EFFECT OF SOIL PROPERTIES ON THE KINETICS OF PHOSPHATE RELEASE

The kinetics of the phosphate release by 35 non-calcareous soil sample s were investigated by consecutive extraction of the soil. The soil sa mples covered a wide range of texture, pH and degree of phosphate satu ration. For all soils, phosphate release initially followed first orde r kinetics. With time, the process became linear to the logarithm of t ime. These relationships were used to distinguish between the initial fast process of phosphate desorption and the much slower diffusion-dis solution process. The influence of physico-chemical soil properties on the parameters describing the two processes was investigated using mu ltiple regression analysis. The total amount of reversibly adsorbed ph osphate was highly significant correlated to the oxalate extractable p hosphate of the soil: the former approximated 33 % of the latter. The rate constant of the initial desorption process was mainly influenced by the pH of the soil: increasing the pH positively affects the desorp tion rate. The kinetics of the final diffusion-dissolution process was most related to the oxalate extractable phosphorus content of the soi l. Under the prevailing experimental conditions, the reversibly adsorb ed phosphate is completely released after eight days only whereas it w ill take almost two years to solubilize all the precipitated P. Using the degree of phosphate saturation (DPS) instead of P-ox for estimatin g the reversibly adsorbed phosphate results in regression models with about the same determination coefficients.