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land study, the committee considered primarily one resource function: the production of different kinds, amounts, and arrangements of vegetation. The characterization of resource functions is mostly a scientific endeavor, but the development of standards requires explicit value judgments. So, although soils perform many functions, the three selected as a basis for measuring performance were those considered most important for sustaining agricultural productivity and protecting water quality. The selection of performance standards depends, fundamentally, on the economic and noneconomic values placed on the use and existence of the natural resource in question.
As embodied in such laws as the Clean Air Act and the Clean Water Act, national policy has recognized the importance of air and water quality to the country's well-being. However, there is no equivalent federal "Soil Quality Act," despite the critical role soil plays in mediating both water and air quality. Soil and Water Quality urges that soil quality be a national environmental priority:
The quality of a soil depends on attributes such as the soil's texture, depth, permeability, biological activity, capacity to store water and nutrients, and the amount of organic matter contained in the soil. Soils are living, dynamic systems that are the interface between agriculture and the environment. High-quality soils promote the growth of crops and make farming systems more productive. High-quality soils also prevent water pollution by resisting erosion, absorbing and partitioning rainfall, and degrading or immobilizing agricultural chemicals, wastes, or other potential pollutants. (National Research Council, 1993a, p. 2)
Traditionally, soil quality has been equated with soil productivity, a measure of promotion of plant growth. Soils perform a much broader range of functions in the environment, however, including regulation of water flow in watersheds and of greenhouse gas emission, attenuation of natural and artificial wastes, and regulation of air and water quality (National Research Council, 1993a). Consequently, measures of soil quality will have to be altered to reflect these aspects.
No comprehensive index of soil quality yet exists that captures fully soil's function in the ecosystem. Soil and Water Quality notes that it would be "impossible and unnecessary to monitor changes" in all of the soil attributes that relate to critical ecosystem functions (National Research Council, 1993a, p. 205). Moreover, the set of relevant indicators can be expected to change with geographic variation in soil types. The report does suggest a set of indicators that includes the most relevant physical, chemical, and biological attributes of the soil: nutrient availability, organic carbon, labile carbon, texture, water-holding capacity, soil