The plow is a potent tool of agriculture for the same reason that it has degraded productivity. Plowing turns over soil, mixes it with air, and stimulates the decomposition of organic matter. The rapid decomposition of organic matter releases a flush of nutrients that stimulates crop growth. But over time, plowing diminishes the supply of soil organic matter and associated soil properties, including water holding capacity, nutrient holding capacity, mellow tilth, resistance to erosion, and a diverse biological community.
As European and American settlers spread across North America in the 1800's, great expanses of land were exposed to the plow for the first time. After several decades of farming using intense tillage and much lower inputs of new organic matter, much of that land lost up to half of its original organic matter. Sandy lands that once contained 2% organic matter are now below 1%. Rich prairie loam that carried 8% or more, is now at 4%. On the semiarid U.S. Great Plains soil productivity decreased 71% during the 28 years following sod breaking (Flach et al., 1997).
The initial rise in atmospheric CO2 levels can be explained largely by the conversion of soil organic matter into CO2 and by the conversion of forests to other land uses. In the U.S., cultivation and land clearing emitted more CO2 than the burning of fossil fuels until the late 1950's.
After 1940, soil organic matter in some regions began to stabilize or rise slightly as farmers began to use higher yielding varieties, apply fertilizers, and remove less straw at harvest (Cole et al., 1990). Improved yields have been important in improving soil organic matter levels over the past few decades (Allmaras et al. 1998, 2000).
Allmaras, R.R., D.E. Wilkins, O.C. Burnside, and J.D. Mulla. 1998. Agricultural technology and adoption of conservation practices. In: F.J. Pierce and W.W. Frye, eds. Advances in Soil and Water Conservation. Chelsea: Ann Arbor Press.
Allmaras, R.R, H.H. Schomberg, C.L. Douglas Jr., and T.H. Dao. 2000. Soil organic carbon sequestration potential of adopting conservation tillage in U.S. croplands. Journal of Soil and Water Conservation 55:365-373.
Cole, C.V., I.C. Burke, W.J. Parton, D.S. Schimel, D.S. Ojima, and J.W.B. Stewart. 1990. Analysis of historical changes in soil fertility and organic matter levels of the North American Great Plains. In: P.W. Unger, T.V. Sneed, and R.W. Jensen, eds. Proc. International Conference on Dryland Farming, Amarillo, TX, 15-19 Aug. 1988. Texas A&M University, college Station, TX.
Flach, K.W., T.O. Barnwell, Jr., and P. Crosson. 1997. Impact of agriculture on atmospheric CO2. In: E.A. Paul, K. Paustian, E.T. Elliott, and C.V. Cole, eds. "Soil Organic Matter in Temperate Agroecosystems: Long-term Experiments in North America." CRC Press, Boca Raton, FL.