What can you do?
Irrigating with saline water on duplex or clayey soils can result in dangerous levels of salinity building up in the root zones of crops.
In such cases, after two years of irrigation with water with conductivities of 0.7 -1.4 ds/m, the water ponding above the clayey B horizon has been found to have salinity values ranging from 3.0 to 5.0 dS/m. These levels reduce yields significantly and if the soil dries out, will kill crops.
The observations indicate that the depth of this "perched water" varies between about 20 and 30cm and therefore is at least 10cm thick. A layer of perched water like this represents about 1000 - 2500 kg/ha (1-2.5 tonnes/ha) of salt in the upper soil profile, the root zone of all crops.
Where is the salt coming from?
If in each cropping year about two megalitres or about six applications per year of irrigation water at a salinity of 1.0 dS/mare added per hectare then about 1300 kg salt are added pe rhectare per year.
In addition, salt is added as fertiliser. For example if about 600kg/ha/year is applied and about 30-50% of this fertiliser is being used by the crop then about 300 - 450 kg of salt would be added to the soil each year from fertilisers. Thus inputs from irrigation and fertiliser could total almost 1800 kg/ha/year. Therefore, in two years of cropping, assuming minimal leaching, the highest level of salt accumulation (2500 kg/ha) mentioned above could be accounted for from management inputs of fertiliser and saline irrigation alone. Under these conditions, each year the salinity will progressively become worse, becoming
serious in the second year of cropping and even worse in the third and later years of cropping.
In situations where there is sloping ground the situation could be worse as some of the salty "perched" will move laterally through the topsoil and accumulate down slope. As a result salt will build up even quicker in "bottom of slope" locations.
Where saline water is being added to soils with clay subsoils, the natural drainage is often NOT sufficient to allow leaching of the salt being added and salt will build up in the root zone and place crops at risk.
What can you do?
If you are in an area where there may be a soil salinity risk, or where your irrigation water may be saline, there are six critical actions you should take:
- Determine if there is salinity hazard from salt stored already in the soil or regolith or if a saline watertable is already present. This can be done by a consultant experienced in a salinity assessment.
- Determine if your water source contains significant salinity and measure your water salinity regularly. A chemical analysis of the water will also be needed. These tests will tell you if you are at risk from inducing salinity from your applied irrigation water.
- Measure your soil salinity before and during your irrigation year. Measure at the surface and in the zone just above the clayey B horizon. If you are adding salt, the levels at these two locations can increase rapidly.
- Improve your drainage. This may mean subsurface drainage to be effective.
- Ensure you leach out any salt build-up by using additional irrigations, and rainfall. Check that leaching has occurred before you plant again.
- Maintain your calcium and sodium balance using gypsum or lime as appropriate.
Water containing some salt can be used for irrigation but will not be sustainable unless you take active steps to manage it.Adapted from:
Doyle, R. & Bastick, C. 1998. "The Risks of Irrigating with Saline Water" Saltline Issue No 14