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/warer ratios of the suspensions ranging from saturated paste to 1:5 are used in research and commercial sod 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 sods between +/-0.10 pH units of the measured pH(1:1w). Results for individual sods 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-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) CaCl(2) solution at a soil/water ratio of 1:1 is a more robust measurement for soils of North America.