EVALUATION OF CONTACT TIME EFFECT ON AVAILABLE PHOSPHORUS SORPTION AND DESORPTION OF BASEMENT SOILS IN SOUTHWEST NIGERIA

ABSTRACT Contact time of phosphorus (P) sorption and desorption had been reported as a major chemical phenomenon that influence P availability in soil. This study investigates the phosphorus sorption properties of basement complex soils in Southwest Nigeria at various contact times. This research was carried out in the Soil Science and Land Management Laboratory and Soils of Forest Island in Africa (SOFIIA ) Laboratory at Federal University of Agriculture, Abeokuta (FUNAAB). Soil samples were collected from three different locations which are, Imeko, FUNAAB and Owode. The sorption experiment was done by weighing three (3) grams of each soils in triplicate into extracting bottles of 100 ml capacity and the soils was quantitatively equilibrated with 30 ml of CaCl2 solution containing 0, 20, 40, 60, 80, 100 mg L-1 of Phosphorus in the form of KH2PO4. Desorption was done after the sorption study. The data collected was subjected to analysis for phosphorus sorption and desorption using the Microsoft excel. The sorption revealed that adsorption increases with both contact time and P concentration while the trend in desorption showed that the higher the concentration, the higher the desorbed value. Sorption at 100 mg P kg-1 was 72.3, 68.1 and 74.7% higher than 20 mg P kg-1 at 30 mins, while desorption was 91.7, 73.6 and 78.5% higher at 100 mg P kg-1 in Imeko, FUNAAB and Owode respectively. In conclusion, this study showed that in all samples, adsorption and desorption increased with contact duration, indicating that 30 minutes of contact time provides a more valid estimate of the soil's phosphorus retention capacity, though, desorption values are consistently lower than adsorption. This study recommends 30 minutes contact time for optimum estimation of phosphorus availability in basement soils of southwest Nigeria.