Comparison of Soil pH Methods on Soils of North America

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 (pHsp), 1:1 and 1:2 soil/water suspensions (pH1:1w and pH1:2w), and 1:1 and 1:2 soil/0.01 mol L−1 CaCl2 suspensions and . In addition, the effects of soil suspension electrical conductivity (EC1:1w) 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 pH1:1w and was highly significant (r2 = 0.984). Similar regressions were found between pHsp and , and between pH1:2w and The EC1:1w explained 85.1% of the variation in the difference between pH1:1w and pH1:1CaCl A model comprised of a linear term of pH1:1CaCl and a nonlinear term EC1:1w explained 99.5% of the variation in soil pH1:1w, with the estimated pH of 88% of the soils between ±0.10 pH units of the measured pH1:1w 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 CaCl2 It is our assertion that the current practice of determining soil pH in soil–water suspensions of low ionic strength results in a significant analytical measurement error associated with liquid junction potentials of a glass reference–H+ electrode system. For soil diagnostic analysis purposes, we concluded that the measurement of soil pH using 0.01 mol L−1 CaCl2 solution at a soil/water ratio of 1:1 is a more robust measurement for soils of North America.