The Loam Ranger – Salinity classes of irrigation water – how can they be used?
Dear Loam Ranger,
I cannot get access to enough clean water for irrigation. All I have is groundwater with some salt in it. Can I still irrigate with it?
The good news is that it’s possible to irrigate with moderately saline water if you are careful.
Irrigation water is conventionally divided into irrigation suitability classes according to the salinity content:
Salinity (mS/cm) |
Class |
Interpretation |
0–0.28 |
1 |
Low-salinity water suitable for use on most crops except on soils with very low permeability |
0.28–0.80 |
2 |
Medium-salinity water posing few restrictions if reasonably drained soils allow some leaching. Under high evaporation and poor drainage, some precautions are needed |
0.80–2.3 |
3 |
High-salinity water. Cannot be used on soils with poor drainage. Even with good drainage, leaching is essential, and only salt-tolerant crops can be grown |
2.3–5.5 |
4 |
Very-high salinity water. Can be used only on well-drained soils with leaching, constant monitoring and salt-tolerant crops |
>5.5 |
5 |
Extremely high-salinity water. Almost totally unusable even with strict precautions. Occasional emergency use in mild weather only |
Classes 1 and 2 don’t require much thought, because the water quality is considered excellent and good, respectively. Class 5 water is pretty much regarded as emergency irrigation water only, and the use of water of this quality is strongly discouraged.
That leaves classes 3 and 4. So what are the special management requirements, techniques and considerations required for their use?
Leaching
Two of the main considerations in using classes 3 and 4 irrigation water safely are soil permeability and access to a reliable “clean” water source to leach the salts down through the soil profile and so prevent accumulation. The salinity of classes 3 and 4 water is much less than that of sea water, so the actual salt content of the irrigation water is not the limiting factor. Of greater importance is the salt concentration in the soil water. When the water is initially applied, the salt concentration can be considered “low”. But as the plants draw up the water and the sun begins to dry the soil, the salt concentration starts to increase, as the same amount of salts is now diluted in less water. When this happens, a fresh irrigation will both wash the accumulated salts further down into the soil profile, beneath the reach of the roots, and dilute any remaining saline soil water back to the “low” level.
Key to this management technique is keeping the soil moisture content high. This is tied in with the concept of leaching requirement – supplying water in excess of the soil’s field capacity to ensure that salts are moved down the profile. An irrigation has exceeded field capacity when water starts moving below the root zone (something you can examine for yourself in a trench).
Salt accumulation
The salt that is leached down the profile can end up in one of two places. If groundwater is within a reasonable distance, the salt enters that and drains off site (with the danger that it will enter waterways). Alternatively, if there is never enough irrigation water supplied to leach down into the groundwater, the salt simply builds up at a certain depth, in a layer called a salt pan. Deeper-rooted crops and trees can then be killed. Either way, you must take care to ensure that the salt never causes environmental damage. Regular testing will enable you to keep an eye on this.
Can gypsum help?
Gypsum is a well known treatment for sodic soils. A soil in which sodium makes up more than about 5% of all cations (positively charged ions) bound to clay particles is considered to be sodic. Sodium ions tend to force clay particles apart, destroying soil structure and permeability to water. Calcium ions displace sodium ions, restoring soil structure. Gypsum (calcium sulphate) is an excellent source of calcium, which is why is it routinely used to restore structure to sodic soils.
Anything that adds sodium to a soil increases the risk of sodicity and therefore loss of soil structure. Saline irrigation water poses such a risk. If the only irrigation water available to you is saline, then you need regular soil and water tests to keep an eye on the risk of sodicity (and salinity). Gypsum applied to the soil will resolve any sodicity problems, but save your money until testing shows that you need it.
Further reading
Dairy Australia. 2008. Effluent and Manure Management Database for the Australian Dairy Industry.
McMullen B. 2000. SOILpak. Chapter D5, Sodic Soil Management. NSW Agriculture, Orange.
SESL. 2007. Make best use of recycled water.
SESL. 2007. Glossary of water quality terms.
SESL. 2008. Salinity (incl. classes).
SESL. 2008. How salinity is measured.
SESL. 2008. Gypsum – what is it good for?
SESL. 2008. Managing irrigation water quality.
SESL. 2009. Irrigation in drought.
SESL. 2009. Recycling irrigation runoff.
SESL. 2009. Turfgrass irrigation – principles and practices.