Potassium buffer capacity in subtropical soils with high organic matter content

ABSTRACT Subtropical soils can have medium to high soil organic matter levels, which directly affects cation exchange capacity (CEC) and potassium (K) availability to plants. This generates the need to define K buffer capacity (KBC) values and K corrective rates to improve the fertilization recommendation system for these soils, ensuring high yields and reducing the likelihood of K losses in the environment. This study aimed to define KBC values and the K rates to be applied in corrective fertilization of acidic soils with high SOM and CECpH7.0 in a subtropical climate, and evaluate K corrective rates in vineyard soils in relation to the regional fertilization recommendation. Thirty-one native forest soils from the subtropical climate of Serra Gaúcha (RS), Southern Brazil, were collected. The samples were incubated with ten rates of K2O for 30 days, with three replicates. After the incubation period, available K (K_M1) contents were obtained by the Mehlich-1 extractor. The K_M1 contents of the 0.00-0.20 m layer of 209 vineyards in the region were evaluated in the 2021/22 and 2022/23 crop seasons. Natural K_M1 contents in 97 % of the forest soils evaluated were classified as "high" and "very high" availability. Average KBC value obtained was 3.1 kg ha-1 K2O, indicating the nutrient rate required to increase the K_M1 content by 1.0 mg dm-3. Correction rates for the low and medium K availability classes were, respectively, 170 % higher and 78 % lower than the current regional fertilization recommendation for fruit trees. The K_M1 contents in 38 % of the vineyard soils in the study region were classified as "very high" availability, indicating the possibility of reducing the use of K fertilization. We recommend adjusting the K2O rates based on KBC values, aiming to maintain K_M1 contents in the "high" availability class, reducing production costs and unnecessary K nutrient applications.