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Introduction

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 can be 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 take up moisture through membranes in root cells by osmosis. Water passes through a semi-permeable membrane, and moves from a solution of low levels of dissolved salts to one with higher salts.

This process 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, the symptoms of chloride toxicity 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.

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.

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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.

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

EC is measured in milliSiemens per metre (mS/m) in DPIRD. Some laboratories use different units for 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 TDS, multiply a measurement in mS/m by 5.7, or a measurement in mS/cm or dS/m or mS/cm by 570. These conversion figures are approximate, suitable for EC readings of less than 1000mS/m and for the common salts found in WA irrigation water.

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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.

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Guidelines for critical salinity

Tables 5 to 8 show the tolerance of plants to irrigation with saline water. These values should only be used as a guide because the extent of salinity damage depends on the factors described previously.

If the salinity of the water is near the upper recommended limit, conduct preliminary trials under the specific conditions present to determine if crop damage will occur.

Tables 5 to 8 also show the threshold salinity at which yield begins to decline (0% yield loss) and the salinity at which 10% and 25% of yield is lost. Changes of water salinity of 20% above or below the indicated salt tolerance value may have little effect because of the modifying effect of soil, climate and management. The yield loss data depends on several  assumptions.

The crop tolerance figures relate to a loamy soil, with good drainage and with at least 15% of the applied water percolating below the root zone (leaching fraction 15% or more). These figures are applicable to sprinkler irrigation systems in which there is an extended drying period between irrigations. Crops can usually tolerate higher salinity under higher frequency irrigation.

These guidelines are likely to be too restrictive for sprinkler irrigation on very permeable sands of the Swan Coastal Plain. Irrigation on these soils is frequent, often with a leaching fraction over 15%. Sprinkler irrigation of crops with water high in chlorine or sodium may result in damage via absorption through the leaves, even though the salinity concentration is below the critical level listed in Tables 5 to 8.

The guidelines apply mainly to sprinkler irrigation. Trickle irrigation is applied frequently which reduces salinity concentrations in the root zone and increases in salinity due to evaporation are minimal.

For crops where yield loss data is not available, a maximum recommended concentration or range of concentrations is given.

Recycling of salts

Groundwater below horticultural properties on the Swan Coastal Plain may become more saline over time. The longer an area is irrigated, the higher the risk. Large amounts of water are pumped from the shallow aquifer in some areas. As excess irrigation water infiltrates back to the aquifer, the salt level increases because of evaporation and addition of fertiliser salts. Good irrigation management should, in most cases, overcome these problems. Excessive pumping from an aquifer can also result in the intrusion of salty water.

If several sources of differing quality water are available, blend the poorer quality with better quality to reduce or prevent salinity damage.

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Analysis of water samples

A number of laboratories in Western Australia will analyse water for electrical conductivity. Check the Yellow Pages phone book for contact details.

Use a glass or plastic bottle that is about 500mL capacity. Rinse the bottle in the water to be sampled before filling. Seal the bottle and mark it with the sender’s name and address, and date of sampling.

When sampling from bores or wells, run the pump for a few minutes to ensure a representative sample is taken. Large variations in the salinity of surface irrigation water can occur throughout the year, usually highest from the end of summer until the first rains. Collect the water sample at the time of year when water will be pumped for use.

Crop tolerance tables

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In Tables 5, 6 and 7 detailed yield loss data is not available for some crops. A maximum recommended concentration or range of concentrations is given. The data should serve only as a guide. Absolute tolerances vary depending on climate, soil conditions and cultural practices.