Agricultural and Environ. Services Labs

Soil pH, one of the most routine measurements performed, is used to interpret chemical reactions, nutrient availabilities, and the rates of many biological processes in soils. Predominantly, soil pH is measured in soil–water suspensions using a glass H +-sensing electrode and a reference electrode. Soil/water ratios of the suspensions ranging from saturated paste to 1:5 are used in research and commercial soil testing laboratories. The objective of this study was to evaluate relationships between pH measured in saturated paste extract (pH sp), 1:1 and 1:2 soil/water suspensions (pH 1:1w and pH 1:2w), and 1:1 and 1:2 soil/0.01 mol L −1 CaCl 2 suspensions (pH 1:1CaCl2 and pH 1:2CaCl2). In addition, the effects of soil suspension electrical conductivity (EC 1:1w) on measured values of soil pH were assessed. Analytical results for 120 agricultural soil samples from a laboratory proficiency program database indicated that the relationship between pH 1:1w and pH 1:1CaCl2 was highly significant (r 2 = 0.984). Similar regressions were found between pH sp and pH 1:1CaCl2, and between pH 1:2w and pH 1:2CaCl2. The EC 1:1w explained 85.1 % of the variation in the difference between pH 1:1w and pH 1:1CaCl. A model comprised of a linear term of pH 1:1CaCl and a nonlinear term EC 1:1w explained 99.5 % of the variation in soil pH 1:1w, with the estimated pH of 88 % of the soils between ±0.10 pH units of the measured pH 1:1w. Results for individual soils indicated that a nonlinear function using matrix EC explained>98 % of the variation in soil pH across a range of soil/water ratios and suspension matrices ranging from 0.00 to 0.01 mol L −1 CaCl 2. It is our assertion that the current practice of determining soil pH in soil–wate