The Indicator

Soil organic matter would be reported as the percentage of organic matter (dry weight) in the upper soil profile (4–6 inches). The data would be presented as a percentage of all croplands having several ranges of percent organic matter, on a national basis and on a regional basis for the latest year for which data are available. It should be noted that it may prove difficult to discern trends in organic matter using the coarse ranges chosen (less than 2%, 2 to 4%, and greater than 4%); an approach addressing change on the regional or local level may be necessary.

Soil organic matter content in the upper soil profile (4–6 inches) was chosen because human activity, particularly management practices, has had its greatest impact here. Soil organic matter content is related to the cation exchange capacity of the soil, soil water-holding capacity, nitrogen mineralization rates, and microbial activity.

Soil organic matter content is also related to biogeochemical processes, and the cycling of carbon and nitrogen within the upper soil profile is related to soil carbon content. Measurement of changes in the soil organic matter content over time provides a quantitative assessment of the soil capacity to support crops and other plant and animal life.

Soil organic matter content is a critical component of soil structure and is vital to all soil processes. Soil organic matter provides the chemical and biological basis for soil components (sand, silt, and clay) to form soil aggregates and is critical in key physical processes (such as water and gas exchange, penetration resistance, and compaction). Differences in climate, parent material, and management history have produced large regional differences in soil organic matter content.

In addition, since soil organic matter is about 60% carbon, the amount of organic matter is a predictor of the amount of carbon in soils. Storage of carbon in soils has become important in international negotiations on the management of greenhouse gas emissions, as increased carbon storage can be useful in offsetting emissions of carbon from fuel burning and other sources. In order to be of use to such negotiations, this indicator would probably need to measure carbon in the upper 3 feet of soil, not just the upper 4–6 inches. While this is not the current focus of the indicator, such a presentation would make this indicator analogous to those in the forest and grasslands and shrublands chapters.

The Data Gap

U.S. Department of Agriculture soil survey data (contained within the State Soil Geographic Database [STATSGO] and Soil Survey Geographic [SSURGO] datasets) provide an initial county-level estimate of soil organic matter content, but there are no programs in place to monitor and report soil organic matter content on a national basis. Universities and other research institutions have carried out observations of the changes in soil organic matter content under different management practices, but the results of these investigations do not provide national coverage. The STATSGO database is available at http://www.ftw.nrcs.usda.gov/stat_data.html and SSURGO is available at http://www.ftw.nrcs.usda.gov/ssur_data.html.