The hypothesis that exchangeable potassium is the only available source of this element for Brachiaria decumbens Stapf. was tested in a pot experiment. Surface and B horizon samples of Latosols and Podzolic soils were used. Soil exchangeable potassium in the beginning and at the end of the experiment was measured and the amount of potassium taken up by the above-ground parts of the plants was calculated. Potassium uptake by the plants was generally higher than the change in the amount of soil exchangeable potassium by about 50% for the surface soil samples and more than twice for samples of the B horizon of the Podzolic soils. Nevertheless, a high correlation was observed between uptake of potassium and exchangeable potassium for 23 out of the 24 soil samples, with a correlation coefficient of 0.91. It was concluded that part of the non-exchangeable potassium is available for plants in the soils studied.