Measuring soil acidity
Soil pH is the most widely used and simplest method for assessing soil acidification and when to apply lime.
The pH of a solution is the negative logarithm of its hydrogen ion concentration. As the concentration of hydrogen ions increases, pH values decline.
Soil pH was traditionally measured in a suspension of 1 weight of soil to 5 volumes of water. However, the salt content of the soil has an influence on soil pH measured by this method. If salt is added to a soil-water suspension, the pH decreases. To overcome this problem, a salt solution of 0.01 molar calcium chloride in now used instead of water. All pH values quoted in this Bulletin were measured in calcium chloride — pHCa.
The standard depth used in WA when sampling for soil nutrients and acidity is 10cm. If subsurface acidity is considered important, samples should also be collected from 10–20cm and from 30–40cm.
Measuring aluminium concentrations when measuring soil pH
When commercial soil testing laboratories in WA measure soil pH in calcium chloride they will, if requested, also measure the concentration of soluble aluminium present in the extract solution used to measure the pH.
Aluminium toxicity is likely when the concentration of aluminium measured by this procedure is greater than 8–10mg/kg soil. This is not a measure of exchangeable aluminium in soil, which requires a more sophisticated procedure. It is a relatively crude test but allows paddocks with the largest aluminium concentration measured in the extract solution to be given top priority for liming.
Subsoil acidity
Research in the cropping areas of south-western Australia has shown that the decline in soil pH since the start of agriculture is often greater in the subsoil than in surface soil, with the greatest decline from about 10–40cm depth. Extensive research in eastern Australia has shown that subsoil acidity is a major problem for high rainfall pastures in that region.
Until the mid-1990s, there was little evidence for subsoil acidity in the high rainfall areas of WA, probably because these areas were developed much more recently than the high rainfall pastures in eastern Australia. However, since the early 2000s, there is increasing local evidence for subsoil acidity in high rainfall pastures, particularly where surface soil pHCa has been allowed to decline to 4.0 or less.
Whether or not a low subsoil pH reduces the productivity of our generally shallow-rooted pastures has yet to be determined. Subsoil acidity is more difficult and costly to ameliorate than topsoil acidity.