Effects of irrigating with saline water on soil structure
By applying irrigation water of a high salinity, you not only increase the risk of soil salinity, but the level of sodium within the soil. This is known as sodification of the soil.
Sodic soils in the Shepparton irrigation region
Sodic soils are wide-spread in the Shepparton irrigation region. They are often identified by:
- poor infiltration of irrigation water or rainfall
- increased run-off
- poor seedling emergence
- surface crusting.
What is sodicity?
Sodicity is the name given to water or soil that has a high concentration of sodium compared to calcium and magnesium.
Water is said to be sodic when the ratio of sodium to calcium and magnesium, known as the sodium adsorption ratio (SAR), is greater than 3.
In the Shepparton irrigation region, groundwater is commonly saline and sodic with an average salinity of 2340EC units (i.e. μS/cm) and a SAR of 15 respectively.
Soil is said to be sodic when the sodium exchangeable percentage (ESP) is greater than 6.
Exchangeable sodium percentage is the amount of exchangeable sodium that can be replaced by another ion, such as calcium. Above this value, soils are likely to break apart (disperse) in water if soil salinity is low.
These values are not definitive and should be used only as a general guide.
Salinity, clay content and other soil chemical measures also influence the extent to which sodicity will affect soil structure. Soil structural decline is more pronounced on heavier soil types such as Goulburn clay loams.
Ironically salinity actually reduces the effect sodium has on the development of poor soil structure. It provides a stabilising influence on the soil. This means that with increasing salinity, higher concentrations of sodium can be tolerated by the soil. However, too high a salt or sodium content can be toxic to plants.
Causes of sodicity
When irrigating with saline water, the sodium ions displace other more useful ions such as calcium and magnesium. The higher the salinity of irrigation water, the greater the impacts on soil structure. If irrigation with saline water is continued, over time the soil becomes concentrated with sodium.
When fresh, less saline water is applied, clay bonds become weak. The weakening of the bonds results in the swelling of clay particles, which then disperse forming a soil with little or no structure. The clay particles move through the soil clogging pores and reducing the volume of water that can move into or through the soil profile.
The difference between salinity and sodicity
Sodicity is often confused with salinity. Salinity is the total concentration of salts dissolved in water or soil. It affects the ability of plants to extract moisture from the soil. Soil salinity develops through the accumulation of salts, predominantly sodium chloride, and is measured by their electrical conductivity (EC).
Sodicity affects plant growth by altering the soil structure. Poor soil structure occurs when clay particles clog soil pores leading to surface crusting, reduced water infiltration and low aeration of the soil profile. As a result there is less water available for plant growth.
Irrigating pastures with saline groundwater (greater than 800 EC on a Lemnos loam) over the summer period. This results in high soil salinity and sodicity. When channel water irrigation or winter rain follows, the salts are washed down the profile and soil salinity can be reduced to the point where the soil particles disperse.
Irrigation with saline water is stopped after a number of years and is followed by fresh water irrigation or rainfall. This results in low soil EC and potentially poor soil structure. This will most likely occur when groundwater salinity becomes too salty or there is a change in crop.
Identifying a sodicity problem on your farm
Sodic soils may develop slowly over a number of years. Visual signs that sodicity is present on your farm may include:
- increased run-off from pastures, due to decreased infiltration
- poorer emergence of seedlings
- increased surface crusting
- increased soil strength in dry sodic soils.
Good groundwater practices means:
- less money and time spent applying gypsum
- a larger amount of water available for plant growth
- increased plant productivity
- more efficient irrigation.
Mixing of groundwater and surface water supplies through the whole irrigation season creates a balance between:
- leaching salts from the soil profile
- maintaining the right soil salinity to stabilise sodic soils.
Applying gypsum and increasing soil organic matter are additional methods used to stabilise fragile sodic soils.
Shandying your groundwater
Shandying groundwater to 800 EC units at every irrigation will ensure that soil structure is maintained. Alternating between straight groundwater and channel water irrigations, or starting the season with straight groundwater irrigation will increase the chances of soil structural problems developing over time.
It is recommended that soils receiving groundwater be tested on a regular basis to assess soil structure.
A typical soil test will determine:
- exchangeable sodium percentage (ESP)
- soil salinity
- clay dispersion
- soil organic matter content
- soil texture
- soil Ca/Mg ratio.
Land forming on sodic soils
Land forming on sodic soils increases the risk of reducing soil structure. Sodic soils are less able to tolerate physical disturbance. Applying gypsum to the soil during land forming aids in stabilising the soil.
Using gypsum or lime to improve soil structure
Short to medium term improvement in soil structure is achieved by applying gypsum (calcium sulphate) to the soil:
- works by replacing the sodium on the surface of clays with calcium
- tends to reduce surface clay dispersion and subsoil swelling
- the sodium released in the chemical reaction is leached below the root-zone.
Gypsum is best applied to soils during land forming and before sowing pastures:
- It should also be applied on a more regular basis after the summer irrigation season before the onset of winter rains.
- The amount and regularity of applying gypsum is best determined by soil tests every 1-2 years.
Lime (calcium carbonate) acts in the same way as gypsum:
- improves soil structure, but may act more slowly.
- lime is best suited on soils with a pH less than 6.5.
Organic matter content
Increasing the amount of organic matter in your soil profile will help to bind the soil together.
You should aim to have more than 2% organic carbon levels in your topsoil (0-10 cm).
For more information on managing your groundwater to prevent the development of unstable soils contact Nick O'Halloran at Tatura on (03) 5483 1101.
- Surapaneni A, Olsson K and Hall R. Understanding soil sodicity. Soil Sense Note Number C-14.
- Bethune M.G. and Batey, T.J. (2001). Impact of soil hydraulic properties resulting from irrigating saline sodic soils with low salinity water. Australian Journal of Experimental Agriculture, 2002, 42, 273-279.
Thanks to the Natural Heritage Trust, Goulburn-Broken Catchment Management Authority and the Department of Natural Resources and Environment.