The use of cation activity ratios to estimate the intensity of soil acidity

Soil solution extracts can be relatively difficult to obtain and to handle. The aim of this study was to investigate whether a simple 0.01 M CaCI, extraction could be used to estimate the intensity factor of soil acidity by employing the Schofield Ratio Law. Activity ratios of H, Ca and A1 were compared in soil solutions and in 0.01 M CaCI, extracts on four soils adjusted to a range of pH values with acid and lime. The ratios pH-1/2pCa (0.01 M CaCl₂) and pH-1/2p(Ca + Mg) (soil solution) were not statistically significantly different. The ratio pH-1/3pAl was less strongly correlated, but again was not significantly different between the soil solution and extractant. The Schofield Ratio Law did not hold over the entire range of data for pCa-+2/3pAl, but the values in the two solutions correlated closely. Cation activity ratios in 0.01 M CaCl₂ can be used to estimate the intensity factors of acidity in a soil, even though the extractant has a higher ionic strength than many soil solutions in variable charge soils. Therefore pH and A1 as measured in 0.01 Cacl₂ extracts may be useful tests for routine diagnostic purposes and soil surveys.