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

The equilibrium phosphate sorption and release behavior and the kineti cs of adsorption were studied in some acidic soils of South and Southe ast Asia. Phosphate sorption data at 25-degrees-C were fitted to the T empkin, Freundlich, and Langmuir adsorption isotherms. The soils varie d widely in their capacity to sorb and retain P. During desorption, th e 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 fit ted to a modified Freundlich type of kinetic equation to obtain a para meter related to the sorption rate coefficient. The effect of temperat ure on the kinetics of sorption was small, suggesting the process is d iffusion controlled. Equilibrium and kinetic P sorption parameters wer e 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 compone nts 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 therm odynamic equilibrium constant for P sorption by the given soils. The r esults suggest that the stability of the P sorption reaction products in the given soils contribute to the P-fixing characteristics of these soils.