Soil Salinity
Introduction
Soil salinity is one of the biggest problems that Australian soils are currently facing. Before the exploitation by European settlers of the soil, the water table was very stable, resulting in neutral salt levels in the soil. The vegetation on the surface allowed the water table to remain stable as the plants required water. However since the exploitation by European settlers such as the removing of natural vegetation and the creation of farms, the water table has been consistently rising and soil salinity has also been rising. Without the aid of naturally adapted native vegetation which have deep roots and tolerance of highly variable climate, the problem of soil salinity in Australia is spiraling out of control and costing the economy almost AU$600 million in profits and this does not include the costs of damages.
Excessive soil salinity in the soil has now resulted in the salt contaminating the waterways, leading to dissolved salts in water. This affects further affects agriculture, drinking water supplies and the health of the ecosystem. Soil salinity is a major issue in South-Western Australia and regions near the Murray-Darling Basin including but not limited to New South Wales, Victoria and South Australia. It is estimated 5.7 million hectares of land is at risk of salinitization and this number is expected to rise to 17 million hectares by 2050 if this problem is not treated. The state that is most at risk from salinitization is Western Australia which has 70% of the affected area in Australia. South Australia and New South Wales is affected to an extent however the problem is getting worse in these states.
Excessive soil salinity in the soil has now resulted in the salt contaminating the waterways, leading to dissolved salts in water. This affects further affects agriculture, drinking water supplies and the health of the ecosystem. Soil salinity is a major issue in South-Western Australia and regions near the Murray-Darling Basin including but not limited to New South Wales, Victoria and South Australia. It is estimated 5.7 million hectares of land is at risk of salinitization and this number is expected to rise to 17 million hectares by 2050 if this problem is not treated. The state that is most at risk from salinitization is Western Australia which has 70% of the affected area in Australia. South Australia and New South Wales is affected to an extent however the problem is getting worse in these states.
This oblique photograph shows salt floating on top of the water. The amount of salt shows the extent of the problem and that the salt is now contaminating water sources such as the Murray Darling Basin.
Source: http://www.environment.gov.au/water/quality/publications/factsheet-salinity-and-water-quality
Causes
There are two types of salinity:
Primary Salinity: Primary salinity develops naturally. This occurs mainly in areas that receive insufficient amounts of rainfall to leach the salts that accumulate in the soil profile. The low rainfall allows the evaporation of soil moisture to be very high, further allowing the soil salinity to rise. Primary Salinity occurs throughout the world in arid areas and in Australia, an estimated 29 million hectares face this problem. This normally occurs in salt marshes, salt lakes and slat flats (14 million hectares) and naturally saline subsoils (15 million hectares) due to limited groundwater to take the salts to the surface.
Secondary Salinity: Secondary Salinity develops due to interference of the environment by humans. It is a result of changed land use and management. There are two main types of secondary salinity: "dryland salinity" and "irrigation-induced salinity".
In Australia, the main cause of dryland salinity is due to agriculture as native vegetation is replaced by crops and pastures thus altering the water balance. Unlike native crops, these crops have very shallow roots which do not allow the crops to regulate the salinity levels or efficiently use all the moisture in the soil. As a result of this imbalance, more water is allowed to pass through the soil, as groundwater and this raises the water table which contains stores of salt. The rise of salt to the surface via the water table is then left behind when the water evaporates, increasing the soil salinity.
Irrigation-induced salinity occurs when excess levels of water is applied to crops which then travels past the soil reaching the water table. This excess water then mobilizes the salt deposits in the water table and bringing the salt to the surface gradually. This occurs much more rapidly then dryland salinity and the consequences are also very severe. The diagram below demonstrates what happens when the water table rises.
Primary Salinity: Primary salinity develops naturally. This occurs mainly in areas that receive insufficient amounts of rainfall to leach the salts that accumulate in the soil profile. The low rainfall allows the evaporation of soil moisture to be very high, further allowing the soil salinity to rise. Primary Salinity occurs throughout the world in arid areas and in Australia, an estimated 29 million hectares face this problem. This normally occurs in salt marshes, salt lakes and slat flats (14 million hectares) and naturally saline subsoils (15 million hectares) due to limited groundwater to take the salts to the surface.
Secondary Salinity: Secondary Salinity develops due to interference of the environment by humans. It is a result of changed land use and management. There are two main types of secondary salinity: "dryland salinity" and "irrigation-induced salinity".
In Australia, the main cause of dryland salinity is due to agriculture as native vegetation is replaced by crops and pastures thus altering the water balance. Unlike native crops, these crops have very shallow roots which do not allow the crops to regulate the salinity levels or efficiently use all the moisture in the soil. As a result of this imbalance, more water is allowed to pass through the soil, as groundwater and this raises the water table which contains stores of salt. The rise of salt to the surface via the water table is then left behind when the water evaporates, increasing the soil salinity.
Irrigation-induced salinity occurs when excess levels of water is applied to crops which then travels past the soil reaching the water table. This excess water then mobilizes the salt deposits in the water table and bringing the salt to the surface gradually. This occurs much more rapidly then dryland salinity and the consequences are also very severe. The diagram below demonstrates what happens when the water table rises.
Effects
The effects of soil salinity becomes more severe the longer the problem is present in a particular area. High concentrations of salt poses many threats to the environment, animals and humans. It also affects agriculture and infrastructure, thus affecting the economy as well as the well-being of the environment. Excess levels of salinity in the soil may lead to the poisoning of native vegetation and crops which may lead to decline in biodiversity of flora and fauna in Australia and the increase of more salt-resistant species. Increase of another species may lead to disruption to the structure of the ecosystem.
The lack of ground cover which will be the result of decrease of vegetation will make the soil more prone to water and wind erosion. This will cause increased pollution of water from both the salt and rogue soil particles. This makes the water unsuitable for drinking by both animals and humans leading to highly complex ecosystem being threatened. Increased soil salinity also reduces crop yields as it impairs the growth and health of crops that are salt-intolerant. Salt also accelerates the time it takes for metal to corrode, so farm machinery are also being impacted by the increased soil concentrations. This comes at a huge cost for the maintenance of both the crops and the machinery.
Although the increase of soil salinity levels directly and indirectly affects environments and to some level, the economy, some ecosystems have adapted to the increased levels of salt but there is a limit to how far they can adapt. Steps have to be taken by the government and groups to ensure that soil salinity becomes less of a problem.
The lack of ground cover which will be the result of decrease of vegetation will make the soil more prone to water and wind erosion. This will cause increased pollution of water from both the salt and rogue soil particles. This makes the water unsuitable for drinking by both animals and humans leading to highly complex ecosystem being threatened. Increased soil salinity also reduces crop yields as it impairs the growth and health of crops that are salt-intolerant. Salt also accelerates the time it takes for metal to corrode, so farm machinery are also being impacted by the increased soil concentrations. This comes at a huge cost for the maintenance of both the crops and the machinery.
Although the increase of soil salinity levels directly and indirectly affects environments and to some level, the economy, some ecosystems have adapted to the increased levels of salt but there is a limit to how far they can adapt. Steps have to be taken by the government and groups to ensure that soil salinity becomes less of a problem.
Effective Solutions
There are quite a number of solutions that can minimize the risk or excess soil salinity or treat the problem. The more effective and more commonly used solutions and preventive measures are:
Planting salt-tolerant plants: There are two main plants currently being used to treat soil salinity; Swamp Saltbush (Atriplex amnicola) and Saltgrow. The swamp saltbush, a bush native to Western Australia (bottom left) is highly resistant to salt and is often used in the process of rehabilitation of land that has been severely affected by soil salinity. The popularity of this bush is also due to the long-term survival of this bush in very saline soils. A study has also shown that sheep like the bush however the bush is quite difficult to plant. This drawback has enabled the development of a tree eucalyptus hybrid called the saltgrow (bottom right). It was crossbred with the properties of river red gum (tolerance to salt, drought, waterlogging), flooded gum and Tasmanian blue gum (fast growth and commercial properties). It is often planted in areas with excess salt or waterlogging as a result of the rising water table. A trial has found that it has only taken ~15 years to allow normal operations in a area of 13 hectares where the water used to be 50 cm deep in some areas with signs of salt clumps forming on the water.
Planting salt-tolerant plants: There are two main plants currently being used to treat soil salinity; Swamp Saltbush (Atriplex amnicola) and Saltgrow. The swamp saltbush, a bush native to Western Australia (bottom left) is highly resistant to salt and is often used in the process of rehabilitation of land that has been severely affected by soil salinity. The popularity of this bush is also due to the long-term survival of this bush in very saline soils. A study has also shown that sheep like the bush however the bush is quite difficult to plant. This drawback has enabled the development of a tree eucalyptus hybrid called the saltgrow (bottom right). It was crossbred with the properties of river red gum (tolerance to salt, drought, waterlogging), flooded gum and Tasmanian blue gum (fast growth and commercial properties). It is often planted in areas with excess salt or waterlogging as a result of the rising water table. A trial has found that it has only taken ~15 years to allow normal operations in a area of 13 hectares where the water used to be 50 cm deep in some areas with signs of salt clumps forming on the water.
Deep Drainage and Pumping is a vital precautionary step preventing the rise of the water table which contains most of the salt however it is quite expensive and most farms do not have the financial power to construct such a system. The primary type of drainage is the horizontal drainage system. The system consists of buried pipes wrapped with water absorbing materials at the level that is wanted for the water table. There is normally an open ditch drain connected to the pipe drain for surface water to enter and this reduces the amount of water entering the water table. The pipes then leads to a collector which holds and discharges the water into the main waterways. However as figure 2 shows, the effectiveness of these pipes are reduced the further away from the drain thus to maintain a highly effective system, there has to be lots of pipes.
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Establishing salt interception schemes to divert saline water to evaporation basins is similar to establishing deep drainage however the piping leads to an evaporation basin. A bore and pump system pumps the water from the water table to the evaporation basin which is a large area designed to use evaporation to leave salt behind while the water is being evaporated. This left over salt can be processed and sold, this effectively reduces the salinity levels in the water and soil. Such system has been implemented in the Murray-Darling to reduce the very saline waters. As figure 3 shows, before the implementation of the network all the groundwater (which is very salty) eventually ends up in the river, increasing the salinity levels. After the implementation of the system, the groundwater is pumped to a evaporation bond. This has been hugely successful as it has approximately reduced the salt levels by 200 EC*.
*EC (Electrical Conductivity)- Based on the principle of the fact that salt conducts electricity.
Total Dissolved Salts (mg/L) = EC (μS/cm at 25oC) x 0.6 This equals to 120mg/L. This is a lot of salt.