and their management are of broad societal concern.
Great civilizations have almost invariably had good soils as one of their chief natural resources. The ancient dynasties of the Nile were made possible by the food-producing capacity of the fertile soils of the valley and its associated irrigation systems. Likewise, the valley soils of the Tigris and Euphrates rivers in Mesopotamia and of the Indus, Yangtze, and Hwang Ho rivers in India and China were habitats for flourishing civilizations. Subject to frequent replen-ishing of their fertility by natural flooding, these soils provided continued abun-dant food supplies. They made possible stable and organized communities and even cities, in contrast to the nomadic, shifting societies associated with upland soils and with their concomitant animal grazing. It was not until the value of manures and crop residues was discovered that people were able to make extensive use of upland soils for sustained crop culture.
Soil destruction or mismanagement was associated with the downfall of some of the same civilizations that good soils had helped to build. The cutting of timber in the watersheds of the rivers encouraged erosion and topsoil loss. In the Euphrates and Tigris valleys, the elaborate irrigation and drainage sys-tems were not maintained. This resulted in the accumulation of harmful salts, and the once productive soils became barren and useless. The proud cities that had occupied selected sites in the valleys fell into ruin, and the people migrated elsewhere.
History provides lessons that modern nations have not always heeded. The wasteful use of soil resources in the United States during the first century of intensive agricultural production by the early settlers and the following generations provides such an example. Even today there are many who do
not fully recognize the long-term significance of soils. This may be due in part to widespread ignorance of what soils are, what they have meant to past generations, and what they mean to us and will mean to future generations.
1.1
What Is Soil?
Concepts of SoU. Part of the lack of concern for soils may be attributed to different concepts and viewpoints concerning this important product of nature. For example, to a mining engineer the soil is the debris covering the rocks or minerals that he must quarry. It is a nuisance and must be removed. To the highway engineer the soil may be the material on which a roadbed is to be placed. If its properties are unsuitable, that particular soil will need to be removed and replaced with rock and gravel.
The average homeowner also has a concept of soil. It is good if the ground is mellow or loamy (Figure 1.1). The opposite viewpoint is associated with "hard clay," which resists being spaded into a good seedbed for a flower garden. The homemaker can differentiate among variations in the soil, espe-cially those relating to its stickiness or tendency to cling to the shoe soles and eventually to carpets.
The farmer, along with the homeowner, looks upon the soil as a habitat for plants. The farmer makes a living from the soil and is thereby forced to pay more attention to its characteristics. To the farmer the soil is more than useful-it is indispensable.
A prime requisite for learning more about the soil is to have a general concept of what it is. This concept must encompass the viewpoints of the engineer, the homeowner, and the farmer. It has developed through the practical and scientific discoveries of the past.
FIGURE 1.1 A hard soil clod (left) that would be difficult to work into a good seedbed. A friable soil intermixed with organic matter (right) is much more easily handled. [Courtesy USDA National Tillage Machinery Laboratory.]
1.2
E
volution of Modern Concepts of Soil
There are two basic sources of our current knowledge of soils. First, there is the practical knowledge gained by farmers through centuries of trial and error. This was the only information available before the advent of modem science, which now provides the second source of facts about soils and their manage-ment.
Experience of the Cultivator. The earliest recorded history contains evidence that, through trial and error, humans learned to distinguish differences in soils. They also learned the value of treating soils with plant and animal wastes. More than 42 centuries ago the Chinese used a schematic soil map as a basis for taxation. Homer, in his Odyssey, said to have been written about 1000
B.C., makes reference to the use of manure on the land. Biblical references are made to the "dung hill" and to the beneficial practice of "dunging" around plants. Greek and Roman writers described a reasonably elaborate system of farming that involved leguminous plants and the use of ashes and sulfur as soil amendments. This evidence suggests that by the time of the early Roman civilization many of the practical principles governing modern agriculture, in-cluding soil management, had been discovered and put to use by farmers and livestock owners.
Further development and application of these principles were halted by the barbarian invasions of Rome. Even so, Roman agriculture was the founda-tion for most European agriculture during the feudal Dark Ages. The cultural practices were passed from generation to generation even though the farmers were ignorant about why the practices were necessary. When in the seven-teenth and eighseven-teenth centuries there was a blossoming of scientific inquiry, the stage was set for the application of science to the improvement of agricul
-tural systems, including those involving soils.
Early Scientific Investigations and Soil Productivity. From the seventeenth century until the middle of the twentieth century the primary occupation of soil scientists was to increase the production of crop plants. Jan Baptista van Helmont, a Flemish chemist, in his famous five-year willow tree experiment concluded that 164 pounds of dry matter came primarily from the water supplied since the soil lost no weight while producing the tree. This concept was altered by John Woodward, an English researcher, who found that muddy water pro-duced more plant growth than rainwater or river water, leading him to conclude that the fine earth was the "principle" of growth. Others concluded that the principle was humus taken in by the plants from soil. Still others assumed that the principle was somehow passed on from dead plants or animals to the new plant. Jethro Tull early in the eighteenth century demonstrated the benefits of cultivation, thinking erroneously that stirring the soil would make it easier for plants to absorb small quantities of fine earth.
It remained for the French agriculturist J. B. Boussingault, through a series
4
1 The Soil in Perspectiveof field experiments starting in 1834, to provide evidence that the air and rain were the primary sources of carbon, hydrogen, and oxygen in plant tissues. But his investigations were largely disregarded until 1840, when the eminent German chemist Julius von Liebig reported findings that crop yields were di-rectly related to the content of "minerals" or inorganic elements in the manures applied to the soil. Liebig's reputation as an outstanding physical chemist was instrumental in convincing the scientific community that the old theories were wrong. He proposed that mineral elements in the soil and in added manures and fertilizers are essential for plant growth.
Testing Liebig's Concepts. Liebig's work revolutionized agricultural theory and opened the way for numerous other investigations. Those of J. B. Lawes and
J.
H. Gilbert at the Rothamsted Experiment Station in England are most noteworth since they put Liebig's theories to the test in the field. While they confirmed many of his findings, they identified two errors. Liebig had theorized that nitrogen came primarily from the atmosphere rather than the soil. He further assumed that salts could be fused before being added as fertilizers; apparently he gave no thought to what this drastic action would do to the solubility and "availability" to plants of the nutrient elements. The Rothamsted field research proved that nitrogen applications to the soil markedly benefited plant growth. It showed further that mineral elements must be in an "available" form for optimum uptake by plants. The investigations led to the development of acid-treated phosphate rock or "superphosphate," which is still an important commercial fertilizer source of phosphorus.While the work of Gilbert. Lawes. and others pinpointed weaknesses in components of Liebig's theory. the primary concept is still considered basically sound. For example. Liebig stated what has since been called the law of the minimum: "By the deficiency or absence of one necessary constituent. all the others being present. the soil is rendered barren for all those crops to the life of which that one constituent is indispensable." The significance of this finding will be more apparent later as soil fertility and plant nutrition interac-tions are considered (see Section 1.16).
After Liebig. unraveling the complexities of nitrogen transformations in soil awaited the emergence of soil bacteriology. J. T. Way. who demonstrated the cation adsorption properties of soils. discovered in 1856 that nitrates are formed in soils from ammonia-containing fertilizers. Twenty years later
R. W. Warington demonstrated that this process was biological. and in 1890 S. Winogradski isolated the two groups of bacteria responsible for the transfor-mation of ammonia to nitrate. Coupled with the discovery in the 1880s that nitrogen-assimilating bacteria grow in nodules of the roots of legumes, these findings provided background information for sound soil and crop management practices.
Early Researcb in the United States. The European investigations on soil fertil-izers were found to be quite applicable to the United States when they were
tested in the late nineteenth century. Upon being tilled, the soils along our eastern and southern seaboards were easily depleted of nutrients by the perco-lation of rainfall and by crop removal. Edmond Ruffin, a Virginia farmer, grasped the concepts of nutrient depletion and in his writings was especially critical of those who did not properly care for and replenish their soil. Unfortu-nately, the abundance of open land to the west encouraged the abandonment of the "worn-out" soils in the east rather than the adoption of more realistic management systems.
The establishment of the U.S. Department of Agriculture in 1862 and the state Agricultural Experiment Stations in 1886 gave a decided boost to both field and laboratory investigations on soils. Numerous field trials were initiated to test the applicability of findings of the European investigators. Unfortunately, in most of the trials soil was not considered to be a dynamic medium for crop growth. Instead the soil was considered merely as a "storage bin," in keeping with Liebig's concepts. Exceptions existed, such as the work of men like F. H. King of Wisconsin, who studied the movement and storage of water in soils in relation to root penetration and crop growth. Also C. G. Hopkins of Illinois developed effective soil-management systems based on limestone, rock phosphate, and legumes. Milton Whitney of the U.S. Department of Agri-culture urged greater consideration of properties of soils in the field and initiated the first national soil survey system.
Field SoillnvestigatioDs. Liebig's concepts thoroughly dominated the thinking of soil scientists in the late nineteenth and early twentieth centuries. Further-more, except for the field testing of crop response to fertilizer, much of the research was done in the laboratory and greenhouse. Inadequate attention was given to the variable characteristics of the soils as found in the field. Nor was much significance given to the differences in soils as related to the climate in which they were found. Soils were considered as geological residues on the one hand and as reservoirs of nutrients for plant growth on the other.
As early as 1860, E. W. Hilgard, then in Mississippi, published his findings, which called attention to the relationships among climate, vegetation, rock materials, and the kinds of soils that develop. He conceived of soils not merely as media for plant growth but as dynamic entities subject to study and classifi-cation in their field setting. Unfortunately, Hilgard was ahead of his time. It was necessary for many of his concepts to be rediscovered before they were accepted.
Parallel to Hilgard's investigations were those made by the brilliant team of soil scientists in Russia led by V. V. Dokuchaev. These scientists found unique horizontal layerings in soils-Iayerings associated with the climate, vegetation, and underlying soil material. The same sequence of layering was found in widely separated geographical areas provided the areas had similar climate and vegetation. The concept of soils as natural bodies was well
1.3
oped in the Russian studies. as were concepts of soil classification based on field soil characteristics.
The Russian studies were under way as early as 1870. Unfortunately. lan-guage barriers prevented effective communication of the Russian concepts to scientists in Western Europe. Asia. and the Americas until 1914. when they were published in German by K. D. Glinka. a member of the Russian team of soil scientists. These concepts were quickly grasped by C. F. Marbut of the
u.s.
Department of Agriculture. who had been placed in charge of theu.s. National Soil Survey by Dr. Whitney. Marbut and his associates developed
a nationwide soil classification system based to a great extent on the Russian concepts. Consideration of soils as natural bodies has led to further modifica-tions in soil classification systems. which will receive attention in later chap-ters.The
Approach-Edaphological Versus Pedological
The previous section suggests that two basic concepts of soil have evolved through two centuries of scientific study. The first considers soil as a natural entity. a biochemically weathered and synthesized product of nature. The sec-ond conceives of the soil as a natural habitat for plants and justifies soil studies primarily on that basis. These conceptions illustrate the two approaches that can be used in studying soils-that of the pedologist and that of the
edaphologist.
The origin of the soil. its classification. and its description are involved in pedology (from the Greek word pedon, which means soil or earth). Pedology considers the soil as a natural body and does not focus primarily on the soil's immediate practical utilization. A pedologist studies. examines. and classifies soils as they occur in their natural environment. These findings may be as useful to highway and construction engineers as to the farmer.
Edaphology (from the Greek word edaphos, which also means soil or
ground) is the study of the soil from the standpoint of higher plants. It considers the various properties of soils as they relate to plant production. The edapholo-gist is practical. having the production of food and fiber as an ultimate goal. Simultaneously. the edaphologist must be a scientist to determine the reasons for variation in the productivity of soils and to find means of conserving and improving this productivity.
In this textbook the dominant viewpoint will be that of the edaphologist. Pedology will be used. however. to the extent that it gives a general understand-ing of soils as they occur in nature and are classified. Furthermore. since studies of the basic physical. chemical. and biological characteristics of soils contribute equally to edaphology and pedology. it is not possible to separate these ap-proaches fully. This is illustrated in the following section. which deals with soils as they are found in the field.
1.4
A Field View of Soil
Someone has said that the soil is to the earth as the peel is to the orange. This analogy is acceptable but should be modified to stress the great variability in soil from site to site on the earth's surface. Even a casual examination of road cuts from one geographic area to another suggests differences in soil depth. color. and mineral makeup. The trained eye of the soil scientist. however. can identify common properties of soils from areas as distant as Hawaii. India. and the continental United States. The common properties will receive immedi-ate attention; the variations will be treimmedi-ated in succeeding chapters.
Soil Versus Regolith. Views such as that shown in Figure 1.2. in which uncon-solidated materials are found on underlying rocks. are quite familiar. Above bedrock some unconsolidated debris is present almost universally. This mate-rial. known as regolith, may be negligibly shallow or hundreds of feet thick. It may be material that has weathered from the underlying rock or it may have been transported by the action of wind, water. or ice and deposited upon the bedrock or upon other material covering the bedrock. As might be expected. the regolith tends to vary in composition from place to place.
An examination of the upper 1-2 meters of the regolith shows that it differs from the material below. It is higher in organic matter since plant residues FIGURE 1.2 Relative positions of
the regolith. its soil, and the underlYing country rock. Sometimes the regolith is so thin that it has been changed entirely to soil; in such a case. soil rests directly on bedrock.
[Photo courtesy Tennessee VaJley Authority.]
8
1 The Soil in PerspectiveSoil
.
.
.
.'
.
• '. • • • . . . . D '. ' 0 . . ' • • tI I • • • •.
..
.
.
• • • ., ' . 0 • " • ,;:II • • o9':
t i l ' ~: • • . • • t:J p . 0 ' Q ' I ' • "', . ' ~, .. ,~'o' . ' ." • ' 0 ' • D ' r . . . . I ' c:::. .t:JfD J 0 \ ' : = 0 " .O,'.c;:,' '. .' t . r--.,.., '. & "i:::> ," · •• ·a'.O.c:: 0 ' 0 • ~ , ' .:=;:, • • c:::::> <:::>. Ie CI •• Regolith Bedrockdeposited originally on the surface have been incorporated by earthworms and other animals and have been decomposed by microorganisms. Being near the surface. it has been more subject to weathering. Products of this weathering. especially if they have moved vertically. give rise to characteristic layering. This upper and biochemically weathered portion of the regolith 1 is the
soil. It is the product of both destructive and synthetic forces. Weathering and microbial decay of organic residues are examples of destructive processes. whereas the formation of new minerals. such as certain clays. and the develop-ment of characteristic layer patterns are synthetic in nature. These forces have given rise to a distinctive entity in nature called the soil. which. in turn. is comprised of a large number of individual soils.
The Soil Versus A Soil. Characteristics of the soil vary widely from place to place. For example. the soil on steep slopes is generally not as deep and productive as soil on gentle slopes. Soil that has developed from sandstone is more sandy and less fertile than soil formed from rocks such as limestone. The properties of a soil that has developed in tropical climates are quite
differ-ent from those of a soil found in temperate or arctic areas.
Scientists have recognized these soil variations from place to place and have set up classification systems in which the soil is considered as composed of a large number of individual soils. each having its distinguishing characteris-tics. Therefore. a soil. as distinguished from the soil. is merely a well-defined subdivision having recognized characteristics and properties. Thus. a Cecil clay loam. a Marshall silt loam. and a Norfolk sand are examples of specific soils, which collectively make up the overall soil covering the world's land areas. The term "soil" is a collective term for all soils. just as "vegetation" is used to designate all plants.
1.5
The
Soil
P
rofile
Examination of a vertical section of a soil in the field reveals the presence of more or less distinct horizontal layers (Figure 1.3). Such a section is called a profile, and the individual layers are regarded as horizons. These horizons above the parent material are collectively referred to as the solum (from the Latin legal term solum meaning soil. land. or parcel of land). Every well-devel-oped. undisturbed soil has its own distinctive profile characteristics. which are utilized in soil classification and survey and are of great practical impor-tance. In judging a soil one must consider its whole profile.
Soil Horizons. The upper layers or horizons of a soil profile generally contain considerable amounts of organic matter and are usually darkened appreciably 1 Where the original regolith was relatively uniform. the material below the soil is considered
to have a composition similar to the parent material from which the soil was formed.
FIGURE 1.3 Field view of a road cut that reveals the underlying layers of a soil. The closeup emphasizes soil layering and the distinctive character of the soil profile. The surface layer is darker in color because of its higher organic matter content. One of the subsurface horizons (point of pick) is characterized by a distinctive structure. The existence of layers such as those shown is used to help differentiate one soil from another.
because of such an accumulation. When a soil is plowed and cultivated, these layers are included in what is termed the surface soil or the topsoil. This is
sometimes referred to as the furrow slice because it is the portion of the
soil turned or "sliced" by the plow.
The underlying layers (referred to as the subsoil) contain comparatively
less organic matter than the topsoil. The various subsoil layers, especially in mature, humid region soils, present two very general belts: (a) an upper transi-tion zone characterized by the loss of minerals and by some organic matter accumulation and (b) a lower zone of accumulation of compounds such as iron and aluminum oxides, clays, gypsum, and calcium carbonate.
The solum thus described extends a moderate depth below the surface. A depth of 1-2 meters is representative for temperate region soils. Here, the noticeably modified lower subsoil gradually merges with the less weathered portion of the regolith whose upper portion is geologically on the verge of becoming a part of the lower subsoil and hence of the solum.
The various layers comprising a soil profile are not always distinct and well defined. The transition from one to the other is often so gradual that the establishment of boundaries is rather difficult. Nevertheless, for any particu-lar soil the various horizons are characteristic and their properties greatly influence the growth of higher plants.
1.6
Topsoil and Subsoil
Topsoil, being near the surface, is the major zone of root development. It carries much of the nutrients available to plants, and it supplies a large share of the water used by crops. Also, as the layer that is plowed and cultivated, it is
FIGURE 1.4 Root system of a com plant growing in a deep open soil. Roots of crops such as alfalfa or of trees probably penetrate even further. [Courtesy USDA National
Tillage Machinery Laboratory.]
subject to manipulation and management. By proper cultivation and the incor-poration of organic residues. its physical condition may be modified. It can be treated easily with chemical fertilizers and limestone. and it can be drained.
In short. its fertility and to a lesser degree its productivity2 may be raised. lowered. or satisfactorily stabilized at levels consistent with economic crop production.
This explains why much of the soil investigation and research has been devoted to the surface layer. Plowing. cultivation. liming. and fertilization are I The term fertility refers to the inherent capacity of a soil to supply nutrients to plants in adequate amounts and in suitable proportions. Productivity is related to the ability of a soU to yield crops and Is the broader term since fertility is only one of a number of factors that determine the magnitude of crop yields.
applied essentially to the furrow slice. In practice. then. the term "soil" usually denotes the surface layer. the "topsoil." or the furrow slice.
Even though the subsoil cannot be seen from the surface. there are few land uses that are not influenced by subsoil characteristics. Certainly crop production is affected by root penetration into the subsoil (Figure 1.4) and by the reservoir of moisture and nutrients held therein. Likewise. the selection of building sites and the location of roadways are influenced by subsoil charac-teristics. These observations are of practical significance since. unlike the top-soil. the subsoil is subject to little human alteration except by drainage. As a consequence. land-use decisions are often more dependent on the nature of the subsoil than on topsoil characteristics.
1.7
Mineral (Inorganic) and
Organic
Soils
The profile generalizations just described relate to soils that are predominantly
mineral or inorganic in composition. Even in their surface layers mineral soils
are comparatively low in organic matter. generally ranging from 1 to 10%. In contrast. soils in swamps. bogs. and marshes commonly contain 80-95% organic matter. Organic soils include all soils with more than 20% (by weight) organic matter along with soils that are continually saturated with water and contain
12-18% organic matter (depending on clay content). When drained and cleared. organic soils are most productive. especially for high-value crops such as fresh market vegetables. Organic deposits may also be excavated. bagged. and sold as organic supplements for home gardens and potted plants. The economic significance of organic soils is considerable in localized regions.
Because they occupy such a high proportion of the total land area. mineral soils are of greater importance than organic soils and will deservedly receive major attention in this text. The origin. character. and agricultural use of organic soils are considered as a unit in Chapter 14. Until then the discussion will be concerned primarily with mineral soils.
1.8
General Definition of Mineral Soils
Mineral soils have already been denoted as the "upper biologically. chemically. and physically weathered portion of the regolith." When expanded by profile and horizon study. this statement presents a pedological concept of soil origin and characterization. Keeping in mind the role of soils in plant production. we may define the soil as "a collection of natural bodies developed in the unconsolidated mineral and organic material on the immediate surface of the earth that (a) serves as a natural medium for the growth of land plants and (b) has properties due to the effects of climate and living matter acting upon parent material. as conditioned by topography. over a period of time."
1.9
Four Major Components of Soils
Volume Composition of Mineral Soils. Mineral soils consist of four major components: mineral materials, organic matter, water, and air. Figure 1.5 shows the approximate proportions of these components in a representative silt loam surface soil in optimum condition for plant growth. Note that this soil contains about half solids and half pore space (water and air). Of the total soil volume, a bout half is solid space, 45% mineral matter and 5% organic matter. At optimum moisture for plant growth, the pore space is divided roughly in half; 25% of the volume if;! water space and 25% is air. The proportions of air and water are subject to rapid and great fluctuations under natural conditions, depending on the weather and other factors.
It should be emphasized that the four major components of a typical soil exist mainly in an intimately mixed condition. This encourages interactions within and between the groups and permits marked variation in the environ-ment for the growth of plants.
The volume composition of subsoils is somewhat different from that just described. Compared to topsoils they are lower in organic matter content, are somewhat lower in total pore space, and contain a higher percentage of small pores. This means they have a higher percentage of minerals and water and a considerably lower content of organic matter and air.
FIGURE 1.5 Volume composition of a silt loam surface soil when in good condition for plant growth. The air and water in a soil are extremely variable, and their proportions determine in large degree the soil's suitability for plant growth.
1.10
1.11
Mineral (Inorganic) Constituents in Soils
A casual examination of a sample of soil illustrates that the inorganic portion is variable in size and composition. It is normally composed of small rock fragments and minerals3 of various kinds. The rock fragments are remnants of massive rocks from which the regolith and, in turn, the soil have been formed by weathering. They are usually quite coarse (Table 1.1). The minerals, on the other hand, are extremely variable in size. Some are as large as the smaller rock fragments; others, such as colloidal clay particles, are so small that they cannot be seen" without the aid of an electron microscope.
TABLE 1.1 Four Major Size Classes of Inorganic Particles and Their
General Properties
Common Means of Dominant
Size fraction name observation composition Very coarse Stone. gravel Naked eye Rock fragments
Coarse Sands Naked eye Primary minerals
Fine Silt Microscope Primary and secondary
minerals
Very fine Clay Electron Mostly secondary
microscope minerals
Quartz and some other primary minerals such as micas and feldspars have persisted with little change in composition since they were extruded in molten lava. Other minerals, such as the silicate clays and iron oxides, have been formed by the weathering of less resistant minerals as soil formation progressed. These minerals are called secondary minerals. In general, the pri-mary minerals dominate the coarser fractions of soil, whereas secondary miner-als are most prominent in the fine materiminer-als, especially in clays. Clearly, mineral particle size has much to do with the properties of soils in the field.
Soil Organic Matter
Soil organic matter represents an accumulation of partially decayed and par-tially synthesized plant and animal residues. Such material is continually being broken down as a result of the work of soil microorganisms. Consequently, 3 The word "mineral" is used in this book in two ways: (1) as a general term to describe soils dominated by inorganic constituents and (2) as a more specific term to describe distinct minerals found in nature. such as quartz and feldspars. More detailed discussions of the common soil-forming minerals and the rocks in which they are found are given in Section 1.19 and in Chapter 12.
it is a rather transitory soil constituent and must be renewed constantly by the addition of plant and/or animal residues.
The organic matter content of a soil is small, varying from 2 to 6% by weight in typical well-drained mineral soils. Its influence on soil properties and consequently on plant growth, however, is far greater than the low percent-age would indicate. Organic matter functions as a "granulator" of mineral particles, being largely responsible for the loose, easily managed condition of productive soils. Also, it is a major soil source of two important mineral elements, phosphorus and sulfur, and essentially the sole soil source of nitrogen. Through its effect on the physical condition of soils, organic matter also in-creases the amount of water a soil can hold and the proportion of this water available for plant growth (Figure 1.6). Finally, organic matter is the main source of energy for soil organisms, both plant and animal. Without it, biochemi-cal activity would come nearly to a standstill.
Soil high in organic matter Soil high in organic matter Soil low in organic matter in organic matter
FIGURE 1.6 Soils high in organic matter are darker in color and have greater water-holding capacities than do soils low in organic matter. The same amount of water was applied to each container. As the lower photo shows, the depth of water penetration was less in the soil at the left because of its greater water-holding capacity.
1.12
Soil organic matter consists of two general groups: (a) original tissue and its partially decomposed equivalents and (b) humus. The original tissue includes the undecomposed roots and the tops of higher plants. These materials are subject to vigorous attack by soil organisms. which use them as sources of energy and tissue-building rna terial.
The more resistant products of this decomposition. both those synthesized by the microorganisms and those modified from the original plant tissue. are collectively known as humus. This material, usually black or brown in color. is colloidal in nature. Its capacity to hold water and nutrient ions greatly ex-ceeds that of clay. its inorganic counterpart. Small amounts of humus thus augment remarkably the soil's capacity to promote plant production.
Soil Water-A Dynamic Solution
Two major concepts concerning soil water emphasize the significance of this component of the soil in relation to plant growth.
1. Water is held within the soil pores with varying degrees of tenacity depend-ing on the amount of water present and the size of the pores.
2. Together with its dissolved salts. soil water makes up the soil solution, which is so important as a medium for supplying nutrients to growing plants. The tenacity with which water is held by soil solids determines to a marked degree the movement of water in soils and its use by plants. For example. when the moisture content of a soil is optimum for plant growth (Figure 1.5).
plants can readily absorb the soil water. much of which is present in pores of intermediate size. As some of the moisture is removed by the growing plants. that which remains is present in only the tiny pores and as thin films around the soil particles. The attraction of the soil solids for this water is great. and they can compete successfully with higher plants for it. Consequently. not all the soil water is available to plants. Much of it remains in the soil after plants have wilted or died as a consequence of water shortage.
The soil solution contains small but significant quantities of dissolved salts. many of which are essential for plant growth. Nutrients are exchanged between the soil solids and the soil solution and then between the soil solution and plants. These exchanges are influenced to a degree by the concentration of salts in the solution. which. in turn. is determined by the total salts in the soil. by the makeup of the soil solids. and by the content of soil water. Such are the dynamic nature and importance to plant life of this solute-bearing water.