Using saline water for irrigation

Salinity has long been recognised as a problem in many parts of Australia, and many irrigators have to consider using marginal quality water.

If you do have to use saline water for irrigation you need to understand how salinity affects a crop. You need to monitor salinity levels constantly to ensure they stay within the acceptable range, and you need to be prepared to accept lower-than-average yields.

How does salt affect the plant?

Plant roots generally take up moisture by a type of osmosis through membranes in root cells.

Osmosis is a natural process where water, passing through a semi-permeable membrane, moves from a solution of low levels of dissolved salts to one with a higher salt level.

This process allows water to move from a solution of relatively low concentration (the irrigation water) into a solution of relatively high concentration (in the plant root cells) in an attempt to establish equilibrium in the two solutions.

This continues until the plant cells become full, or turgid.

Moderately saline water

If the irrigation water is moderately saline, the plant has to work harder to absorb water from the soil.

With lack of water, the plant soon begins to wilt, and growth is slowed, with reduced yields.

Highly saline water

If highly saline irrigation water is used, the process of osmosis can become reversed. Where the solution outside the plant roots is higher in salt concentration than that of the root cells, water will move from the roots into the surrounding solution.

The plant loses moisture, and so suffers stress.This is why symptoms of high salt damage are similar to those from high moisture stress:

  • leaf tip dieback
  • margins yellowing, scorched and turning brown or black, followed by
  • leaf fall of dead leaves.

If water is sprayed directly on leaves, it can cause salt scorch and leaf damage even at lower salinities.

Another danger: specific ion toxicity

Irrigation water is only moderately saline, but if it contains high concentrations of specific ions, it can still damage the crop.

  • High chloride or sodium ion levels (the most common) cause symptoms similar to high salinity.
  • High bicarbonate levels can make calcium and magnesium unavailable to the plant.
  • Some effluent waters that may be used for irrigation can have high boron levels.

Acceptable salinity levels

There will be some variation in how salinity affects the plant, depending on crop, variety, rootstock, leaching ability of the soil and also method of irrigation (spray, drip or furrow).

Most crops, including salt-sensitive crops, should accept salinity levels of up to 700 µS/cm without loss of yield. (See How salinity is measured).

With salinities over 700 µS/cm, we could expect to see reduced yields from some salt-sensitive plants.

The water salinity levels acceptable to each crop (that is, the levels that do not affect crop yields), and levels that will cause about 10% loss of yield.

Acceptable salinity levels for horticulture

Using the table on fruit crops (Table 2), for example, we see that most grapevines should accept salinity levels of up to 1000 µS/cm without loss of yield. Above this figure, yields generally start to decline, and we could expect up to 10% yield loss at 1700 µS/cm.

Acceptable salinity levels for pasture

Similarly, using the table on forage crops (Table 4), we would expect lucerne, for example, to accept salinity levels of up to 1300 µS/cm without loss of yield. Above this figure, yields generally start to decline, and there might be up to 10% yield loss at 2200 µS/cm.

Soils and salinity

High salt levels in soils also reduce yield.

High levels of sodium from common salt can also, over time, damage soil structure, causing

  • surface crusting
  • reduced infiltration
  • restricted subsoil drainage and root development in 'blocky' soils.

Soils with high proportions of sodium are known as sodic soils.

Management options for saline irrigation water

For horticultural crops

  • Rootstocks: Choosing saline-resistant rootstocks in new plantings can minimise the detrimental effect of salinity. (See comment on grapevines.)
  • Mulching: As saline water evaporates from the soil it leaves behind salts. A good mulch under the crop helps reduce surface evaporation, maintains moisture near the soil surface and lessens the build-up of soil salinity.

Monitoring salinity levels

If you have a regular irrigation water salinity problem, you must have a meter for monitoring salinity levels.

Small hand-held meters are readily available at reasonable cost. Calibrate meters against a known buffer solution at least twice each season, as all meters tend to drift in accuracy over time. This calibration is a simple procedure: the meter should come with instructions for this.

Using leaching

The main method of reducing the effect of saline water is to apply extra water to leach salts below the rootzone.

Water with an EC as low as 800 µS/cm will add over ½ tonne of salt for each megalitre of water applied. At this rate, unless leaching and drainage occurs, soil salinity may quickly increase to unsustainable levels.

Leaching often occurs with rainfall. In other cases, irrigation water beyond the crop's water requirement may need to be applied.

The extra irrigation water needed to leach salts is termed the leaching fraction, and this can be calculated for various crops and soil types.

The formula for determining the minimum leaching requirement (MLR) is:

MLR = ECWA÷ [(7.5 x ECWY %) – ECWA]

where

  • ECWA is the EC of the available irrigation water
  • ECWY % is the salinity level of the irrigation water that results in a specified percentage yield loss.

Example: Say we needed to use water with an EC of 1800 µS/cm. We are prepared to accept a 10% yield loss, and we know that water of 1700 µS/cm (1.7 dS/m) EC would cause this (acceptable) yield loss.

MLR = 1.8 ÷ [(7.5 x 1.7) – 1.8] = 0.16

This means that, in this example, 16% more water (than is strictly required by the crop) should be applied when irrigating.

Note on drainage: If you apply additional leaching water, you must have good subsoil drainage to ensure the leached saline water can regularly be removed from the soil.

Checking the salt index of fertilisers

All fertilisers have a salt index which indicates what the fertiliser contributes to soil salinity.

If your irrigation water or soils are saline, changing to fertilisers with similar nutrients but with a lower salt index may help. For example, potassium chloride has a salt index of 114 but potassium sulphate has a lower salt index of 46.

Information on the salt index of each fertiliser should be available from your local supplier.

Desalinisation

Desalinisation for saline waters is technically possible, but its use is limited by cost (initial capital cost of the equipment, and high operation and maintenance costs) and the problem of disposing of the residual saline concentrate.

General problems to consider with desalinisation treatments:

  • Pre-treatment of the water may require sand filtration, micro-filtration, and ultraviolet treatment to kill bacteria.
  • Water with high iron, silica or manganese is difficult to treat.
  • It takes 2-3 days to properly sanitise membranes.
  • Membranes are operating under high pressure, and have a 3-year life.

Normal agricultural uses would not warrant the cost and maintenance of desalinisation. For example, for a reasonable quantity of water at 800 µS/cm salinity, a system to recover 75% of the water would have an initial capital cost of about $160,000 (1998 values), and operating and maintenance costs of about 50-60 cents per kilolitre of water treated. [Horlocks, personal communication]

At a 75% recovery rate, every megalitre of water treated would leave most of the salt in the remaining 250,000 litres: this extremely high saline concentrate would need to be disposed of in an environmentally acceptable manner.

Further reading

*Grapevine rootstocks for saline conditions are discussed in several publications including
Using Grapevine Rootstocks - The Australian Perspective by Peter May and
Viticulture - Volume 1 edited by B. Coombe and P. Day.

A web site offering information on water quality, part of the National Water Quality Management Strategy, is http://www.environment.gov.au/water/quality/nwqms/index.html

References

Awad, A. S. 1984, Water quality assessment for irrigation.

Australian National Committee on Irrigation & Drainage 2001, Rural Water Industry Terminology and Units, 2nd edn, ANCID/Sinclair Knight Merz, Armadale.

Handreck, K. and Black, N. 1991, Growing Media for Ornamental Plants and Turf, University of NSW Press, Kensington.

K. Horlocks, Memtec Pty Ltd.