PHOSPHATE SORPTION CHARACTERISTICS OF SELECTED MAJOR GHANAIAN SOILS

Abstract

Using the Langmuir isotherm, we determined the phosphate sorption of 16 important soil series developed over major parent materials in the different ecological zones of Ghana. The phosphate sorption maximum was highest (222 to 286 ppm, mean 254) in forest soils and in one Vertisol, followed by the forest/savanna intergrade (112 to 147 ppm, mean 133), and was lowest (39 to 93 ppm, mean 69) in savanna soils. With the exception of two soil series, Akuse and Volta, all the soils had low standard P requirements (i.e., the amounts of P required to attain 0.2 ppm P in equilibrium solution). The standard P requirement followed the same trend as and was significantly correlated with sorption maximum (r = 0.854). The influence of parent material and pH, organic matter, clay, free Fe2O3, and Al2O3 content on the sorption maximum was studied by linear, partial, and multiple-regression analysis. The phosphate sorption maximum was highly significantly correlated with soil properties in the order: free Al2O3 (X5; r = 0.912), clay content (X3; r = 0.894), free Fe2O3 (X4; r = 0.830), and organic carbon (X2; r = 0.790); the relationship with soil pH (X1 per se did not attain significance. This relative importance of the effects of soil properties of P sorption was confirmed by partial regression analysis. Generally, combining two or more soil properties in a multiple-regression equation improved the prediction of the sorption maximum. However, the best prediction of P sorption could be made using the multiple-regression equation Y = 13.40 + 30.83X2 + 4.04X3 + 0.003X4 + 0.18X5 (R = 0.984) and by the more easily and frequently determined soil properties: organic carbon and clay, Y = 13.93 + 56.65X2 + 6.61X3 (R = 0.935).