Soil pH Survey

The pH provides a general guide to the nature of possible corrosion. Acidic soils are corrosive. Neutral soils are optimal for the development sulphate-reducing bacteria. Alkaline soils are generally benign, however, exceedingly high pH values can lead to low electrical resistivities.

Development of acidity in soils is a result of the natural processes of weathering under humid conditions. In regions of moderate rainfall, soluble salts do not accumulate except where soil waters seep to lower levels and collect in depressions. However, in regions of high rainfall, not only are soluble salts removed from the soil but the absorbed bases normally present in the colloidal materials of the soil are partially removed, and result in increased acidity. The processes eventually give rise to the condition known as soil acidity. The depth to which this leaching of the bases occurs varies with rainfall, drainage, type of vegetation, and nature of the material present.

The degree of acidity or alkalinity of a soil is expressed as the pH, a value that represents the logarithm of the reciprocal of the hydrogen ion concentration. A pH value of 7 indicates neutrality; lower values, acidity; and higher values, alkalinity. Terms used for soil classification based on pH are defined as follows:

High alkalinity lowers electrical soil resistivity and increase soil corrosivity. Certain corrosive substances in the medium (e.g., chloride ions) and mechanical effects can destroy surface films locally, leading to intensive local corrosion such as pitting and stress corrosion. Hence, maintaining adequate cathodic protection is always recommended.