Salinity

Salinity is one of the most important environmental factors affecting plant growth and productivity, particularly in arid and semi-arid regions of the world. The negative effects of salinity are associated with a reduction in water availability in the soil solution and the accumulation of specific ions in the leaves. This is a serious problem since the salinisation of arable lands is increasing throughout the world.

Salinity effects depend on the salt concentration and plant tolerance. In this sense, the olive is considered a moderately salt-tolerant plant that may be cultivated in saline soils where other fruit trees cannot grow. The olive may develop well with a minor reduction of yield with an electrical conductivity (EC) of the saturated soil extract ranging from 4 to 6 dS m^-1. In soils with high calcium, like those rich in gypsum, these values could be increased in 2 dS m^-1. Salt tolerance, however, is a cultivar-dependent characteristic.

Even at low contents of total soluble salts, plant growth can be affected by the excess of some specific ions in the soil solution, particularly chloride, sodium and boron. The olive is very tolerant to an excess of chloride, so this ion does not represent a problem in olive culture. In fact, KCl or CaCl₂ can be used without any problem in olive cultivation. Excess sodium may negatively affect both the soil and the plant. Exchangeable sodium percentage (ESP) in the soil above 15% causes a general deterioration of soil structure resulting in poor soil aeration and reduced permeability.

Many fruit tree species can develop toxicity symptoms even at lower ESP values, but the olive may be affected at levels between 20% and 40%. Calcium supply to the saline soil solution regulates sodium uptake by plants and can prevent the accumulation of toxic levels of sodium. In the olive, it has been reported that the application of CaCl₂ to the irrigation water prevents sodium transport from the root to the shoot, which may be an important ability for survival under saline conditions. Boron is an essential element for plant growth, but, in excess, can be toxic and even cause death, particularly in young olive plants. The olive is also moderately tolerant to boron and can tolerate up to 2 ppm in the soil solution. It should be noted, however, that excess boron in the soil solution is usually associated with a high concentration of this ion in the irrigation water.

Few studies carried out with a limited number of varieties reveal differences in the tolerance of olives to sodium and boron excess, but a comprehensive screening of olive varieties for tolerance to these ions is not available.

Irrigated olive orchards are being expanded in many areas of the world, and many of these orchards are being irrigated with saline waters. These waters act as salinisation agents by providing the salts they contain to the soil. In irrigated olive orchards, salinity caused by low-quality irrigation water can be a serious problem if irrigation volumes are insufficient. To prevent an excess of soil salinisation, it is necessary to increase irrigation volumes above crop needs to leach salts through the soil profile.

Different studies indicate that olives can be irrigated with water with an EC of up to 5 dS m^-1, and even up to 10 dS m^-1 in tolerant cultivars such as Picual. The risk of soil sodification due to the irrigation water is measured by the sodium adsorption ratio (SAR). It is generally established that a SAR < 3 indicates good quality water. However, the risk of sodicity also depends on the EC, so that for the same value of SAR the risk decreases with increasing EC of the irrigation water. Also, several studies indicate that olives can tolerate concentrations of 2 g L^-1 of NaCl and up to 4 g L^-1 (and even 8 g L^-1) in tolerant cultivars. Finally, the olive can tolerate 1-2 mg L^-1 of boron in the irrigation water.

In conclusion, growing a tolerant olive variety, appropriately using saline water, supplying calcium to the irrigation water to prevent sodium toxicity, and employing a high-frequency irrigation system such as drip irrigation to maintain high soil humidity, can allow the use of highly saline irrigation water without affecting growth and yield in olive trees. Table 4 presents an overall classification of olive varieties in terms of their tolerance to high salinity stress.

Table 4. Tolerance of olive varieties to salinity.

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