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In most of Southern Africa, crop yields are marginal because of the low intrinsic soil fertility, the limited use of inorganic fertilizers and the declining use of traditional fallows. To increase soil fertility and food production sustainably, soil fertility must be increased. The inclusion of agroforestry trees in agro-ecosystems is one of the ways to provide the essential organic matter and mineralisable nutrients. In Eastern Zambia ICRAF researchers and their counterparts in the national agricultural system have developed a promising agroforestry alternative to traditional fallows. It involves planting Sesbania sesban as 2 to 3 year improved tree fallows in N-depleted fields. Thereafter, the trees are clearfelled and all the twigs and leaf litter incorporated in the soil. The result is a doubling or even a quadrupling of maize yields. In addition, these planted leguminous tree increase biologically active pools of soil organic matter and the rate of release of plant-available nitrogen from soil organic matter. The results suggest that tree species differ greatly in their effects on soil organic matter pools and nitrogen availability to subsequent maize crops.
Transactions of the Zimbabwe Scientific Association Volume 72 (Supplement) 1998, pp. 72-83