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.
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.