Evaluation of forage legumes for soil fertility improvement in maize/cassava production systems

Soil fertility decline is a major factor limiting crop production in smallholder farms in sub-Saharan Africa. This research examined intercropping of Clitoria ternatea and Lablab purpureus forage legumes as potential nutrient sources in maize and cassava production systems. The legumes accumulated nutrients (2-yr avg.) of 50 to 101 kg N ha -1, 4 to 8 kg P ha-1, 33 to 83 kg K ha -1, 7 to 32 kg Ca ha-1, and 5 to 7 kg Mg ha-1 during the long rain season, with dolichos producing the greater values. Nutrient contents of clitoria were greater than dolichos during the short rain season, with values 20 to 40% lower than the long rain season. Intercropping reduced nutrient contents of the legumes to <80% of monoculture, and was most pronounced for clitoria intercropped with cassava. Delayed harvest of dolichos from two to four mo after planting increased dolichos nutrient accumulation between two and fourfold. Fertilizer inputs increased maize and stover yields by 70% over maize grown without fertilizer inputs. Intercropping maize with clitoria increased grain and stover nutrient accumulation by values ranging from 50 to 80%, compared to maize monoculture without fertilizer inputs, while intercropping maize with dolichos yielded less grain and stover. Cassava monoculture resulted in the greatest tuber yields (9 Mg ha -1), while intercropping of cassava with a legume reduced tuber yield by 21%. Clitoria intercropping with maize and/or cassava resulted in area by time equivalent ratios ranging from 1.2 to 1.6, while those of dolichos ranged from 1.1 to 2.0, indicating that intercropping of legumes with maize or cassava improved biological efficiency of the systems.;An asymptotic model best described clitoria and dolichos dry matter disappearance and nutrient release across different harvest management strategies, resulting in rates of 0.2 and 0.5 k wk-1 for clitoria and dolichos residues, respectively. Nitrogen release was generally slowest in clitoria and in dolichos 4-mo cut foliage. The k-values for nutrient release of dolichos ranged from 0.2 to 2.5 wk-1, and those for clitoria 0.3 to 1.0 wk-1. Across harvest management strategies, the general order of nutrient release was K > P > Mg > N, while that of dolichos cut at 2 mo was K > Mg > N > P and K > N > P > Mg for dolichos cut at 4 mo. These results demonstrated the legumes potential to supply nutrients for maize grain and cassava tuber production, and indicated that biological efficiency may be improved by intercropping maize with legumes. The need to develop N and K replenishment recommendation for cropping systems with cassava is suggested. Further research aimed at reducing nutrient losses by synchronizing nutrient release with principal crop demand is recommended.