Land Salinisation

Surface salinity refers to an excess of soluble salts (primarily sodium chloride) in the soil solution which adversely affects plant growth. Surface salinity assesses the current salinity status of the soil. Salinity risk refers to the likelihood of the soil becoming saline given current land use patterns and management practices. Salinity risk is an estimate of the extent of salinisation likely to develop once the water balance reaches a new (post- clearing) equilibrium (van Gool et al. 2005).

Salinity may be classified as either primary salinity or secondary salinity (also called dryland salinity). Primary salinity refers to salinity present within the landscape in its virgin or non-cleared state and is generally confined to certain soil types within the Zone of Ancient Drainage and areas where annual rainfall is <350 mm.

Secondary salinity refers to that which has developed after clearing. Surface salinity is mainly concerned with secondary salinity. Predicting where secondary salinity may develop is extremely difficult because it involves complex interactions among geology, geomorphology, degree of clearing, hydrology, climate, land use history and management (Moore 1998).

Salinity risk has been recently mapped by DAFWA for the South West NRM Region, see here.

Causes

Secondary salinity is the result of a disturbance of the hydrological balance. The primary cause has been the clearing of perennial native vegetation and its replacement with shallower-rooted annual pastures and crops. This has caused groundwater levels to rise and subsequent mobilisation of ancient salt deposits stored within the soil profile. These salts are present naturally, brought in with rainfall and accumulated over a long time. Consequently secondary salinity is most closely related to watertable height. However, landform, site drainage, geology and vegetation also have an effect. Developments which obstruct the natural flow of surface drainage, or irrigation for agriculture (e.g. market gardens) may also result in surface salinity.

Effect

Salinity reduces a plant’s ability to absorb water by adversely affecting the osmotic potential of the root zone solution. It can also induce nutrient deficiencies by affecting plants’ metabolism or by inhibiting the uptake of necessary nutrients. High sodium salt concentrations, for instance, can cause calcium and magnesium deficiencies. The salt tolerance of agricultural plants varies considerably between species and stage of growth. In general, crops tolerate salinity to a threshold level, but above this yields decrease approximately linearly with increasing salt concentrations (Maas and Hoffman 1977).

High exchangeable sodium and magnesium ion concentrations are also often associated with adverse physical soil conditions, such as soil structure decline and soil swelling and dispersion, leading to reduced permeability aeration and root penetration (Smolinski and Scholz 1997). Surface salinity has also resulted in many major rivers becoming too salty for irrigation or human consumption.

Management 

Effective management of secondary salinity must include both management of the catchment and the saltland. The primary aim is to lower the groundwater. This can be done by increasing evapo-transpiration or decreasing surface recharge. Planting perennials such as trees, fodder shrubs and pastures, or re-establishing native vegetation will increase evapo-transpiration. Collecting surface water via interceptor drains and dams will decrease recharge. Groundwater can also be managed by installing deep drainage, however cost is usually prohibitive for dryland agriculture.

Areas suffering primary salinity are not suitable for agriculture and should not be cleared. If they have been cleared, management options will be restricted unless the surface soil has low permeability, when improving infiltration may increase leaching of the salt (Moore 1998).

Key messages across south-west Western Australia (DAFWA Report Card 2013)

Condition and trend

• More than one million hectares of agricultural land in the south-west of WA is severely salt-affected.
• Dryland salinity has expanded throughout most of the south-west of WA since 1998, especially following episodic rainfall events, such as occurred in 1999, 2000, 2005, 2006 and 2007.
• In areas cleared and developed for agriculture after 1960, most watertables continue to rise, despite a decline in annual rainfall.
• As these areas approach a ‘new’ hydrological equilibrium, climate impacts will become the dominant controller of groundwater level trends and the extent of dryland salinity.
• Hydrological equilibrium and the potential extent of dryland salinity may take many decades to develop, especially in drier areas.

Management implications

• The implications of dryland salinity to the agricultural industry are widespread and include reductions in crop yield, area of arable land and land capability.
• The opportunity cost of lost agricultural production is at least $344 million per year. Salinity also physically and financially impacts rural infrastructure, public and private water resources and biodiversity, with costs exceeding those to agriculture.
• Dryland salinity is a major cause of land degradation and remains a potential threat to 2.8–4.5 million hectares of highly productive, low-lying or valley soils, across the south-west of WA.
• Management to contain or adapt to salinity is technically feasible using plant-based and engineering options, though recovery is economically viable in only a few areas.

Projects addressing Land Salinisation

SWCC supports a number of landholders and partners to undertake projects addressing agricultural practices including land salinisation. Two projects addressing land salinisation are:

• Messina: an annual legume for salt-lands

• Enrich Forage Shrubs

Further information on these and many other projects can be found on the Agtrialsites website at www.agtrialsites.com.

The Agtrialsites website is a cross regional NRM WA initiative that aims to provide information on sustainable agriculture projects across Western Australia.

Source:

• Simons J, George R and Raper P (2013). ‘Dryland salinity’. In: Report card on sustainable natural resource use in agriculture, Department of Agriculture and Food, Western Australia.
• Van Gool, D., Vernon, L. and Runge, W. (2008). Land Resources in the South-West Agricultural Region. A shire-based summary of land degradation and land capability. Department of Agriculture and Food Western Australia.