ABSTRACT
Soil pH is a simple measurement that requires little equipment and that can give some insight into soil composition through acidity and alkalinity. The measure of acidity or alkalinity of soils in a solution of distilled/deionized water is given as pH, which is a measure of the hydrogen ion concentration. pH is defined as the logarithm of the reciprocal of the hydrogen ion concentration, i.e., pH = log(1/H+); where H+ = moles of H+ per L. For each whole number decrease in pH, there is an increase in the number of hydrogen ions present by a factor of 10. The pH of soils may provide a preliminary indication of the soils’ corrosivity as it might affect steel, such as used in driven piles, buried pipes, anchors, etc. Solutions with a pH less than 7 are acidic and solutions with pH greater than 7 are basic. Pure water has a pH of 7 or neutral:
pH = log(1/0.0000001 = log(1000000) = 7
These tests are simple to perform, and results are obtained in about an hour. pH is measured on a 1:1 soil:water mixture and should be measured at room temperature, approximately 20°C. The complete test procedure measures pH in both a soil-water solution and a soil-0.01M calcium chloride solution. According to ASTM D4972, both liquids are necessary to fully define a soil’s pH; however, many laboratories only perform pH using distilled water. A standard test method for determining the pH of water is given in ASTM D1293.
