Browsing by Author "Nel, Teneille"
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- ItemConversion of soil pH values and exchangeable base cation quantities determined by different methods(Stellenbosch : Stellenbosch University, 2021-12) Nel, Teneille; Hardie-Pieters, Ailsa G.; Clarke, Catherine E.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.ENGLISH ABSTRACT: Soil management recommendations based on chemical test results rely on comparisons with norm values developed using specific analytical methods and soil taxonomic classes. Conversions of soil pH and exchangeable base cation concentration between different methods are required for the standardization of data from scientific literature, soil test reports and soil databases. The combined effects of soil physicochemical properties on the accuracy and parsimony of soil pH and cation concentration conversions are unknown. Rapid soil analyses can be obtained by infrared (IR) spectroscopy, but few studies have evaluated the accuracy of IR spectral-based models to determine soil pH and exchangeable base cations in Western Cape soils. The aim of this study was to develop equations for conversion of soil pH values and exchangeable base cation concentrations between different methods of measurement. An additional aim was to develop IR spectral-based models for determination of soil pH and exchangeable base cation content of Western Cape soils. Soil pH was measured in different matrices (water, 0.1M KCl and 0.01M CaCl2) of varying soil: solution ratios (1:1, 1:2.5, 1:4 and 1:5). The exchangeable base cation content was measured according the ammonium acetate (1M, pH = 7), compulsive exchange, ammonium acetate (0.2M, pH = 7), Ambic- I, Mehlich-III, Bray-II and Citric acid (1%) methods. Models converting soil pH and cation content between values obtained by different methods were developed using simple linear regression (SLR). In addition, multivariate linear regression (MLR) equations incorporating soil physicochemical properties as parameters were developed. Spectral measurements of the soil samples were taken with three spectroscopic instruments in the near- to mid-IR range. IR spectral-based models predicting exchangeable base cation content and soil pH were calibrated using partial least squares regression. SLR equations for predicting soil pH (RMSE = 0.12-0.32) were more accurate than the conversion factors. MLR equations constructed by stepwise regression to convert exchangeable base cation content (RMSE = 1.4-1.5 cmolc kg-1, 0.4 cmolc kg-1, 16-20 mg kg-1 and 27-39 mg kg-1 for Ca, Mg, Na and K, respectively) were more accurate than SLR models. Exchangeable K content was accurately converted between the most extraction methods. Models converting base cation content may be chosen based on the purpose of application and relative size of the prediction error. IR spectroscopy is recommended to determine exchangeable Ca and Mg content in Western Cape soils (RMSE = 1.2-1.4 cmolc kg-1 and 0.37 cmolc kg-1 for Ca and Mg, respectively). Subsetting soil sample sets by similarities in texture and organic carbon content led to more accurate predictions of soil pH and exchangeable Ca content (RMSE decreased by up to 0.19 pH units and 0.79 cmolc kg- 1, respectively). In summary, use of the developed SLR as well as MLR models are recommended to convert soil pH and exchangeable base cation concentrations of Western Cape soils. Models should only be as complex as practical context requires. Moreover, IR spectroscopy may be used for determination of exchangeable Ca and Mg content in Western Cape soils.