The Laws of Diminishing Yields in the Tropics

The key problem of conventional agriculture in the tropics and subtropics is the steady decline in soil fertility, which is closely correlated with the duration of soil use. The reason for this can be found primarily in the occurrence of soil erosion, the loss of organic matter, leaching of nutrients into deeper soil layers, and soil physical degradation associated with conventional tillage practices that leaves the soil bare and unprotected in times of heavy rainfall and heat (Derpsch and Moriya 1998). The result of soil degradation is not only that farm land has to go out of the production process, but also that there is an increasing need for more inputs and investments to maintain high levels of productivity. In the United States for instance, 50 percent of fertilizer needs is applied only to compensate for the losses in soil fertility due to soil degradation, and in Zimbabwe, soil nutrient losses by erosion are three times higher than the total quantity of fertilizers applied (Stocking 1988, cited by Steiner 1996). The GLASOD project (Global Assessment of Soil Degradation), which is a United Nations program for the environment (UNEP) that aims to determine worldwide soil degradation, distinguishes four processes of degradation caused by man: degradation by water erosion, by wind erosion, chemical and physical soil degradation (Oldeman and others 1990).