NITROGEN USE IN MAIZE GRAIN LEGUME CROPPING SYSTEMS IN SEMIARID KENYA

Locally suitable cultivars of maize, beans, and cowpeas were grown in field experiments for four seasons in semi-arid Kenya. For three seasons, the dry matter production and grain yield of maize and beans were not increased by N fertilizer additions up to 120 kg N ha(-1). Fertilizer recoveries measured by N-15 isotope dilution techniques were low less than 20%. Inoculated and uninoculated beans failed to fix N-2. By contrast the cowpea derived 50% of its N from fixation, equivalent to 197 kg N ha(-1). The N content of the grain generally exceeded 40 kg N ha(-1), and the N content of the seeds from the grain legumes were greater than those from the cereals. Large inputs of N fertilizer or N by fixation are required if maize-grain legume cropping system in semiarid Kenya are to be sustained in the long term.