This investigation was conducted with surface horizon samples from 7 south-western Australian soils and their 3 size fractions ( sand, silt, and clay). The K release of these materials was measured for several extractants; the highest amounts of K were released from the clay (<2 μm) fraction. The presence of sand-size feldspars and incomplete removal of attached organic matter resulted in sand releasing significant amounts of K. The proportions of total K released in boiling 1 M HNO3 by the sand, silt, and clay fractions ranged from 0.4 to 3.4%, 2.6 to 36.3%, and 11.2 to 51.4%, respectively, and from 2.0% to 22.9% for the whole soils. Cumulative K uptake by 6 harvests of ryegrass over 260 days ranged from 0.26 to 1.23 cmol/kg soil.The clay fraction released higher proportions of total K to acid compared with the sand and silt size fractions because of the high specific surface area of the clay and because it contained proportionately higher amounts of illite, which releases K by both ion exchange and dissolution, whereas K release from feldspars requires congruent dissolution of the silicate structure. The differences in contents of StepK ( relatively available fraction of the non-exchangeable K) and CRK (constant rate K) for 1 M HNO3 dissolution of these soils and size fractions reflect differences in mineralogical composition between the soils and size fractions. The low contents of StepK for the sand fraction indicated that K was strongly retained by feldspars. The soils with high CRK values had significant amounts of illite in the clay fraction. Values of CRK were positively related to cumulative K uptake and cumulative dry matter yield of ryegrass.