Soil Salinity

What is Salinity?

In broad scientific terms, salinity is a measure of salt content.  Salinity is of interest to observers in many areas.  Oceanographers measure the weight ratio between dissolved salts and water in seawater.  Salinity is also measured in the air and the effects of its deposition can be seen in the corrosion of cars and other metallic objects, particularly in coastal areas.  When discussing land-based salinity, we typically refer to the presence of high levels of salt in the surface soils, affecting both irrigated and dryland (non-irrigated) areas.  The focus of this article is soil salinity.

Where does Salinity pose greatest threat?

Figure 1:
National Salinity Hazard Map
 
Source:  http://www.brs.gov.au/land&water/salinity/a4map.jpg

At least 2.5 million hectares (5% of cultivated land) is currently affected by dryland salinity and approximately 5.7 million hectares of Australian farmland has a high salinity risk.  The wheat belt in southwest Western Australia is most afflicted.  Among other areas, the Murray-Darling Basin, which extends from Queensland through New South Wales, Victoria and South Australia, is also affected.  Salinity due to rising water tables affects 326,000 hectares of land in South Australia.

What are the causes of Soil Salinity?

Numerous factors contribute to salinity in the soils of Australia.  In our earlier, wetter history, the country’s interior hosted inland seas.  As those seas evaporated they left vast salt deposits in their wake.  However, the most significant factor is the airborne salt that is mobilised via wind over the oceans and has, for millions of years, deposited over inland areas in rain and dust.  Airborne salt affects coastal areas and stretches far inland in parts of Western Australia and South Australia. 

Figure Two:
Typical changes in water and salt balances following clearing
(for a catchment with rainfall of approximately 700 mm per annum)
 

Source:  http://www.wrc.wa.gov.au/protect/Salinity/

Irrigation and land clearance for agriculture has compounded the problem of increasing salt content in soils by creating an imbalance in the volume of water deposited (recharge) relative to that which is used (discharge).  Rainwater percolates into and is suspended in the subsoil.  The surface of that groundwater is known as the water table.  The extensive root structures of trees trap much of the water falling on the land in the upper layers of the soil, using it for growth and returning the water to the atmosphere via transpiration and evaporation.  In that environment, there is less recharge to the subterranean groundwater.  Further, trees and shrubs exhibit deep root structures that are sometimes able to tap into groundwater throughout the year, even during droughts, effectively increasing the discharge of groundwater supplies.  Agricultural crops replacing those native trees typically do not trap as much rainfall water in their shallow root structures, nor do they develop deep root structures during their brief life cycle from planting to harvest. These features combine to increase recharge and decrease discharge with the result that groundwater rises.

As the water table rises to the surface, salts suspended in the soil add to that already disolved in the water.  The increasingly saline groundwaters enter the plant root zone and challenge the survival of freshwater-dependent crops.  In such situations, livestock can be at risk from both undersupply of necessary grasses and salt-polluted water resources.

Source:  http://www.brs.gov.au/land&water/salinity/index.html

What is the long-term effect of soil salinity?

Analysts project that, by 2050, areas affected by salinity in Australia may spiral to 17.1 million hectares. This would not only have a serious negative impact on our agricultural output, but also on our watercourses and wetlands, water reserves, roads and buildings.  Unimpeded, salinity will make Adelaide's water supply exceed World Health Organisation salinity limits 2 days in 5 by 2020.  It will also reduce irrigation water supplies and degrade existing irrigation areas, at the same time damaging rural infrastructure and many country towns.

Unless preventative measures are taken, soil salinity in South Australia is expected to increase by 60% over the next 50 years, affecting over 520,000 hectares by the middle of the century.  In South Australia alone, costs associated with soil salinity are estimated to rise from a year 2000 base of $44.4 million to $91.8 million per annum by 2050.  These costs include: losses in agricultural production; road, rail and building maintenance; and costs associated with an increase in River Murray salinity.

How may we arrest this devastation?

Replanting appropriate, healthy, highly productive trees to manage surplus groundwater is one possible measure.  However, unless those trees are adapted to salty environments, they will not be able to survive or rehabilitate badly affected salty areas.  Strategic placement of trees in the higher-catchment recharge areas is generally a preferred option, at least in areas where recharge can be reliably identified. 

Viable alternatives to planting trees include perennial crops that develop deep root structures, such as lucerne.  In areas that are likely to remain salty for some time, planting salt-tolerant plants like saltbush may result in improvement.  Agronomists and scientists are developing other salt-tolerant plant species for future release.

Drainage can sometimes be employed to combat salinity in situations where the saline water can be safely disposed.  A commercial approach includes actively removing salt from the land.  By pumping saline groundwater into evaporation ponds, the salt may be removed and sold for commercial production of chemicals or as table salt. 

Source:  http://www.brs.gov.au/land&water/salinity/index.html

Clearly, entrepreneurial farmers will be pleased to find some positive returns in this frustrating environmental issue, which proves that, although these problems are severe, they can be substantially overcome with determined and sustained leadership, the right incentives and scientifically sound interventions.  If you would like to know more about how you can help stem physical and financial losses associated with salinity in your local area, please contact:

Bruce Munday
Communication Coordinator for South Australia
Phone and fax: 08 85 387075
E-mail: bcmunday@senet.com.au

This article was produced by Jennifer Norman (normanj2@az1.bp.com), BP Australia Pty Ltd.

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