Soil Acidity

Soil acidification is a major soil degradation problem in the South Coast Region (DAFWA, 2013). A high proportion of soils are either below optimum soil pH or at high risk of acidity for productive agriculture because current commercial crop and pasture species require a near neutral soil pH for optimum production.

Causes of soil acidification

Acidification is a natural process and part of normal weathering. However it can be accelerated under agriculture practices by the incomplete cycling caused by (Bolan and Hedley, 2003). The causes of incomplete cycling include:

• The excretion of hydrogen ions from plants due to greater update of captions than anions

• Removal of plant and animal products that typically contain high levels of anions

• Nitrate leaching by the use of ammonium-based fertilisers

• Nitrogen leaching from the root zone in legume pastures

• The accumulation of organic matter as humic acids in the soil

Diagnosing soil acidity

The only way to diagnose soil acidity is to measure soil pH. Soil samples should be taken at 0-10, 10-20 and 20-30 cm depth to determine a soil pH profile.

Subsurface soil acidity is a growing problem but top soil pH can be quite difference from the subsurface pH and acidity in the subsurface soil cannot be detected or estimated by knowing the topsoil pH (Gazey et al., 2014)

Reducing the impact of soil acidity

Where soils are at risk of becoming acidic, the future impact of soil acidity can be reduced by reducing the nitrate leaching by using nitrate-based fertilisers rather than ammonium based fertilisers but the removal of agricultural products as alkaline plant material will decrease soil pH over time.

Once the soil became acidic, the most practical way to ameliorate acidic soils is applying lime to neutralise soil acidity. Without on-going amelioration, agricultural soils can continue to acidify to the point where they become too expensive to recover.

If untreated, acidification is likely to affect a larger area of land than any other soil degradation problem (O’Connell et al., 1999)

Lime Application

Developing a lime program

One of the best ways to manage soil acidity is through application of lime. Carbonate ions from lime neutralise excess hydrogen ions in the acid soil to generate carbon dioxide and water released into the soil air and soil water.

The liming materials most commonly used are lime sand or limestone (calcium carbonate) and dolomite (calcium/magnesium oxide). Burnt lime or quick lime (calcium oxide) is not recommended for use as an agricultural lime as storage and handling of these products is difficult. To develop a lime program, we need to know what our soils are, what sort of lime we can manage to use and to get down lime in the right place.

Checking soil regularly will help detect any changes in soil pH and allow liming rates to be adjusted accordingly. This way the lime will be applied where it is needed most.

Before purchasing lime, check on the quality of lime that’s available and the costs associated with transport and spreading. The key indicators of agricultural lime quality are neutralising value and particle size, regardless of the source.

Neutralising value (NV): The carbonate content of lime sand, limestone and dolomitic lime determines the capacity of the lime to neutralise acidity. Neutralising value (NV) indicates the capacity of lime to neutralise soil acidity relative to pure calcium carbonate (100%).

Particle size: The size of the lime particle determines how quickly the lime can neutralise acid. The finer the particle of lime the faster they react with soil.

Total cost to be considered should include the purchase cost at pit, transport cost to paddock and spreading cost. As lime quality varies from pit to pit, it can actually be more cost-effective to transport a higher quality product further.

A useful tool for calculating and comparing the cost effectiveness of agricultural lime, Lime comparison calculator is available at the Soil Quality Website.

Once you decide which lime to apply, it is recommended that you read your supplier’s latest product information sheet. You can find out the laboratory results which include information on neutralising value and particle size. As lime is a natural product there will be variability within each pit. It is important to check the pit range which shows how consistent the product is. Lime WA suppliers present the latest information with a range based on the approved laboratory test.

Sourcing lime on the South Coast

Because of its geological history, the South Coast Region has much fewer and lesser quality lime supplies, than the west coast. The economics of applying more South Coast lime versus transporting lime from the west coast has to be carefully considered.

SCNRM’s review of this trade off reflected the complexity surrounding the decision making for producers on the application of lime but suggested it is a more cost effective option to extract and crush on-farm secondary carbonate, even at low neutralising values than to transport lime sand from the west coast especially to the Esperance port region, despite the lime sands higher neutralising values. Several variables including seasonal conditions and prices, which vary from year to year, increase the complexity associated with understanding response to lime.

The report assists land managers to consider the cost of transport, the neutralizing value of the lime source, the initial soil pH to determine the amount of lime required, soil testing here is a key management tool.

On-farm lime extraction

An option for growers on the South Coast is on-farm extraction if lime can be found on private land where extraction is acceptable based on environmental and amenity values and competitive in an economic sense to sourcing lime off-farm.

It is strongly recommended that all on-farm lime extraction projects begin with identifying what you want to achieve during the life of the pit and the results you seek when the pit is expired. Considerations include successful approvals, extraction, sound financial management, sound natural resource management and successful site rehabilitation, restoration and also social consideration.

For growers to access the opportunity and potential impact of developing on-farm lime sources, read our practical guidelines.

Extent of potential lime resources

Tenure of the South Coast of Western Australia

This map shows the extent of potential lime resources and tenure of the South Coast of Western Australia versus the land tenure of the same extent. The information for resources is based on record 2015/7 Lime sand and Limestone Resources of Southern Western Australia, published by the Geological Survey of Western Australia.

Land tenure information was sourced from a range of resources published by Land gate and the Department of Biodiversity, Conservation and Attractions Western Australia.

This information is accessed under license through data.wa.gov.au, you will need to sign up to access the map. MAP: https://www.arcgis.com/home/webmap/viewer.html?webmap=8a750f283a1f4b6bab848c63cdcb5927&extent=115.0041,-36.5564,126.4958,-31.4127