Field Research & Agronomy

Soil acidity is a major constraint to farming in the Western Australia wheatbelt. An estimated two thirds of the area are affected by soil acidification, which is accelerated by the removal of produce, leaching of nitrogen from the root zone and using ammonium-based fertilizers.

With agriculture aiming to maximise produce removal, it is acidifying by nature and therefore requires strategic planning to limit the cost to production.

Measuring Soil pH

Acidity increases with the increasing presence of hydrogen ions (H+). It is measured using the pH scale, which is a logarithm of hydrogen ion concentration in moles/L. The scale ranges from 1 to 14 with a value of 7 being neutral, values above 7 being alkaline and values below 7 being acidic. A one-unit pH change indicates a ten-fold increase in the number of hydrogen ions (e.g. pH 5.0 has 10 times the amount of hydrogen ion than pH 6.0 and 100 times the amount of pH 7.0).

Effect on crop

The availability of nutrients is affected by the pH of a soil. With increasing acidity, the macronutrients will become less available and the micronutrients will become more available.

Acidity does not affect crop growth as such, but it does increase the presence of soluble aluminium in the soil. As a result, the root growth is hindered. Aluminium will start to dissolve at a pH of 5.5 (calcium chloride) and become toxic for many species at around pH 4.5 (calcium chloride). The degree to which the plant is affected depends on its tolerance to acidity (or aluminium) and can be seen in this critical pH range graph.

Many Western Australian soils have a more acidic sub-soil and therefore greater concentration of soluble aluminium at depth. The reduction of root growth due to aluminium toxicity reduces the ability of the roots to source water and nutrients, leading to premature haying-off or droughting of the crop.

Balancing acidic soil

To balance the pH levels, first the extent of the problem must be identified. The best way to do this is to:

a) Soil test the topsoil (0-10cm), and

b) test the pH of the subsoil (10-30cm).

Subsoil acidity will take longer to rectify, as the ameliorant has to travel through a greater profile. Research presented at the 2003 Agribusiness Crop Updates by Chris Gazey (AgWA) would suggest that the topsoil has to be greater than pH 5.5 (CaCl2) for lime sand to act on subsoil acidity. Once the state of soil acidity is defined, then priorities for ameliorant application can be set to bring paddocks back to a desired pH level.

Application of ameliorant

There are a few different forms of ameliorant available in Western Australia, including lime and dolomite.

The best form to use would depend on the quality, freight rates and overall cost of the product. The quality is measured by a function of the neutralising value, particle size and hardness of the product. It is expressed as the ‘effective neutralising value’ and is used by all registered lime suppliers.

Lime sand is the most commonly used ameliorant and has also been the most researched in WA. Lime sand should be applied at a maximum of 1 to 1.5 tonne per hectare to avoid causing imbalances in the soil with other nutrients such as copper, zinc, manganese and potassium.

Calculate lime use equivalent

Once paddocks have been restored to the desired pH range (5.5 to 6.5 CaCl2), then a lime use equivalent can be calculated. This is calculated on the rotation, soil type and fertilizer regime employed to give an estimate on the lime required to neutralise the nett acidity increase. One can then incorporate an ongoing liming program into the rotation to maintain desired pH levels.

Crop selection

The selection of acid tolerant plants and species can also help minimise the cost of soil acidity. Their use should be used in conjunction with an amelioration program, as the soil is likely to continue to acidify.