The potassium status of 22 soils with low reserves of non-exchangeable potassium was assessed by measuring: (a) the potassium potential (the free energy of exchange of potassium for calcium plus magnesium); (b) the soil’s capacity to resist a change in potential (“buffering capacity”); (c) the amount of exchangeable potassium and its percentage saturation. The ability of these measures to predict the supply of potassium was tested by growing subterranean clover (Trifolium subterraneum L.) in pots and determining its potassium content at successive harvests. The soil’s buffering capacity was found to affect the supply of potassium even when uptake was small. It is suggested that this was partly because buffering capacity affects the rate of supply by diffusion to plant roots. As the uptake of potassium became progressively larger, the potential became increasingly unsuitable as a single index of availability, but a multiple regression containing terms for potential and for buffering capacity continued to account for a large proportion (89%) of the variation. The relationship obtained can be used to indicate the amount of potassium required to bring a deficient soil to a desirable status.