Selectivity of K adsorption was studied using an approach of K-Ca exchange equilibrium on soils with a range of chemical and physical properties,including Latosol,Red soil,Lou soil,Black soil,and Paddy soil.The results indicated that the values of K_G and K_V varied with the ratios of K to Ca in adsorbed phase.The phases of curves suggested that sites differing in affinity for K existed on soil colloids.At low K saturation,the high select ivity for K can be attributed to wedge sites existing in the clay minerals.The number of wedge sites increased in the order of Black soil > Lou soil > Paddy soil > Red soil > Latosol.The number of sites was related to the occurrence of weathered mica and vermiculite.At high K saturation,the K G increased in the order of Latosol > Red soil > Black soil > Lou soil > Paddy soil,which was greatly influenced by the origin of charges on colloids.