The total K content of the hilly upland soils in Zhejiang Province ranged from 0.57% to 3.13% and increased with increasing contents of the feldspar and mica in parent rock. The contents of readily available K and slowly available K were 33-216 ppm and 80-838 ppm respectively which were correlated with the relative contents of kaolinite and illite in the soil clay.By means of chemical successive extraction, electro-ultrafiltration (EUF) and quantity-intensity (Q/I) studies, it was found that the potential bufferring capacity (PBC) and supplying power of potassium were lower in those soils with clay minerals dominated by kaolinite, bw higher in the soils dominated by illite. The ryegrass pot experiments revealed that the soil potassium supplying power was more important than the reserve of the available K.