Annual soil improving legumes: agronomic effectiveness, nutrient uptake, nitrogen fixation and water use

Annual soil improving legumes have a role in the management of soil fertili ty under the low-input management conditions of resource poor farmers. Seve ral species have been identified as promising for the sub-humid tropical ar eas of eastern Africa having bi-modal rainfall. Most of these can be interc ropped with species used for food and/or have alternative uses for food, fo rage or weed suppression. We compared five annual legumes for fixation of a tmospheric nitrogen, soil water uptake, soil P and nitrate recovery, effect s on subsequent crops and for phosphorus recovery from Busumbu P rock. Cana valia [Canavalia ensiformis (L.) DC] produced the most biomass, fixed the m ost N, was most efficient in extraction of soil nitrate, and supplied the m ost N to subsequent food crops. Not unexpectedly, it was also most effectiv e in improving soil productivity. Mucuna [Mucuna pruriens (L.) DC var. util is] produced less biomass than canavalia but derived a greater proportion o f plant N from the atmosphere, while crotalaria [Crotalaria ochroleuca G. D on.] and lablab [Lablab purpureus (L.) cv. Rongai] fixed little nitrogen. L ablab and soybean [Glycine max (L.)] produced the least biomass. All legume s and food crops failed to acquire significant amounts of P from Busumbu so ft rock on this moderately acidic soil. The ratios of C:P in the legume bio mass were high enough to cause an early net immobilization of P. Profile so il water status was highest under soybean and lowest under canavalia, refle cting differences in biomass production by the legumes and the subsequent m aize-bean intercrop. Surface soil water was similar for all species, but di fferences were evident at depth. All legumes except soybean extracted water below 1.3-m depth. (C) 2000 Elsevier Science B.V. All rights reserved.