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 (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 modd 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-water suspensions oflow 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.