A kinetic-mechanistic model is P sorption in calcareous soils was developed. Model construction was based on batch sorption experiments in which three forms (phases) of sorbed P were defined: Pwe, water-extractable sorbed P; Pbic, bicarbonate-extractable sorbed P; and Pn, nonextractable sorbed P. The form Pwewas associated with equilibrium sorption and described by a Freundlich-type equation. Pbicwas shown to consist of a fast, irreversible sorption term and a time-dependent precipitation reaction of slightly soluble Ca-P compounds (e.g., DCPD, DCP, or OCT). Pnswas also shown to consist of a fast sorption term and a time-dependent term associated with precipitation of sparingly Ca-P compounds, e.g., HA. The precipitation reactions in the calcareous soil were assumed to be controlled by the Ca-P-CO2system.