Calculating soil titratable acidity from routine soil analyses of cultivated South African soils using various model structures

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Soil titratable acidity (TA) is determined through extraction with an acetate-buffered salt solution (potassium sulphate, K2SO4) and is routinely used to predict the soil lime requirement using the Eksteen method. Soil TA is not always reported by commercial soil testing laboratories, but rather exchangeable acidity (1M KCl) is determined. Therefore, a need exists to use other routinely determined soil properties in a model to derive TA. In this study, routine soil-analysis results from a commercial laboratory (n = 5 915 measurements) were used to generate various model structures to calculate a theoretical value of TA and to evaluate the accuracy thereof. Measured TA was found to be significantly correlated to the calculated TA using the Eksteen R-value, soil pH, organic carbon percentage, exchangeable Ca2+ and Mg2+, and a function of the interaction between pH and organic carbon. Soil TA could be calculated most accurately using these factors in multivariate adaptive regression splines model (r 2 = 0.69, mean absolute error = 0.16) at TA values of < 2 cmolc kg−1. However, the exponential model calculations remained stable over a larger range of actual TA values up to 4 cmolc kg−1. Given the ease of use and interpretability, it is recommended that an exponential function model is used to calculate TA.