Soil acidity in the Western Australian wheatbelt is considered to be one of the largest yield limiting factors faced by farmers. An estimated two-thirds of the area is effected by soil acidity costing in excess of $70 million annually. Unless addressed, the problem will continue to grow.

Soil acidification is accelerated by;

  • Removal of produce
  • Leaching of nitrogen from the root zone
  • Using ammonium based fertilisers.

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

What do we mean?

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; ie pH 5.0 has 10 times the amount of hydrogen ion than pH 6.0 and 100 times the amount of pH 7.0.

Soil pH is most commonly measured in a Calcium chloride solution, but can be measured in water. Measurements taken in water will reflect a higher pH than if taken in Calcium chloride (Typically, water pH will be 0.6 to 1.0 unit higher than CaCl2). The lower the pH, the greater the difference. Care should therefore be taken when reading pH levels as to the form of test results.

Calcium Chloride was chosen as it more accurately mimics the soil solution, which is not pure water, but a mix of salts.

Effect on crop

Nutrient availability

The availability of nutrients is effected by the pH of a soil. It is best illustrated in the diagram below. In general, the macro nutrients will become less available with increasing acidity and the micro nutrients will become more available.


Figure 1. Influence of pH on nutrient availability

Soluble aluminium

Acidity, or the amount of hydrogen ions present in the soil, do not effect crop growth as such, but increases the presence of soluble Aluminium in the soil which retards root growth. 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 effected depends on its tolerance to acidity (or aluminium) and can be seen in the table below.

Critical pH values for broadacre crops

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. This often leads to a premature haying off or droughting of the crop. Subsoil acidity, in effect forms a chemical hardpan similar to the physical hardpan caused by compaction. The effect on root growth can be seen below.


Picture of plant root growth affected by Al toxicity.


Close up of root hairs affected by Aluminium toxicity

Causes

Product Removal

Plant matter excluding roots is generally alkaline and its removal from the system, even in the form of wool, will have a nett acidifying action.

Leaching of Nitrogen

When nitrate nitrogen is leached below the root zone, being negatively charged (anion) it will take with it another positively charged (cation) element. This is usually not hydrogen as it binds very strongly with the soil surface and therefore hydrogen ions will accumulate and thus increase soil acidity.

Ammonium based fertilisers

Addition of ammonium fertilisers also has an acidifying effect, although not to the same degree as leaching nitrate. This is due to the nitrogen cycle where by ammonium converts to nitrate which has a nett acidifying effect.

It is also important to note that many of our native Western Australian soils are naturally acidic, particularly in the sub soil.

How do we fix it?

Identify the extent of the problem

The best ways to do this is to soil test the topsoil (0-10cm) and also 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

Lime sand is the most commonly used and has also been the most researched form of ameliorant in Western Australia.

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 etc.

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 fertiliser 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.

Pictures used in this article Courtesy of Department of Agriculture, Western Australia.
There are numerous booklets and FarmNotes on aspects of Soil Acidity on the Ag Depts web site