Soil salinity

Many geological formations beneath soils used for agriculture have marine origins and often contain high levels of salts (particularly sodium chloride).

In many regions of Australia, potential evaporation is higher than the amount of rainfall. Consequently, most of the water that infiltrates into the soil is extracted by plants to be transpired, or it evaporates from the soil surface. This tends to leach salt contained in the rain down the profile and helps reduce salinity in the surface soil while increasing the salinity of subsoils. However, there are many dry inland regions of Australia where the soil is saline to the surface and do not receive sufficient rainfall to leach salt out of the soil profile. These tend to be areas in the lowest parts of the landscape where runoff water accumulates during rare rainfall events, and the water evaporates to leave the salts behind. This has occurred over millions of years.

Fluctuations in water table depth are influenced by the amount of recharge (water entry) and discharge (water loss). Groundwater levels rise when the input of water to the groundwater system (recharge) exceeds the amount of groundwater leaving the system (discharge).

Permeable soil profiles aid rapid rainfall infiltration, allowing water to soak in quickly and enter the groundwater. The influence of changes in the amount of groundwater, as well as contributing factors such as vegetative cover, soil management, disruption of natural drainage lines, irrigation and topography can contribute to the formation of potential recharge areas. Discharge areas can be associated with barriers to the movement of groundwater (for example, clay layers and non-permeable rock) and groundwater pressure, forcing water to the surface. However, they may also be associated with local groundwater systems, where shallow groundwater will flow and discharge into areas that can then be affected by salinity. Water loss from a groundwater system can occur through evaporation, pumping, seepage, springs and baseflow to rivers and lakes.

Where groundwater levels are about one metre from the surface, the capillary rise of saline groundwater causes salts to accumulate at the soil surface and in the root zone. This salt may emerge as a ‘saline seep’ or where a flat, bare area of naturally saline clay subsoil is exposed by erosion. Evaporation at the surface leaves behind concentrated salt deposits known as ‘saline scalds’. Clay soils with a high bulk density in the subsoil and slow drainage tend to accumulate salt.

Leaching (downward movement) of salts down the soil profile largely removes salt accumulated in the root zone. The lateral movement (parallel to the slope) of water can result in salt accumulation in downslope soils. Land management changes such as clearing and grazing can lower water use and increase the movement of salts towards the soil surface, as well as lower parts of the landscape – increasing the risk of salinity.

Subsurface drainage in leaching soils helps remove localised salt beyond the rooting zone of plants, draining into groundwater and/or waterways. However, in soils that are not freely drained, the lateral movement of water may cause saline water to seep to the surface elsewhere. In a confined system, water cannot drain away, resulting in rising groundwater levels, bringing salt to the surface. In a hill and valley landscape, a rising water table causes the area of naturally saline soils to gradually extend upslope, bringing salt towards the soil surface. 

Replacement of deep-rooted native vegetation (often trees, including summer-active species) with shallower-rooted annual crops and pastures results in less evapotranspiration over the year. Consequently, there is greater percolation of water below the root zone. As a greater proportion of the rainfall moves down through the soil as deep drainage, the groundwater begins to rise. The response time of a regional groundwater system to this increased deep drainage varies from a few years to decades. Sometimes, runoff is also increased. Droughts greatly reduce deep drainage losses from agricultural systems, causing the expansion of salt-affected land to be slowed.

Water storage and delivery channels associated with irrigation often leak and cause local water tables to rise and surface salinity to develop. However, most of the salinisation under irrigation comes from inefficient irrigation practices whereby water is applied in excess to crop demand over a number of years. In much the same way that clearing deep-rooted vegetation changes the amount of rainfall percolating to groundwater in dryland agriculture, too much water percolating below the root zone of irrigated crops causes the groundwater to rise gradually.

Water quality is important when applying irrigation water to a crop. It is vital to know its salt content and the proportion of the salt that is sodium chloride. Sodium chloride is undesirable firstly because sodium and chloride can be toxic above certain concentrations and secondly because sodium can make the soil more sodic.

The following equation can be used to estimate the amount of salt imported via irrigation water:

Salt input (t/ha) = Megalitres irrigation water (per ha) x EC (dS/m) x 0.64

Causes of salinity include barriers to the movement of water which can include roads, railways and land reformation.

Use crops and pastures (e.g. cotton, barley, couch grass) that are less prone to the adverse effects of salinity than others (e.g. beans, apples, clovers).

Where saline areas have been identified, fence them off to control stock access. Often improvements in pasture cover can be achieved just through the exclusion of grazing animals. Establish salt land pasture species. Excess water with the potential to aggravate salinity problems can be intercepted and re-directed via ground works such as broad surface drains. Subsoil drainage systems can be installed to capture deep drainage water and evaporation basins constructed to store it.  Such works generally require formal authorisation from government agencies.

Once water levels are within two metres of the surface, the capillary rise of saline groundwater causes salts to accumulate at the soil surface and in the root zone.

Use rainwater more efficiently to minimise deep drainage losses. This could be done by growing higher biomass crops, opportunity cropping, permanently fencing off pasture areas, stock exclusion and avoiding long fallows. Replacing long fallow systems with opportunity cropping or cover crops in Queensland should be considered as a means of increasing water use and reducing salinity problems, as well as protecting soil from erosion.

Intercept excess water and re-direct via ground works such as broad surface drains and reverse interceptor drains. Install surface, subsurface drainage systems and evaporation basins to capture and store deep drainage water. Evaporation basins should be sealed with impermeable clay or a synthetic liner to minimise leakage.

Protect and manage remnant vegetation. Plant perennial pastures and trees/shrubs in recharge areas. Establish pastures on poor producing land or if a mixed farming system, consider alternate components of rotations such as lucerne. 

Identify and remove subsoil constraints so roots are able to access water deep in the profile, increasing water use. Introduce deep rooted perennials (i.e. trees, shrubs, lucerne) or pastures capable of growing throughout the year. Improve drainage where temporary ponding occurs after rainfall. The adoption of zero tillage combined with stubble retention has shown increased leaching of chloride salts and movement of the salt ‘bulge’ deeper in the profile.

Water has to be applied to flush out the salts that would otherwise accumulate to the point where crop growth is retarded. This is referred to as the ‘leaching fraction’ and varies for a range of water qualities and crop tolerances. Drainage systems with appropriate drainage capacity should be installed.

Introduce appropriate irrigation scheduling and monitoring, and reduce water loss through evaporation from open water ducts and leakage from unlined channels. Assess the quality of irrigation water. In general, water is considered a low salinity risk if EC is below 0.75 dS/m.

• Monitor the salinity status of the root zone.
• Manage subsoil constraints so that deep and vigorous root growth is encouraged.
• Monitor the depth and quality of groundwater using piezometers.
• Monitor the quality of irrigation water using an EC meter.
• When excess salt has to be leached from the root zone, aim to use the minimum possible amount of water (the leaching requirement).
• Install surface or subsurface drain systems where appropriate.
• Maximise the use of rainwater, which usually has much less dissolved salt than irrigation water.
• Where irrigation water is stored in dams, do everything possible to minimise evaporation losses. Evaporation leads to an increase in the concentration of dissolved salts in the water that remains in the storage.
• Minimise seepage losses from storage dams and evaporation basins, which may cause water tables to rise.

• Use crop varieties that maximise water use.
• Grow healthy crops and pastures that will use more soil water. This includes controlling crop/pasture diseases, pests and weeds.
• Introduce conservation farming techniques such as no-till, controlled traffic, and crop/pasture rotations, which will improve soil health and increase organic matter. Maintain soil fertility, pH and structure to encourage growth of high yielding crops.
• Avoid long fallows and bare ground.
• Mulch exposed ground where possible to help retain soil moisture.
• Keep all remnant vegetation and establish new native vegetation.
• Work with your neighbours, Landcare and irrigation groups to develop sub-catchment and catchment strategies to manage groundwater.
• Identify potential saline areas (can be done through an electromagnetic survey or a survey of pasture composition).
• Monitor the watertable beneath the farm to determine if the watertable is rising and whether irrigation practices are efficient.
• Plant salt-tolerant species/varieties in the problem areas and avoid over-grazing.
• Where possible, remove subsoil constraints so that deep and vigorous rooting is encouraged. This is particularly important for recharge areas.
• Maximise perennials in the farming system.