'Insalting' the earth

Salt is a natural part of soil, however, when the amount becomes so great that non salt-tolerant plants are adversely affected, the soil is termed saline. Western Australia has over 70 per cent of Australia's reported dryland salinity. In 1982, 264 000 ha of Western Australia was affected by salt. In 1990, nearly 443 000 ha of once productive agricultural land was affected by salinity. In 1996, nearly 1 804 000 ha was salt-affected. The 1990 figures have been estimated as representing a loss of $44.2 million in gross production. This figure represents only the loss of agricultural production. Losses of fresh water supplies, habitats and amenities are also substantial. If left unchecked, salinity could affect up to 30% of the agricultural region. Data collected in 1996 has an estimated 1.8 million hectares of farmland salt-affected to some extent and this area could double in the next 15 to 25 years and then double again before reaching an equilibrium.

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Where does the salt come from?

Salt in soils may come from weathering of rocks and from rainfall. Salts have accumuluated mainly come from rainfall. Measurements of salt content in rainfall have established that annual salt deposition via rainfall ranges from more than 200 kg/ha (about 18 kg on a suburban block) near the west coast, to 18 kg/ha at inland sites such as Merredin. The native vegetation uses the rainfall, but leaves the salt behind in the soil.

Where rainfall is high enough, water passes through the soil to leach out salt, thus preventing its accumulation. In low rainfall areas, however, large amounts of salt accumulate in the soil. Even in these areas, a balance is eventually reached between the amount of salt entering the soil and the amount that leaves it. The amount of salt stored in our soils is staggering. CSIRO scientists have measured salt storage in soils in areas of the Darling Ranges. An average of 170 tonnes of salt per hectare was measured in areas with more than 1000 mm average annual rainfall, and this rose to 810 tonnes per hectare in areas with less than 800 mm average annual rainfall. The majority of this salt is stored in the subsoil clay.


How does soil salinity develop?

It needs to be understood that Western Australia's native vegetation is extremely efficient at using the rainfall. Measurements in an uncleared catchment near Newdegate indicated that less than 1 mm of rainfall in 10 years managed to get past the root zone to add to the water table.

The cause of salinity in Western Australia has long been known. An article published in 1924 first identified the removal of the native vegetation as the prime environmental disturbance that caused the salinity problem. Removal of the deep-rooted native vegetation changes the water cycle. In farming it is replaced by annual crops and pastures. These are shallow-rooted, and by comparison, they use much less rainwater than does the native vegetation. This results in more of the rain water reaching the water table causing it to rise and bringing the stored salts closer to the surface.

Once the saline water table rises to one or two metres below the soil surface in valley floors, capillary action transports enough salts into the root zone to have an adverse effect on plant growth. At other sites the saline water table may seep from the soil when forced to the surface by high points in the bed-rock (for example, dolerite dykes) or where it intersects a change in the surface slope.

What can we do about salinity?

Those parts of the catchment that allow water to enter the water table may be nearby or some kilometres away. This has implications for designing a strategy to combat soil salinity that is equitable among neighbouring farmers.

Salinity is a catchment problem and any measures to remedy and prevent it must be a community decision. A catchment is considered to be an area that sheds water to a common drainage. With respect to salinity problems, there are two broad catchment units: when water comes to the surface, (discharge area), when the water table rises, and where water enters the system, (recharge area), and is added to the water table. Management of land varies depending on its location within the catchment. This may involve farmers carrying out landcare activities for someone else's benefit further down the catchment.

In intake areas, landholders aim to use more water by:

• Not clearing native vegetation in areas which are predisposed to causing salinity elsewhere;
• Replanting areas to high-water-use trees and shrubs;


• Planting perennial pasture species;


• Alley farming systems; and


• Tillage practices, fertiliser regimes and choice of crop varieties to maximise crop water use and minimise run-off.


• Management in discharge areas aims to seek production while stabilising and reducing the area affected by salinity. Procedures include:
• Using trees to lower the water table;
• Planting salt-tolerant plants;
• Surface water control into dams;
• Earthworks to control flooding;
• Forming Catchment Groups to lead to better district planning;
• Alley Farming;
• Minimum tillage;
• Waterway revegetation;
• Alternative crops;
• Saltland fodder shrubs/trees; and
• Stubble retention.

References:
Anon., (1996) Salinity: A situation statement for Western Australia; A Report to the Minister for Primary Industry and Minister for the Environment, p.1-6.
Hooper, Kate and George, Richard, (1995), Healthy Farmland Healthy Bushland, Landscope, Vol.10., No.3.,CALM, Perth, Western Australia

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