Soil Nutrient Status and Cowpea Biological Nitrogen Fixation in a Maize-Cowpea Rotation Under Conservation Farming

This paper reports the impacts of a four-year maize-cowpea rotation under conservation farming on selected soil chemical, physical and biological properties with or without 100 kg/ha of Nitrogen: Phosphorus: Potassium: Sulphur (10:20:10:65) compound fertilizer. The study took advantage of an already established 4-year maize-cowpea rotation site and a maize monocrop field from which soil samples were collected for selected chemical, physical and biological analyses, and for setting up a greenhouse experiment for the determination of biological nitrogen fixation capabilities of cowpea. The results suggest that maize-cowpea rotations and addition of fertilizer influence particular chemical, physical and biological attributes of the soil in a varied manner. Rotating maize and cowpea has no influence on soil reaction, soil organic carbon, micronutrients, and exchangeable bases except for potassium. However, the addition of fertilizer to the maize-cowpea rotation reduces total nitrogen, while increasing the levels of sulphur and phosphorus in both the rotation and maize monocrop. Soil bulk density, total porosity and infiltration rate are not influenced by the maize-cowpea rotation with or without fertilizer amendment. Rotating maize with cowpea without the addition of fertilizer can result in an increase in plant available water, an observation needing further study. It can also be concluded that maize-cowpea rotations can reduce microbial biomass, regardless of fertilizer amendment, thus suggesting a need to understand maize and cowpea rhizopheric attributes affecting microbial biomass levels. Under the current conditions, the amount of biologically fixed nitrogen by cowpea is reduced by the application of fertilizer but not influenced by the rotation. Since these findings are based on four-year crop rotation, it is being recommended that further work be conducted to continue monitoring soil so as to factor in the effect of length of time.