Soil pH

The pH scale

Soil pH is used to indicate the acidity (or alkalinity) of soil, and is a measure of the concentration of hydrogen ions (H+) in the soil solution. pH is measured from 1 (acidic) to 14 (alkaline), with 7 being neutral and is measured on a negative logarithmic scale (base 10).

The lower the pH, the higher the acidity (Figure 1). Most plants are favoured by a pH_Ca (measured in a 0.01M CaCl₂ solution and denoted as pH_Ca) between 5.5 and 8. Changes in soil chemistry and microbiology when pH is below or above this range adversely impact plant processes resulting in reduced growth and yield.

Figure 1 Examples of where common substances fit on the pH scale

Because of the logarithmic scale, soil with a pH of 4 is 10 times more acidic than a soil with a pH of 5, 100 times more acidic than a soil with a pH of 6 and 1000 times more acidic than a soil with a pH of 7. This means that a small decrease in soil pH results in a large increase in acidity. For example, there is 2.5 times more acid at pH_Ca 4.4 than at 4.8. This small, 0.4 of a unit drop from the recommended minimum subsurface pH of 4.8 would result in aluminium toxicity to plant roots in most Western Australian soils.

Soil pH buffering

Sandy soils acidify quicker because of the lower buffering capacity but the pH can be recovered faster with the application of less lime compared to clay soils.

The buffering capacity of a soil indicates the capacity of the soil to resist pH change. Hydrogen ions in soil are present both in the soil solution and adsorbed onto the soil surfaces. pH measures the concentration of hydrogen ions in the soil solution. Soils differ in the number of surface sites able to accommodate hydrogen ions. Soils with large numbers of sites able to hold hydrogen ions are able to resist change in the concentration of hydrogen ions in the soil solution and therefore have a high buffering capacity.

Soils with a high proportion of clay or organic matter have a larger number of surface sites able to hold hydrogen ions and are able to resist a decrease in pH. However, once acidic, highly buffered soils are able to resist an increase in pH. When hydrogen ions in the soil solution are neutralised by lime, hydrogen ions from the soil surfaces are release into the soil solution to maintain equilibrium and resist increase in pH. Better buffered soils are slower to acidify but require more lime to lift pH when they do acidify.

Clays are generally better buffered than loams, which in turn are better buffered than sands (Table 1). Poorly buffered sandy soil types comprise more than 40% of the agricultural land in the south-west of WA Department of Primary Industries and Regional Development (DPIRD) soil-landscape map unit database, 2013.

The naturally acidic peaty sands of the south coast have a high buffering capacity and would require more lime to increase pH than other wheatbelt soils.

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 pH_Ca.

Figure 2 The most accurate soil pH measurements will be achieved in a commercial laboratory. Photo: CSBP

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 (pH_w) 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.

Figure 3 Comparison between pH measured one part soil to five parts water and pH measured one part soil to five parts 0.01M calcium chloride in WA wheatbelt soils