The amount of organic matter in soils is a result of the balance between the gains and losses of organic materials. Let’s use the abbreviation SOM as shorthand for soil organic matter. Then the change in soil organic matter during one year (the SOM change) can be represented as follows:
SOM change = gains – losses [equation 1]
If gains are greater than losses, organic matter accumulates and the SOM change is positive. When gains are less than losses, organic matter decreases and SOM change is negative. Remember that gains refer not to the amount of residues added to the soil each year but rather to the amount of residue added to the more resistant pool that remains at the end of the year. This is the fraction (f) of the fresh residues added that do not decompose during the year multiplied by the amount of fresh residues added (A), or gains = (f)(A). For purposes of calculating the SOM percentage estimates in table 3.2 we have assumed that 20% of annual residue additions remain at the end of the year in the form of slowly decomposing residue.
If you follow the same cropping and residue or manure addition pattern for a long time, a steady-state situation usually develops in which gains and losses are the same and SOM change = 0. Losses consist of the percentage of organic matter that’s mineralized, or decomposed, in a given year (let’s call that k) multiplied by the amount of organic matter (SOM) in the surface 6 inches of soil. Another way of writing that is losses = k(SOM). The amount of organic matter that will remain in a soil under steady-state conditions can then be estimated as follows:
SOM change = 0 = gains – k(SOM) [equation 2]
Because in steady-state situations gains = losses, then gains = k(SOM), or
SOM = gains/k [equation 3]
A large increase in soil organic matter can occur when you supply very high rates of crop residues, manures, and composts or grow cover crops on soils in which organic matter has a very low rate of decomposition (k). Under steady-state conditions, the effects of residue addition and the rate of mineralization can be calculated using equation 3 as follows:
If k = 3% and 2.5 tons of fresh residue are added annually, 20% of which remains as slowly degradable following one year, then the gains at the end of one year = (5,000 lbs. per acre)0.2 = 1,000 lbs. per acre.
Assuming that gains and losses are happening only in the surface 6 inches of soil, then the amount of SOM after many years when the soil is at equilibrium equals gains/k = 1,000 lbs./0.03 = 33,333 lbs. of organic matter in an acre to 6 inches.
The percent SOM = 100 (33,000 lbs. organic matter/2,000,000 lbs. soil).
The percent SOM = 1.7%.
Angers, D.A. 1992. Changes in soil aggregation and organic carbon under corn and alfalfa. Soil Science Society of America Journal 56: 1244–1249.
Carter, M. 2002. Soil quality for sustainable land management: Organic matter and aggregation– Interactions that maintain soil functions. Agronomy Journal 94: 38–47.
Carter, V.G., and T. Dale. 1974. Topsoil and Civilization. Norman: University of Oklahoma Press.
Gale, W.J., and C.A. Cambardella. 2000. Carbon dynamics of surface residue and root-derived organic matter under simulated no-till. Soil Science Society of America Journal 64: 190–195.
Hass, H.J., G.E.A. Evans, and E.F. Miles. 1957. Nitrogen and Carbon Changes in Great Plains Soils as Influenced by Cropping and Soil Treatments. U.S. Department of Agriculture Technical Bulletin No. 1164. Washington, DC: U.S. Government Printing Office. This is a reference for the large decrease in organic matter content of Midwest soils.
Jenny, H. 1941. Factors of Soil Formation. New York: McGraw Hill. Jenny’s early work on the natural factors influencing soil organic matter levels.
Jenny, H. 1980. The Soil Resource. New York: Springer-Verlag.
Khan, S.A., R.L. Mulvaney, T.R. Ellsworth, and C.W. Boast. 2007. The myth of nitrogen fertilization for soil carbon sequestration. Journal of Environmental Quality 36: 1821–1832.
Magdoff, F. 2000. Building Soils for Better Crops, 1st ed. Lincoln: University of Nebraska Press.
Magdoff, F.R., and J.F. Amadon. 1980. Yield trends and soil chemical changes resulting from N and manure application to continuous corn. Agronomy Journal 72: 161–164. See this reference for further information on the studies in Vermont cited in this chapter.
National Research Council. 1989. Alternative Agriculture. Washington, DC: National Academy Press.
Puget, P., and L.E. Drinkwater. 2001. Short-term dynamics of root and shoot-derived carbon from a leguminous green manure. Soil Science Society of America Journal 65: 771–779.
Schertz, D.L., W.C. Moldenhauer, D.F. Franzmeier, and H.R. Sinclair, Jr. 1985. Field evaluation of the effect of soil erosion on crop productivity. In Erosion and Soil Productivity, pp. 9–17. Proceedings of the National Symposium on Erosion and Soil Productivity, New Orleans, December 10–11, 1984. St. Joseph, MI: American Society of Agricultural Engineers, Publication 8–85.
Tate, R.L., III. 1987. Soil Organic Matter: Biological and Ecologycal Effects. New York: John Wiley.
Wilhelm, W.W., J.M.F. Johnson, J.L. Hatfield, W.B. Voorhees, and D.R. Linden. 2004. Crop and soil productivity response to corn residue removal: A literature review. Agronomy Journal 96: 1–17.