Effect of annual legumes on soil nitrogen and on the subsequent yield of maize and grain sorghum

Abstract

The objective of this study was to quantify the nitrogen and yield advantage of cowpea, soybean and groundnuts to succeeding maize and grain sorghum crops. Treatments at six localities consisted of a crop rotation with annual legumes and maize or grain sorghum. Monoculture maize or grain sorghum with five nitrogen fertilisation levels was also included as treatments at all localities. Nitrate nitrogen in the soils was determined annually just before planting. Maize yields were determined at four localities and grain sorghum yields at two localities. The average nitrogen advantages after cowpeas, soybean and groundnuts were 32, 22 and 7 kg ha-1 respectively. For cowpeas and soybean, 65% and 54% of the variation in nitrogen advantage between years and localities could be accounted for by the yield of the respective legumes. Approximately 10 kg ha-1 nitrogen advantage resulted from both 1 ton ha-1 soybean seed yield and also from 1 ton ha-1 cowpea dry matter yield. Maize yields increased by 27, 51 and 90% after rotation with cowpea, soybean and groundnut, respectively, compared to monoculture maize (with zero N application). For cowpeas the total yield increase could be accounted for by the measured nitrogen enrichment of the soil, but after soybean and groundnuts an additional average maize yield increase of 10% was found. Average grain sorghum yields after cowpeas and soybeans were respectively 7 and 13% higher than the control (monoculture grain sorghum and zero N application). These increases could not be ascribed to nitrogen enrichment of the soil, since no reaction to nitrogen fertilisation levels was found at either locality.