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The changing concept of soil quality.

Soil quality is again a topic of interest to soil scientists, agriculturists, biologists, agricultural and environmental policy decision makers, and to readers of the semi-popular press. Soil quality is the key to sustainable agriculture. An understanding of soil quality is also one of the keys to understanding natural ecological processes.

Soil quality concerns are not new, but each time we come to them we use a slightly different approach because our understanding of soils has increased and the questions that scientists and society ask have changed. Productivity and suitability for different beneficial uses of soil have been major themes in soil quality, but there are other approaches. It is useful to consider the changing concept of soil quality so we can put our present concerns in the context of the other ideas as we seek to reverse soil degradation and improve soil quality.

Suitability for various beneficial uses is probably the oldest and one of the most frequently used concepts of soil quality. Often this concept is related to the quantity of crops produced, but it can also be related to quality. Soil health, as it determines human health, is expressed through crop quality. Because soil is a unique habitat for a wide diversity of biota, various biological parameters have been used in defining soil quality. A further refinement is to define soil quality in terms of the various functions that soils perform in ecosystems, such as recycling of nutrients, partitioning of rainfall, and buffering. Finally, there are subjective concepts of soil quality that relate to intrinsic value and uniqueness (Warkentin and Fletcher).

Review of concepts

Suitability for different uses. Rating soils for crop growth appears in the first written literature, and most certainly predates the written records we have. Both Asian and European literature exists on this topic. Columella gathered information that was available to the Romans, and set out advice on soil suitability (Olson). Terms such as "rich" soils were used to indicate soil quality.

Closely related to crop growth were concepts of draft requirements for tillage, where "heavy" soils were often more difficult to farm than "light" soils. Soil quality was related to the cost of inputs required to change soils; for example, the draining of clay soils. The adage, "Between the farmer and prosperity stands his land," is a soil quality statement.

In North America, in the first half of this century, much emphasis was placed on regional suitability of soils for different crops (Hilgard). Huddleston reviewed two types of soil productivity ratings that have been used--deductive ratings based on crop yield data, and inductive ratings based on inferences about the effects of soil properties on crop yield. Properties such as soil texture, profile morphology, soil depth, and drainage are used. Soil quality standards to maintain productivity are discussed in papers from the USDA Forest Service. Land capability classification systems use soil properties and other land factors to determine the most suitable sustained use of a soil without degradation. Soil productivity and land capability comparisons obviously need to be put within the limitations of climate.

A major concern currently is the suitability of soils for disposal of various "wastes." Part of the suitability is for effective recycling, but another part is long-term storage and inactivation of "pollutants." Quality would then depend upon factors determining safe storage.

Soil health for human health. Soil health is used synonymously with soil quality in some writing. The relationship is drawn between soil health and the health of animals and humans eating the crops produced by the soil (Haberern). The components of soil health are the biological processes that produce a balance of major and minor nutrients, the trace organics that have enzymatic functions, and the freedom from plant diseases and from various pests that attack unhealthy crops growing in unhealthy soils. Soil health is achieved through promoting biological activity in the soil through additions of organic matter, and through avoiding addition of potentially toxic materials. Until recently, these ideas were largely the concern of organic farming. As we learn more about the dependence of our health on foods we eat, soil health ideas are being examined more widely in soil science research and practice.

Biomass and biological activity. Measurements of biological activity and the relationship of soil productivity to biological activity is a theme that has run through soil science for the past 100 years. But the use of biological activity in concepts of soil quality has been a relatively minor theme in soil science thinking in the last 50 years. Soil productivity has been related more to additions of nutrients and management of water. Soil biology was more prominent in the previous half century, after the discoveries of soil bacterial functions and nitrogen fixation by symbiotic and free living bacteria. Biological processes were not understood until the latter part of the 1800s, well after physical and chemical properties of soils were investigated in the mid years of the century.

In the last 15 years there has been increased interest in measuring biological parameters to characterize soil functions. Measurements of biomass and specific components of biomass, biological activity measured by respiration, enzymatic activity, and diversity of organisms are all becoming much more important (Doran et al.). These biological parameters are being related to soil management--to determine the effects, for example, of different tillage practices on soil biota.

Functions of soil in ecosystems. Another approach is to identify the functions of soil in an ecosystem (Doran and Parkin). Much as water quality is now seen as meeting ecosystem functions in a watershed, soil quality can be seen in terms of optimum functioning of soil in an ecosystem. What are the roles of soil in ecosystem processes, and what characteristics of soil make it particularly suited to carry out these roles? These soil functions can form the basis for evaluating soil quality. The concept then is not suitability for different uses, but whether the functions are optimum within the constraints of the specific ecoregion. While these functions have been studied individually, their use in specifying soil quality is recent.

Some of these functions include the following: (a) recycling of organic materials in soils to release nutrients for further synthesis into new organic materials; (b) partitioning of rainfall at the soil surface into runoff and infiltration; (c) maintaining habitat diversity of pore sizes, surfaces, and water and gas relative pressures; (d) maintaining habitat stability, including a stable structure, resistance to wind and water erosion, and buffering of habitat against rapid changes of temperature, moisture, and concentration of potentially toxic materials; (e) storage and gradual release of nutrients and water; and (f) the partitioning of energy at the surface, which is important in global circulation processes.

Cycling of carbon and nutrients is probably the best known soil function in ecosystems. Carbon and nitrogen cycles have been measured and modelled. Nutrient cycling has been studied more in non cultivated soils (e.g., forests or rangeland). The levels of cycling activity that can be expected in specific ecoregions are known.

The partitioning of water at the soil surface is an equally important function in ecosystems. This partitioning determines both quantity and quality of surface and groundwater. Water running over the surface can carry sediment and other pollutants, and quickly reaches drainages. Water that infiltrates into the soil and moves through the soil is generally purified, decreases in temperature, and slowly reaches the surface through base flow.

The diversity of soil at different scales from micrometer to kilometer allows the different soil processes to work. Both oxic and anoxic conditions can occur in the soil in relative proximity. Water partitioning varies with the nature of the surface.

Buffering is one of the important characteristics of soil, a characteristic that sets it apart from other habitats. Part of the quality of soil habitat is the fact that changes in temperature or concentration of either toxic or beneficial chemicals are slow. Cumulative effects are particularly worrisome in soils because the high buffering capacity can mask undesirable effects until a threshold is reached beyond which changes may be irreversible. Stability of soil structure is increasingly being recognized as a basic soil concern because it controls many of the ecosystem functions. Deterioration of soil structure is a major factor in soil degradation, although crop production improvements in fertilizer and water management and in plant breeding have masked this deterioration.

The storage functions of soils are also major components in the concept of suitability of soils for plant growth, where the amount of water and nutrients retained become important aspects. Much of our soil science information relates to these functions of storage and gradual release.

Energy partitioning, along with water partitioning, is important in land surface processes that drive the global circulation of air masses. We are just beginning to appreciate the importance of these functions of soils.

Intrinsic value. Does the soil have intrinsic value apart from its uses in crop growth or its functions in the ecosystem? Uniqueness and irreplacability would lead to intrinsic values. This is a very subjective concept of soil quality. It is not widely explored by professional soil scientists, but such concepts are held in various forms by naturalists and people who see a special relationship with the earth (Leopold). Our economic models exclude intrinsic values of resources.

Applying the concepts

Related ideas. Soil quality is not a soil property easily amenable toe concise definition. In consequence, ideas with different names are used for related concepts. Soil degradation is a change that has left the soil with impaired levels at which its functions are carried out. This may be an impaired ability for infiltration or a decreased diversity of habitat, a well as the more obvious changes caused by erosion or increases in salinity.

Soil resilience, in soil quality discussions, is the ability of a soil toe recover its normal functions after a natural or human-induced stress (Eswaran). The time scale for recovery will vary with the stress. Cumulative effects of many seemingly insignificant individual stresses must be included.

For various beneficial uses, we modify the rates at which these functions proceed. An ecosystem that completely recycles biomass does not produce food that can be exported from the system, so we increase the rates of recycling by increasing nutrient levels, making the soil eutrophic. This affects other processes, often decreasing rates, for example of natural buffering or organic matter storage. For engineering use of soils we decrease infiltration, porosity and gas exchange to get increased strength (i.e., resistance to applied stress).

Management for soil quality. How can these soil quality concepts be used? A soil quality index (Smith et al.) would be useful to compare different soils or to determine for a soil whether quality is increasing or decreasing. The latter is probably the easiest use, because other factors are more nearly constant. Indexes can be calculated in various mathematical ways, but the difficulty lies in drawing conclusions from comparisons across broad regions. The ecological functions of soils are limited by climate, so optimum functioning would need to be calculated for an ecoregion. Another limitation is in understanding soils--we do not yet understand how to relate, for example, biological diversity to soil quality. The exciting work of using soil quality concepts for sustainable production and environmental protection has just begun.

REFERENCES CITED

Doran, J.W., and T.B. Parkin. 1994. Defining and assessing soil quality. In: J.W. Doran et al. (eds.) Defining Soil Quality for a Sustainable Environment. Soil Science Society of America, Madison, WI. Special Publication 35.

Eswaran, H. 1994. Soil resilience and sustainable land management in the context of Agenda 21. In: D.J. Greenland and I. Szabolcs (eds.) Soil Resilience and Sustainable Land Use. CAB International, Wallingford, UK.

Haberern, J. 1992. A soil health index. Journal of Soil and Water Conservation. 47(6).

Hilgard, E.W. 1892. USDA Agricultural Weather Bureau Bulletin 3:1-59.

Huddleston, J.H. 1984. Development and use of soil productivity ratings in the United States. Geoderma 32:297-317.

Leopold, A. 1947. A Sand County Almanac and Sketches Here and There. Oxford University Press, New York.

Olson, L. 1943. Columella and the beginning of soil science. Agricultural History 17:65-72.

Smith, J.L., J.J. Halvorson, and R.I. Papendick. 1993. Using multiple-variable indicator kriging for evaluating soil quality. Soil Science Society of America Journal 57:743-749.

USDA Forest Service. 1992. Proceedings of the Soil Quality Standards Symposium. Publication WO-WSA-2, Washington, DC.

Warkentin, B.P., and H.F. Fletcher. 1977. Soil quality for intensive agriculture. Proceedings of the International Seminar on Soil Environment and Fertilizer Management in Intensive Agriculture. Society of Science of Soil and Manure. Japan. pp. 594-598.

Benno P. Warkentin is with the Department of Crop and Soil Science, Oregon State University, Corvallis 97331-7306.
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Author:Warkentin, Benno P.
Publication:Journal of Soil and Water Conservation
Date:May 1, 1995
Words:2118
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