Potassium (K) availability to plants is estimated based on the exchangeable K fraction. Multielement methods increase the analysis efficiency in soil laboratories. The cation exchange capacity (CEC) of soils may affect the prediction of available K. Thus, the purpose of this study was to estimate the available K for corn and soybean in soils in the Rio Grande do Sul (RS) state using Mehlich-1 (M1), Mehlich-3 (M3), ion-exchange resin (resin), ammonium acetate pH 7.0 (AcNH4), and verify whether the CTC affects the estimated availability of soil K to plants. The experiment was carried out in microplots in the field and after 45 days of growth the shoot dry mass was quantified and K determined in the tissue. The K quantities estimated by the distinct methods were correlated, the relationship with K uptake by plants in all soils established and the soils separated in CTC classes. Resin-extracted K was about 50 % lower than that obtained by M1, M3 and AcNH4. There close correlations between the K quantities extracted by the methods. The relationship between the quantities of K uptake by the crops and the amounts extracted by the tested methods, which did not differ from each other regardless of soil CTC class. K availability to plants in soils in RS can be estimated by the methods tested in this study. There was no difference between the evaluation methods of K availability in the CTC classes. Despite the resin method being as effective as other methods in estimating available K, it has the drawback of a lower extraction capacity, which results in lower values for K availability classes and thus may favor analytical errors.
soil test methods; correlation; potassium fertilizer; soybean; corn
