Effects of Seasons and Vigna unguiculata Cultivation on Soil Physical and Chemical Properties in Port Harcourt, Nigeria

This research was conducted to evaluate and compare seasonal variations and Vigna unguiculata cultivation on soils physiochemical properties. Samples of topsoil (0-15cm) and subsoil (15-30cm) were collected from the mangrove and rainforest vegetation in Port Harcourt, Nigeria during the dry and rainy seasons, and soil analysis was done using standard methods. Results on soil physical properties showed that soils in the rainforest vegetation had higher bulk densities (1.42-1.53gcm-3) and saturated hydraulic conductivity (15.88-22.30cmh-1), during the seasons than the mangrove vegetation (0.56-0.94gcm-3) and (10.51-18.37cmh-1), while total porosity values were higher for the mangrove vegetation (46.7-58.2%) than in the rainforest (28.3-45.7%). Soil chemical properties were generally higher in top soils than in the sub soils except for available phosphorus which had higher levels in the sub soil than at the top soil. Soils in the mangrove vegetation were strongly acidic (pH = 3.1), while soils in the rainforest vegetation were slightly acidic (pH = 5.0). The mangrove soils had higher organic carbon (5.0-7.5%), total nitrogen (0.3-0.6%), available phosphorus (47.1-80.2mg/kg), calcium (11.3-21.1cmol/kg) and magnesium (4.0-8.6cmol/kg), than the soils in the rainforest vegetation with ranges of (0.40.9%), (0.1-0.3%), (15.5-51.6mg/kg), (2.4-4.2cmol/kg), (1.0-2.0cmol/kg) respectively. Mean organic carbon value increased from 0.8% before cultivation of cowpea, to 1.0% after cultivation of cowpea; and levels were higher in rainy season (1.1%) than in dry season (0.8), total nitrogen mean value was the same (0.3%) before and after cultivation of cowpea, Ca+ and Mg+ values reduced from 4.1cmol/kg and 2.6cmol/kg before planting to 2.3cmol/kg and 1.1cmol/kg after planting, while Na+ increased from 0.2cmol/kg, to 0.5cmol/kg after planting. K+ had same value of 0.4cmol/kg before and after planting. Available Phosphorus increased slightly from 37.0mg/kg before planting to 37.5mg/kg after planting, among others. Seasonal variations and cowpea cultivation had a significant effect on soil physiochemical properties.