The relationships between different pools of K, i.e. exchangeable, HNO3-extractable, mineral and total, were investigated as a function of clay mineralogy and soil development in soils of Kharkeh Research Station, Kurdestan Province, western Iran. Samples from different horizons of ten pedons were selected and analysed for clay mineralogy and K fractionation. X-ray diffraction patterns revealed that clay minerals in the soils studied were similar in type, while their abundances were different. The smectite content was significantly greater in Vertisols than in the other soils. The results of K fractionation showed that mineral K and HNO3-extractable K (exchangeable and non-exchangeable, respectively) and the clay content of the soils containing lesser illite (10–30%) were significantly different from those with more illite (30–50%). Moreover, the regression slopes between water-soluble and NH4OAc-extractable K were lower in soils with more smectite due mainly to the presence of larger specific surface areas for K sorption in smectitic soils. Based on soil evolution and different forms of K, the soils studied were grouped in two major categories: (1) Vertisols and (2) Entisols, Inceptisols and Mollisols. There were greater contents of all forms of K in Vertisols than in the other soil orders. This was mainly related to the greater clay content and the dominance of smectite in the clay fraction of Vertisols.