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At first sight, we could ask ourselves why irrigation is required for olive trees, which are typically Mediterranean plants, and thus perfectly adapted to withstand dry periods. This is all the more applicable given that we are located at the northern limit of their acclimatization zone and generally speaking (in theory) these trees flourish in a climate with a comparatively high rainfall.
As was the case with all drought-tolerant plants, farmers in ancient times would only grow them in arid or poor areas, where other more delicate plants would not have survived. If the olive tree produced little, it would be put down to the “alternate bearing cycles” that were deemed inevitable.
Nowadays, we are well aware that the olive tree is just like any other tree and that, if it is given water to drink during the critical periods when it is thirsty, there will be a noticeable improvement in its fruit production, both in terms of quantity and quality.
The climate of Nice region, which is so attractive for tourists, is characterised by long periods of fine weather, broken by short periods of rain, which is sometimes very intense. Whether or not this is due to global warming, we have to realise that these weather patterns have become more accentuated over the last few years.
The water requirements
As with all plants, these trees are faced with a cruel dilemma:
• they have to open the stomata on their leaves (their skin pores) enough to achieve maximum inhalation of carbonic gases and the extraction of the carbon they need…
• but if they are opened too much the water contained in the cells will be allowed to evaporate.
The trick is therefore to provide enough water to enable them to achieve maximum respiration, but not to over-irrigate, which would lead to health problems, poor quality yield and wastage.
In theory, he water requirements of the olive tree consist of the potential evapotranspiration (ETP) minus the effective rainfall, i.e. water that falls from the clouds and water returned to the atmosphere through evaporation and plant transpiration.
The water application must, therefore, be adjusted according to the amount of rain that falls, air temperature, solar radiation, soil retaining capacity and the requirements of the olive tree during the critical period.
The olive tree’s water requirements are particularly important in the spring, during the pre-flowering period and in September/ October, when the fruit fills out.
Between the setting and sclerification of the pit or stone (hardening), i.e. between mid-July and mid-August, the tree should be rationed to prevent the olives from having large pits.
With nature being what it is, we can see that the olive trees require water precisely during the times of the year when there is most rainfall. Good use of irrigation will, therefore, be applied exclusively to make up for any deficit in rainfall. Any more than this will only lead to health problems, such as the fatal root rot, sooty mould and olive leaf spot and, without actually increasing the yield, will produce large tasteless olives, with a large pit, which keep badly and mature late.
Generally speaking, the olive grower who has an irrigation system set up in his olive grove will always tend to apply too much water. Very fortunately, it is no longer necessary to note down religiously the dates and amounts of the latest rainfalls and make complicated calculations to determine the olive trees’ water requirements.
The latest technology gives us very precise information about the amounts of water available in the soil. This relates, in particular, to soil moisture sensors and the “Watermark” sensors. In slight contradiction to the above, water applications are most important during the months of July and August, so as to maintain, in spite of the rationing, an optimum moisture level in the soil.
It is during this period that soil moisture sensors will be most useful. They allow the tree to be rationed (exposed to water stress) without actually being deprived of water and without any wastage of water, which is often a scarce and precious resource during that time of the year.
The irrigation processes
The oldest of irrigation methods is also a system that is most labour-intensive, least efficient and wastes the most water, with risky results.
This is a well-known system that uses irrigation channels and gravity flow of water into furrows dug around the olive trees. Unless you are fortunate enough to have access to an unlimited amount of water and a lot of time to spare, this method is unworthy of further discussion. Another method, which is not advised but, nevertheless, is conceivable, is the use of sprinklers. There are many types of sprinklers: rotators, spinners and wobblers, perforated pipes, pop-up fixed spray heads, etc.
These systems are very inconvenient and have few advantages:
• By watering the foliage and producing a humid atmosphere below the tree, they can generate a number of diseases such as sooty mould, olive leaf spot and root rot, as well as the proliferation of harmful insects, such as black scale and olive fruit flies.
• This system promotes the growth of grass cover to the detriment of the olive tree’s roots
• It wastes a large part of the water through excessive evaporation. i.e. 30% to 50%.
• Not all of the ground is covered properly unless more sprinklers are used,
• However, it is impossible to prevent the wetting of the olive tree’s trunk and stump, which is not recommended.
• The sprinklers are very sensitive to (changes in) water pressure. In order to have a uniform flow rate in all parts of the olive grove, particularly those that are on a slope, more pressure regulators have to be used.
• The pressure required to make the installation work well is far higher than that required for a drip system. It is often necessary to invest in a pump.
• Finally, it is impossible to use this method for applying fertilisers directly into the olive tree’s root zone.
The advantages are:
• Having an irrigated area covering practically the whole of the tree’s root zone and thus a lot less localised than with a drip irrigation system. This is particularly applicable for light and permeable soils.
• The system is considerably less sensitive to poorly filtered water than the drip method. Blockages are very rare.
• Moving the installation is, generally speaking, a lot less complicated when tillage is required.
The irrigation method best adapted to olive groves and currently the most efficient, while saving the most on water, is the drip or trickle irrigation system.
Each drop that leaves the emitter is immediately absorbed into the ground, always forming a bulb of wet soil in the very same place, which the olive tree’s rootlets or radicels will soon access and use.
The shape of this wet bulb depends on the type of soil. In light and permeable formations it will be narrow and deep with water lost through infiltration. In sticky and heavy formations it will be wide and shallower with the risk of water-logging, which the olive trees certainly do not like.
In the first scenario, the number of emitters has to be increased, and in the second, care must be taken not to over-irrigate. In any case, in both scenarios the best solution is to have 4 emitters per adult olive tree.
The first thing you obviously need for irrigating olive trees is water. The ideal solution is to have a constant source of water at no cost, which flows into a tank of sufficient capacity to cover the number of olive trees to be irrigated. This capacity must correspond to a good month’s irrigation in order to avoid shortages at the critical time. Knowing that an olive tree requires 80 litres of water per day during the dry months, when water requirements are greatest, it is quite simple to calculate the size of the tank. Ideally, once again, this reservoir should be built at an elevation that is ten metres higher than the highest elevation olive tree.
As the ideal solution is usually difficult to achieve, you often have to open an account with the local water company or local council and ensure that this water is not treated with chlorine. If the mains pressure is insufficient, then a pump will have to be used, either to fill up the tank situated at a higher level than the highest elevation olive tree, or to pressurise the irrigation system.
The irrigation equipment
The emitters. The emitters will be of the constant flow type, i.e. whatever the water pressure between 1 and 3 bar, the flow rate will be constant. This will allow each olive tree to be given the same amount of water, whatever the gradient of the land. Above 3 bar, i.e. with a 30 metre elevation difference, a pressure regulator will be required.
Depending on the type of emitter, the flow rate varies between 1 and 4 litres/hour. Certain emitters are adjustable within the range (of flow), but they are more expensive and are not really justified.
The liquid fertiliser
Drip irrigation offers the possibility of applying compound and easily absorbable fertilisers directly and effortlessly into the root zone. A dosing pump injects a predetermined quantity of fertiliser solution into the water system.
It is possible to make up this fertiliser solution yourself or buy it ready-made at your local wholesale store.
If you wish to mix the solution yourself, it is important to ensure that you only use products that fully dissolve in water at an ambient temperature, without crystallizing in the tank or even worse in the irrigation system.
Generally speaking, nitric acid (N), ammonium nitrate (N), phosphoric acid (P), monopotassium phosphate (P), potassium nitrate (N, K) and calcium nitrate are used.
Some products also contain trace elements, such as magnesium sulphate or magnesium nitrate.
As far as the doses are concerned, the same rules apply as for ‘solid’ fertilisers. We have already said that the benefits are: an appreciable saving on time and effort and the fact that fertilisers are made available more quickly and directly applied into the root zone, being specifically placed in the wetted soil area (wetted bulb).
The main disadvantage of this system is the cost. In fact, the prices are distinctly higher than those of solid fertilisers. Another drawback is that not all of the land around the olive tree is fertilised. After a few years the areas where there are no emitters become barren while the wetted areas could become too ‘heavy’ and unstable.
Nevertheless, this system is without equal in terms of giving these olive trees their (correct) springtime application of nitrogen fertiliser. In fact, certain solid fertilisers, such as sulphate of ammonia, dissolve in water without any problem and may be applied in this manner at no extra cost. You will thus avoid any losses caused when the rainfall takes too long to penetrate the soil or, adversely, when too much rain is washed down into the aquifer or water table.
Irrigation of the olive tree makes it possible:
• To protect against the risk of drought with the possibility of excellent yields materialising;
• To counteract alternate bearing cycles, thus allowing the trees to produce simultaneously both the fruit and the branches (vegetation) that will bear the fruit the following year;
• To improve production in terms of quantity and quality; in particular, by allowing the tree to produce a maximum of perfect flowers and ensure their setting;
• To allow fertilisers, diluted into the irrigation water and applied very precisely, to be made available to the olive tree, directly into the root zone;
• To accelerate the growth of young olive plants and thus be able to have a harvest within 5 or 6 years as opposed to 10 to 15 years when they are left to manage by themselves.