Potassium Available to Corn Plants Extracted by Ammonium Acetate, Ammonium Chloride, Mehlich-1 and Mehlich-3 Solutions in Southern Brazilian Soils

The main Brazilian soil testing laboratories use Mehlich-1 solution for the determination of available potassium in the soil. However, this procedure has chemical and operational limitations. In search of alternatives for Mehlich-1, this study was to compare different extraction solutions, vis: Mehlich-1 and 3, and 1.0 mol L-1 ammonium chloride (NH4Cl) and 1.0 mol L-1 ammonium acetate (NH4OAc), as soil-K index to predict K availability to corn plants in southern Brazilian soils. The greenhouse pot experiment consisted of 30 soils grown with 2 corn crops. All soil-K index presented high correlation and determination coefficients between K uptake by corn plants and the K extracted from the soil. The Mehlich-1 solution was consistently better correlated with K uptake by corn plants than NH4OAc and NH4Cl but extracted less exchangeable K of the soils. Grouping soils with similar mineralogy and physicochemical properties enhances and improves the prediction of K availability for corn plants. Moreover, the K saturation index can be used as a sole indicator of K availability to plants. Overall, the results indicate that it is feasible to replace the traditional solution Mehlich-1 in the extraction of soil exchangeable K by solution of NH4Cl mol L-1 in laboratories that have an Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES).