Phosphate adsorption in the major soil groups in Thailand was examined over the wide range of pH and equilibrium P concentration, and the characteristics of these soils in P adsorption were discussed in terms of Langmuir adsorption model and equilibrium diagrams for phosphate compounds.
　P adsorption increased steeply at pH values where buffer intensity of the soils was at its maximum. Thus, P adsorption is greatly affected by kinds and amounts of soil components responsible for H⁺ or OH^- acceptance.
　P adsorption by Grumusol increased steeply due to precipitation of calcium phosphate compounds above pH 6 when more than 75 ppm of phosphate was added. The product of actual ion concentration, Q_ST= (Ca_T)^x×(P_T)^y, also exceeded conditional solubility product, P_s= ^c*K_so(H⁺)^z/(α_ca)^x(α_H2PO4)^y around 6, and adsorption isotherm indicated that phosphate was precipitated as calcium phosphates.
　P adsorption by ASS reached its maximum around pH 5, but adsorption isotherm at this pH value showed that the precipitation of alumino-phospho compounds in the bulk solution was not significant even at 150 ppm, therefore, this is due to the formation of surface chelating compounds with hydroxy-aluminium. Rock phosphate with high content of Ca is highly recommendable as a phosphate fertilizer to ASS. P adsorption in FWA increased greatly in acidic pH region when high concentration of P was added and then decreased smoothly with increasing the equilibrium pH. This reflects structural fragility of the soil, and indicates that clay with ill-defined structure and amorphous materials are responsible for P adsorption in this soil.
　P adsorption in RBL and NCB is much less than in Grumusol and ASS. Fe contained in RBL seems to be inactive unless P concentration is high and pH is very low. So that there is no need to pay much attention in ordinary levels of phosphate fertilization.