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INTRODUCTION TO SOIL CHEMISTRY
No one regards what is at his feet; we all gaze at the stars. Quintus Ennius (239-169 BCE)
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 Nation that destroys its soil destroys itself. Franklin Delano Roosevelt (1937)
1.1 The soil chemistry discipline
The above quotations illustrate how differently humans see the soil that gives them life and sustenance. In recent decades, great strides in understanding the importance of soils for healthy ecosystems and food production have been made, but the need for preservation and improved utilization of soil resources remains one of society's greatest challenges. Success requires a better understanding of soil processes.
Soil is a complex mixture of inorganic and organic solids, air, water, solutes, microorganisms, plant roots, and other types of biota that influence each other, making soil processes complex and dynamic (Figure 1.1). For example, air and water weather rocks to form soil minerals and release ions; microorganisms catalyze many soil weathering reactions; and plant roots absorb and exude inorganic and organic chemicals that change the distribution and solubility of ions. Although it is difficult to separate soil processes, soil scientists have organized themselves into subdisciplines that study physical, biological, and chemical processes, soil formation and distribution, and specialists that study applied soil science topics such as soil fertility.
The discipline of soil chemistry has traditionally focused on abiotic transformations of soil constituents, such as changes in oxidation state of elements and association of ions with surfaces. Chemical reactions in soils often lead to changes between solid, liquid, and gas states that dramatically influence the availability of chemicals for plant uptake and losses from soil that in turn are important aspects of fate and transport of nutrients and contaminants in the environment. With the ever-increasing pressures to produce more food and extract resources such as timber, oil, and water from the environment, pressures on soil resources are increasing. Addressing these pressures and challenges requires detailed knowledge and understanding of soil processes. Modern soil chemistry strives to understand interactions occurring within soils, such as interactions between soil microbes and soil minerals.
Figure 1.1 Soils are composed of air, water, solids, ions, organic compounds, and biota. The soil in the microscopic view shows soil particles (e.g., aggregates of minerals and organic matter), air and water in pore spaces, microbes, and a plant root. Fluxes of material or energy into and out of the soil drive biogeochemical reactions, making soils dynamic. Fluxes can be to the atmosphere, eroded or leached offsite into surface water, or percolated to groundwater.
The focus of soil chemistry is chemical reactions and processes occurring in soils. A chemical reaction defines the transformation of reactants to products. For example, potassium availability for plant uptake in soils is often controlled by cation exchange reactions on clay minerals, such as:
(1.1) where reactants are aqueous K+ and Na+ adsorbed on a clay mineral (Na-clay), and products are aqueous Na+ and K+ adsorbed on a clay mineral (K-clay). The adsorption reaction exchanges ions between aqueous solution in the soil pore and the soil solids (clay mineral in this case) and is thus a solid-solution interface reaction. Cation exchange reactions are a hallmark of soil chemistry.
A goal of soil chemistry is predicting whether a reaction will proceed, which can be done using thermodynamic calculations. Soils are complex, however, and predicting the fate of chemicals in the environment requires including multiple competing reaction pathways occurring simultaneously. In addition, many soil reactions are slow and fail to reach equilibrium before the system undergoes a perturbation, making prediction of chemical species a moving target. The complexity and dynamic aspect of soils make understanding chemical reactions in nature a challenging problem, but, over the past 150 years, great advances have been made. The goal of this book is to present the current state of knowledge about soil chemical processes so that students can use them to understand the environmental fate of chemicals.
1.2 Historical background
About 2500 years ago, the senate of ancient Athens debated soil productivity and voiced the same worries about sustaining and increasing soil productivity heard today: Can this productivity continue, or is soil productivity being exhausted?
In 1790, Malthus noticed that the human population was increasing exponentially, whereas food production was increasing arithmetically. He predicted that by 1850 food demands would overtake food production, and people would be starving and fighting like rats for morsels of food. Although such predictions have not come to fruition, there are real challenges to feeding the world's increasing population, especially considering predicted changes in climate that will have significant impacts to food production systems and regional populations.
It is encouraging that food productivity has increased faster than Malthus predicted. 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, is an open question. One part of the answer lies in wisely managing soil resources so that food production can continue to increase and ecosystem functions can be maintained. Sustainable management requires careful use of soil and knowledge of soil processes. Soil chemistry is an important subdiscipline required for understanding soil processes.
Agricultural practices that increase crop growth, such as planting legumes, application of animal manure and forest litter, crop rotation, and liming were known to the Chinese 3000 years ago. These practices were also learned by the Greeks and Romans, and appeared in the writings of Varro, Cato, Columella, and Pliny, but were unexplained. Little progress on technology to increase and maintain soil productivity was made thereafter for almost 1500 years because of lack of understanding of plant-soil processes, and because of undue dependence on deductive reasoning. Deduction is applying preconceived ideas, broad generalities, and accepted truths to problems without testing if the preconceived ideas and accepted truths are valid. One truth accepted for many centuries and derived from the Greeks was that all matter was composed of earth, air, fire, and water; a weak basis, as we later learned, on which to increase knowledge.
In the early fifteenth century, Sir Francis Bacon promoted the idea that the scientific method is the best approach to gaining new knowledge: observe, hypothesize, test and measure, derive ideas from data, test these ideas again, and report findings. The scientific method brought progress in understanding our world, but the progress in understanding soil's role in plant productivity was minimal in the ensuing three centuries.
Palissy (1563) proposed that plant ash came from the soil, and when added back to the soil could be reabsorbed by plants. Plat (1590) proposed that salts from decomposing organic matter dissolved in water and were absorbed by plants to facilitate 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." While these early theorists proposed reasonable relationships between plants and soils, accurate experimental design and proof was lacking, and their proposals were incomplete and inaccurate.
Van Helmont, a sixteenth-century scientist, tried to test the ideas of plant-soil nutrient relationships. He planted a willow shoot in a pail of soil and covered the pail so that dust could not enter. He carefully measured the amount of water added. After five years, the tree had gained 75.4 kilograms. The weight of soil in the pot was still the same as the starting weight, less about two ounces (56 g). Van Helmont disregarded the 56 grams as what we would today call experimental error. He concluded that the soil contributed nothing to the nutrition of the plant because there was no loss of mass, and that plants needed only water for their sustenance. Although he followed the scientific method as best he could, he came to a wrong conclusion. Many experiments in nature still go afoul because of incomplete experimental design and inadequate measurement of all essential experimental variables.
John Woodruff's (1699) experimental design was much better than Van Helmont's. He grew plants using rainwater, river water, and sewage water for irrigation, and added garden mould to the soils. The more solutes and solids in the growth medium - the "dirtier" the water - the better the plants grew, implying that something in soil improved plant growth. The idea developed that the organic fraction of the soil supplied the plant's needs.
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