Potassium uptake by plants is generally higher than the initial amount of exchangeable potassium, indicating that non-exchangeable potassium forms participate in supplying plants with this nutrient. The objective of this study was to evaluate the potassium depletion by successive cultivations in a sandy-textured soil, with and without potassium fertilization in the past. The greenhouse experiment was carried out at the Universidade Federal de Santa Maria, in southern Brazil. Samples (0-0.10 m) of a Typic Hapludalf, without and with previous potassium fertilization, in pots, received doses of 0, 90 and 270 mg kg-1 of potassium and were cultivated for five successive crop cycles in pots. At each harvest, the shoott was cut at the soil surface and a soil sample taken from the 0-10 cm layer. The shoot was oven-dried, weighed and the potassium concentration determined. The soil was oven-dried and exchangeable potassium extracted by Mehlich-1; non-exchangeable potassium by NaBPh4, and boiling 1 mol L-1 HNO3; and total potassium by HF. The capacity of the soil to provide plants with potassium depends more on the soil characteristics themselves than on the previous sequence of potassium fertilization. Potassium uptake by plants triggers a continuous depletion process of potassium in different forms in the soil; potassium depletion is more marked when the available potassium concentration is lower.
potassium fertilization; potassium forms; potassium extractors
