Theme commentary
Tony Gleeson, Synapse Research & Consulting
Alex Dalley, Ministry of Agriculture Fisheries and Forestry, Dili, East Timor
prepared for the 2006 Australian State of the Environment Committee, 2006

Salinity and acidity

Salinity  occurs naturally, even in healthy catchment areas. Salt borne from the sea by wind and rain is deposited across the landscape. Naturally occurring salts are leached downwards into groundwater where they are concentrated by the transpiration of plants. This naturally-occurring salinity is known as primary salinity.

Secondary salinity is the salinisation of land and water resources due to the impacts of human activities (NLWRA 2000a). Secondary salinity takes the form of irrigated salinity due to rises in groundwater resulting from irrigation, and dryland salinity caused by the removal of vegetation that otherwise keeps saline groundwater at levels below the root zone.

Of particular concern is the condition of riparian vegetation , which is severely affected as it occupies the lowest parts of the landscape where much of the saline groundwater is released to the surface; but there has been no significant increase in vegetated stream length since 1989. This is of major concern as riparian vegetation plays a key function in stopping the movement of salt through river systems.

In SoE2001, salinity was described as a keystone indicator that could inform about disturbances to hydrological and nutrient balances. The importance of accurate and publicly accepted information about salinity hazard is clear when considering the broad range of impacts that it may have on human societies. At the farm level, salinity will result in the loss of production and income. Other effects include the decline in capital value of land, damage to infrastructure, salinisation of water storages, loss of farm flora and fauna, and loss of shelter and shade. These effects are magnified at the regional scale. Salinity will have a substantial impact on resources such as biodiversity, water supplies and infrastructure (DEH 2005).

Despite acceptance of the importance of salinity hazard as an indicator, no new data on salinity hazard have been collected since the Australian Dryland Salinity Assessment 2000. The maps of areas forecast to contain land of high hazard risk of dryland salinity in 2050 (NLWRA 2000a) that caused much contention in regional Australia have not been updated. Nor have these data been combined with other datasets, such as the area of land under perennial vegetation, overall land cover change, or area of land under high to medium grazing pressure. The timeframe of five-year reporting, in the context of salinity, is a short one and responses need not be constrained by this lack of data.

In 2003–04 the National Action Plan for Salinity and Water Quality (NAP) and the regional component of the Natural Heritage Trust (NHT) invested $33 million in actions that have a major focus on land salinity. The majority of these funds were disbursed in New South Wales, Victoria and South Australia, primarily for on ground activities.

Whilst salinity problems have a high profile, soil acidity currently affects eight to nine times more land than that affected by dryland salinity. Approximately 50 million hectares of agricultural land (around half the total area) has a surface soil pH value less than 5.5 (NLWRA 2001b). In the next ten years this is expected to increase to a total of 99 million hectares with a pH value of 5.5 or lower.  Acidic soils are those with a pH less than 5.5 and they are usually found in areas of high rainfall. Acid soils are toxic to plants because they can release toxic levels of aluminium and other mineral elements. Acid soil conditions also restrict the availability of nutrients and trace elements vital to plant growth. It is estimated that acidity affects half of all agricultural land in NSW (NSW Agriculture 1999).  The four main causes of soil acidity are:

  • removal of product from the farm
  • leaching of nitrogen below the plant root zone
  • inappropriate use of nitrogenous fertilisers
  • build-up in organic matter (NSW Agriculture 1999).

Soil acidity is a particular problem in many of Australia's low-lying coastal regions, especially in areas where mangrove swamps have been cleared for agriculture or urban development. The exposure of coastal acid sulphate soils  (pH less than 3.5) to the atmosphere results in the manufacturing of huge quantities of sulphuric acid, which reduces water quality in rivers and estuaries and can result in fish kills.

To raise the pH of all soils in Australia to 5.5, it is estimated that Australia would need to apply a one-off application of 66 million tonnes of lime to its acidic soils (NLWRA 2001b). The Audit estimates current agricultural lime use is nearly two million tonnes of lime per year (NLWRA 2001b). This is clearly insufficient to deal with existing acidity problems let alone take into account continuing soil acidification.

There are alternative management options open to land managers wishing to deal with acid soils without using lime. For instance, timing fertiliser applications to match plant demand, avoiding long fallows, and retaining crop residues rather than burning. Data describing the national-scale extent and effectiveness of these responses are unavailable for this commentary.

Some $9.5 million of funding has been supplied through NAP and NHT for major activities to improve soil condition. Despite the magnitude of the soil acidity problem, it seems to receive little attention compared with the attention given to the problem of salinity.