The semi-arid tropics stretch over 48 counties on four continents, covering around 2,000 million hectares. It is inhabited by 700 million people. The soil types in the region are usually low in organic matter, N and P. The mean annual temperature is above 18oC and precipitation exceeds potential evapotranspiration for only 2 to 7 months (Fig. 8.1), with moisture balance being kept at a deficit for the rest of the year.4 Annual precipitation ranges from 400 to 1000 mm with erratic distribution, which often results in either no rainfall for extended periods or heavy local downpours. Because of repeated wet/dry periods and periodical heavy rainfall, nutrient depletion from the soil is the main biophysical problem in most of the SAT.
Despite the harsh environmental conditions, the rate of population increase has been high in the past and will be kept high(er) in the near future. To feed the growing population, more foods should be produced, by improving productivity of the lands and crops and by developing more sophisticated and sustainable ways of resource management. Due to the unpredictable weather and to poor accessibility of agricultural resources such as fertilizers, the cropping options for farmers are restricted to low input farming systems. High rainfall, which often comes at the onset of each growing season, greatly reduces the efficiency of fertilizer for crop uptake. The fear of a fatal crop failure due to drought makes the farmers reluctant to apply sufficient fertilizer for crop growth. Most crops in the region are grown with no fertilizer or insufficient amounts of fertilizer, due to the consideration of safe investment.
Sorghum and pearl millet are the two major cereals grown in the region, followed by rice, wheat and maize. Rice is grown either irrigated or upland. Pulses and oilseeds are important cash crops, on which the household largely depends for its cash income. There is a variety of cropping systems practiced by the farmers for these crops. Mono-cropping is not the farmers' only choice. They move flexibly to other cropping systems. The cropping options available to the SAT farmers include shifting cultivation, pioneer cultivation, alley cropping, monocropping, multiple cropping, intercropping, mixed cropping, mixed row cropping and relay cropping.
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