Potassium buffering capacity and corrective potassium fertilizer recommendations in soils from Southern Brazil

Abstract

: Soils with low potassium (K) availability require corrective fertilization for grain crops. The recommended rate to increase K availability up to the critical level depends on the soil K buffering capacity (KBC). This study aimed to quantify the KBC and recalculate the rates necessary to reach the K critical level in 23 soils from Southern Brazil. Soil samples were incubated with six K rates, that were thus grouped according to CEC pH7.0 : below 7.5 cmol c dm -3 (0, 30, 60, 90, 120 and 180 kg K 2 O ha -1 ); from 7.6 to 15 cmol c dm -3 (0, 30, 60, 120, 180 and 240 kg K 2 O ha -1 ) and from 15.1 to 30 cmol c dm -3 (0, 30, 60, 120, 240 and 360 kg K 2 O ha -1 ). The soil test K (STK) was extracted by Mehlich-1 and the fertilizer rates necessary to increase the STK by 1 mg dm -3 were quantified. The KBC values were correlated with a suite of soil testing metrics. The relationship between the cation exchange capacity (CEC pH7.0 ) and KBC was modeled, and a linear-plateau equation presented the best fit. The K rates were calculated using the following equation: K rate = (Critical Level - STK) × KBC and were compared with the current rate indicated by the Local Soil Fertility Committee. The K fertilizer rates by KBC were about 2.5 times higher than the values currently indicated, and the largest differences were observed in soils with low STK and high CEC pH7.0 .