Water salinity and plant irrigation

Irrigating with water of higher salinity than a crop can tolerate results in yield loss and decreased quality.

Plants vary greatly in their tolerance to saline water. The extent of yield loss when plants are irrigated with saline water depends on a number of factors including soil type, drainage and the frequency, method and time of irrigation.

Introduction

The types of salts in irrigation water are mainly common salt (sodium chloride), calcium and magnesium bicarbonates, chlorides and sulphates. In most areas of Western Australia, about three-quarters of the total soluble salt is sodium chloride, though this may vary in coastal and pastoral areas. For example, in irrigation water at Carnarvon, only about half the total soluble salt is sodium chloride.

Crop yields are usually markedly reduced before visual symptoms of salinity damage become apparent.

The first of sign of salinity is usually stunted growth, with plant leaves often having a bluish-green colour. As salt levels in the soil increase to more toxic levels, scalding or burning on the tip and edges of the older leaves occurs. The leaf dies and falls off and finally, the plant dies. In other cases, the youngest leaves may appear yellow, or the crop may show signs of wilting, even though the soil appears adequately moist.

Salty irrigation water can affect plant growth in two ways: salinity effect and toxicity effect.

Salinity effect

Plant roots generally take up moisture through membranes in root cells by osmosis. This 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 higher salts.

This continues until the plant cells become full. If the irrigation water is moderately saline, the plant has to work harder to absorb water from the soil and growth is slowed, with reduced yields.

If highly saline irrigation water is used, the process of osmosis can reverse. 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 suffers stress. This is why symptoms of high salt damage are similar to those of high moisture stress.

Toxicity effect

Excessive concentrations of sodium and chloride ions in irrigation water can cause toxicities in plants. These ions can be taken up either by the roots or by direct contact on the leaves. More damage is caused by direct absorption through the leaves.

Sodium

Typical sodium toxicity symptoms are leaf burn, scorch and dead tissue along the outside edges of leaves, in contrast to the symptoms of chloride toxicity which normally occur initially at the extreme leaf tip. High concentrations of sodium in irrigation water can induce calcium and potassium deficiency in soils low in these nutrients and crops may respond to fertilisation with these nutrients. Another effect of sodium is that if sodium is high in relation to calcium and magnesium,  waterlogging may result due to the degradation of well-structured soils.

The direct toxic effects of sodium concentrations in irrigation water on different plants are shown in Table 1, which lists the effect of the sodium absorption ratio (SAR) of the irrigation water. The SAR measures the relative percentage of sodium ions in water to calcium and magnesium ions. A high SAR indicates there is potential for sodium to accumulate in the soil. This can degrade soil structure by breaking down clay aggregates, which results in waterlogging and poor plant growth.

Table 1 Tolerance of crops to sodium
Tolerance Sodium adsorption ratio of irrigation water Crops
Very sensitive 2-8 Avocado, citrus, deciduous fruits and nuts
Sensitive 8-18 Beans
Moderately tolerant 18-46 Clover, oats, tall fescue, rice
Tolerant 46-102 Barley, beets, lucerne, tomatoes, wheat

Chloride

The chloride ion can be taken up by plant roots and accumulate in the leaves. Excessive accumulation may cause burning of the leaf tips or margins, bronzing and premature yellowing of the leaves. In general, most fruit trees are sensitive to chloride, whereas most vegetable, forage and fibre crops are less sensitive. Table 2 shows the tolerance of some crops to chloride damage by root uptake.

Crops, and even varieties and rootstocks, vary greatly in their tolerances to chloride and sodium. If irrigation water has a total salinity close to the critical concentration, then test its chloride and sodium concentrations.

Chemical analysis of soil or leaves can be used to confirm probable chloride toxicity. Fruit leaves usually suffer from toxicity when the dried leaves contain more than 0.2% sodium or 0.5% chloride.

Table 2 Chloride upper tolerance limits for some fruit crops, cultivars and rootstocks by root uptake
Crop (variety/rootstock) Chloride concentration in irrigation water
(mg/L)
Citrus rootstocks  
trifoliata 120
rough lemon 200
troyer citrange, sweet orange 300
Rangpur lime, Cleopatra mandarin 600
Stone fruit rootstocks

Marianna plum (for budding plums and apricots) 600
Myrobolan plum (for budding plums and apricots) 370
Peach 235
Avocado rootstocks

Mexican 120
West Indian 190
Grape rootstocks  
Ramsey 950
Dog Ridge 700
Sultana 600
Soft fruit varieties

Blackberry, boysenberry 235
Raspberry 120
Strawberry 120-190

Direct adsorption through leaves

Some crops which are not sensitive to root uptake of chloride or sodium ions develop symptoms of leaf burn when sprinkled with saline water.

Damage is most severe during hot dry conditions because evaporation concentrates the salts on leaf surfaces. Table 3 shows chloride and sodium concentrations in irrigation water that will damage the leaves of certain crops.

Table 3 Chloride and sodium concentrations in irrigation water causing damage to leaves
Sensitivity Chloride (mg/L) Sodium (mg/L) Affected crop
Sensitive

<178

<114

Almond, apricot, citrus, plum
Moderately sensitive 178-355 114-229 Capsicum, grape, potato, tomato
Moderately tolerant 355-710 229-458 Barley, cucumber, sweetcorn
Tolerant

>710

>458

Cauliflower, cotton, safflower, sesame, sorghum, sunflower

Leaf injury is influenced by cultural and environmental conditions such as drying winds, low humidity, speed of rotation of sprinklers and timing and frequency of irrigations. Data presented are only general guidelines for summer daytime sprinkling.

Measuring salinity

Salinity of water is measured by its electrical conductivity (EC), which may be converted to total dissolved salts (TDS). The EC does not identify the dissolved salts, or the effects they have on crops and soil, but gives a fairly reliable measure of salinity problems. Table 4 shows a general salinity classification for water.

EC is measured in milliSiemens per metre (mS/m). However, laboratories can use different units for measuring salinity.

To convert mS/m to milliSiemens per centimetre (mS/cm), deciSiemens per metre (dS/m) or millimhos per centimetre (mmhos/cm), multiply by 0.01. To change mS/m to microSiemens per centimetre (µS/cm), multiply by 10.

To convert EC to milligrams per litre (mg/L) or parts per million (ppm) of TSS, multiply a measurement in mS/m by 5.5, or a measurement in mS/cm or dS/m or mS/cm by 550. These conversion figures are approximate and slightly different conversion figures may be used in some areas.

Table 4 General salinity classification for water
EC
(mS/cm, dS/m or mmhos/cm)
EC
(mS/m)
Approximate total soluble salts
(mg/L or ppm)
Status
0-0.80 0-80 0-440 Low salinity
0.80-2.50 80-250 440-1375 Moderately salty
2.50-5.00 250-500 1375-2750 Salty

>5.00

>500

>2750

Very salty

Factors affecting damage

The extent of plant yield loss when irrigated with saline water depends on a number of factors including:

Soil type and drainage

The key to irrigating successfully with saline water is to leach or move salts downwards away from the root zone.

In well drained sandy soils, irrigation water can readily flush salts out of the root zone but this is less successful on poorly drained, heavy soils. The amount of leaching to maintain acceptable growth depends on:

  • salinity of the irrigation water
  • salt tolerance of the crop
  • climatic conditions
  • soil type
  • water management.

The amount of additional water required to leach salt from the root zone is called the leaching fraction.

Frequency and timing

Salt concentration in the root zone continually changes following irrigation. As the soil dries, the salt concentration in the soil solution increases and this reduces the moisture available to the plant. Frequent, light irrigations increase salt concentrations in the topsoil and should be avoided.

High rainfall and heavy irrigations will remove salts from within the root zone.

Watering during hot dry conditions will increase evaporation and therefore increase the concentration of salt.

Fertilising

If salinity is a problem, avoid fertilisers containing chloride.

Replace muriate of potash (potassium chloride) with sulphate of potash and use nitrogen, phosphorus and potassium (NPK) fertilisers which contain sulphate of potash.

Growth stage

Plants are generally more susceptible to salinity damage during germination and at the seedling stage than when established.

The best quality water should be used at this stage.

Rootstocks and varieties

Rootstock and variety differences are important factors affecting salt tolerances of tree and vine crops, especially with avocado, citrus, grapes and stone fruit (see Table 2).

Irrigation method

Drip irrigation allows water with higher salt content to be used than other delivery methods, as evaporation losses are minimal.

Drip irrigation can alsoreduce the effects of salinity by maintaining continuously moist soil around plant roots and providing steady leaching of salt to the edge of the wetted zone.

Sprinkler irrigated crops are potentially subject to additional damage caused by salt uptake into the leaves and burn from spray contact with the leaves.

If using saline water for sprinkler irrigation, irrigate when temperatures are coolest. Watering in the heat of the day concentrates the salts due to high evaporation. Watering during high winds also concentrates salts.

Do not use sprinklers which produce fine droplets and misting. Avoid knocker sprinklers if possible, especially slow revolution sprinklers which allow drying periods, causing salt to build up on the leaves.

Contact information

Rohan Prince
+61 (0)8 9368 3210
Page last updated: Thursday, 27 October 2016 - 9:39am