POTASSIUM SUPPLY AS AFFECTED BY RESIDUAL POTASSIUM FERTILIZATION IN A CERRADO OXISOL

POTASSIUM SUPPLY AS AFFECTED BY RESIDUAL POTASSIUM FERTILIZATION IN A CERRADO OXISOL In some soils with low levels of exchangeable potassium, non-exchangeable forms can participate in plant supply. There is evidence that K supply in non-exchangeable forms depends more on plant nutrient demand than on soil properties. The purpose of this study was to evaluate the supply and depletion of K forms in a Cerrado Oxisol, as affected by residual K fertilization and successive cultivation of Brachiaria ruziziensis (Syn. Urochloa ruziziensis). Soil samples were collected in the 0-20 cm layer in plots of a field experiment installed in 2001, where soil had been fertilized for 10 years with 0, 60, 120 and 180 kg ha(-1) of K2O. In a greenhouse, 0, 150 and 300 mg dm(-3) K was applied to the soil samples and B. ruziziensis was grown and cut five times. The K supply of the plants depended more on the newly added fertilizer than on the residual effect of previous fertilization. The non-exchangeable K is responsible, over time, for the maintenance of exchangeable K levels and becomes the main source of K for B. ruziziensis plants in the absence of K fertilization. Brachiaria ruziziensis plants have the capacity to absorb large amounts of K from the soil, confirming the importance of this cover crop for soil nutrient cycling.