Controlled deficit irrigation is a method of water management whereby the water needs of the tree are not completely met during the growing season. It originates from the fact that the olive tree adapts well to different conditions of water availability and that the processes underlying the development of the various organs are not influenced in the same way by water shortage. Furthermore, it is possible to act selectively on the development of the different tissues of the fruit by dosing the water.
Le main strategies deficits consist of:
- limit distribution of water in some periods up to a certain level of water deficit. Usually the phase of hardening of the endocarp until almost the end of summer is the most used as a period of restriction or interruption of the water supply;
- reduce by a predetermined percentage the amount of water throughout the irrigation season, allowing the deficit to gradually increase over the course of the irrigation season;
- maintain the level of constant deficit;
- alternate periods of deficit and repayment of water so as to expose the trees to cycles of stress and recovery. In all these cases the trees will use the available water reserve of the soil at least partially to meet their needs. It is important to avoid the deficit becoming excessive or too prolonged because this could affect not only the development of the fruit and the accumulation of oil, but also the restoration of the soil reserve.
Il main advantage of deficit irrigation in olive growing is given by water saving. Experimental tests conducted in various Italian areas show that the quantity of water that must be administered, in order not to have negative effects on the quantity and quality of production, is on a seasonal basis from 40 to 70% of the complete needs of the olive tree. With these levels of water return the quantity of oil per tree and per hectare is less than 20% lower compared to fully irrigated trees, an acceptable reduction in production largely offset by positive effects on product quality. It has also recently been seen that a certain degree of water stress in the early stage of fruit development stimulates the accumulation of phenolic compounds in the fruit and in the oil at harvest.
The reduction of water volumes also determines benefits from the point of view of the vegetative-reproductive balance of the tree. With deficit irrigation you have better control of the vegetative growth of the canopy and one more balanced development of the root system, which tends to deepen and expand laterally. An extensive root system offers advantages both from the point of view of absorption of mineral elements and water and in terms of anchoring the tree. However, during the breeding phase, the use of deficit strategies produces a reduction in growth proportional to the lower water availability compared to fully irrigated plants and is therefore advisable only in particular cases.
Regardless of the volumes and periods of restriction, the adoption of controlled deficit irrigation requires that:
a) the uniformity of water distribution is high;
b) the operator has considerable sensitivity in interpreting the water status of the tree, as it is more likely that the tree suffers in drought years or that it exhausts the water reserve in the soil;
c) the operator understands the principles of the deficit strategy and is able to act promptly to adapt the cultivation technique if necessary;
d) that the water status of the tree is periodically evaluated also through monitoring of the soil potential or bio-indicators.
(Taken from: National Olive and Oil Academy informative series – Vol.9, Irrigation: R.Gucci, T.Caruso, G.Palai)
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