INFLUENCE OF SOIL PHYSICAL AND CHEMICAL PROPERTIES ON PHOSPHORUS FIXING CAPACITY OF SELECTED SOIL SERIES IN SOUTHWESTERN NIGERIA

Selected soil series (Jago, Mamu, Apomu, Itagunmodi, Egbeda, Ondo, Iwaji, Iwo and Gambari Series) from Southwestern Nigeria were evaluated for their Phosphorus (P) sorption characteristics as influenced by soil physical and chemical properties. The samples were equilibrated in 50 ml of 0.01M CaCl2 containing various amounts of P as KH2PO4 to give 0, 1, 2, 3, 5, 8, 10, 15, 20 and 25 ppm for seven days at room temperature. The physical and chemical properties as well as P sorption characteristics of the soil series were determined. The adsorption isotherm of the samples showed different curves for samples of each soil series. The rates of P adsorption increased with increased concentrations of P, but at a certain level of higher concentration, the level of P became almost constant and had no more capacity to adsorb P. Phosphorus adsorption capacity decreased in the following order Itagunmodi > Gambari > Ondo > Egbeda > Iwaji > Mamu > Jago > Iwo > Apomu. Percent clay, Ca, Effective Cation Exchange Capacity (ECEC) and total exchangeable bases (TEB) were significantly positive correlated with P fixation while silt, soil pH, soil organic matters were positively insignificant correlated with P fixation. Percent sand, available P, extractable Fe, total exchangeable acidity (TEA) were negatively correlated with P fixation. The binding energy have negative relationship with percent sand, available P, extractable Fe and TEA while positive relationship was observed with % clay, pH, organic matter, TEB and ECEC. The result showed that Itagunmodi soil series required large amount of P fertilization for productive cultivation activities. The reduction of P sorption will require management practices like regular application of P fertilizers to improve the P solution concentrations and improvement of soil structure through the application of organic manure.