Fifteen soil samples were collected from Zhejiang and Jiangsu provinces to study the mechanism of phosphate sorption by soils in relation to the kinds of canon of supporting electrolyte and the change of soil pH values through equilibrium.Increasing the pH of strong acidic soil by adding base resulted in the,hydrolysis of exchangeable Al and formation of polymeric hydroxy-Al species which increased the sorption of added P,The reaction of phosphate with acidic soil might promote the hydrolysis of exchangeable Aland release lmoton,and result in drop in the pH of the system.There might be calcium phosphates formation in soils with P addition n 0.01 mol CaCl₂ when pH>6,causing the equilibriatrn concentration of P to decrease markedly.Organic matter had an important effect on P sorption by soils.At iow pH,organic matter might inhibit the hydyolysis of Al⁸⁺ in solution through Al-organci complex formation.Mceanwhile,the organci anions might compete with phosphate for reaction sires on the surface of hydroxy-Al species.Both of the mechanisms meantioned above leaded to the decrease of P sorption in a cidic soils.