Water erosion can carry away the fertilized upper soil layer and deposit it in rivers and lakes. Some part of sorbed phosphate ions will release into the water and potentially can cause environmental pollution and eutrification of surface waters. How phosphate ions are desorbed from the soil particles - sooner or later - fertilized with P - are discussed in this paper. A calcareous loamy soil of pHH20 7.9 was incubated with 0, 80 and 320 mg.kg-1 P and then equilibrated with several rates of P. After shaking and centrifugation the solid phase was shaken 12 times with distilled water and the desorbed quantity of P was calculated. Using the adsorption equation, a new model was constructed to describe the one-step and multi-step desorption. Parameters of the model help to calculate the reversible and irreversible fraction of sorbed P. Higher fertilizer P rates produced higher rates of desorbable P. This model, developed by combining a modified Freundlich equation and the mass balance equation, makes it possible to describe multi-step desorption and the calculation of potentially desorbable P in soil.