Effect of intercrops and fertilizer on root growth, nodulation and yield of cowpea (Vigna unguiculata), and on soil microbial population

The effects of different crop combinations on root growth and nodulation of cowpea [Vigna unquiculata (L.) Walp.] were investigated on a soil under continuous cropping. Fertilizer application significantly increased nodule counts at 10 weeks after planting in cowpea sole and in cowpea-okra (Hibiscus esculentus L.)-tomato (Lycopersicon esculentum Mill. nom, cons.) and in cowpea-maize (Zea mays L.)-okra-tomato intercropping systems. Cowpea in cowpea-okra-tomato intercropping had the highest nodule weight at 10 weeks. The lowest weight was obtained from sole cowpea without fertilizer. Fertilizer significantly increased root weight in sole and intercropped cowpea. Soils carrying sole okra and tomato, including their combinations with 1,2 or 3 crops possessed reduced microbial populations due to fertilizer application, except cowpea-maize-tomato and maize-tomato. Fertilized and unfertilized tomato soils contained the highest microbial population, while soils carrying 4 crop combinations had the lowest. Simple correlation analysis indicated that cowpea yield, root weight, nodule count and nodule weight were highly and positively correlated and the correlation was highest at 10 weeks after planting. Nodule counts and weight were highly significantly correlated at each sampling stage. Fertilizer application increased greater nodulation in the intercrops but resulted in a lower seed yield of cowpea.