Solution potassium (So-K) and exchangeable K (Ex-K) extracted by NH4OAc are the major sources of potassium used by plants, but initially non-exchangeable K (NEX-K) can also contribute significantly to plant uptake. Perennial ryegrass (Lolium Perenne, cv. Roper) was successively planted in ten surface soil samples with different clay mineralogical composition belonging to five soil types to determine the contribution of Ex-K and NEX-K to K availability in the greenhouse during 260 d (five cuttings). Before planting, soil samples were leached to remove So-K and Ex-K so that the plants would use more K from the NEX-K pool. The leaching was performed using 0.1M chloride solution containing Ca and Mg in the molar ratio of 4: 1. The soil types having initially high NEX-K contents showed a high K uptake. A highly significant positive correlation was observed between total K uptake and illite content for the unleached soils (r=0.81, P<0.01) and the Ca/Mg leached soils (r=0.75, P<0.01). Significant declines in the plant K concentration values were observed between the first and the last cutting: from 4.1 to 2.3 g kg-1 (a drop of 45%) for the unleached soils and from 3.4 to 2.1 g kg-1 (a drop of 39%) for the leached soils. By the five cuttings of ryegrass, the concentrations of NH4OAc-K had significantly (P<0.01) declined in all unleached and leached soils. A pronounced decline in NH4OAc-K concentrations (a drop of 86% for unleached soil and 89% for leached soil) were detected in the soils belonging to Typic Calcixerepts containing high amount of illite. The NEX-K uptake by ryegrass ranged from 421 to 859 mg kg-1 for the unleached soils and from 410 to 574 mg kg-1 for the leached soils. Among the soils, NEX-K uptake accounted for up to 96% of the total uptake removed by five harvests of ryegrass herbage. For the unleached soils, NEX-K uptake was significantly (P<0.01) related to total K uptake. There were large differences in NEX-K uptake from soils that had similar NH4OAc-K values. Hence, K buffer power and soil types coupled with available K must be taken into account when planning any application of K.