The critical potassium level for fertilizer recommendation for soils in the State of Rio Grande do Sul (RS) and Santa Catarina (SC) with CEC from 5.1 to 15 cmol c dm-3 is 60 mg dm-3. However, until 2002 concentrations of 80 mg dm-3 had been used. Two experiments were carried out on an experimental area of the Department of Soil Science of the Federal University of Santa Maria (RS-Brazil) on a Sandy Typic Hapludalf under no-tillage. The objective of the study was to evaluate critical potassium levels for fertilizer recommendations for soybean, maize and sorghum. The first experiment was set up in 1991 and carried out until 2002. The main plot treatments were the application of 0, 60, 120, and 180 kg ha-1 K2O every four years, and split-plot treatments were the reapplication of 60 kg of K2O in 0, 1, 2, or 3 years. The second experiment was carried out from 1995 to 2002 and the treatments were 0, 50, 100, 150, and 200 kg ha-1 year-1 K2O. Results showed that the critical potassium level extracted with Mehlich-1 solution is 42 mg dm-3. When using the critical potassium level established by the Regional Soil Chemistry and Fertility Commission in these soils it is possible to reach over 95% of the maximum crop yield.
maximum relative yield; potassium fertilization; soybean; maize; sorghum; cover crops
