Agronomic measure and strategies for soil conservation use the protective effect of plant cover to reduce erosion and soil loss. In considering effectiveness of crop management strategies, there are many factors that need to be considered such as type of crop or vegetation used in a crop rotation, tillage practices, landscape characteristics, rainfall magnitude and intensity. In general, row crops are less effective and may cause some erosion. In a 100 year study, land under continuous maize has only 44% as much of the top soil compared to land kept under grass, whereas the land under a rotation of maize and pasture had 70 % of the top soil (Gantzer et al. 1990). Legumes and grasses used on a rotation scheme with row crops can provide good ground cover and also help maintain the organic content of the soil, contributing to the soil fertility and stability of aggregate structure, ttis type of rotation can also potentially increase the yields of the main crop.
Maize grown on 2-4° slopes as a row crop with conventional tillage and clean weeding, can result in soil loss between 10 and 120 t ha4, taking data from India (Singh et al. 1979) and Zimbabwe (Hudson 1981). Soybeans are often used in the rotation with maize because of their apparent ability to reduce soil loss by intercepting a higher percentage of the rainfall and that it requires minimal nitrogen fertilizer. However, studies in the U.S. Midwest indicate that soybean cultivation can result in as much if not more erosion than maize (Laflen and Moldenhauer 1979). tte mean soil loss over a seven year period for maize in a silt-loam soil with a 4% slope was 7 t ha4 for continuous maize, 6.5 t ha4 for maize followed by soybean. Lai (1976) found that when using sequential cropping near Ibadan, Nigeria, on a slope of 6 maize crop followed by cowpeas with no tillage produced a soil loss of only 0.21 ha4 but when growing cowpeas followed by maize but with tillage gave a loss of 6.21 ha4. Although the effects of tillage and no till are difficult to isolate, it seems that a maize-cowpeas sequence produces less erosion than a cowpeas-maize because maize is a soil-depleting crop and when grown second, is planted into an already partially exhausted soil.
Fallow crops can be useful only for grazing or fodder, but have no immediate value to the farmer who does not have a cattle farm, ttus this type of management strategy is not followed by the main cereal-growing areas and is not a practical to control soil erosion. An alternative approach in the temperate regions is to minimize the period of bare ground, for example, by leaving crop residue on the land after harvest and delaying ploughing until the following spring. Another practice is to have a winter cover crop that is planted late in the fall and ploughed in to form a green manure prior to sowing the main crop, tte cover crops are rye, oats, mustard, sweet clover and other similar crops. Although the cover crop grows rapidly and retains nutrients in the soil that would be otherwise lost to leaching, the cost of growing a cover crop may outweigh the benefits an individual farmer receives, ttis is especially true on small holdings where farmers do not have sufficient cash reserves. Depending on the seasonal rainfall pattern, the cover crop could complete for the available soil moisture and, in dry areas, adversely affect the growth of the main crop.
An area that is receiving significant attention now as a coping strategy for soil erosion and also carbon sequestration is mulching or covering the soil with crop residue and sometimes standing stubble, tte cover protects the soil from raindrop impact and reduces the velocity of runoff and wind, ttis is a useful alternative form of cover crop in dry areas where insufficient rain prevents the establishment of ground cover before the onset of heavy rains or strong winds or where a cover crop completes for soil moisture with the main crop. In semi-humid tropics, one of the benefits of mulching is to lower the soil temperature, and prevents surface evaporation to maintain the soil moisture which may increase crop yields. However, in cool climates, a reduction in soil temperature may shorten the germination and the growing season, whereas in wet areas, higher soil moisture maintained in mulch soils may induce anaerobic conditions. Furthermore, crop yields can be reduced because fungal and bacterial decomposition of mulch competes for nitrogen with the main crop so mulching generally requires added nitrogen fertilizer for compensation, tterefore one has to be selective in applying mulches or maintaining crop residue and aware of the potential changes in soil environment while reducing the rate of soil erosion. Numerous studies have successfully demonstrated the benefits of mulches and crop residue in minimizing soil loss under different landscape and slope conditions (Lai 1976; Khybri 1989; Sherchan et al. 1990).
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