Measurement of pH
In WA, soil pH should be measured in a calcium chloride solution. Water pH values will be higher.
The most accurate method of soil pH measurement will be achieved in a professional laboratory (Figure 2). The Australasian Soil and Plant Analysis Council Inc can provide a list of accredited laboratories. The accepted standard technique is to measure the pH of soil in a weak solution of calcium chloride (CaCl2). A ratio of one part soil to five parts 0.01M CaCl2 is used. pH measured by this method is commonly indicated as pHCa.
This method overcomes the problems of seasonal variation in soil pH when measured in water, especially in soils with low total salts. Soils vary in the concentration of salts such as calcium, magnesium, sodium and potassium chlorides, nitrates and sulfates.
The concentration of salts also varies as the moisture content of the soil varies. The impact of these variations on pH is minimised when measured in 0.01M CaCl2 and allows valid comparisons of soil pH between years.
Soil pH measured in water (pHw) can be 0.6-1.2 pH units higher than in calcium chloride (Figure 3). If conversion is necessary, 0.7 is usually deducted from the water value.
In this page and other DPIRD pages on soil acidity all references to soil pH are measured in calcium chloride, unless otherwise stated.
Appropriate sampling of soil for pH testing is vital for meaningful results. For further information on soil sampling techniques see Diagnosing soil acidity.