Greener Pastures: soil acidity

Ameliorating soil acidity

The simplest way to ameliorate soil acidification is to add lime to the soil. When lime is added to moist soil, alkali dissolves from the lime and consumes hydrogen ions. The lime can be left on the soil surface, incorporated into the surface 2cm using heavy harrows or disced into the surface 10cm. Generally speaking, the lower the surface pH, the more necessary incorporation becomes.

Ameliorating subsoil acidity

If the pH_Ca in the 10–20cm and 20–30cm zones is less than 5.0, subsoil acidity may be a problem. The only practical way to raise the subsoil pH is to apply sufficient lime to raise the pH_Ca of the top 10cm of soil to 5.5 or greater. When the pH_Ca of the top soil is above 5.5, the alkali dissolved from the lime moves rapidly into the subsoil. If the subsoil soil pH_Ca is maintained above about 5.5, subsoil acidity should not be an issue.

Sources of lime

There are three sources of agricultural lime in WA:

Limestone

Limestone (calcium carbonate) is the most common lime source south of Perth; it is found naturally as rock and needs to be crushed to be effective.

Limesand

Limesand (calcium carbonate) is mostly found north of Perth and occurs as sand dune deposits. Unlike limestone, limesand is not usually ground before application because much of it exists as fine, porous, sand-grain sized material.

Dolomite

Dolomite is a mixture of calcium and magnesium carbonates, with most deposits usually comprising about 60% calcium carbonate and 40% magnesium carbonate. It occurs naturally near salt lakes in the cropping areas of the south-west. Dolomite often contains large rocks so is usually ground before application. Because dolomite deposits are usually a long way from high rainfall areas, high transport costs result in dolomite being more expensive than limestone or limesand.

Although magnesium deficiency has not been detected in pastures in WA, dolomite is often recommended to increase soil magnesium levels. To ensure magnesium deficiency does not occur in animals, they can often be supplemented directly.