Land salinity

Many of South Australia’s soils are inherently saline, due to:

• the presence of inland seas during the Pliocene-Miocene eras, which left behind marine sediments and salts
• natural salt accumulation in the soil from rainwater when it evaporates
• topography, with low-lying basins accumulating salt over time.

Dry saline land is a type of land salinity that occurs in soils with high levels of naturally occurring salt but is not associated with a shallow watertable. In mild situations, dry saline land can also be referred to as transient salinity, where salts are trapped within the soil profile (e.g., due to low permeability clay subsoil) and salts move up and down (with varying influence on plant rootzones) depending on seasonal conditions. In extreme cases, where naturally high salt concentrations extend to the soil surface and visibly affect plant growth, these areas of dry saline land are commonly called magnesia ground or magnesia patches. These are mainly found in the Eyre Peninsula.

Watertable-induced salinity is caused by the movement of salts from shallow, saline ground water into the plant root zone through capillary action. This is a natural feature in some of South Australia’s soils, but this process can be intensified through land management actions (secondary salinity) which increases groundwater recharge.

Dryland salinity is the term given to land salinisation associated with elevated watertables in non-irrigated areas. Historical clearance of native vegetation and replacement with dryland farming (shallow-rooted annual crops and pastures) means that less rainfall is used by plants and this can cause groundwater levels to rise in some situations. Elevated watertables can cause the movement and accumulation of salts towards low-lying areas where discharge of water and salt occurs at or near the soil surface, in plant rootzones, or to adjacent waterways. These changes to the landscape water balance can increase the intensity or extent of land salinisation and/or increase saline groundwater discharge to streams and rivers, including the River Murray.

Irrigation-induced salinity can occur where excess irrigation water is applied, relative to the crop water use and deep drainage, and rising shallow groundwater brings salts up into the plant rootzone.

Mallee seeps have been an emerging issue over the last 10 years. Concern is growing with the formation of new seeps and existing seeps increasing in size. Mallee dune seepage is caused by water imbalances in dune-swale ‘mallee' landscapes utilised for rain-fed annual cropping. Water that drains beyond the crop root zone in sandy soils can be lost to deep drainage, or can form local perched watertables where restrictive subsoil layers limit downward movement. The perched water can discharge at the soil surface such as in lower lying areas, forming seeps. This process has the potential to result in soil salinity by accumulation of dissolved soil salts in the seepage discharge areas. Where restrictive subsoil layers are absent, deep drainage to regional water tables in the Murraylands region contributes to salts ultimately flowing into the River Murray.

Soils with the highest potential for formation of dune seepage are sand over clay soils, and deep sands. The ‘Blanchetown Clay’ geological formation commonly causes the perched water-tables in mallee dune soils in SA.