The release of phosphorus from secondary phosphate rock is influenced by the Ca-P-adsorbing layer on surface of the rock. Washing with NaCl, KCl or NH₄Cl to remove adsorbed Ca on the surface of phosphate rock resulted in a large increase in P extracted subsequently with water. It was suggested that the removal of adsorbed Ca could accelerate the dissolution of apatite by creating a sink for Ca at the solid surface.