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 K₂O ha^-1); from 7.6 to 15 cmol_c dm^-3 (0, 30, 60, 120, 180 and 240 kg K₂O ha^-1) and from 15.1 to 30 cmol_c dm^-3 (0, 30, 60, 120, 240 and 360 kg K₂O 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: $K\text{rate}=(\text{Critical Level}-STK)\times KBC$ 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.

potassium fertilization; critical potassium level; rate adjustment; southern Brazil