Use of models for phosphorus adsorption on some sodic soils of Punjab [Pakistan]

Langmuir, Freundlich, two surface Langmuir and Linear adsorption model were applied on seven different textured sodic soils varying in their P adsorption capacity, in order to characterize the P adsorption isotherms for these sodic soils. Langmuir and Freundlich models prove equally good for these sodic soils based on correlation coefficient values. In soil 1, 2, and 3 Langmuir models showed its superiority over the Freundlich model. While in soil 4, 6, and 7, Freundlich model proved better. However, in soil 5 both the Langmuir and Freundlich model came at par with each other. Langmuir models showed two straight lines in each soil. Two surface Langmuir model proved better in the region II (average r=0.7268), while in region I (average r = 0.6900) results were non-significant except soil 4 and 5 with r-values 0.9270** and 1.000**. Bonding energy constants calculated by two surface Langmuir model were high in region I while low in the region II and vice versa. It means that a small amount of P in region I held firmly while a slightly greater amount of P held less firmly in region II. This indicated that two group of sites exits in these sodic soils. One group was got activated at lower P concentration while other group became active at slightly higher P concentration. A parallel relationship between the pH of the equilibrium P solution (EPS) and percentage adsorption was noted. This indicated an inverse relationship between pH and soil P saturation. Similarly, an inv relationship was inferred between soil P saturation and % adsorption of P. It was observed that with increasing P saturation of the soil components, OH may be released. Langmuir and Freundlich adsorption parameters (KL, b, Kf, and 1/n) significantly correlated with sand, silt and organic matter percent contents. These soil properties may be responsible for P adsorption in these sodic soils. Freundlich Kf significantly correlated with Langmuir adsorption maximum (b) value (0.898*), which may made possible for one to be able to calculate adsorption maximum value where Langmuir model got failed to justify itself Iike in soil 6 in this study.