The kinetics of phosphorus (P) release from a calcareous sandy soil amended with fertilizer or sewage sludge (SS) was studied using batch and column-leaching experiments. Activities of free and ion-associated species were computed from their measured concentrations using VMINTEQ software. Double-function plots of P potential versus Ca, Al, or Fe potentials indicated that time of equilibration and the soil to solution ratio, strongly affected the solution status in respect to various P minerals. The trends of P potential compared to the stability lines of these systems indicated that increase in equilibration time could compensate for the dilution effect of soil. The simultaneous changes in the Gibbs free energies of Ca-P, Al-P and Fe-P systems well described the dynamics of the P-leaching process: there were often reversals in the increase of the Gibbs free energy of the Ca-P during P release, and each temporal decrease was accompanied by an increase in the Gibbs free energy of Al-P and/or Fe-P, regardless of the source of the soil-P amendment. These results confirmed that no single phosphate mineral controlled the entire process of P leaching: both Al-P and Fe-P played a significant role, even in the high-pH calcareous soil.