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Overview

Comprehensive, up-to-date coverage of the basics of soil chemistry

Although only a meter in depth over the earth's surface, soil is key to sustaining life-affecting air and water quality, the growth of plants and crops, and the health of the entire planet. The complex interplay among organic and inorganic solids, air, water, microorganisms, and plant roots in soil is the subject of Soil Chemistry, a reference pivotal to understanding soil processes and problems.

Thoroughly reorganized for ease of use, this updated Third Edition of Soil Chemistry summarizes the important research and fundamental knowledge in the field in a single, readily usable text, including:

  • Soil-ion interactions
  • Biogeological cycles and pollution
  • Water and soil solutions
  • Oxidation and reduction
  • Inorganic solid phase and organic matter in soil
  • Weathering and soil development
  • Cation retention (exchange)
  • Anion and molecular retention
  • Acid and salt-affected soils

New to the Third Edition is an enhanced emphasis on soil solution chemistry and expanded coverage of phosphate chemistry and the chemical principles of the aqueous phase. At the same time, the book has retained the clear examination of the fundamentals of the science of soil that has distinguished earlier editions. Complete with SI units and end-of-chapter study questions, Soil Chemistry is an excellent introductory resource for students studying this crucial topic.

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Product Details

  • ISBN-13: 9780471363392
  • Publisher: Wiley
  • Publication date: 6/13/2001
  • Edition description: REV
  • Edition number: 2
  • Pages: 320
  • Product dimensions: 6.40 (w) x 9.30 (h) x 1.10 (d)

Meet the Author

DR. HINRICH L. BOHN is Professor Emeritus in the Department of Soil, Water, and Environmental Science at the University of Arizona in Tucson.

DR. BRIAN L. McNEAL is a professor in the Soil Science Department of the University of Florida in Gainesville.

DR. GEORGE A. O'CONNOR is also a professor in the Soil Science Department of the University of Florida in Gainesville.

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Read an Excerpt

Soil Chemistry


By Hinrich L. Bohn Brian L. McNeal George A. O'Connor

John Wiley & Sons

ISBN: 0-471-36339-1


Chapter One

INTRODUCTION

No one regards what is at his feet; we all gaze at the stars. -Quintus Ennius (239-169 BC)

Heaven is beneath our feet as well as above our heads. -Henry David Thoreau (1817-1862)

The earth was made so various that the mind of desultory man, studious of change and pleased with novelty, might be indulged. -William Cowper (The Task, 1780)

The quotations illustrate how differently humans see the soil that gives them life and feeds them. Those opinions have been held for a long time. Most people are still at the knowledge level of Quintus Ennius who lived more than 2000 years ago. They take for granted the food that the soil produces, the clean water and air that the soil provides. Thoreau's and Cowper's wonder and fascination of soils is rarely expressed or felt. Yet the soil is wondrous if one looks closely. The soil-the solid but porous surface of the earth to about one meter depth, the depth that roots penetrate-has many mysteries. The soil is as mysterious and exciting as any other science and any other part of the universe.

Soil is a mixture of inorganic and organic solids, air, water, microorganisms, and plant roots. All these phases influence each other: Weathering and adsorption by the soil affect air and water quality, air and water weather the soil, microorganisms catalyze many of the reactions, and plantroots absorb and exude inorganic and organic substances. Soil chemistry considers all these reactions but emphasizes the reactions of the soil solution, the thin film of water and its solutes (dissolved substances) on the surfaces of soil particles.

1.1 THE SOIL SOLUTION

The soil solution is the interface between soil and the other three active environmental compartments-atmosphere, biosphere, and hydrosphere (Fig. 1.1). The boundaries are dashed lines to indicate that matter and energy move actively from one compartment to another; the environmental compartments are closely interactive rather than isolated. The interface between marine sediments and seawater, and between groundwater and subsoils, is chemically much the same as the interface between surface soils and the soil solution. Sediments remove and release ions from the bodies of water they contact by the same processes as the interface between the soil and the soil solution.

The soil solution is the source of mineral nutrients for all terrestrial organisms. As the soil solution percolates below the root zone, it becomes groundwater or drains to streams, lakes, and the oceans, and strongly affects their chemistry. The amounts of matter transferred are much greater and the rates of these reactions are much faster in the soil than in the other environmental compartments. The soil solution is the most important transfer medium for the chemical elements that are essential to life.

The soil solution differs from other aqueous solutions in that it is not electrically neutral and usually contains more cations than anions. The net negative charge of soil clay particles in most soils extends electrically out into the soil solution, and the charge is balanced by an excess of cations in the soil solution. These cations belong to the solid but are present in the soil solution. Soils in old and heavily weathered soils, as in parts of Australia, Africa, and South America, or in soils of volcanic origin, as in Japan and New Zealand, may have a net positive charge. There the soil solution has an excess of anions.

The interactions of ions and electrical charge at the soil particle-soil solution interface happens at all particle interfaces. In cases outside of the soil, this interaction is generally negligibly small. The soil is unique because the soil's surface area is so large that this interaction becomes so extensive. Because of this interaction, the boundary between soil solids and the soil solution is diffuse. The water and ions at the interface belong to both the aqueous phase and to the soil solids.

The diffuse layer extends out as much as 50 nm into the aqueous solution from the particle surface. For clay (colloidal)-size (<2 µm) particles with their large surface area, this interaction is great enough to significantly affect the composition of the soil solution next to the colloidal particles. Because soils contain considerable clay, a large part of the soil solution is affected by colloids. At the so-called field capacity water content, most of the soil solution is in the < 10 µm contacts and pores between sand (50-2000 µm) and silt (2-50 µm) particles. Clay particles and microbes congregate at these contacts so the soil solution interacts closely with these reactive bodies in these contact zones. The soil solution on open sand and silt surfaces is only 10-100 nm thick, so much of this water is also affected by the particle's charge. The portion of soil solution affected by soil colloids increases as the soil dries.

Most soil reactions occur at the soil solution/soil interface. Ions in water can move and react fast enough to measure easily. Slower but still measurable reactions occur in the weathered surfaces of soil particles. These poorly understood surfaces contain considerable water. Reaction rates in the truly solid phase at soil temperatures, however, are too slow to be measured in our lifetimes.

Because the mass and reactivity of soils are great, the chemistry of the atmosphere and fresh water are largely controlled by the chemistry of the soil solution. Reactions that require days and years in air, and hours in water, require only seconds and minutes in soils. The compositions of the air, water, and biomass compartments in the environment evolved from, and still respond to, the chemistry of the soil. The soil came first and as it changed, it changed the others. The change in the others also changed the soil, but to a lesser extent because of the soil's mass.

The soil solution contains a wide variety of solutes, including probably every element in the periodic table. This book discusses those solutes that are active in the environment-the solutes that affect plant and animal life, or have been of concern in pollution.

Figure 2.1 shows the chemical elements that are essential to living organisms, those that are reactive, and those that are in significant amounts in each environmental compartment. Chapter 2 discusses the life-essential and other important elements in more detail. All of the major and minor essential elements are stored and available for transfer in soils; the amounts in the other compartments, although important, are generally much, much smaller. Major and minor refer only to the amounts needed by organisms; all are essential and all are needed in their proper amounts. Too little is deficiency, too much is toxicity. The essential elements are also called essential nutrients, but elements is a better term because nutrients implies energy content, such as in the carbohydrates and fats of food.

With few exceptions, the soil supplies these elements to living organisms in about the right amounts. Although not perfect, the soil's supplying power is remarkably effective. This is not simply a happy coincidence; life evolved in response to the availability of these elements in soils.

For most people the major reason to study soil chemistry is to insure and to increase production of food and fiber crops. The soil is and will be the main source of human nutrition. The oceans supplement our food supply but their productivity is limited by the osmotic potential of the water, the limited availability of essential elements, the low temperature of ocean water, and the long food chain between photosynthetic organisms and those large enough to harvest. The oceans can provide only a small amount of society's needs and wants. The soil's productivity per unit area is many times that of the oceans. Terrestrial plants remain the cheapest and best means of converting solar energy into life support for this planet. The growth of plants on soils is the basis of most of the world's economy and of a nation's well being.

Soil chemistry is only one of many factors that affect plant growth. In contrast to climate and other uncontrollable factors, however, agriculturalists can influence and modify soil chemistry to considerable extent. The amounts of essential elements needed by plants over a season are small enough that supplementing the soil supply is feasible. Increasing the efficiency of that fertilization is a continuing soil chemistry challenge. The toxicity of materials that harm plant growth can also be controlled by soil chemistry.

People are now learning to appreciate the soil's large role in the biogeochemical cycling of the elements. Soils can mitigate many undesirable human-caused changes (pollution) of the environment. Safe removal of wastes from the environment has been recognized to be as important for continued civilization as food production. The retention, exchange, oxidation, and precipitation of waste in soils make them unequaled as recycling media.

In earlier times when the population was less dense and industries were few and small, wastes were distributed widely on and in the soil and could readily return to their natural biogeochemical cycles. By concentrating wastes in urban areas, industrial facilities, landfills, feedlots, and sewer outfalls-releasing wastes to the air and water rather than allowing them to react in soils-by fertilization, and by creating synthetic chemicals that react slowly, humanity has occasionally and in local areas exceeded the rate at which these materials can return to their biogeochemical cycles. "Advanced" societies sometimes overlook the degradative functions of soil and look instead for expensive, and only partially satisfactory, technological methods of waste disposal. Humanity creates pollution, which has awakened a new awareness of the importance of soil chemistry.

Soil chemistry is closely related to colloid (surface) chemistry, geochemistry, soil fertility, soil mineralogy, and soil microbiology or biochemistry. Soil fertility considers soil as a medium for plant growth. Soil mineralogy examines the structural chemistry of the solid phase. Soil microbiology studies soil biochemical reactions. Such subdivision is necessary to study the soil thoroughly, but these subdivisions sometimes obscure the interaction between soil components, and this interaction is often as important as the properties of the components alone.

Soil chemistry traditionally has had two branches: inorganic and organic, but strict separation of the two fields is difficult and pointless in many cases. The direction of biochemical soil reactions is largely based on the inorganic phase. Soil organic processes affect primarily the rate of soil chemical reactions. Biochemical reactions are carried out by soil microorganisms, whose vast numbers in soils influence many reactions. For several elements, notably carbon, nitrogen, and sulfur, the microbial role almost totally determines soil reaction rates. Biochemical and microbial reactions are primarily catalytic processes affected by the independent variables of soil mineral composition, climate, gas exchange with the atmosphere, and energy from photosynthesis. Despite the importance of biochemical reactions, research in soil chemistry historically has been more oriented toward inorganic processes.

1.2 BACKGROUND

Food and fiber production were already important before agriculture began. After fear, food is the dominant concern of every animal. The senate of ancient Athens debated soil productivity 2500 years ago and voiced the same worries about sustaining and increasing soil productivity that are heard today: Can this productivity continue or is soil productivity being exhausted?

In 1790, Malthus noticed that the human population was increasing exponentially and that food production was increasing arithmetically. He predicted that by 1850 the demand for food due to population growth would overtake food production, and people would be starving and fighting like rats for morsels of food. Similar apocalyptic predictions continue to crop up and cannot be disregarded. It is encouraging, however, that productivity has increased since the Greek senate debates and faster than Malthus predicted. In recent history food productivity has been increasing faster than ever. The earth now feeds the largest human population ever and a larger fraction of that population is better fed than ever before. Whether this can continue and at what price to the environment and other organisms is an open question. One encouraging part of the answer is the rapidly declining human birth rate on most continents in recent decades, thus putting less stress on the soil's resources in the future. Another part of the answer lies in soil chemistry, and much progress is still to be made in our understanding of the soil and its potential.

Agricultural practices that increase crop growth-planting legumes, manuring with animal dung and with litter from forests, rotating crops, and liming-were known to the Chinese 3000 years ago. These practices had also been learned by the Greeks and Romans and appeared in the writings of Varro, Cato, Columella, and Pliny. The reasons for their effectiveness, however, were unknown. Little or no further progress was made in the Western world for almost 1500 years because of ignorance and deductive reasoning. Deduction is applying preconceived ideas, broad generalities, and accepted truths to particular problems, without testing if the preconceived ideas and accepted truths are valid. One accepted truth, derived from the Greeks, was that matter was composed of earth, air, fire, and water-a weak basis, as we later learned, on which to increase knowledge.

In the early 1500s, Sir Francis Bacon pointed that inductive reasoning, the scientific method, is a much more productive approach to gaining new knowledge-observe and measure, derive broader ideas from the data, and test these ideas again. The scientific method brought progress, but the progress in soil chemistry was slow.

Palissy (1563) thought that plant ash came from the soil and when added back to the soil could be reabsorbed by plants. Plat (1590) thought that salts from decomposing organic matter dissolved in water and absorbed by plants were responsible for plant growth. Glauber (1650) thought that saltpeter (Na, K nitrates) was the key to plant nutrition by the soil. Kuelbel (ca. 1700) believed that humus was the principle of vegetation. Boerhoeve (ca. 1700) believed that plants absorbed the "juices of the earth." Others have found this idea in Pliny's writings. None of them, however, had experimental proof.

Van Helmont (1592) tried to test these ideas.

Continues...


Excerpted from Soil Chemistry by Hinrich L. Bohn Brian L. McNeal George A. O'Connor Excerpted by permission.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

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Table of Contents

Preface.

Introduction.

Important Ions.

Water and Solutions.

Oxidation and Reduction.

Inorganic Solid Phase.

Soil Organic Matter.

Weathering and Soil Development.

Cation Retention (Exchange) in Soils.

Anion and Molecular Retention.

Acid Soils.

Salt-Affected Soils.

Index.

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