How the University of Georgia Soil, Plant, and Water Laboratory
Measures and Reports Soil pH Results

How we measure pH.

Beginning November 1, 2004, the University of Georgia Soil, Plant, and Water Laboratory implemented a new procedure for soil pH. We now measure soil pH in a 0.01 molar calcium chloride solution, rather than in deionized water. All other aspects of the procedure are the same as before. In the procedure, 20 cm³ of soil is placed in a 3-ounce wax-paper cup with 20 ml of 0.01 molar calcium chloride solution, which is allowed to equilibrate for at least 15 minutes. The soil pH is then measured using a LabFit AS-3000 Dual pH Analyzer.

Why we changed methods.

Soil pH measurements using deionized water are unstable and unreliable when soil salts are low. We have found the measurement of pH in 0.01 molar calcium chloride to provide more stable pH readings between years and during a season, because it eliminates the effect of changing salt levels on the pH reading that result from fertilizer and manure application and from leaching rains. The effect of naturally occurring salt on soil pH is explained in more detail in University of Georgia Cooperative Extension Circular 875, Soil Testing: Soil pH and Salt Concentration.

What we found in our research.

In a study of over 3000 soil samples submitted to our laboratory, we found the pH measured with the new method (0.01 molar calcium chloride) to be 0.6 pH unit lower (on average) than when measured in our previous method using deionized water.

How we report soil pH.

We report soil pH by adding the value of 0.6 to the value we measure by the new method. For example, when the measured value of pH by the new method is 5.4, we add 0.6 to the measured value, and report 6.0 on the soil test report. Reporting pH this way allows the new method values to be compared to the traditional method for measuring pH and on the same basis as the recommended pH ranges for various crops and horticultural plants.