People have different ideas regarding what makes a quality soil. For example, for producers in production agriculture, it may mean high-yielding land, sustaining or enhancing productivity, maximizing profits, or maintaining the soil resource for future generations; for consumers, it may mean plentiful, healthful, safe and inexpensive food for the present and for future generations; for naturalists, it may mean soil in harmony with the landscape and its surroundings; for the environmentalist, it may mean soil functioning at its potential in an ecosystem with respect to maintenance or enhancement of biodiversity, water quality, nutrient cycling and biomass production.
Understanding soil quality means assessing and managing soil so that it functions optimally now and is not degraded for future use. Soil has both inherent and dynamic qualities. Inherent soil quality is a soil’s natural ability to function. For example, clay soils have a tendency to pond water more than sandy soils. Inherent soil qualities tend not to change much over time. Dynamic soil quality, on the other hand, is how soil changes depending on how it is managed. Management choices affect the amount of soil structure, soil depth and soil organic matter, as well as its water- and nutrient-holding capacity (Dam et al., 2004). Much like air and water, the quality of soil has a profound effect on the health and productivity of a given ecosystem and the environments related to it. However, unlike air and water for which we have quality standards, the definition of soil quality is complicated because it is not directly consumed by humans and animals (Doran et al., 1996).
Historically, farmers have viewed soil quality/health as a function of yield potential and nutrient levels. In recent years they have also come to the realization that the biological health of the soil is profoundly important (Roming et al. 1996). Soil quality cannot be measured directly, so we evaluate indicators. Indicators are measurable properties of soil or plants that provide clues about how well the soil can function. Indicators can be a variety of physical, chemical and biological characteristics. However, descriptive indicators and quantitative means to monitor soil quality are at best difficult to define. Arshad and Conan (1992) gave possible descriptive indicators to characterize soil quality, which include evidence of erosion, soil structure, friability, crusting of the soil surface and ponding of water. All of these are physical attributes of the soil and to a certain extent can be controlled by best management practices and are somewhat qualitative in nature. Quantitative measures to monitor soils -- such as soil pH and extractable N-P-K -- are more developed and are still being explored as to how they affect yield, nutrient levels and the biological health of the soil. Many decades of research have consistently shown that the best means of improving and/or restoring soil quality/health is by proper and regular additions of organic matter -- primarily through the use of cover crops, crop residues, manures and reduced-tillage practices.
Precision agriculture provides a tremendous opportunity to aid in the monitoring and maintenance of soil quality. Current soil sampling routines -- either on a grid system or zone management -- have the potential to help farmers monitor the condition of their soil over time. On-the-go soil sensors such as soil electro-conductivity (EC) measurements and near-infrared soil-color sensing all add pieces to the puzzle in the assessment of soil quality. Also, yield monitor data, along with as-applied fertilizer, herbicide/pesticide maps and site-specific manure application, all document what was done, where it was done and how it was done. This documentation is essential in identifying possible soil-quality indicators that may be used to identify reasons for poor-yielding areas that cannot be explained by routine whole-field soil testing. Also, environmental monitoring/documentation of inputs will help characterize how these inputs move through the ecosystem.
The agricultural inputs that are supplied do not disappear; they are simply redistributed over time. The goal should be to control their movement and protect fragile resources – including soil quality. Environmental consciousness must not be considered a nuisance but a requirement for survival.
Arshad, M.A. and G.M. Coen. 1992. Characterization of Soil Quality: Physical and Chemical Criteria. American J of Alternative Agriculture. 7: 25-30.
Dam, R.F., BB.Mehdi, M.S.E. Burgess, C.A. Madramootoo, G.R. Mehuys, I.R. Callum. 2004. Soil bulk density and crop yield under eleven consecutive years of corn with different tillage and residue practices in a sandy loam soil in central Canada. Soil and Tillage Research 84: 41–53.
Doran, J.W. and T.B. Parkin. 1996. Methods for Assessing Soil Quality. SSSA Spec.Publ. 49. Soil Science Society of America, Madison, WI.
Roming, D.E., M.J. Garlynd, and R.F. Harris. 1996. Farmer-Based Assessment of Soil Quality: A Soil Health Scorecard. SSSA Spec. Publ. 49. Soil Science Society of America, Madison, WI.