Fertilizer recommendations in Rio Grande do Sul and Santa Catarina (RS and SC) states are based on studies of crops grown under conventional tillage and knowledge accumulated over the years. In this period, the crop system and soil sampling depth were altered and crop yields increased; this can affect the critical values of K, the fertility status and fertilizer quantity required by crops. This study aimed to determine the levels of available K extracted by Mehlich-1, Mehlich-3 and ion-exchange resin from no tillage soils under soybean, wheat and corn. For this purpose, the K levels of soil samples from 17 experiments were used as well as grain yield data of soybean, wheat and corn grown under no-tillage in 48 studies carried out in RS. Based on the production functions the calibration curve and fertility status were determined. Results indicated that the plant response to K fertilization followed the order: corn> wheat > soybean. The critical K values extracted with Mehlich-1, Mehlich-3 and resin were 89.5, 91; 6 and 83.8 mg kg-1 K in the 0-20 cm layer and 117.2; 116.1 and 102.5 mg kg-1 K in the 0-10 cm layer, respectively, and exceeded those recommended by CFSRS / SC (2004) for soybean, wheat and corn grown under no-tillage.
soil fertility status; critical soil K; K fertilization; soil analysis method
