MODELING PHOSPHATE-SORPTION KINETICS IN ACID SOILS

A model of P-sorption kinetics was developed, that accounts for the de pendency of reaction rate on concentration, sorbed P content, sorption maximum and time. The sorption phenomena predicted by the model agree with available observations in the literature. The model was tested w ith batch experiments for 17 acid top- and subsoils. The experiments r evealed a significant correlation between the sorption maximum, F-m, a nd the sum of amorphous iron and aluminium in soils. Plotting the dime nsionless P-sorption ratio, F(c, t)/F-m, which may be interpreted as t he fractional saturation of the sorption capacity of a soil, against t he natural logarithm of the exposure variable, I, gave S-shaped curves . Different parts of the S-shaped curve can be experimentally assessed , depending on the initial fractional saturation of the P-sorption cap acity of soils. Apart from this dependency, one set of parameter value s sufficed to describe the sorption kinetics of 10 different sandy top - and subsoils. For non-sandy soils, the parameter values differed and depended on the initial P content. The model enables extrapolation to long times, which is necessary for applications to field conditions.