This document provides an overview of the 8th edition of the textbook "Fundamentals of Soil Science" by Henry D. Foth. The preface outlines major revisions made to the textbook, including careful revision of topics, changes in depth of coverage, and the inclusion of many new figures and tables. The book is organized into 19 chapters covering topics such as soil physical properties, soil water, soil fertility, soil taxonomy, and the relationship between soils and plant growth, erosion, and environmental quality. Color photos of soil profiles are also included. The preface expresses the author's hope that readers will find the book both informative and interesting.

4. FUNDAMENTALS OF
SOIL SCIENCE

5. FUNDAMENTALS OF
SOIL SCIENCE
EIGHTH EDITION
HENRY D. FOTH
Michigan State University
JOHN WILEY & SONS
New York • Chichester • Brisbane • Toronto • Singapore

6. Cover Photo
Soil profile developed from glacio-fluvial sand in a balsam fir-black spruce
forest in the Laurentian Highlands of Quebec, Canada. The soil is classified as
a Spodosol (Orthod) in the United States and as a Humo-Ferric Podzol in
Canada.
Copyright © 1943, 1951
by Charles Ernest Millar and Lloyd M. Turk
Copyright © 1958, 1965, 1972, 1978, 1984, 1990, by John Wiley & Sons, Inc.
All rights reserved. Published simultaneously in Canada.
Reproduction or translation of any part of
this work beyond that permitted by Sections
107 and 108 of the 1976 United States Copyright
Act without the permission of the copyright
owner is unlawful. Requests for permission
or further information should be addressed to
the Permissions Department, John Wiley & Sons.
Library of Congress Cataloging In Publication Data:
Foth, H. D.
Fundamentals of soil science / Henry D. Foth.-8th ed.
p. cm.
I ncludes bibliographical references.
ISBN 0-471-52279-1
1. Soil science. I. Title.
S591.F68 1990
631.4-dc20
90-33890
CIP
Printed in the United States of America
1098765432
Printed and bound by the Arcata Graphics Company

7. PREFACE
The eighth edition is a major revision in which and water flow is discussed as a function of the
there has been careful revision of the topics hydraulic gradient and conductivity. Darcy's Law
covered as well as changes in the depth of cover- is used in Chapter 6, "Soil Water Management,"
age. Many new figures and tables are included. i n regard to water movement in infiltration, drain-
Summary statements are given at the ends of the age, and irrigation. Chapter 6 also covers dis-
more difficult sections within chapters, and a posal of sewage effluent in soils and prescription
summary appears at the end of each chapter. athletic turf (PAT) as an example of precision
Many nonagricultural examples are included to control of the water, air, and salt relationships in
emphasize the importance of soil properties when soils used for plant growth. "Soil Erosion," Chap-
soils are used in engineering and urban settings. ter 7, has been slightly reorganized with greater
The topics relating to environmental quality are emphasis on water and wind erosion processes.
found throughout the book to add interest to many Chapters 8 and 9, "Soil Ecology" and "Soil Or-
chapters. Several examples of computer applica- ganic Matter," are complimentary chapters relat-
tion are included. i ng to the biological aspects of soils. The kinds
The original Chapter 1, "Concepts of Soil," was and nature of soil organisms and nutrient cycling
split into two chapters. Each chapter emphasizes remain as the central themes of Chapter 8. An
an important concept of soil-soil as a medium expanded section on the rhizosphere has been
for plant growth and soil as a natural body. Topics i ncluded. The distinctions between labile and sta-
covered in Chapter 1 include the factors affecting ble organic matter and the interaction of organic
plant growth, root growth and distribution, nutri- matter with the minerals (especially clays) are
ent availability (including the roles of root inter- central themes of Chapter 9. Also, the concept of
ception, mass flow and diffusion), and soil fertil- cation exchange capacity is minimally developed
i ty and productivity. The importance of soils as a i n the coverage of the nature of soil organic matter
source of nutrients and water is stressed in Chap- i n Chapter 9.
ter 1 and elsewhere throughout the book. Chapter Chapter 10, "Soil Mineralogy," and Chapter 11,
2 covers the basic soil formation processes of "Soil Chemistry", are complimentary chapters re-
humification of organic matter, mineral weather- l ating to the mineralogical and chemical proper-
i ng, leaching, and translocation of colloids. The ties of soils. The evolution theme included in
i mportant theme is soil as a three-dimensional Chapter 2 is used to develop the concept of
body that is dynamic and ever-changing. The con- changing mineralogical and chemical properties
cepts developed in the first two chapters are used with time. Soils are characterized as being mini-
repeatedly throughout the book. mally, moderately, and intensively weathered,
The next five chapters relate to soil physical and these distinctions are used in discussions of
properties and water. The material on tillage and soil pH, liming, soil fertility and fertilizer use, soil
traffic was expanded to reflect the increasing ef- genesis, and land use.
fect of tillage and traffic on soils and plant growth Chapters 12 through 15 are concerned with the
and is considered in Chapter 4. The nature of soil general area of soil fertility and fertilizer use.
water is presented as a continuum of soil water Chapters 12 and 13 cover the macronutrients and
potentials in Chapter 5. Darcy's law is developed micronutrients plus toxic elements, respectively.
V

8. vi PREFACE
Chapters 14 and 15 cover the nature of fertilizers taxonomy is covered in Chapter 17. This allows a
and the evaluation of soil fertility and the use of consideration of soil classification after soil
fertilizers, respectively. Greater stress has been properties have been covered. This arrangement
placed on mass flow and diffusion in regard to also makes the book more desirable for use in
nutrient uptake. The interaction of water and soil two-year agricultural technology programs and
fertility is developed, and there is expanded cov- overseas, in countries where Soil Taxonomy is not
erage of soil fertility evaluation and the methods used.
used to formulate fertilizer recommendations. The final chapter, "Land and the World Food
Recognition is made of the increasing frequency Supply," includes a section on the world grain
of high soil test results and the implications for trade and examines the importance of nonagro-
fertilizer use and environmental quality. Greater nomic factors in the food-population problem.
coverage is given to animal manure as both a Both English and metric units are used in the
source of nutrients and a source of energy. Infor- measurement of crop yields, and for some other
mation on land application of sewage sludge and parameters. Using both kinds of units should sat-
on sustainable agriculture has been added. isfy both United States and foreign readers.
Throughout these four chapters there is a greater Special thanks to Mary Foth for the artwork and
emphasis on the importance of soil fertility and to my late son-in-law, Nate Rufe, for photographic
fertilizers and on the environmental aspects of contributions. Over the years, many colleagues
growing crops. have responded to my queries to expand my
The next four chapters (Chapters 16, 17, 18, and knowledge and understanding. Others have pro-
19) relate to the areas of soil genesis, soil taxon- vided photographs. The reviewers also have pro-
omy, soil geography and land use, and soil survey vided an invaluable service. To these persons, I
and land use interpretations. In this edition, the am grateful.
subjects of soil taxonomy (classification) and of Finally, this book is a STORY about soil. The
soil survey and land use interpretations have re- story reflects my love of the soil and my devotion
ceived increased coverage in two small chapters. to promoting the learning and understanding of
The emphasis in the soil geography and land use soils for more than 40 years. I hope that all who
chapter is at the suborder level. References to read this book will find it interesting as well as
l ower categories are few. Color photographs of i nformative.
soil profiles are shown in Color Plates 5 and 6. No Henry D. Foth
reference to Soil Taxonomy (USDA) is made until East Lansing, Michigan

9. BRIEF CONTENTS
VII

10. DETAILED CONTENTS
CHAPTER 1 CHAPTER 3
SOIL AS A MEDIUM FOR SOIL PHYSICAL PROPERTIES 22
PLANT GROWTH 1
SOIL TEXTURE 22
FACTORS OF PLANT GROWTH 1 The Soil Separates 22
Particle Size Analysis 24
Support for Plants 1
Soil Textural Classes 25
Essential Nutrient Elements 2
Determining Texture by the Field Method 25
Water Requirement of Plants 3
Influence of Coarse Fregments on
Oxygen Requirement of Plants 4
Class Names 26
Freedom from Inhibitory Factors 5
Texture and the Use of Soils 26
PLANT ROOTS AND SOIL RELATIONS 5
SOIL STRUCTURE 27
Development of Roots in Soils 5
I mportance of Structure 28
Extensiveness of Roots in Soils 7
Genesis and Types of Structure 28
Extent of Root and Soil Contact 8
Grade and Class 29
Roles of Root Interception, Mass Flow,
and Diffusion 8 Managing Soil Structure 29
SOIL FERTILITY AND SOIL PRODUCTIVITY 9 SOIL CONSISTENCE 31
Soil Consistence Terms 31
DENSITY AND WEIGHT RELATIONSHIPS 32
Particle Density and Bulk Density 32
Weight of a Furrow-Slice of Soil 33
Soil Weight on a Hectare Basis 34
CHAPTER 2 SOIL PORE SPACE AND POROSITY 34
SOIL AS A NATURAL BODY 11 Determination of Porosity 34
Effects of Texture and Structure on Porosity 35
THE PARENT MATERIAL OF SOIL 12 Porosity and Soil Aeration 35
Bedrock Weathering and Formation of
Parent Material 12 SOIL COLOR 36
Sediment Parent Materials 13 Determination of Soil Color 37
Factors Affecting Soil Color 37
SOIL FORMATION 13 Significance of Soil Color 37
Soil-Forming Processes 14
Formation of A and C Ho zons
ri
14 SOIL TEMPERATURE 38
Formation of B Horizons 14 Heat Balance of Soils 38
The Bt Horizon 15 Location and Temperature 39
The Bhs Horizon 17 Control of Soil Temperature 39
Permafrost 40
Formation of E Horizons 17
Formation of 0 Horizons 18
CHAPTER 4
SOILS AS NATURAL BODIES 18 TILLAGE AND TRAFFIC 42
The Soil-Forming Factors 18
Soil Bodies as Parts of Landscapes 19 EFFECTS OF TILLAGE ON SOILS AND
How Scientists Study Soils as Natural Bodies 19 PLANT GROWTH 42
I mportance of Concept of Soil as Natural Body 20 Management of Crop Residues 42
ix

11. X DETAILED CONTENTS
CHAPTER 6
SOIL WATER MANAGEMENT 73
Tillage and Weed Control 43
Effects of Tillage on Structure and Porosity 43
Surface Soil Crusts 44
Minimum and Zero Tillage Concepts 44 WATER CONSERVATION 73
Tilth and Tillage 45 Modifying the Infiltration Rate 73
Summer Fallowing 75
TRAFFIC AND SOIL COMPACTION 46 Saline Seep Due to Fallowing 76
Compaction Layers 46 Effect of Fertilizers on Water Use Efficiency 77
Effects of Wheel Traffic on Soils and Crops 47
Effects of Recreational Traffic 47 SOIL DRAINAGE 78
Effects of Logging Traffic on Soils and Water Table Depth Versus Air and Water Content
Tree Growth 48 of Soil 79
Controlled Traffic 49 Benefits of Drainage 80
Surface Drainage 80
FLOODING AND PUDDLING OF SOIL 50 Subsurface Drainage 80
Effects of Flooding 50 Drainage in Soil of Container-Grown Plants 81
Effects of Puddling 50
Oxygen Relationships in Flooded Soils 51 I RRIGATION 82
Water Sources 82
I mportant Properties of Irrigated Soils 82
,/
Water Application Methods 83
CHAPTER 5
Flood Irrigation 83
SOIL WATER 54
Furrow Irrigation 83
Sprinkler Irrigation 83
SOIL WATER ENERGY CONTINUUM 54 Subsurface Irrigation 85
Drip Irrigation 85
Adhesion Water 55
Cohesion Water 55 Rate and Timing of Irrigation 85
Gravitational Water 56 Water Quality 86
Summary Statements 56 Total Salt Concentration 86
Sodium Adsorption Ratio 86
ENERGY AND PRESSURE RELATIONSHIPS 57 Boron Concentration 87
Pressure Relationships in Saturated Soil 57 Bicarbonate Concentration 87
Pressure Relationships in Unsaturated Soil 58
Salt Accumulation and Plant Response 89
THE SOIL WATER POTENTIAL 59 Salinity Control and Leaching Requirement 89
The Gravitational Potential 59 Effect of Irrigation on River Water Quality 93
The Matric Potential 59 Nature and Management of Saline and
The Osmotic Potential 60 Sodic Soils 93
Measurement and Expression of Saline Soils 93
Water Potentials 60 Sodic Soils 93
Saline-Sodic Soils 94
SOIL WATER MOVEMENT 61
Water Movement in Saturated Soil 62 WASTEWATER DISPOSAL 94
Water Movement in Unsaturated Soil 63 Disposal of Septic Tank Effluent 94
Water Movement in Stratified Soil 63 Land Disposal of Municipal Wastewater 96
Water Vapor Movement 66
PRESCRIPTION ATHLETIC TURF 97
PLANT AND SOIL WATER RELATIONS 66
Available Water-Supplying Power of Soils 66
CHAPTER 7
Water Uptake from Soils by Roots 67
SOIL EROSION 100
Diurnal Pattern of Water Uptake 68
Pattern of Water Removal from Soil 69
Soil Water Potential Versus Plant Growth 69
Role of Water Uptake for Nutrient Uptake 71 WATER EROSION 100
Predicting Erosion Rates on Agricultural Land 100
SOIL WATER REGIME 71 R = The Rainfall Factor 101

12. DETAILED CONTENTS XI
K = The Soil Erodibility Factor 102 Mycorrhiza 126
LS = The Slope Length and Slope Nitrogen Fixation 127
Gradient Factors 103
C = The Cropping-Management Factor 104 SOIL ORGANISMS AND ENVIRONMENTAL
P = The Erosion Control Practice Factor 105 QUALITY 128
Pesticide Degradation 128
Application of the Soil-Loss Equation 106
The Soil Loss Tolerance Value 107 Oil and Natural Gas Decontamination 128
Water Erosion on Urban Lands 108 EARTH MOVING BY SOIL ANIMALS 130
Water Erosion Costs 109 Earthworm Activity 130
Ants and Termites 130
WIND EROSION 110
Types of Wind Erosion Rodents 131
110
Wind Erosion Equation 111
Factors Affecting Wind Erosion 111
Deep Plowing for Wind Erosion Control 113
CHAPTER 9
Wind Erosion Control on Organic Soils 113
SOIL ORGANIC MATTER 133
CHAPTER 8
THE ORGANIC MATTER IN ECOSYSTEMS 133
SOIL ECOLOGY 115 DECOMPOSITION AND ACCUMULATION 133
Decomposition of Plant Residues 134
Stable Soil Organic Matter
THE ECOSYSTEM 115 Labile Soil Organic Matter 134
Producers 115 135
Consumers and Decomposers 116 Decomposition Rates 136
Properties of Stable Soil Organic Matter 136
MICROBIAL DECOMPOSERS 116 Protection of Organic Matter by Clay 137
General Features of Decomposers 116
Bacteria 117 ORGANIC SOILS 139
Fungi 117 Organic Soil Materials Defined 139
Actinomycetes 118 Formation of Organic Soils 139
Vertical Distribution of Decomposers in Properties and Use 140
the Soil 119 Archaeological Interest 140
SOIL ANIMALS 119 THE EQUILIBRIUM CONCEPT 141
Worms 120 A Case Example 141
Earthworms 120 Effects of Cultivation 142
Nematodes 121 Maintenance of Organic Matter in
Arthropods 121 Cultivated Fields 143
Springtails 121 Effects of Green Manure 144
Mites 122 HORTICULTURAL USE OF ORGANIC MATTER 144
Millipedes and Centipedes 122 Horticultural Peats 145
White Grubs 122 Composts 145
Interdependence of Microbes and Animals
i n Decomposition 123
NUTRIENT CYCLING 123
CHAPTER 10
Nutrient Cycling Processes 123
SOIL MINERALOGY 148
A Case Study of Nutrient Cycling 124
Effect of Crop Harvesting on Nutrient Cycling 124
SOIL MICROBE AND ORGANISM CHEMICAL AND MINERALOGICAL COMPOSITION OF
I NTERACTIONS 125 THE EARTH'S CRUST 148
The Rhizosphere 125 Chemical Composition of the Earth's Crust 148
Disease 126 Mineralogical Composition of Rocks 149

13. DETAILED CONTENTS
WEATHERING AND SOIL MINERALOGICAL MANAGEMENT OF SOIL pH 178
COMPOSITION 149 Lime Requirement 179
Weathering Processes 150 Lime Requirement of Intensively
Summary Statement 150 Weathered Soils 180
Weathering Rate and Crystal Structure 151 Lime Requirement of Minimally and Moderately
Mineralogical Composition Versus Soil Age 153 Weathered Soils 180
Summary Statement 155 The Liming Equation and Soil Buffering 181
SOIL CLAY MINERALS 155 Some Considerations in Lime Use 182
Mica and Vermiculite 156 Management of Calcareous Soils 182
Smectites 158 Soil Acidulation 183
Kaolinite 159 EFFECTS OF FLOODING ON CHEMICAL
Allophane and Halloysite 160 PROPERTIES 183
Oxidic Clays 160 Dominant Oxidation and Reduction Reactions 183
Summary Statement 161 Effect on Soil pH 184
CHAPTER 12
ION EXCHANGE SYSTEMS OF SOIL CLAYS 161
161
PLANT-SOIL MACRONUTRIENT
Layer Silicate System
Oxidic System 162
Oxide-Coated Layer Silicate System 162 RELATIONS 186
DEFICIENCY SYMPTOMS 186
CHAPTER 11
SOIL CHEMISTRY 164
NITROGEN 186
The Soil Nitrogen Cycle 187
Dinitrogen Fixation 187
CHEMICAL COMPOSITION OF SOILS 164 Symbiotic Legume Fixation 187
Nonlegume Symbiotic Fixation 192
Nonsymbiotic Nitrogen Fixation 192
ION EXCHANGE 165
Summary Statement 192
Nature of Cation Exchange 165
Cation Exchange Capacity of Soils 166
Cation Exchange Capacity Versus Soil pH 167 Mineralization 192
Kinds and Amounts of Exchangeable Cations 168 Nitrification 193
I mmobilization 194
Carbon-Nitrogen Relationships 195
Exchangeable Cations as a Source of Plant
Nutrients 169
Anion Exchange 169 Denitrification 195
SOIL pH 170 Human Intrusion in the Nitrogen Cycle 196
Determination of Soil pH 170 Summary Statement on Nitrogen Cycle 197
Sources of Alkalinity 170 Plant Nitrogen Relations 197
Carbonate Hydrolysis 170 PHOSPHORUS 197
Mineral Weathering 171 Soil Phosphorus Cycle 198
Sources of Acidity 171 Effect of pH on Phosphorus Availability 199
Development and Properties of Acid Soils 171 Changes in Soil Phosphorus Over Time 199
Role of Aluminum 172 Plant Uptake of Soil Phosphorus 200
Moderately Versus Intensively Plant Phosphorus Relations 201
Weathered Soils 173
Role of Strong Acids 174
POTASSIUM 202
Soil Potassium Cycle 202
Acid Rain Effects 174
Summary Statement 203
Soil Buffer Capacity 174
Plant Uptake of Soil Potassium 204
Summary Statement 176
Plant Potassium Relations 205
SIGNIFICANCE OF SOIL pH 176 CALCIUM AND MAGNESIUM 205
Nutrient Availability and pH 177 Plant Calcium and Magnesium Relations 206
Effect of pH on Soil Organisms 178
Soil Magnesium and Grass Tetany 206
Toxicities in Acid Soils 178
pH Preferences of Plants 178 SULFUR 207

14. xiii
DETAILED CONTENTS
CHAPTER 13 APPLICATION AND USE OF FERTILIZERS 237
MICRONUTRIENTS AND
Time of Application 238
TOXIC ELEMENTS 210
Methods of Fertilizer Placement 238
Salinity and Acidity Effects 240
IRON AND MANGANESE 210 ANIMAL MANURES 241
Plant "Strategies" for Iron Uptake 211 Manure Composition and Nutrient Value 241
Nitrogen Volatilization Loss from Manure 242
COPPER AND ZINC 212
Plant Copper and Zinc Relations 213 Manure as a Source of Energy 243
BORON 214 LAND APPLICATION OF SEWAGE SLUDGE 244
Sludge as a Nutrient Source 244
CHLORINE 214 Heavy Metal Contamination 245
MOLYBDENUM 214 FERTILIZER USE AND ENVIRONMENTAL
Plant and Animal Molybdenum Relations 215 QUALITY 246
Phosphate Pollution 246
COBALT 216 Nitrate Pollution 246
SELENIUM 217 Nitrate Toxicity 247
POTENTIALLY TOXIC ELEMENTS SUSTAINABLE AGRICULTURE 247
FROM POLLUTION 217
CHAPTER 16
RADIOACTIVE ELEMENTS 218
SOIL GENESIS 250
CHAPTER 14 ROLE OF TIME IN SOIL GENESIS 250
FERTILIZERS 221 Case Study of Soil Genesis 250
Time and Soil Development Sequences 252
FERTILIZER TERMINOLOGY 221 ROLE OF PARENT MATERIAL IN SOIL GENESIS 253
Grade and Ratio 221 Consolidated Rock as a Source of
General Nature of Fertilizer Laws 222 Parent Material 253
Types of Fertilizers 222 Soil Formation from Limestone Weathering 253
FERTILIZER MATERIALS 222 Sediments as a Source of Parent Material 254
Nitrogen Materials 222 Gulf and Atlantic Coastal Plains 255
Phosphorus Materials 224 Central Lowlands 256
Potassium Materials 226 Interior Plains 258
Micronutrient Materials 228 Basin and Range Region 258
Volcanic Ash Sediments 258
MIXED FERTILIZERS 228
Effect of Parent Material Properties on
Granular Fertilizers 228
Soil Genesis 258
Bulk Blended Fertilizers 229
Stratified Parent Materials 259
Fluid Fertilizers 229
Parent Material of Organic Soils 260
NATURAL FERTILIZER MATERIALS 230
ROLE OF CLIMATE IN SOIL GENESIS 260
Precipitation Effects 260
CHAPTER 15
Temperature Effects 262
Climate Change and Soil Properties 263
SOIL FERTILITY EVALUATION AND
FERTILIZER USE 232
ROLE OF ORGANISMS IN SOIL GENESIS 263
Trees Versus Grass and Organic
Matter Content 263
SOIL FERTILITY EVALUATION 232 Vegetation Effects on Leaching and Eluviation 264
Plant Deficiency Symptoms 232 Role of Animals in Soil Genesis 265
Plant Tissue Tests 232
Soil Tests 233 ROLE OF TOPOGRAPHY IN SOIL GENESIS 265
Computerized Fertilizer Recommendations 235 Effect of Slope 265

15. xiv DETAILED CONTENTS
Effects of Water Tables and Drainage 266 ANDISOLS 294
Topography, Parent Material, and Genesis and Properties 294
Time Interactions 267 Suborders 294
Uniqueness of Soils Developed in Alluvial
ARIDISOLS 294
Parent Material 267
Genesis and Properties 295
HUMAN BEINGS AS A SOIL-FORMING FACTOR 269 Aridisol Suborders 295
Land Use on Aridisols 296
CHAPTER 17 ENTISOLS 297
SOIL TAXONOMY 271
Aquents 297
Fluvents 297
DIAGNOSTIC SURFACE HORIZONS 271 Psamments 298
Mollic Horizon 271 Orthents 298
Umbric and Ochric Horizons 272 HISTOSOLS 299
Histic Horizon 272 Histosol Suborders 299
Melanic Horizon 272 Land Use on Histosols 300
Anthropic and Plaggen Horizons 273
INCEPTISOLS 300
DIAGNOSTIC SUBSURFACE HORIZONS 273 Aquepts 300
Cambic Horizon 273 Ochrepts 301
Argillic and Natric Horizons 274 Umbrepts 301
Kandic Horizon 274
Spodic Horizon 275 MOLLISOLS 302
Albic Horizon 275 Aquolls 302
Oxic Horizon 275 Borolls 303
Calcic, Gypsic, and Salic Horizons 275 Ustolls and Udolls 303
Subordinate Distinctions of Horizons 276 Xerolls 304
SOIL MOISTURE REGIMES 276 OXISOLS 304
Aquic Moisture Regime 276 Oxisol Suborders 306
Udic and Perudic Moisture Regime 276 Land Use on Udox Soils 306
Ustic Moisture Regime 277 Land Use on Ustox Soils 307
Aridic Moisture Regime 277 Extremely Weathered Oxisols 307
Xeric Moisture Regime 278 Plinthite or Laterite 308
SOIL TEMPERATURE REGIMES 279 SPODOSOLS 308
Spodosol Suborders 308
CATEGORIES OF SOIL TAXONOMY 279 Spodosol Properties and Land Use 309
Soil Order 279
Suborder and Great Group 282 ULTISOLS 311
Subgroup, Family, and Series 283 Ultisol Suborders 311
Properties of Ultisols 311
AN EXAMPLE OF CLASSIFICATION: Land Use on Ultisols 312
THE ABAC SOILS 283
VERTISOLS 312
THE PEDON 284 Vertisol Suborders 313
Vertisol Genesis 313
CHAPTER 18
Vertisol Properties 314
SOIL GEOGRAPHY AND LAND USE 285
Land Use on Vertisols 315
CHAPTER 19
SOIL SURVEYS AND LAND-USE
ALFISOLS 285
I NTERPRETATIONS 318
Aqualfs 285
Boralfs 286
Udalfs 286
Ustalfs 293 MAKING A SOIL SURVEY 318
Xeralfs 293 Making a Soil Map 318

16. DETAILED CONTENTS XV
Writing the Soil Survey Report 320 FUTURE OUTLOOK 332
Using the Soil Survey Report 321 Beyond Technology 333
The World Grain Trade 333
SOIL SURVEY INTERPRETATIONS AND
LAND-USE PLANNING 322 Population Control and Politics 334
Examples of Interpretative Land-Use Maps 322
APPENDIX I
Land Capability Class Maps 323
Computers and Soil Survey Interpretations 323
SOIL TEXTURE BY THE
FIELD METHOD 337
SOIL SURVEYS AND AGROTECHNOLOGY
TRANSFER 324
CHAPTER 20 APPENDIX II
LAND AND THE WORLD TYPES AND CLASSES OF
FOOD SUPPLY 326 SOIL STRUCTURE 339
POPULATION AND FOOD TRENDS 326
APPENDIX III
Development of Agriculture 326
The Industrial Revolution 326
Recent Trends in Food Production 327 PREFIXES AND THEIR CONNOTATIONS
FOR NAMES OF GREAT GROUPS IN THE
Recent Trends in Per Capita Cropland 328
U.S. SOIL CLASSIFICATION SYSTEM
Summary Statement 329
POTENTIALLY AVAILABLE LAND AND (SOIL TAXONOMY) 341
SOIL RESOURCES 329
GLOSSARY 342
World's Potential Arable Land 329
I NDEX 353
Limitations of World Soil Resources 332
Summary Statement 332

17. CHAPTER 1
SOIL AS A MEDIUM FOR
PLANT GROWTH
SOIL. Can you think of a substance that has had serve as channels for the movement of air and
more meaning for humanity? The close bond that water. Pore spaces are used as runways for small
ancient civilizations had with the soil was ex- animals and are avenues for the extension and
pressed by the writer of Genesis in these words: growth of roots. Roots anchored in soil suppport
And the Lord God formed Man of dust from the
plants and roots absorb water and nutrients. For
ground.
good plant growth, the root-soil environment
should be free of inhibitory factors. The three
There has been, and is, a reverence for the ground essential things that plants absorb from the soil
or soil. Someone has said that "the fabric of hu- and use are: (1) water that is mainly evaporated
man life is woven on earthen looms; everywhere it from plant leaves, (2) nutrients for nutrition, and
smells of clay." Even today, most of the world's (3) oxygen for root respiration.
people are tillers of the soil and use simple tools
to produce their food and fiber. Thus, the concept
of soil as a medium of plant growth was born in Support for Plants
antiquity and remains as one of the most impor- One of the most obvious functions of soil is to
tant concepts of soil today (see Figure 1.1). provide support for plants. Roots anchored in soil
enable growing plants to remain upright. Plants
grown by hydroponics (in liquid nutrient culture)
FACTORS OF PLANT GROWTH
are commonly supported on a wire framework.
Plants growing in water are supported by the
The soil can be viewed as a mixture of mineral buoyancy of the water. Some very sandy soils that
and organic particles of varying size and composi- are droughty and infertile provide plants with little
tion in regard to plant growth. The particles oc- else than support. Such soils, however, produce
cupy about 50 percent of the soil's volume. The high-yielding crops when fertilized and frequently
remaining soil volume, about 50 percent, is pore irrigated. There are soils in which the impenetra-
space, composed of pores of varying shapes and ble nature of the subsoil, or the presence of water-
sizes. The pore spaces contain air and water and saturated soil close to the soil surface, cause shal-
1

18. z SOIL AS A MEDIUM FOR PLANT GROWTH
FIGURE 1.1 Wheat harvest
near the India-Nepal border.
About one half of the world's
people are farmers who are
closely tied to the land and
make their living producing
crops with simple tools.
l ow rooting. Shallow-rooted trees are easily blown
over by wind, resulting in windthrow.
some plants. Plants deficient in an essential ele-
ment tend to exhibit symptoms that are unique for
that element, as shown in Figure 1.2.
Essential Nutrient Elements
More than 40 other elements have been found
FIGURE 1.2
Plants need certain essential nutrient elements to Manganese deficiency symptoms on
complete their life cycle. No other element can kidney beans. The youngest, or upper leaves, have
completely substitute for these elements. At least light-green or yellow-colored intervein areas and
16 elements are currently considered essential for dark-green veins.
the growth of most vascular plants. Carbon, hy-
drogen, and oxygen are combined in photosyn-
thetic reactions and are obtained from air and
water. These three elements compose 90 percent
or more of the dry matter of plants. The remaining
13 elements are obtained largely from the soil.
Nitrogen (N), phosphorus (P), potassium (K), cal-
cium Ca), magnesium (Mg), and sulfur (S) are
required in relatively large amounts and are re-
ferred to as the macronutrients. Elements re-
quired in considerably smaller amount are called
the micronutrients. They include boron (B),
chlorine (Cl), copper (Cu), iron (Fe), manganese
(Mn), molybdenum (Mo), and zinc (Zn). Cobalt
(Co) is a micronutrient that is needed by only

19. FACTORS OF PLANT GROWTH 3
i n plants. Some plants accumulate elements that Cations are positively charged ions such as Ca t
are not essential but increase growth or quality. and K + and anions are negatively charged ions
The absorption of sodium (Na) by celery is an such as NO3- (nitrate) and H 2 PO4 - (phosphate).
example, and results in an improvement of flavor. The amount of cations absorbed by a plant is
Sodium can also be a substitute for part of the about chemically equal to the amount of anions
potassium requirement of some plants, if po- absorbed. Excess uptake of cations, however, re-
tassium is in low supply. Silicon (Si) uptake may sults in excretion of H+ and excess uptake of
i ncrease stem strength, disease resistance, and anions results in excretion of OH - or HCO 3 - to
growth in grasses. maintain electrical neutrality in roots and soil.
Most of the nutrients in soils exist in the miner- The essential elements that are commonly ab-
als and organic matter. Minerals are inorganic sorbed from soils by roots, together with their
substances occurring naturally in the earth. They chemical symbols and the uptake forms, are listed
have a consistent and distinctive set of physical i n Table 1.1.
properties and a chemical composition that can I n nature, plants accommodate themselves to
be expressed by a formula. Quartz, a mineral the supply of available nutrients. Seldom or rarely
composed of SiO2, is the principal constituent of is a soil capable of supplying enough of the essen-
ordinary sand. Calcite (CaCO 3) i s the primary tial nutrients to produce high crop yields for any
mineral in limestone and chalk and is abundant is reasonable period of time after natural or virgin
many soils. Orthclase-feldspar (KAISi 3 O8 ) is a l ands are converted to cropland. Thus, the use of
very common soil mineral, which contains po- animal manures and other amendments to in-
tassium. Many other minerals exist in soils be- crease soil fertility (increase the amount of nutri-
cause soils are derived from rocks or materials ent ions) are ancient soil management practices.
containing a wide variety of minerals. Weathering
of minerals brings about their decomposition and
the production of ions that are released into the
soil water. Since silicon is not an essential ele- Water Requirement of Plants
ment, the weathering of quartz does not supply an A few hundred to a few thousand grams of water
essential nutrient, plants do not depend on these are required to produce 1 gram of dry plant mate-
minerals for their oxygen. The weathering of cal- rial. Approximately one percent of this water be-
cite supplies calcium, as Ca t+ , and the weather- comes an integral part of the plant. The remainder
i ng of orthoclase releases potassium as K+. of the water is lost through transpiration, the loss
The organic matter in soils consists of the re- of water by evaporation from leaves. Atmospheric
cent remains of plants, microbes, and animals conditions, such as relative humidity and temper-
and the resistant organic compounds resulting ature, play a major role in determining how
from the rotting or decomposition processes. De- quickly water is transpired.
composition of soil organic matter releases es- The growth of most economic crops will be
sential nutrient ions into the soil water where the curtailed when a shortage of water occurs, even
i ons are available for another cycle of plant though it may be temporary. Therefore, the soil's
growth. ability to hold water over time against gravity is
Available elements or nutrients are those nutri- i mportant unless rainfall or irrigation is adequate.
ent ions or compounds that plants and microor- Conversely, when soils become water saturated,
ganisms can absorb and utilize in their growth. the water excludes air from the pore spaces and
Nutrients are generally absorbed by roots as creates an oxygen deficiency. The need for the
cations and anions from the water in soils, or the removal of excess water from soils is related to the
soil solution. The ions are electrically charged. need for oxygen.

20. 4 SOIL AS A MEDIUM FOR PLANT GROWTH
Chemical Symbols and Common Forms Oxygen Requirement of Plants
of the Essential Elements Absorbed by Plant Roots
TABLE 1.1
Roots have openings that permit gas exchange.
from Soils Oxygen from the atmosphere diffuses into the soil
and is used by root cells for respiration. The car-
bon dioxide produced by the respiration of roots,
and microbes, diffuses through the soil pore
space and exits into the atmosphere. Respiration
releases energy that plant cells need for synthesis
and translocation of the organic compounds
needed for growth. Frequently, the concentration
of nutrient ions in the soil solution is less than that
i n roots cells. As a consequence, respiration
energy is also used for the active accumulation of
nutrient ions against a concentration gradient.
Some plants, such as water lilies and rice, can
grow in water-saturated soil because they have
morphological structures that permit the diffusion
of atmospheric oxygen down to the roots. Suc-
cessful production of most plants in water culture
requires aeration of the solution. Aerobic micro-
organisms require molecular oxygen (O 2) and use
oxygen from the soil atmosphere to decompose
FIGURE 1.3 The soil in which these tomato plants organic matter and convert unavailable nutrients
were growing was saturated with water. The stopper i n organic matter into ionic forms that plants can
at the bottom of the left container was immediately reuse (nutrient cycling).
removed, and excess water quickly drained away.
Great differences exist between plants in their
The soil in the right container remained water
ability to tolerate low oxygen levels in soils. Sensi-
saturated and the plant became severely wilted
within 24 hours because of an oxygen deficiency.
FIGURE 1.4 Soil salinity (soluble salt) has seriously
affected the growth of sugar beets in the foreground
of this irrigated field.

21. PLANT ROOTS AND SOIL RELATIONS 5
tive plants may be wilted and/or killed as a result etc.) in the shoot via photosynthesis and translo-
of saturating the soil with water for a few hours, as cation of food downward for root growth, and
shown in Figure 1.3. The wilting is believed to (2) the absorption of water and nutrients by roots
result from a decrease in the permeability of the and the upward translocation of water and nutri-
roots to water, which is a result of a disturbance of ents to the shoot for growth.
metabolic processes due to an oxygen deficiency. After a root emerges from the seed, the root tip
elongates by the division and elongation of cells
i n the meristematic region of the root cap. After
Freedom from Inhibitory Factors the root cap invades the soil, it continues to elon-
Abundant plant growth requires a soil environ- gate and permeate the soil by the continued divi-
ment that is free of inhibitory factors such as toxic sion and elongation of cells. The passage of the
substances, disease organisms, impenetrable root tip through the soil leaves behind sections of
layers, extremes in temperature and acidity or root that mature and become "permanent" resi-
basicity, or an excessive salt content, as shown in dents of the soil.
Figure 1.4. As the plant continues to grow and roots elon-
gate throughout the topsoil, root extension into
the subsoil is likely to occur. The subsoil environ-
PLANT ROOTS AND SOIL RELATIONS
ment will be different in terms of the supply of
water, nutrients, oxygen, and in other growth fac-
Plants utilize the plant growth factors in the soil by tors. This causes roots at different locations in the
way of the roots. The density and distribution of soil (topsoil versus subsoil) to perform different
roots affect the amount of nutrients and water that functions or the same functions to varying de-
roots extract from soils. Perennials, such as oak grees. For example, most of the nitrogen will
and alfalfa, do not reestablish a completely new probably be absorbed by roots from the topsoil
root system each year, which gives them a distinct because most of the organic matter is concen-
advantage over annuals such as cotton or wheat. trated there, and nitrate-nitrogen becomes avail-
Root growth is also influenced by the soil environ- able by the decomposition of organic matter. By
ment; consequently, root distribution and density contrast, in soils with acid topsoils and alkaline
are a function of both the kind of plant and the subsoils, deeply penetrating roots encounter a
nature of the root environment. great abundance of calcium in the subsoil. Under
these conditions, roots in an alkaline subsoil may
absorb more calcium than roots in an acid top-
Development of Roots in Soils soil. The topsoil frequently becomes depleted of
A seed is a dormant plant. When placed in moist, water in dry periods, whereas an abundance of
warm soil, the seed absorbs water by osmosis and water still exists in the subsoil. This results in a
swells. Enzymes activate, and food reserves in the relatively greater dependence on the subsoil for
endosperm move to the embryo to be used in water and nutrients. Subsequent rains that rewet
germination. As food reserves are exhausted, the topsoil cause a shift to greater dependence on
green leaves develop and photosynthesis begins. the topsoil for water and nutrients. Thus, the man-
The plant now is totally dependent on the sun for ner in which plants grow is complex and changes
energy and on the soil and atmosphere for nutri- continually throughout the growing season. In
ents and water. In a sense, this is a critical period this regard, the plant may be defined as an inte-
i n the life of a plant because the root system is grator of a complex and ever changing set of
small. Continued development of the plant re- environmental conditions.
quires: (1) the production of food (carbohydrates, The root systems of some common agricultural

22. SOIL AS A MEDIUM FOR PLANT GROWTH
FIGURE 1.6 Four stages for the development of the
shoots and roots of corn (Zea maize). Stage one
(left) shows dominant downward and diagonal root
growth, stage two shows "filling" of the upper soil
layer with roots, stage three shows rapid elongation
of stem and deep root growth, and stage four (right)
shows development of the ears (grain) and brace
root growth.
FIGURE 1.5 The tap root systems of two-week old
soybean plants. Note the many fine roots branching
off the tap roots and ramifying soil. (Scale on the systems and shoots during the growing season
right is in inches.) revealed a synchronization between root and
shoot growth. Four major stages of development
crops were sampled by using a metal frame to were found. Corn has a fibrous root system, and
collect a 10-centimeter-thick slab of soil from the early root growth is mainly by development of
wall of a pit. The soil slab was cut into small roots from the lower stem in a downward and
blocks and the roots were separated from the soil diagonal direction away from the base of the plant
using a stream of running water. Soybean plants (stage one of Figure 1.6). The second stage of root
were found to have a tap root that grows directly growth occurs when most of the leaves are de-
downward after germination, as shown in Figure veloping and lateral roots appear and "fill" or
1.5. The tap roots of the young soybean plants are space themselves uniformily in the upper 30 to 40
several times longer than the tops or shoots. Lat- centimeters of soil. Stage three is characterized by
eral roots develop along the tap roots and space rapid elongation of the stem and extension of
themselves uniformily throughout the soil occu- roots to depths of 1 to 2 meters in the soil. Finally,
pied by roots. At maturity, soybean taproots will during stage four, there is the production of the
extend about 1 meter deep in permeable soils ear or the grain. Then, brace roots develop from
with roots well distributed throughout the topsoil the lower nodes of the stem to provide anchorage
and subsoil. Alfalfa plants also have tap roots that of the plant so that they are not blown over by the
commonly penetrate 2 to 3 meters deep; some wind. Brace roots branch profusely upon entering
have been known to reach a depth of 7 meters. the soil and also function for water and nutrient
Periodic sampling of corn (Zea maize) root uptake.

23. PLANT ROOTS AND SOIL RELATIONS 7
There is considerable uniformity in the lateral
distribution of roots in the root zone of many
crops (see Figure 1.7). This is explained on the
basis of two factors. First, there is a random distri-
bution of pore spaces that are large enough for
root extension, because of soil cracks, channels
formed by earthworms, or channels left from pre-
vious root growth. Second, as roots elongate
through soil, they remove water and nutrients,
which makes soil adjacent to roots a less favor-
able environment for future root growth. Then,
roots grow preferentially in areas of the soil de-
void of roots and where the supply of water and
nutrients is more favorable for root growth. This
results in a fairly uniform distribution of roots
throughout the root zone unless there is some
barrier to root extension or roots encounter an
unfavorable environment.
Most plant roots do not invade soil that is dry,
nutrient deficient, extremely acid, or water satu-
rated and lacking oxygen. The preferential devel-
opment of yellow birch roots in loam soil, com-
pared with sand soil, because of a more favorable
FIGURE 1.8 Development of the root system of a
yellow birch seedling in sand and loam soil. Both
soils had adequate supplies of water and oxygen,
but, the loam soil was much more fertile. (After
Redmond, 1954.)
FIGURE 1.7 Roots of mature oat plants grown in
rows 7 inches (18 cm) apart. The roots made up 13
percent of the total plant weight. Note the uniform,
lateral, distribution of roots between 3 and 24 inches
(8 and 60 cm). Scale along the left is in inches.
Extensiveness of Roots in Soil
Roots at plant maturity comprise about 10 percent
of the entire mass of cereal plants, such as corn,
wheat, and oats. The oat roots shown in Figure 1.7
weighed 1,767 pounds per acre (1,979 kg/ha) and
made up 13 percent of the total plant weight. For
trees, there is a relatively greater mass of roots as
compared with tops, commonly in the range of 15
to 25 percent of the entire tree.

24. SOIL AS A MEDIUM FOR PLANT GROWTH
combination of nutrients and water, is shown in Roles of Root Interception, Mass Flow,
Figure 1.8. and Diffusion
Water and nutrients are absorbed at sites located
Extent of Root and Soil Contact
on or at the surface of roots. Elongating roots
directly encounter or intercept water and nutrient
A rye plant was grown in 1 cubic foot of soil for 4 ions, which appear at root surfaces in position for
months at the University of Iowa by Dittmer (this absorption. This is root interception and accounts
study is listed amoung the references at the end of for about 1 percent or less of the nutrients ab-
the chapter). The root system was carefully re- sorbed. The amount intercepted is in proportion
moved from the soil by using a stream of running to the very limited amount of direct root and soil
water, and the roots were counted and measured contact.
for size and length. The plant was found to have Continued absorption of water adjacent to the
hundreds of kilometers (or miles) of roots. Based root creates a lower water content in the soil near
on an assumed value for the surface area of the the root surface than in the soil a short distance
soil, it was calculated that 1 percent or less of the away. This difference in the water content be-
soil surface was in direct contact with roots. tween the two points creates a water content gra-
Through much of the soil in the root zone, the dient, which causes water to move slowly in the
distance between roots is approximately 1 cen- direction of the root. Any nutrient ions in the water
timeter. Thus, it is necessary for water and nutri- are carried along by flow of the water to root
ent ions to move a short distance to root surfaces surfaces where the water and nutrients are both in
for the effective absorption of the water and nutri- position for absorption. Such movement of nutri-
ents. The limited mobility of water and most of the ents is called mass flow.
nutrients in moist and well-aerated soil means The greater the concentration of a nutrient in
that only the soil that is invaded by roots can the soil solution, the greater is the quantity of the
contribute significantly to the growth of plants. nutrient moved to roots by mass flow. The range
TABLE 1.2 Relation Between Concentration of Ions in the Soil Solution and
Concentration within the Corn Plant
Adapted from S. A. Barber, "A Diffusion and Mass Flow Concept of Soil
Nutrient Availability, "Soil Sci., 93:39-49, 1962.
Used by permission of the author and The Williams and Wilkins Co., Baltimore.
a Dry weight basis.

25. SOIL FERTILITY AND SOIL PRODUCTIVITY 9
of concentration for some nutrients in soil water is nutrient also depends on the amount needed. Cal-
given in Table 1.2. The calcium concentration cium is rarely deficient for plant growth, partially
(Table 1.2) ranges from 8 to 450 parts per million because plants' needs are low. As a consequence,
(ppm). For a concentration of only 8 ppm in soil these needs are usually amply satisfied by the
water, and 2,200 ppm of calcium in the plant, the movement of calcium to roots by mass flow. The
plant would have to absorb 275 (2,200/8) times same is generally true for magnesium and sulfur.
more water than the plant's dry weight to move the The concentration of nitrogen in the soil solution
calcium needed to the roots via mass flow. Stated tends to be higher than that for calcium, but be-
i n another way, if the transpiration ratio is 275 cause of the high plant demand for nitrogen,
(grams of water absorbed divided by grams of about 20 percent of the nitrogen that plants ab-
plant growth) and the concentration of calcium in sorb is moved to root surfaces by diffusion. Diffu-
the soil solution is 8 ppm, enough calcium will be sion is the most important means by which phos-
moved to root surfaces to supply the plant need. phorus and potassium are transported to root
Because 8 ppm is a very low calcium concentra- surfaces, because of the combined effects of con-
tion in soil solutions, and transpiration ratios are centration in the soil solution and plant demands.
usually greater than 275, mass flow generally Mass flow can move a large amount of nutrients
moves more calcium to root surfaces than plants rapidly, whereas diffusion moves a small amount
need. In fact, calcium frequently accumulates of nutrients very slowly. Mass flow and diffusion
along root surfaces because the amount moved to have a limited ability to move phosphorus and
the roots is greater than the amount of calcium potassium to roots in order to satisfy the needs of
that roots absorb. crops, and this limitation partly explains why a
Other nutrients that tend to have a relatively l arge amount of phosphorus and potassium is
l arge concentration in the soil solution, relative to added to soils in fertilizers. Conversely, the large
the concentration in the plant, are nitrogen, mag- amounts of calcium and magnesium that are
nesium, and sulfur (see Table 1.2). This means moved to root surfaces, relative to crop plant
that mass flow moves large amount of these nutri- needs, account for the small amount of calcium
ents to roots relative to plant needs. and magnesium that is added to soils in fertilizers.
Generally, mass flow moves only a small
amount of the phosphorus to plant roots. The
phosphorus concentration in the soil solution is Summary Statement
usually very low. For a soil solution concentration The available nutrients and available water are the
of 0.03 ppm and 2,000 ppm plant concentration, nutrients and water that roots can absorb. The
the transpiration ratio would need to be more than absorption of nutrients and water by roots is de-
60,000. This illustration and others that could be pendent on the surface area-density (cm 2 /cm 3 ) of
drawn from the data in Table 1.2, indicate that roots. Mathematically:
some other mechanism is needed to account for
the movement of some nutrients to root surfaces. uptake = availability x surface area-density
This mechanism or process is known as diffusion.
Diffusion is the movement of nutrients in soil
SOIL FERTILITY AND
water that results from a concentration gradient.
SOIL PRODUCTIVITY
Diffusion of ions occurs whether or not water is
Soil fertility is defined as the ability of a soil to
moving. When an insufficient amount of nutrients
is moved to the root surface via mass flow, diffu-
sion plays an important role. Whether or not supply essential elements for plant growth with-
plants will be supplied a sufficient amount of a out a toxic concentration of any element. Soil

26. 10 SOIL AS A MEDIUM FOR PLANT GROWTH
fertility refers to only one aspect of plant growth- and distribution of roots in soils, and (3) the
the adequacy, toxicity, and balance of plant nutri- movement of nutrients, water, and air to root sur-
ents. An assessment of soil fertility can be made faces for absorption. Soils are productive in terms
with a series of chemical tests. of their ability to produce plants.
Soil productivity is the soil's capacity to pro- The concept of soil as a medium for plant
duce a certain yield of crops or other plants with growth views the soil as a material of fairly uni-
optimum management. For example, the produc- form composition. This is entirely satisfactory
tivity of a soil for cotton production is commonly when plants are grown in containers that contain
expressed as kilos, or bales of cotton per acre, or a soil mix. Plants found in fields and forests,
hectare, when using an optimum management however, are growing in soils that are not uniform.
system. The optimum managment system spec- Differences in the properties between topsoil and
i fies such factors as planting date, fertilization, subsoil layers affect water and nutrient absorp-
irrigation schedule, tillage, cropping sequence, tion. It is natural for soils in fields and forests to be
and pest control. Soil scientists determine soil composed of horizontal layers that have different
productivity ratings of soils for various crops by properties, so it is also important that agricultur-
measuring yields (including tree growth or timber i sts and foresters consider soils as natural bodies.
production) over a period of time for those pro- This concept is also useful for persons involved in
duction uses that are currently relevant. Included the building of engineering structures, solving en-
i n the measurement of soil productivity are the vironment problems such as nitrate pollution of
i nfluence of weather and the nature and aspect of groundwater, and using the soil for waste dis-
slope, which greatly affects water runoff and ero- posal. The soil as a natural body is considered in
sion. Thus, soil productivity is an expression of all the next chapter.
the factors, soil and nonsoil, that influence crop
yields.
For a soil to produce high yields, it must be
fertile for the crops grown. It does not follow,
REFERENCES
however, that a fertile soil will produce high Barber, S. A. 1962. "A Diffusion and Mass Flow Concept
yields. High yields or high soil productivity de- of Soil Nutrient Availability." Soil Sci. 93:39-49.
pends on optimum managment systems. Many Dittmer, H. J. 1937. "A Quantitative Study of the Roots
fertile soils exist in arid regions but, within man- and Root Hairs of a Winter Rye Plant." Am. Jour. Bot.
agement systems that do not include irrigation, 24:417-420.
Foth, H. D. 1962. "Root and Top Growth of Corn."
these soils are unproductive for corn or rice.
Agron. Jour. 54:49-52.
Foth, H. D., L. S. Robertson, and H. M. Brown. 1964.
"Effect of Row Spacing Distance on Oat Perfor-
mance." Agron. Jour. 56:70-73.
SUMMARY Foth, H. D. and B. G. Ellis. 1988. Soil Fertility. John
Wiley, New York.
The concept of soil as a medium for plant growth
Redmond, D. R. 1954. "Variations in Development of
i s an ancient concept and dates back to at least Yellow Birch Roots in Two Soil Types." Forestry
the beginning of agriculture. The concept empha- Chronicle. 30:401-406.
sizes the soil's role in the growth of plants. Impor- Simonson, R. W. 1968. "Concept of Soil." Adv . in Agron.
tant aspects of the soil as a medium for plant 20:1-47. Academic Press, New York.
growth are: (1) the role of the soil in supplying Wadleigh, C. H. 1957. "Growth of Plants," in Soil, USDA
plants with growth factors, (2) the development Yearbook of Agriculture. Washington, D.C.

27. CHAPTER 2
SOIL AS A NATURAL BODY
One day a colleague asked me why the alfalfa clay content than the two upper layers. The roots
plants on some research plots were growing so penetrated this layer with no difficulty, however.
poorly. A pit was dug in the field and a vertical Below layer three, the alfalfa tap root encountered
section of the soil was sampled by using a metal a layer (layer four) that was impenetrable (too
frame. The sample of soil that was collected was 5 compact), with the root growing above it in a
centimeters thick, 15 centimeters wide, and 75 l ateral direction. From these observations it was
centimeters long. The soil was glued to a board concluded that the alfalfa grew poorly because
and a vacuum cleaner was used to remove loose the soil material below a depth of 58 centimeters:
soil debris and expose the natural soil layers and (1) created a barrier to deep root penetration,
roots. Careful inspection revealed four soil layers which resulted in a less than normal supply of
as shown in Figure 2.1. water for plant growth during the summer, and
The upper layer, 9 inches (22 cm) thick, is the (2) created a water-saturated zone above the third
plow layer. It has a dark color and an organic l ayer that was deficient in oxygen during wet pe-
matter content larger than any of the other layers. riods in the spring. The fact that the soil occurred
Layer two, at the depth of 9 to 14 inches (22 to naturally in a field raises such questions as: What
35 cm) differs from layer one by having a light-gray kinds of layers do soils have naturally? How do the
color and a lower organic matter content. Both l ayers form? What are their properties? How do
l ayers are porous and permeable for the move- these layers affect how soils are used? The an-
ment of air and water and the elongation of roots. swers to these questions require an understand-
I n layer three, at a depth of 14 to 23 inches (35 to i ng that landscapes consist of three-dimensional
58 cm) many of the soil particles were arranged bodies composed of unique horizontal layers.
i nto blocklike aggregrates. When moist soil from These naturally occurring bodies are soils. A rec-
l ayer three was pressed between the fingers, more ognition of the kinds of soil layers and their
stickiness was observed than in layers one and properties is required in order to use soils effec-
two, which meant that layer three had a greater tively for many different purposes.
11

28. 12 SOIL AS A NATURAL BODY
First is the formation of a parent material from
which the soil evolves and, second, the evolution
of soil layers, as shown in Figure 2.1. Approxi-
mately 99 percent of the world's soils develop in
mineral parent material that was or is derived
from the weathering of bedrock, and the rest de-
velop in organic materials derived from plant
growth and consisting of muck or peat.
Bedrock Weathering and Formation of
Parent Material
Bedrock is not considered soil parent material
because soil layers do not form in it. Rather, the
unconsolidated debris produced from the weath-
ering of bedrock is soil parent material. When
bedrock occurs at or near the land surface, the
weathering of bedrock and the formation of par-
ent material may occur simultaneously with the
evolution of soil layers. This is shown in Figure
2.2, where a single soil horizon, the topsoil layer,
overlies the R layer, or bedrock. The topsoil layer
i s about 12 inches (30 cm) thick and has evolved
slowly at a rate controlled by the rate of rock
weathering. The formation of a centimeter of soil
i n hundreds of years is accurate for this example
of soil formation.
Rates of parent material formation from the di-
rect weathering of bedrock are highly variable. A
weakly cemented sandstone in a humid environ-
ment might disintegrate at the rate of a centimeter
i n 10 years and leave 1 centimeter of soil. Con-
FIGURE 2.2 Rock weathering and the formation of
the topsoil layer are occurring simultaneously. Scale
is in feet.
FIGURE 2.1 This alfalfa taproot grew vertically
downward through the upper three layers. At a depth
of 23 inches (58 cm), the taproot encountered an
impenetrable layer (layer 4) and grew in a lateral
direction above the layer.
THE PARENT MATERIAL OF SOILS
Soil formation, or the development of soils that
are natural bodies, includes two broad processes.

29. SOIL FORMATION 13
versely, quartzite (metamorphosed sandstone) thick alluvial sediments occur in the valley. Very
nearby might weather so slowly that any weath- thick glacial deposits occur on the tree-covered
ered material might be removed by water or wind lateral moraine that is adjacent to the valley floor
erosion. Soluble materials are removed during along the left side. An intermediate thickness of
li mestone weathering, leaving a residue of insolu- parent material occurs where trees are growing
ble materials. Estimates indicate that it takes below the bare mountaintops and above the thick
100,000 years to form a foot of residue from the alluvial and moraine sediments. Most of the
weathering of limestone in a humid region. Where world's soils have formed in sediments consisting
soils are underlain at shallow depths by bedrock, of material that was produced by the weathering
l oss of the soil by erosion produces serious con- of bedrock at one place and was transported and
sequences for the future management of the land. deposited at another location. In thick sediments
or parent materials, the formation of soil layers is
Sediment Parent Materials
not limited by the rate of rock weathering, and
several soil layers may form simultaneously.
Weathering and erosion are two companion and
opposing processes. Much of the material lost
from a soil by erosion is transported downslope SOIL FORMATION
and deposited onto existing soils or is added to
some sediment at a lower elevation in the land- Soil layers are approximately parallel to the land
scape. This may include alluvial sediments along surface and several layers may evolve simulta-
streams and rivers or marine sediments along neously over a period of time. The layers in a soil
ocean shorelines. Glaciation produced extensive are genetically related; however, the layers differ
sediments in the northern part of the northern from each other in their physical, chemical, and
hemisphere. biological properties. In soil terminology, the lay-
Four constrasting parent material-soil environ- ers are called horizons. Because soils as natural
ments are shown in Figure 2.3. Bare rock is ex- bodies are characterized by genetically developed
posed on the steep slopes near the mountaintops. horizons, soil formation consists of the evolution
Here, any weathered material is lost by erosion of soil horizons. A vertical exposure of a soil con-
and no parent material or soil accumulates. Very sisting of the horizons is a soil profile.
FIGURE 2.3 Four distinct soil-
forming environments are
depicted in this landscape in
the Rocky Mountains, United
States. On the highest and
steepest slopes, rock is
exposed because any
weathered material is removed
by erosion as fast as it forms.
Thick alluvial sediments occur
on the valley floor and on the
forested lateral moraine
adjacent to the valley floor
along the left side. Glacial
deposits of varying thickness
overlying rock occur on the
forested mountain slopes at
intermediate elevations.

30. 14 SOIL AS A NATURAL BODY
Soil-Forming Processes animals feeding on the organic debris eventually
Horizonation (the formation of soil horizons) re- die and thus contribute to the formation of hu-
sults from the differential gains, losses, transfor- mus. Humus has a black or dark-brown color,
mations, and translocations that occur over time which greatly affects the color of A horizons. In
within various parts of a vertical section of the areas in which there is abundant plant growth,
parent material. Examples of the major kinds only a few decades are required for a surface layer
of changes that occur to produce horizons are: to acquire a dark color, due to the humification
(1) addition of organic matter from plant growth, and accumulation of organic matter, forming an A
mainly to the topsoil; (2) transformation repre- horizon.
sented by the weathering of rocks and minerals The uppermost horizons shown in Figures 2.1
and the decomposition of organic matter; (3) loss and 2.2 are A horizons. The A horizon in Figure 2.1
of soluble components by water moving down- was converted into a plow layer by frequent plow-
ward through soil carrying out soluble salts; and, i ng and tillage. Such A horizons are called Ap
(4) translocation represented by the movement of horizons, the p indicating plowing or other distur-
suspended mineral and organic particles from the bance of the surface layer by cultivation, pastur-
topsoil to the subsoil. i ng, or similar uses. For practical purposes, the
topsoil in agricultural fields and gardens is synon-
Formation of A and C Horizons
ymous with Ap horizon.
At this stage in soil evolution, it is likely that the
Many events, such as the deposition of volcanic upper part of the underlying parent material has
ash, formation of spoil banks during railroad con- been slightly altered. This slightly altered upper
struction, melting of glaciers and formation of part of the parent material is the C horizon. The
glacial sediments, or catastrophic flooding and soil at this stage of evolution has two horizons-
formation of sediments have been dated quite the A horizon and the underlying C horizon. Such
accurately. By studying soils of varying age, soil soils are AC soils; the evolution of an AC soil is
scientists have reconstructed the kinds and the illustrated in Figure 2.4.
sequence of changes that occurred to produce
soils.
Glacial sediments produced by continental and Formation of B Horizons
alpine glaciation are widespread in the northern The subsoil in an AC soil consists of the C horizon
hemisphere, and the approximate dates of the and, perhaps, the upper part of the parent mate-
formation of glacial parent materials are known. rial. Under favorable conditions, this subsoil layer
After sediments have been produced near a
retreating ice front, the temperature may become
favorable for the invasion of plants. Their growth FIGURE 2.4 Sequential evolution of some soil
results in the addition of organic matter, espe- horizons in a sediment parent material.
cially the addition of organic matter at or near the
soil surface. Animals, bacteria, and fungi feed on
the organic materials produced by the plants, re-
sulting in the loss of much carbon as carbon
dioxide. During digestion or decomposition of
fresh organic matter, however, a residual organic
fraction is produced that is resistant to further
alteration and accumulates in the soil. The resis-
tant organic matter is called humus and the
process is humification. The microorganisms and

31. SOIL FORMATION 15
The Bt Horizon Soil parent materials frequently
contain calcium carbonate (CaCO 3), or lime, and
are alkaline. In the case of glacial parent materi-
als, lime was incorporated into the ice when gla-
ciers overrode limestone rocks. The subsequent
melting of the ice left a sediment that contains
li mestone particles. In humid regions, the lime
dissolves in percolating water and is removed
from the soil, a process called leaching. Leaching
effects are progressive from the surface down-
ward. The surface soil first becomes acid, and
subsequently leaching produces an acid subsoil.
An acid soil environment greatly stimulates
mineral weathering or the dissolution of minerals
with the formation of many ions. The reaction of
orthoclase feldspar (KAISiO 3 ) with water and H+
FIGURE 2.5 A soil scientist observing soil is as follows:
properties near the boundary between the A and B
2 KAISiO3 + 9H 2 O + 2H +
horizons in a soil with A, B, and C horizons. As roots
(orthoclase)
grow downward, or as water percolates downward,
they encounter a different environment in the A, B,
and C horizons. (Photograph USDA.) = H 4AI 2 Si 2 0 9 + 2K+ + 4H 4 Si04
(kaolinite) (silicic acid)
eventually develops a distinctive color and some The weathering reaction illustrates three impor-
other properties that distinguish it from the A hori- tant results of mineral weathering. First, clay parti-
zon and underlying parent material, commonly at cles (fine-sized mineral particles) are formed-in
a depth of about 60 to 75 centimeters. This altered the example, kaolinite. In effect, soils are "clay
subsoil zone becomes a B horizon and develops factories. Second, ions are released into the soil
as a layer sandwiched between the A and a new solution, including nutrient ions such as K + .
deeper C horizon. At this point in soil evolution, Third, other compounds (silicic acid) of varying
i nsufficient time has elapsed for the B horizon to solubility are formed and are subject to leaching
have been significantly enriched with fine-sized and removal from the soil.
(colloidal) particles, which have been translo- Clay formation results mainly from chemical
cated downward from the A horizon by percolat- weathering. Time estimates for the formation of 1
i ng water. Such a weakly developed B horizon is percent clay inn rock parent material range from
given the symbol w (as in Bw), to indicate its 500 to 10,000 years. Some weathered rocks with
weakly developed character. A Bw horizon can be small areas in which minerals are being con-
distinguished from A and C horizons primarily by verted into clay are shown in Figure 2.6.
color, arrangement of soil particles, and an inter- Many soil parent materials commonly contain
mediate content of organic matter. A soil with A, some clay. Some of this clay, together with clay
B, and C horizons is shown in Figure 2.5. produced by weathering during soil formation,
During the early phases of soil evolution, the tends to be slowly translocated downward from
soil formation processes progressively transform the A horizon to the B horizon by percolating
parent material into soil, and the soil increases in water. When a significant increase in the clay
thickness. The evolution of a thin AC soil into a content of a Bw horizon occurs due to clay trans-
thick ABwC soil is illustrated in Figure 2.4. l ocation, a Bw horizon becomes a Bt horizon.

32. 16 SOIL AS A NATURAL BODY
cles are believed to disperse when dry soil is
wetted at the end of a dry season and the clay
particles migrate downward in percolating water
during the wet season. When the downward per-
colating water encounters dry soil, water is with-
drawn into the surrounding dry soil, resulting in
the deposition of clay on the walls of pore spaces.
Repeated cycles of wetting and drying build up
layers of oriented clay particles, which are called
clay skins.
Many studies of clay illuviation have been
made. The studies provide evidence that thou-
FIGURE 2.6
sands of years are needed to produce a significant
Weathering releases mineral grains in i ncrease in the content of clay in B horizons. An
rocks and results in the formation of very fine-sized
particles of clay, in this case, kaolinite. example is the study of soils on the alluvial
floodplain and adjacent alluvial fans in the Cen-
tral Valley of California. Here, increasing eleva-
Thin layers or films of clay can usually be ob- tion of land surfaces is associated with increasing
served along cracks and in pore spaces with a age. The soils studied varied in age from 1,000 to
10-power hand lens. The process of accumulation more than 100,000 years.
of soil material into a horizon by movement out of The results of the study are presented in Figure
some other horizon is illuviation. The t (as in Bt) 2.7. The Hanford soil developed on the floodplain
refers to an illuvial accumulation of clay. The Bt is 1,000 years old; it shows no obvious evidence of
horizon may be encountered when digging holes illuviation of clay. The 10,000-year-old Greenfield
for posts or trenching for laying underground soil has about 1.4 times more clay in the subsoil
pipes. (Bt horizon) than in the A horizon. Snelling soils
Alternating periods of wetting and drying seem are 100,000 years old and contain 2.5 times more
necessary for clay translocation. Some clay parti- clay in the Bt horizon than in the A horizon. The
FIGURE 2.7 Clay distribution
as a function of time in soils
developed from granitic parent
materials in the Central Valley
of California. The Hanford
soil, only 1,000 years old,
does not have a Bt horizon.
The other three soils have Bt
horizons. The Bt horizon of
the San Joaquin is a claypan
that inhibits roofs and the
downward percolation of
water. (After Arkley, 1964.)