Effect of soil properties on the kinetics of phosphate release

The kinetics of the phosphate release by 35 non‐calcareous soil samples were investigated by consecutive extraction of the soil. The soil samples covered a wide range of texture, pH and degree of phosphate saturation. For all soils, phosphate release initially followed first order kinetics. With time, the process became linear to the logarithm of time. These relationships were used to distinguish between the initial fast process of phosphate desorption and the much slower diffusion‐dissolution process. The influence of physico‐chemical soil properties on the parameters describing the two processes was investigated using multiple regression analysis. The total amount of reversibly adsorbed phosphate was highly significant correlated to the oxalate extractable phosphate 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 desorption rate. The kinetics of the final diffusion‐dissolution process was most related to the oxalate extractable phosphorus content of the soil. Under the prevailing experimental conditions, the reversibly adsorbed phosphate is completely released after eight days only whereas it will take almost two years to solubilize all the precipitated P. Using the degree of phosphate saturation (DPS) instead of Pox for estimating the reversibly adsorbed phosphate results in regression models with about the same determination coefficients.