The avocado tree is considered to be highly sensitive to salt and particularly chloride ions. There are some differences between the various races of avocado — avocados evolved in three different regions and are grouped accordingly into Mexican, Guatemalan and West Indian races. The Mexican race being the most sensitive, followed by the Guatemalan with the West Indian race being the least sensitive.
One simple and relatively cheap method of determining water quality in terms of total salt, is to carry out an electrical conductivity test (EC) usually provided in microSeimens per centimetre (µS/cm), this figure can be converted to provide an approximate determination of Total Dissolved Solids (TDS) often given in parts per million (ppm). In many locations in the South-West of Western Australia, EC is a fair representation of the levels of the potentially harmful ions sodium and chloride. However, this is not always the case, so it can be of value to get a complete analysis of your water. This will provide accurate levels of sodium and particularly chloride ions. The following information assumes that the bulk of the TDS is sodium and chloride ions and therefore the EC reading is a fair representation.
Preferably, irrigation water for avocados should have an EC of less than 500µS/cm (approximately 320ppm), ideally less than 300µS/cm (190ppm). Poorer quality water of up to about 700µS/cm (450ppm) can be used, and is used by some growers, but management practices must be employed to reduce the impact on tree health and therefore fruit yield and quality. Depending on the severity of the salt levels, various strategies such as the use of more tolerant rootstocks, salt leaching, pulse irrigation and the application of gypsum may be used. At levels higher than 700µS/cm EC, consideration should be given to the suitability of the water for avocado production. It can be done, but it is likely that yield and fruit quality will suffer regardless of the strategies you employ.
The danger with salty irrigation water is that we aim to waste as little water as possible by only applying what the tree uses. The water is used by the tree and also evaporates from the soil surface, leaving behind much of the salts that were in the irrigation water. This starts to cause a significant build-up of salts within the root zone. This can lead to excessive levels of chlorides being taken up by the tree, rates higher than 0.25% in the leaves are considered toxic. Leaching can be used to try and reduce these built up levels of salt in the soil. This is achieved by using extended irrigations that apply sufficient water to the soil to result in saturation of the soil to a point beyond the root zone. The idea is that it will take the salts with the water front to a point beyond the reach of the roots. The frequency of leaching is dependent on water quality, the higher the TSS content, the more frequent you will need to leach. But avoid leaching any more than is necessary as, this is not only wasteful, but will push required plant nutrients beyond the root zone along with the salts.
Leaching is not an exact science and a level of experimentation will be required to determine the best length of irrigation and how frequently you should apply them. Growers with salty water have reported that doing a heavy irrigation once a week or from 'time to time' are useful in removing salts.
Choice of rootstock is important when using salty water. Research in Chile has identified Nabal as a salt tolerant rootstock. Australian research shows that Nabal, Velvick and the relatively new SHSR-03 are good rootstocks in areas with saline water with cumulative three year yields of 120.8 kg per tree for Nabal, 119.1 kg per tree for SHSR-03, and 128.7 kg per tree for Velvick. These rootstocks have Guatemalan and West Indian parentage. For full details of the research see the final report of AV08000 on the Avocados Australia Best Practice Resource (link at end of this article).
Iron can be a significant issue for many avocado growers, particularly those using under-ground water resources. Iron concentrations of greater than 1 milligram per litre (mg/L) should be treated otherwise irrigation blockages are likely to be experienced. At this concentration the iron in the water oxidises upon contact with air and forms precipitates. Evidence of iron precipitating is the red stain often seen on sprinklers and tree trunks. Bacteria in water supplies can also lead to iron precipitating at lower concentrations. The iron precipitate can result in emitter blockage on its own and is mostly a problem for growers using drippers and micro-sprays, though low volume mini-sprinklers can sometimes be affected. The iron and bacteria combination can result in bacterial slime which causes greater issues. The slime grows as a result of the bacteria oxidising the soluble iron and is a great problem for many growers, causing blockages in even the largest of sprinkler emitters.
Iron is most commonly treated by aeration in settling tanks to oxidise the iron and cause it to precipitate followed by quality filtration.
If iron created blockages occur your system will need to be cleaned by either scouring or acid treatment.
Depending on your system, both inorganic and organic sediment can find its way into your irrigation water. If these sediments are larger than your emitters then they can block them resulting in inefficient delivery of water to your trees. The only way to remove sediment is by filtration, ensure your filtration unit is suitable to remove any sediment larger than what can readily flow through your emitters and can handle the flow volume required. Filtration systems should be situated after any aeration systems to oxidise soluble iron and any fertigation systems. Filtration is an integral part of any irrigation system and should be designed by your irrigation engineer.