Optimising crop productivity in legume-cereal rotations through nitrogen and phosphorus management in western Kenya

Abstract

Combined application of organic resources and mineral inputs is integral to sustainable soil fertility management but in-situ production of adequate organic matter is often limited by P availability. An experiment was set up at Nyabeda in Western Kenya aimed at (1) quantifying the contribution of herbaceous and grain legumes to nitrogen supply in a cereal-legume rotation system and (2) quantifying the impact of targeting phosphorus (P) to certain phases of the rotation on overall maize grain yield. In this split-split plot experiment, Mucuna pruriens was used as the herbaceous legume while soybean was used as the grain legume. Results obtained in the two seasons of the study indicated that the use of either mucuna or soybean as previous crop significantly increased maize grain yield with or without the addition of nitrogen fertilizer. More than 5 tons ha(-1) of maize grain yield was realised in season two following the addition of phosphorus fertilizer at both season one and season two compared to about 3 tons ha(-1) of maize grain yields obtained when no P was added. It could be concluded that in this region, the addition of P fertilizer is an integral management option to ensure optimal utilization of the nitrogen fixed by the legume crop. Using P during the legume season may be sufficient to supply P requirements to the succeeding cereal crop. Also, applying P to the mucuna or soybean legume crop was not any different from applying it both to the legume and cereal crops indicating that farmers can save labour and cash by applying P only to the legume. The good performance of maize planted after mucuna was an indication that mucuna could be used by farmers in the region as an N source (Nitrogen Fertilizer Equivalency (NFE) > 100 kg N ha(-1)) thus reducing cost of buying N fertilizers. Although soybean showed a lower NFE of 40 kg N ha(-1), it had higher economic benefits and could thus be more acceptable to the farmers. These findings could be confirmed by using more than two cereals and legume rotation cycles.