SoE 2006 > Land > 4.4 Induced soil acidity

4.4 Induced soil acidity

Soil acidity is the 'hidden' land issue. It is a landscape-scale issue identified by the National Land and Water Resources Audit as the most serious land degradation issue for Australian agriculture, yet it is not widely recognised.

In 2002, the area of NSW affected by acidity was found to be increasing. This trend is likely to have continued, though no comprehensive new assessment of the extent of the problem or the rate of change is available.

Rehabilitation of the land affected by surface soil acidity is possible, but is being held back by the cost relative to the land's productive value. It is unclear whether new investment through CMAs will be more effective than previous statewide programs in preventing the problem from widening. Subsoil acidity is effectively permanent unless new and more cost-effective remediation measures are found.

NSW indicators

Introduction

Soil acidity is now recognised as a serious environmental and economic problem. The National Land and Water Resources Audit (2002) identified soil acidity as the most serious land degradation issue for Australian agriculture. In NSW, the annual income foregone in agriculture as a result of soil acidity was estimated in 2003 to be as high as $378 million – about 60 times higher than for dryland salinity (Lockwood et al. 2003). More recent economic assessments have not been undertaken and the additional costs of damage to ecosystems and community infrastructure have not been quantified.

Many NSW soils are naturally acidic even under native vegetation cover. Acid soils are those having a pH less than 5.5 when measured as 1:5 soil:water. The slow, natural process of soil acidification has been accelerated by modern agricultural farming systems which alter the carbon and nitrogen cycles, such as through the use of improved pasture species, legume-dominant pastures and the application of fertilisers. The four main acidifying processes are: leakage of nitrate below the root depth, oxidation of ammonium fertilisers, accumulation of soil organic matter, and export of plant and animal products from the land (Lockwood et al. 2003). Lighter-textured soils in agricultural areas are at most risk of acidification as they are readily leached and poorly buffered. Surface soil acidity and its associated problems are mostly reversible, but costly. The application of lime can raise surface soil pH levels when used in conjunction with other land-management practices, but subsurface acidification is not practical to treat and is therefore effectively permanent.

If left unchecked, soil acidification reduces pasture and crop growth, and can lead to a decline in vegetative cover and agricultural production. As soil acidity becomes more severe, its impacts begin to extend beyond agricultural production, contributing to waterlogging, groundwater recharge, dryland salinity, nitrate pollution of groundwater, phosphate leakage into streams, and damage to roads from rising water tables (EPA 2003). There is also evidence that the bioavailability of cadmium may have links to soil acidity (SCOPE 2000).

Soil acidification is a different process to the formation of acid sulfate soils, which contain iron pyrites and occur mainly in and around estuaries (see Land 4.5). The two issues are separate and should not be confused.

Current status and trends

Estimates in 2001 put the extent of agricultural land in NSW affected by acidic topsoils at between 16 and 20 million hectares, and acidic subsoils (30–40 cm depth) at 11.7 million hectares (NLWRA 2001) of which 3.1 million hectares was below pH 4.8 (Lockwood et al. 2003). There has been no comprehensive update of the NLWRA survey or more recent analysis on which to base a current assessment of the area affected by either surface or subsoil acidity.

Acid soils are mostly concentrated in a broad band extending southwards from the Central Tablelands, in the prime cropping and pastoral zones of the Central and Southern slopes under irrigation, or in high rainfall areas (Helyar et al. 1990). Significant areas also occur on the North Coast where nitrogen-based fertilisers have been extensively used. Notable acidity occurs on granite soils around Bathurst, Braidwood, Bega and Grafton and areas in the Central West and Northern Tablelands. An additional 5–17 million hectares of NSW agricultural land is predicted to become moderately acidic (pH less than 5.5) within 10 years (NLWRA 2001). A further 8–22 million hectares is expected to reach highly acidic levels (pH less than 4.8) over the same period.

Map 4.5 shows the risk assessment of future acidification in NSW. It has been compiled as part of the Soil Landscape Mapping Program and uses surface-soil buffering capacity to predict acidification hazard. Soils that contain materials that help them to resist acidification are referred to as 'well–buffered'.

Map 4.5: Acidification hazard for agricultural land in NSW

Source: DLWC data 2003

The critical factors in assessing whether predicted acidification rates will occur are the distribution and rate of lime application (Lockwood et al. 2003). The assessment in Map 4.5 and the projections used in the NLWRA assume the absence of lime applications. However, lime application increased markedly from about 100,000 t/yr in the mid-1980s to around 500,000 t/yr by the late 1990s (Hughes 2001). There has been considerable research on liming rates and application techniques for various crops in different soil landscapes (DPI 2006a) but no new data has been collated on the quantities of lime used since 2000. NLWRA (2001) estimated that it would take seven years to bring affected soils back above pH 4.8 by applying lime (based on annual sales figures in 1999 of about 500,000 tonnes of lime in NSW) and 49 years at that rate to raise pH levels to 5.5, assuming no further acidification. Other evidence suggests that about 1.5 million tonnes of lime would be required each year just to maintain current levels of acidity (Fenton 2002). These estimates suggest that even if the growth trend in lime application continued beyond 2000, total lime usage is well below that required to halt the degradation process over the whole of the area affected.

Response to the issue

For farmers, the most common way of treating soil acidity is to add agricultural lime to the soil. An increasing trend in the sales of lime by 2000 (Hughes 2001) suggests that research and extension efforts by government departments under the former Acid Soils Action Program (ASAP) have met with some success.

More recently a decentralised approach to combating soil acidity through CMAs has been pursued. Individual land degradation symptoms like soil acidity are recognised by some CMAs to be a catchment-scale issue of soil health, rather than just a farm or paddock-scale problem (see Land 4.1). Most CMAs have emphasised the adoption of land-management practices that seek to address underlying acidification processes at the same time as other degradation processes, such as salt mobilisation, sodicity and erosion. However, it is too early to tell if this approach is working and to what extent.

The statewide natural resource management targets relevant to soil acidity are 'By 2015 there is an improvement in soil condition' and 'By 2015 there is an increase in the area of land that is managed within its capability'.

Future directions

Continued application of lime is an essential part of the strategy to treat soil acidity. Allowing for some growth in lime usage since 2000 when figures were last available, the targeted application of between 50–66% more lime would be necessary to maintain soil acidity at current levels. While there can be localised and farm-scale successes, to reverse the trend requires a much greater application of lime initially in some areas, followed by maintenance applications. This level of increase appears unlikely.

A clear understanding of acidity trends, in conjunction with lime usage patterns, supply and cost, is fundamental to any strategic attack on soil acidity. Further improvements can be expected to follow from increased scientific understanding and research into better techniques for applying lime and crop/pasture species selection, but the affordability of lime is likely to be critical for meeting targets and improving environmental and economic outcomes.

Lime application does not address the underlying causes of acidification. Improved land-management practices including minimum tillage, reducing legume dominance in pastures, matching nitrogen fertiliser inputs to crop requirements, and replacing annual pastures with perennial species will be needed for long-term prevention once pH has been stabilised at acceptable levels.

A more fundamental change to farming systems may be needed, given the costs of lime use, including consideration of increased agricultural intensification in some areas, balanced by a shift to low input/low yield systems over other wider areas.

Research into, and education about, off-site impacts from soil acidification, framed at a landscape scale, may assist in heightening public awareness of soil acidity issues and help CMAs and governments to determine what priority actions are necessary. Managing from a whole catchment perspective will change the understanding of externalities, such as nitrate pollution of groundwater and phosphate leakage into streams, currently addressed only at the farm scale.