State of the Environment Report 2007
Many soil types are naturally acidic. Problems arise when acidity increases and affects plant growth. Soil acidification occurs due to a gradual increase in the hydrogen ion content of the soil, and is measured by a decrease on the 'pH' scale. This scale varies from pH 0 (strongly acidic) to pH 14 (strong base), with pH 7 being neutral. The most widespread cause of soil acidification is from agriculture practices including application of nitrogenous fertilisers, the leaching of nitrate from legume crops and pastures, and the gradual removal of alkalinity (material that buffers against soil acidity) from soil into harvested or grazed plants. Unfortunately, these are side-effects of agricultural production. Acidification can also be caused by the oxidation of sulfide soil minerals during mining or land development, acid deposition from industrial atmospheric pollutants (e.g. sulfur dioxide) or land contamination.
Soil acidification is difficult to identify, particularly when it occurs below the soil surface. A decline in vegetative condition or agricultural productivity is often the first sign. The critical point for this decline is generally accepted to be about pH 4.5, but this varies among plant and crop species. Soil acidification is primarily managed through lime application, either as lime sand or crushed limestone. Dolomite is also used in some areas. Other practices such as reduced nitrogen fertiliser input, the use of nitrate-based fertilisers, improved timing of fertiliser application with regard to plant growth, and efficient irrigation practices can also be adopted to help reduce acidification. Over time, unchecked acidification can result in nutrient deficient soils and the accumulation of toxic materials, such as aluminium and manganese, which inhibits plant root growth and reduces crop yields. Unmanaged, it may also cause subsurface soil acidification (10-30 cm below the soil surface) which is much more difficult to treat.
Most farmers have a general awareness of soil acidity and knowledge of how to treat acidity problems. While no broadscale monitoring exists, regional modelling for the South West is available to determine areas at risk of surface and subsurface acidification. Most of the South West shows evidence of elevated surface soil acidity risk, although some soils are more susceptible than others (Figure L3.1). It is estimated about two-thirds of the agricultural Wheatbelt is affected by surface soil (topsoil) acidity, or is at risk of acidification. The estimated area of strongly acidic soils (pH < 4.8) is 1-8 million hectares, and of moderately acidic soils (pH 4.8-5.5) is an additional 7-19 million hectares (Commonwealth of Australia, 2001b). This area is much greater than the land affected by dryland salinisation (see 'Land salinisation').
Subsurface soil acidity can have as much effect on plant growth as surface acidity, but is more difficult and costly to treat, and in the long term may be more problematic and threatening. It is estimated there are 0.2-4.8 million hectares of acid subsurface soils in Western Australia (Commonwealth of Australia, 2001b). The areas with, or at highest risk of, subsurface acidification include the northern Wheatbelt and soils from Perth to Geraldton and Augusta to Albany (Figure L3.2). Estimates for the Avon River Basin, indicate that about 93% of surface soils and 83% of subsurface soils have moderate to high risk of acidification (Department of Agriculture and Food, unpublished).
![Figure L3.1: Modelled surface soil acidification risk for the South West. [Data source: Department of Agriculture [ver. 2005]; Presentation: Department of Agriculture.]](/site/files/images/Figure-L3.1_Large.jpg)
![Figure L3.2: Modelled subsurface soil acidification risk for the South West. [Data source: Department of Agriculture [ver. 2005]; Presentation: Department of Agriculture.]](/site/files/images/Figure-L3.2_Large.jpg)
Plants take up nutrients (including acid-buffering chemicals) from soil, resulting in a separation of acidity in the soil and alkalinity in the plant. As agriculture removes plants from land (by harvesting crops or grazing pasture) less alkalinity is returned to the soil and over time it becomes progressively acidic. In the 10 years from 1994-2004, the total area of land dedicated to cropping has increased by about one-third, from 6.1 to 8.1 million hectares (Commonwealth of Australia, 2001b; Australian Bureau of Statistics, 2005). More land dedicated to cropping puts a greater area of land at risk of soil acidification.
Grasses, legumes and wheat typically acidify the soil faster than pasture due to their inefficient use of nitrate. In contrast, annual and perennial pastures are able to establish earlier at the break of the season (when rain starts) and effectively utilise the nitrate thereby reducing the rate of acidification. The area under legume production has decreased by nearly 40% between 1999 and 2005. This may help to alleviate high rates of soil acidification in some areas. The area under wheat production is about 13% higher and has increased in recent years due to favourable growing conditions (Table L3.1).
Fertiliser containing nitrogen is primarily used for enhancing grain and pasture production. Dependency on fertiliser in WA is rising. Between 1989-90 and 2001-02, the amount of nitrogen-based fertiliser applied to soil grew about 450%, from 88 000 tonnes to 399 000 tonnes (McLennan, 1996; Australian Bureau of Statistics, 2002). When these fertilisers leach into soil, 'nitrification' (the natural process of conversion of ammonium to nitrate) occurs, during which acid is produced. If growing plants take up nitrate the potential for acidification is reduced, but if nitrate is leached from the root zone then acidity can build up. All ammonium-based fertilisers cause acidification, whether leached or not. Fertilisers that are not ammonium-based (such as urea) only cause soil acidification if the nitrate, into which they are converted, leaches from the root zone. Superphosphate fertilisers are not directly acidifying, but indirectly add to soil acidity by improving plant growth, and hence the amount of plant material (containing acid buffering chemicals) removed by harvesting or animal grazing. Application of sulfur to soil is also acidifying.
Soil acidification in agricultural areas is primarily managed through lime or dolomite application. Best practice guidelines indicate that lime should be applied at one to 1.5 tonnes per hectare every 3-7 years. If soil pH is below 4.5, then two applications of lime are required within 5 years to lift the pH to within the normal range, which is then maintained by a liming maintenance regime. The best agricultural production responses to lime have occurred when the topsoil pH is very low (i.e. pH < 4.5). Moderately acidic soils (pH 5-5.5) typically show little yield response but depending on the crop, season and yield potential this may not always be the case.
In 2004, the amount of lime and dolomite applied to agricultural soils affected by acidity was 1.03 million tonnes - the highest level of use yet recorded in WA (Figure L3.3). There is evidence of a gradual rise over the past decade in both the area of farmland treated and the quantity of lime being used. While growth in lime use is encouraging from an agricultural perspective, the actual amount of lime required to maintain current acidity in agricultural soils is about double (2 million tonnes per year). Unfortunately, monitoring of lime use has since ceased and was not undertaken in 2005 and 2006.
Treatment of subsurface soil acidity with surface applications of lime can take many years. Subsurface soil acidification under crops has the potential to severely limit crop and pasture production. For long-term soil health, the prevention or minimisation of subsurface soil acidity is vital.
![Figure L3.3: Western Australian lime and dolomite use and area treated over time. [Data source: Australian Bureau of Statistics, 1996 & 2002; O’Connell & Gazey, 2003; C Gazey, Department of Agriculture and Food, pers. comm.]](/site/files/images/Figure-L3.3_Large.jpg)
Estimates of surface soil testing for pH vary from 30% of farmers testing portions of their farms ( Nutrient Management Systems, pers. comm.) to 65-75% of farmers undertaking regular pH testing of surface soils (Department of Agriculture, 2006). It is estimated that only 10% of farmers test acidity levels of subsurface soils (C Gazey, Department of Agriculture and Food, pers. comm.). Surveys also show that farmers have developed an increased awareness of acidification problems and the skills to help correct the problem (Department of Agriculture, 2006).
Draft State Lime Supply Strategy: was initiated in 1998 with the intention of enabling informed decisions about lime production and supply in relation to environmental, conservation, urban and heritage issues. The Department of Industry and Resources oversaw development of the strategy, with guidance from other government agencies (Department of Industry and Resources, 2001). The strategy is currently in draft form and is considered to be an evolving document.
Natural Heritage Trust/National Action Plan for Salinity and Water Quality (NHT/NAP): Through these two Commonwealth Government programs, the South West, Swan, Avon, Northern Agricultural and South Coast regional natural resource management groups have recognised soil acidification as a threat to natural resources. Strategies have outlined specific targets and onground projects to address soil acidification.
Integrated Soil Acidity Research, Development and Extension Program, 1992-2002: The Department of Agriculture and Food in conjunction with University of Western Australia, the Grains Research Development Corporation, and the Natural Heritage Trust coordinated this program. It investigated methods for slowing the rate of soil acidification, establishing the relationship between the level of subsurface acidity and crop yield losses, and developed techniques to add alkalinity back to acidifying soils. It was supported by a promotional extension campaign 'Time to Lime', during which annual lime use by farmers increased by 530% from 150 000 tonnes in 1994 to more than 800 000 tonnes in 2002 (Figure L3.2). The number of farmers using lime rose by 240% from 1353 to 3292 over the same period (Department of Agriculture, 2003).
Across Australia, the economic implication of soil acidification is estimated to be five to six times higher than dryland salinity (Commonwealth of Australia, 2001b). Acidity is insidious, with yield declines of 20-30% occurring over time (Department of Agriculture, 2000). Such losses may go unnoticed if farm productivity is improving for other reasons, such as increased fertiliser use. Plant and crop growth is often limited because of a reduction in the availability of nutrients (calcium, magnesium, boron, molybdenum) or an increase in toxic levels of aluminium, iron or manganese. Toxic levels of aluminium decreases root growth which leads to reduced water uptake by plants and crops, hence contributing to other problems such as waterlogging, erosion and salinisation. Extreme acidification can result in poorly structured or hard-setting topsoils that don't support enough vegetation to prevent soil erosion. Soils may also acidify to the point where acid, nutrients, sediment and heavy metals are exported and impact nearby inland waters (see 'Acidification of inland waters').
Liming is viewed as the major remediation option for soil acidification on farms. Risks of substantial farm losses exist if liming programs are not commenced before critical surface and subsurface pH levels are reached. However, due to the rapid increase in lime use in agricultural, mining and construction industries, significant pressure has been placed on existing and potential limestone stocks. Lime is currently quarried at about 30 coastal sites between Geraldton and Esperance, with many situated in areas of conservation value. These sites are under significant threat from increased lime mining activity as demand increases, and questions are being raised about the sustainability of liming.
3.7 Develop and implement a Soil Acidification Management Strategy as a component of the proposed State Soil Protection Policy, covering all types of acid soils.
3.8 Finalise and implement the draft State Lime Supply Strategy incorporating sustainability principles.
3.9 Develop an agreed baseline of the extent and severity of soil acidification in WA.
