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

Abstract Annual soil improving legumes have a role in the management of soil fertility under the low-input management conditions of resource poor farmers. Several species have been identified as promising for the sub-humid tropical areas of eastern Africa having bi-modal rainfall. Most of these can be intercropped with species used for food and/or have alternative uses for food, forage or weed suppression. We compared five annual legumes for fixation of atmospheric nitrogen, soil water uptake, soil P and nitrate recovery, effects on subsequent crops and for phosphorus recovery from Busumbu P rock. Canavalia [ Canavalia ensiformis (L.) DC] produced the most biomass, fixed the most N, was most efficient in extraction of soil nitrate, and supplied the most N to subsequent food crops. Not unexpectedly, it was also most effective in improving soil productivity. Mucuna [ Mucuna pruriens (L.) DC var. utilis] produced less biomass than canavalia but derived a greater proportion of plant N from the atmosphere, while crotalaria [ Crotalaria ochroleuca G. Don.] and lablab [ Lablab purpureus (L.) cv. Rongai] fixed little nitrogen. Lablab and soybean [ Glycine max (L.)] produced the least biomass. All legumes and food crops failed to acquire significant amounts of P from Busumbu soft rock on this moderately acidic soil. The ratios of C:P in the legume biomass were high enough to cause an early net immobilization of P. Profile soil water status was highest under soybean and lowest under canavalia, reflecting differences in biomass production by the legumes and the subsequent maize–bean intercrop. Surface soil water was similar for all species, but differences were evident at depth. All legumes except soybean extracted water below 1.3-m depth.