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.
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.