Salinity
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'Salinity' describes the salt content of soil or water.
Dryland Salinity
Causes of dryland salinity:
All rain brings in small amounts of salt – typically 20 to 50 kilograms per hectare per year across the agricultural regions. Because of our relatively low rainfall and relatively flat landscape, it can take a long time (sometimes tens of thousands of years) for that salt to get carried back to the ocean via our rivers. When plants use the rainwater, they leave the salt behind which then builds up in the landscape – sometimes in massive amounts.
Link between Clearing native vegetation and Dryland Salinity?
The first clear evidence of the link between clearing of native vegetation and the appearance of dryland salinity was gathered by a West Australian railway engineer, Walter Ernest Wood.Wood noted that the native forests and grasslands that originally occurred across southern Australia used nearly all the rainfall. However, when these were cleared by farmers to grow crops and pastures, some rainwater percolated into the soil profile, and the groundwater rose towards the soil surface bringing with it the salt stored in the soil profile.
When the watertable reaches about 2 m from the soil surface, salt begins to move into the plant root zone, and plant growth and survival became affected.
More recently, additional causes of dryland salinity (especially transient salinity) have been associated with overgrazing and a decline in soil health.
Primary salinity
There are many areas in Australia that were already saline at the time of European settlement – a high proportion of the saline areas in Australia are ‘natural’ and are not the result of agricultural activities.
Secondary salinity
Secondary dryland salinity differs from primary dryland salinity in that it can be directly attributed to human impact – principally the clearing of native vegetation for the growth of annual crops and pastures, and the subsequent soil degradation.
Classic signs of secondary salinity include:
Dryland Salinity
Causes of dryland salinity:
All rain brings in small amounts of salt – typically 20 to 50 kilograms per hectare per year across the agricultural regions. Because of our relatively low rainfall and relatively flat landscape, it can take a long time (sometimes tens of thousands of years) for that salt to get carried back to the ocean via our rivers. When plants use the rainwater, they leave the salt behind which then builds up in the landscape – sometimes in massive amounts.
Link between Clearing native vegetation and Dryland Salinity?
The first clear evidence of the link between clearing of native vegetation and the appearance of dryland salinity was gathered by a West Australian railway engineer, Walter Ernest Wood.Wood noted that the native forests and grasslands that originally occurred across southern Australia used nearly all the rainfall. However, when these were cleared by farmers to grow crops and pastures, some rainwater percolated into the soil profile, and the groundwater rose towards the soil surface bringing with it the salt stored in the soil profile.
When the watertable reaches about 2 m from the soil surface, salt begins to move into the plant root zone, and plant growth and survival became affected.
More recently, additional causes of dryland salinity (especially transient salinity) have been associated with overgrazing and a decline in soil health.
Primary salinity
There are many areas in Australia that were already saline at the time of European settlement – a high proportion of the saline areas in Australia are ‘natural’ and are not the result of agricultural activities.
Secondary salinity
Secondary dryland salinity differs from primary dryland salinity in that it can be directly attributed to human impact – principally the clearing of native vegetation for the growth of annual crops and pastures, and the subsequent soil degradation.
Classic signs of secondary salinity include:
- dead remnants of vegetation that grew on the site before it became saline
- colonisation by salt-loving species(as seen in Canada with coastal species growing inland on the sides of the roads),
- increases in waterlogging and inundation, with fences disappearing into saline lakes.
Secondary dryland salinity is caused by 3 main processes:
- Rising watertables
The lower water use of annual crops and pastures compared to the native vegetation they replace often leads to water draining below the root zone, where it becomes part of the groundwater mixing with the salt stored in the soil profile. The watertable then rises, bringing stored salt to the soil surface. When this salt reaches the root zone and inhibits plant growth and survival, we say that the site is affected by dryland salinity. This has been the most widely accepted cause of dryland salinity in southern Australia, and was the subject of most of the research undertaken through the National Dryland Salinity Program and formed the basis for the Salinity Audit in 2000.
- Transient salinity and Dry saline land
There are increasing areas of ‘dryland salinity’ being identified that are not the result of rising saline groundwater bringing salts into the root zone. It is the result of the seasonal movement of salt into and out of the soil profile. When transient salinity, concentrated by evaporation, occurs within the root zone of crops it can be detrimental to their growth - Irrigation salinity
Excessive irrigation can lead to locally elevated watertables which in time can result in soil salinity if the irrigation water is slightly salty. This is particularly the case if annual rainfall is insufficient or if the subsoil is so impermeable that the salt cannot leach deeper into the soil profile.
Irrigation salinity is a problem for agriculture and horticulture, and also affects parks, gardens and sporting fields in urban areas. Modern irrigation practices have helped reduce the incidence of irrigation salinity, but there are many situations where irrigators are forced to adopt more salt-tolerant crops, pastures or turfs. The growth of salt and waterlogging turfgrasses is being actively pursued for urban areas where irrigation salinity occurs.
Explore your understanding
Q1) Using a flow chart- explain the process of Secondary Salinity
Q2) Explain 3 effects that salinity has on the environment
Q3) What is the difference between primary and secondary salinity?
Q4) What is the link between salinity and erosion?
Q2) Explain 3 effects that salinity has on the environment
Q3) What is the difference between primary and secondary salinity?
Q4) What is the link between salinity and erosion?