KINETIC-STUDY OF PHOSPHORUS DESORPTION BY ALFISOLS AND ENTISOLS
The kinetics of phosphorus (P) desorption by six Greek soils, four Alfisols and two Entisols, have been studied. By the addition of several amounts of P fertilizers at different water/soil rations and different contact times, that amount of P fertilizer required for the two major categories for Greek soils in order to avoid enviromental pollution from overdoses was determined. The experimental data lead to the same desorption model as given by Sharpley (1981): Pd = kPot(partial derivative)w(b); where, Pd is the amount of P desorption from the soil in mug P/g soil, Po the amount of P fertilizers in grams added into the soil, t the contact time of P fertilizers with the soil, w the water/soil rations, and k, partial derivative, and b constants. The values of the partial derivative, k, and b constants for each soil are, nearly the same for all the experimental conditions. For the Entisols, the partial derivative values are lower than for the Alfisols. Calcium carbonate (CaCO3) and exchangeable Ca of the Entisols influenced negatively the desorption-time relationship, while exchangeable Fe and Mn, and the low CEC of the Alfisols had a positive influence. For the Entisols, the b values are higher than for the Alfisols, except for the Agrinio B soil. The clay silty soils with high CECs and low exchangeable Fe and Mn had a positive influence on the the desoption water-soil relationship. Since the values for the partial derivative, K, and b constants for each soil are almost the same, the average of these values can be used in the Shapley model for the determination of desorbed P for each agricultural use for the two major categories of Greek soils.