Salt Concentration and Measurement of Soil pH

The measured value of soil pH depends in part on the laboratory procedures used, such as the soil-solution ratio and soil solution electrolyte composition. One of the most significant factors affecting the measured value of soil pH is the electrolyte concentration of the soil solution. Since electrolyte concentration of agricultural soils can vary greatly during the year and between years, the date of sampling can result in highly variable pH values for samples with the same percentage of base saturation when soil pH is measured in deionized water. For example, we found a different relationship between extractable calcium (Ca) and pH (1:1 in deionized water) for about 18,000 soil samples from the same geographic area taken during winter of 2 years, differing in winter rainfall. On average, samples taken during the wetter year had higher pH for a given value of extractable Ca, consistent with a reduced ionic strength (more leaching) in the wet year. In a comparison of pH in water with pH in 0.01M calcium chloride (CaCl2) for 1,186 soil samples received from clients, the median difference in pH was 0.67. It is notable that 20% of the samples had a difference of 0.8 and 10% had a difference of 0.9pH units. Some samples with differences larger than the median may not receive a lime recommendation when needed because of the erroneously high pH reading in water caused by low ionic strength. The stability of pH readings in 0.01M CaCl2 essentially eliminates this problem.