ABSTRACT
The definitions and classifications of ecogeographical regions termed as semi-arid tropics (SAT) have varied depending on the criteria and purpose. Broadly, considering the agricultural purpose, Vleck and Mokwunye (1990) include zones with 2 to 7 wet months, wherein precipitation exceeds evapotranspiration as SAT. Drier SAT are characterized by mono-model rainfall distribution, with rainy season lasting a mere 3 to 4.5 months, and averaging between 250 mm and 700 mm annually (Troll, 1966). In this book, drier SAT are discussed in chapter 6 titled 'Dryland Agroecosystem'. Hence, the main focus here is on wetter SAT, wherein precipitation ranges between 400 and 1200 mm annually, and rainfall is
bi-model allowing two cropping seasons. Variations in precipitation levels, both in time and space, as well as extended or intermittent droughts are a common feature in SAT. Although not strictly, it is accepted that soil fertility, particularly depreciated levels of N, P and organic C (in sandy soils), and interaction with moisture dictates productivity of this agroecosystem. Of late, continuous cropping without appropriate replenishment of all essential nutrients has caused micronutrient imbalances. The SAT agroecosystem encompasses vast cropping zones in different continents. In Africa, it covers about 485 m ha (Sanchez, 1976), which includes the sub-Saharan belt in West Africa extending from Senegal to Chad. It includes parts of Ethiopia, Kenya in the east, and the fringes of Kalahari in southern Africa. In Asia, SAT cropping systems are conspicuous in western, central and peninsular, India and large tracts in Pakistan. The Brazilian 'Cerrado' is a vast expanse of SAT in Latin America. Larger tracts of SAT also exist in northern Australia. Indeed, SAT is a vast expanse utilized for agricultural activity, wherein nutrient dynamics, ecology and productivity aspects have been studied and reported in great detail. However, within this chapter, only the pearl millet agroecosystem in the Sahelian West Africa, sorghum-based ecosystem in peninsular India, and the legume belt in central and peninsular India are discussed in detail.
