The presence of soluble salts in the soil, groundwater and surface water bodies is a major land degradation problem worldwide. Salinity exacts many economic and environmental costs. These include a reduction in agricultural productivity, a decline in the quality of water supplies for drinking, irrigation and industrial use, damage to urban infrastructure and the loss of biodiversity in both terrestrial and aquatic ecosystems.
Like many land degradation processes, including wind and water erosion, salinity is a natural process. However land use practices, such as clearing and irrigation, have significantly increased the extent of the problem.
The United States Department of Agriculture estimates that, worldwide, 10 million hectares of arable land is lost to irrigation salinity every year. In Australia approximately 2.4 million hectares of land is affected by salinity and 5.7 million hectares of productive land is at risk. It has been estimated that the area of salt-affected land in Australia could increase six-fold in the next 30 to 50 years.
What is salinity?
Salinity refers to the presence of soluble salts in the soil and water, including surface water and groundwater. The salt can be in many forms including sodium chloride, calcium, magnesium, carbonate, bicarbonate and sulphate.
Some soils and landscapes are saline in their natural state, for example inland salt lakes and soils formed from saline parent materials. This is called natural or primary salinity.
Secondary salinity is due to human activities such as land clearing and over-irrigation. These activities result in groundwater rising to the surface, dissolving the salts and then depositing them in the soil.
Groundwater is a layer of soil that is saturated with water that has slowly trickled down through the soil until it cannot go any further because it is stopped by a layer of impermeable soil or rock (bedrock).
Where does the salt come from?
Salt can be found in many old, highly weathered landscapes and originates from:
In undisturbed landscapes, most of the salt is slowly leached into the subsoil, beyond the reach of plant roots
There are two main forms of salinity: dryland salinity and irrigation salinity. In Australia 2.2 million hectares of land is affected by dryland salinity and 160 000 hectares by irrigation salinity.
Causes of dryland salinity
Salinity problems in the soil and surface water bodies occur when more water enters the groundwater system (through a process called recharge) than is discharged from the system. This imbalance causes the water table to rise. As it rises, the groundwater dissolves the soluble salts stored in the subsoil and brings salty water into the reach of plant roots. Evaporation and plant uptake of the water concentrates the salt in the topsoil - where it stays.
The main cause of rising groundwater is the clearing of deep-rooted, perennial native vegetation and its replacement with shallow-rooted, annual crop and pasture species. These introduced species use less water than the native vegetation, resulting in increased groundwater recharge and water table rise.
Land clearing is the past and present cause of dryland salinity. It takes approximately 30 years from the time of clearing for dryland salinity problems to appear, although in some areas they may appear sooner.
Why is dryland salinity such a problem?
The most obvious effect of salinity is the decline in agricultural productivity that is associated with saline soils. High concentrations of salt in the soil are toxic to plants, restrict plant uptake of water and prevent plants from taking up essential nutrients such as calcium.
Aside from declining agricultural productivity, salinity causes many other problems including:
In addition to these economic and environmental costs, salinity also exacts a social cost. This cost includes the emotional and health-related costs of the family farm going bankrupt and the impact of facing the possibility the farm may not be passed onto the next generation.
Management of dryland salinity
Dryland salinity is essentially a water balance problem, since it is the movement of water that controls salt transport. Excess recharge into groundwater stores causes water tables to rise, carrying dissolved salts to the surface. Salinity management, therefore, focuses on reducing groundwater recharge.
The salinity problem is very difficult to solve because:
There are several approaches to salinity control, including protection of remnant vegetation, agronomic measures and engineering solutions. The consensus seems to be that a combination of management practices is the most realistic approach to salinity management.
1. Protection of remnant vegetation
Protecting remnant vegetation can help control recharge and has the added benefit of helping to maintain the biodiversity and heritage values of the landscape.
2. Agronomic measures
Agronomic approaches to dryland salinity management include:
The main problem with agronomic approaches is the introduction of new crop, pasture and tree species into existing farming systems, as this means that new equipment may be needed and new skills developed by the farmers. Additionally, there is uncertainty about the market prospects for new crops and a lack of commercially viable species suitable for low rainfall areas.
Another management option is saltland farming, which involves planting salt-affected land with salt-tolerant grasses or shrubs. Planting discharge areas with salt-tolerant vegetation is an important strategy for decreasing the spread of salinity, reducing the visual impact of saline land, decreasing erosion and obtaining some productivity from salt-tolerant grasses and shrubs. Saltland farming is an important strategy for managing existing saline land, however its contribution to controlling groundwater recharge is minimal.
3. Engineering options
Low flat country is particularly prone to salinity and a combination of paths, channels and contour banks may be used to manage salinity.
Other engineering options include:
Drainage can be advantageous as it effectively removes water from areas where it is problematic and, providing the water is potable (fresh), can supplement farm water supplies. However, engineering works can be expensive to install, operate and maintain, and there is the problem of where to dispose the saline water without causing environmental problems downstream.
Causes of irrigation salinity
Irrigation salinity is the accumulation of salts in the topsoil under irrigation. It is caused by over-irrigation of agricultural land, inefficient water use, poor drainage and the irrigation of unsuitable and 'leaky' soils. All of the above increase groundwater recharge and result in water table rise, bringing salts to the soil surface. The problem is exacerbated by the use of low quality (i.e. salty) irrigation water. Even mildly saline water can cause salinity problems because evaporation and plant uptake remove the water, leaving the salt to accumulate in the soil.
Problems associated with irrigation salinity
Irrigation salinity results in similar problems to dryland salinity including:
Management of irrigation salinity
Like dryland salinity, irrigation salinity is a water balance problem and is managed by reducing groundwater recharge by:
A coordinated approach is generally required with a mixture of engineering and other solutions and over a large area.