Models relating soil pH measurements in H2O, KCl and CaCl2 for volcanic ash soils of Cameroon

Soil pH, apparently the most routinely measured soil property is an important indicator of soil quality, serves as a guide for fertilizer recommendations and liming requirements, and is an index of biogeochemical processes in terrestrial ecosystems. This study was conducted to establish relationships among different pH measurements of surface soils (0-20 cm) derived from volcanic ash parent materials in different electrolytes and soil-to-solution ratios following standard methods. Soil pH increased with increase in solution volume and for a particular soil-to-solution ratio; the trend in pH values was pH H2O > pH 0.01 M CaCl2 > pH 1 M KCl. Electrical conductivity values were very low (mean = 0.04 ± 0.01 dSm−1) and were highly and negatively correlated with pH H2O and pH CaCl2. pH CaCl2 values were on average 0.43 units lower than those measured in water, while those measured in KCl were on average 0.73 units lower than those measured in water. Models relating different pH and EC measurements (n = 38) yielded R2 values ranging from 0.774-0.978. Model validation using a dataset of 59 points gave R2 values ranging from 0.843 to 0.958; RMSE from 0.06-0.09; MAE from 0.05-0.09 and index of agreement from 0.75-0.78. The models developed in this study are useful in quality control of pH measurements, for converting pH values of volcanic ash - derived soils in different electrolytes and soil-to-solution ratios and contribute to information necessary for the development of soil data bases at local and regional levels.