Modeling the movement of phosphorus in some selected agricultural soils in Nigeria

This research investigate clayey, lateritic and sandy soils have been selected from agricultural lands in the South-Eastern Nigeria, for their phosphate sorption characteristics using column experiments, in order ascertain the movement of phosphate in the soils. The soil samples were equilibrated in 25ml of 0.01M CaCl2  containing different concentrations of P as KH2PO4 to produce initial concentrations of 250, 375, 500, 625 and 750µg/L of P for 14 days at room temperature. Six samples and a backup treatment were prepared for each concentration for each soil sample giving a total of 25 samples of each soil. Fitting the adsorption results from all the soil tested into Freundlich, Langmuir and Van Huay linear isotherms revealed that the adsorption of phosphate increased with the concentration of P and contact time. Values of P sorption maximum (Smax)  for all the soils were obtained in day 10 and were 12, 15 and 12 mg/kg soil for clayey, lateritic and sandy soil respectively. The higher sorption of lateritic over clayey soil was due to its high content of clay and clay minerals. The Freundlich isotherm perfectly fitted the clayey soil with the highest accuracy than the Langmuir and Van Huay isotherms, and also gave better description of the lateritic soil but did not adequately describe the sandy soil like the other isotherms. Comparison of the three isotherms showed that the Freundlich isotherm was the best for predicting the phosphorus adsorption behaviour in soils. The leaching of P in the soil column experiment was time-depth dependent and therefore, the use of slow phosphate fertilizer and lime could improve p-adsorption on the soil.