PHOSPHATE SORPTION-DESORPTION BEHAVIOR OF SOME ACIDIC SOILS OF SOUTH AND SOUTHEAST-ASIA

The equilibrium phosphate sorption and release behavior and the kinetics of adsorption were studied in some acidic soils of South and Southeast Asia. Phosphate sorption data at 25-degrees-C were fitted to the Tempkin, Freundlich, and Langmuir adsorption isotherms. The soils varied widely in their capacity to sorb and retain P. During desorption, the amount of sorbed P at a given equilibrium P concentration was always higher than that during sorption, indicating low desorbability of the sorbed P. The kinetic P sorption data at 21 and 35-degrees-C were fitted to a modified Freundlich type of kinetic equation to obtain a parameter related to the sorption rate coefficient. The effect of temperature on the kinetics of sorption was small, suggesting the process is diffusion controlled. Equilibrium and kinetic P sorption parameters were found to be closely correlated with clay and organic matter contents and surface area of soils. A negative correlation between specific P sorption by clay or organic matter and clay or organic matter content of the soils suggests an association between the two colloidal components of the soils, causing loss of effective P-sorbing surface area. An attempt was further made to evaluate the differential change in free energy of P sorption at infinite dilution from the corresponding thermodynamic equilibrium constant for P sorption by the given soils. The results suggest that the stability of the P sorption reaction products in the given soils contribute to the P-fixing characteristics of these soils.