Soil: Salinity and acidification

2016

Soil salinity

Secondary dryland salinity has been one of Australia’s most costly forms of land degradation. Most annual crops, such as wheat, are susceptible to salinity, which reduces grain yields if it exceeds a threshold level. The assessment completed by the National Land & Water Resources Audit (NLWRA) in 2001 (NLWRA 2001) is still the most comprehensive overview of dryland salinity in Australia. Assuming no changes in water balance, the NLWRA expected dryland salinity to increase from 5.7 million hectares to 17 million hectares by 2050. However, the millennium drought appears to have halted the spread of dryland salinity in most of the worst-affected regions, especially in south-western Western Australia and Victoria; the spread is likely to increase with a return to wetter conditions. Large areas of New South Wales along the Great Dividing Range, and in the Liverpool Plains, Hunter Valley and Greater Sydney regions reported soil salinity as their main issue of concern (NSW EPA 2015).

The outlook described by the NLWRA will need further consideration if current projections for a drying of southern Australia are correct. However, the long-term outlook for more recently cleared land in the northern Murray–Darling Basin and central Queensland is unclear. Large areas are yet to reach a new hydrological equilibrium after clearing.

Given the effects of drought over the past decade, this report does not provide an update on previous SoE reports or the NLWRA assessment regarding salinity. However, close surveillance of groundwater systems is essential, particularly in regions that returned to wetter conditions in 2010–15. A key requirement for understanding the state of dryland salinity in Australia will be to maintain the groundwater monitoring network established under the National Action Plan for Salinity and Water Quality.

Soil acidification

Soil acidification is an insidious process that develops slowly. If not corrected, it can continue until the soil is irreparably damaged. Acidification affects about half of Australia’s agriculturally productive soils.

Soil acidification is of greatest concern in situations where:

  • agricultural practices increase soil acidity (e.g. use of high-ammonium nitrogen fertilisers, large rates of product removal)
  • the soil has a low capacity to buffer the decrease in pH (e.g. infertile, light-textured soils)
  • the soil already has a low pH.

The process of acidification considered in this report is distinct from that associated with acid sulfate soils. Such soils occur primarily in coastal settings and naturally contain iron sulfide, which causes severe acidification when it oxidises. This can occur through drainage of coastal wetlands or exposure due to drought, as was the case in the Lower Lakes of South Australia during the millennium drought.

The main onsite effects of acidification include:

  • loss of, or changes in, soil biota involved in nitrification (which fix nitrogen, a key nutrient, within the soil)
  • accelerated leaching of plant nutrients (manganese, calcium, magnesium, potassium and anions)
  • induced nutrient deficiencies or toxicities
  • breakdown and subsequent loss of clay materials from the soil
  • development of subsoil acidity
  • reduced net primary productivity and carbon sequestration
  • erosion as a result of decreased groundcover that may follow acidification.

The potential offsite effects include:

  • mobilisation of heavy metals into water resources and the food chain
  • acidification of waterways as a result of leaching of acidic ions
  • increased siltation (where fine sediments suspended in the water are deposited on the floor) and eutrophication (where a high concentration of nutrients typically triggers excess growth of algae) of streams and water bodies.
Soil acidification in Australia

Soil acidity affects approximately 50 million hectares (50 per cent of Australia’s agricultural land) and about 23 million hectares of subsoil layers, mostly in Western Australia and New South Wales (NLWRA 2001).

Soil acidification restricts options for land management, because it limits the choice of crops and vegetation to acid-tolerant species and varieties. It is relatively straightforward to reverse short-term surface soil acidification through the application of lime. However, it is much harder to reverse the problem if the acidification has advanced deeper into the soil profile, because incorporating lime at depth is more expensive.

Although rates of lime application appear to be increasing, they still fall far short of what is needed to ameliorate existing, and counter ongoing, soil acidification. Western Australia, where more than 70 per cent of surface soils are below appropriate pH levels, has one of the best programs in Australia for combating acidification, but the rates of lime application are still much lower than what is needed to avoid irreparable damage (Gazey et al. 2013; Figure LAN18).

A similar situation exists in South Australia (Figure LAN19). The average quantity of lime sold annually during the past decade is just under half the amount required to balance the estimated annual soil acidification rate (South Australian Government 2013a).

Assessment of state and trends of soil acidification in Australia

In 2011, a group of experts in soil acidification and land resource assessment was convened to provide an assessment of the state and trends of soil acidification across Australia. This assessment has been updated using more recent state and territory SoE reports, where available (Figure LAN20). The assessment summary provides ratings for regions where the most significant issues are apparent. The ratings for all physiographic regions are available on the SoE website.

Only some states (Western Australia, New South Wales and South Australia) have some form of organised monitoring system for soil acidification, which accounts for the significant uncertainty in many regions.

The current assessment is as follows:

  • Soil acidification is widespread in the extensive farming lands of southern Australia.
  • Rates of lime application are well short of the rates needed to arrest the problem.
  • Acidification is common in intensive systems of land use (tropical horticulture, sugar cane, dairying).
  • Acidification is limiting biomass production in some regions, but the degree of restriction is difficult to estimate.
  • Trends in the tropical savannas are uncertain. If acidification is occurring, it will be a difficult problem to solve.
  • Carbon losses are most likely occurring across regions in poor condition, and soil acidification is a major constraint on storing carbon in soils in the future.
Metcalfe D, Bui E (2016). Land: Soil: Salinity and acidification. In: Australia state of the environment 2016, Australian Government Department of the Environment and Energy, Canberra, https://soe.environment.gov.au/theme/land/topic/2016/soil-salinity-and-acidification, DOI 10.4226/94/58b6585f94911