Irrigation changes the balance of soil hydration by generating an extra supply of water; this supply is always always associated with a contribution of salts. Thus, even the best quality soft water contains dissolved salts and, although the amount of salts in this water may seem negligible, the amounts of water brought in over time leads to a cumulative deposit of salts in the soil which can turn out to be considerable. Pure water is lost through evaporation but the salts remain and build up. In arid regions the effect is much more marked: the surface and underground waters are relatively rich in salts (because the water has infiltrated the soil which contains some easily altered minerals) and the dry climate creates a raised evaporative demand, the effect of which is the necessity for large amounts of water for the irrigation of crops. A farmer in a semiarid region may have to apply up to 90 cm of water to respond to the requirements of annual crops. Even if the irrigation water is relatively low in salts, it can lead to the deposition of at least 6 tonnes/ha (2.4 tonnes/acre) of salts per year.

If the applied irrigation water contains relatively significant amounts of sodium ions (Na⁺) compared with the amounts of magnesium ions (Mg²⁺) and calcium ions (Ca²⁺) present, and in particular if the bicarbonate ion (HCO₃^-) is also present, the sodium ions can saturate most of the colloidal exchange sites, this is the origin of the formation of sodic soils. This type of soil has extremely poor fertility.

During the last thirty years, countries with low incomes in the dry regions of the world have developed irrigation considerably in order to be able to produce the food necessary to respond to the needs of a growing population. As a result, the percentage of irrigated arable land has also increased considerably during this period.

In Pakistan, more than 25% of irrigated areas are salt-affected.

In Tunisia 25%, the United States 23%, India almost 17%.

In China almost 15% and in South Africa almost 9%.

Initially, the extension of irrigated zones considerably stimulated food production. Unfortunately, a lot of irrigation projects have not been associated with the putting in place of suitable drainage systems, which has brought about an accelerated salinization of the soil and an accumulation of salts to levels which negatively affect agricultural production. In some zones, sodic soils are formed.

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Today, 10% to 15% of irrigated areas (45 Mha) suffer, to various degrees, from salinization problems. From 0.5% to 1% of irrigated areas are lost each year. Nearly half of all irrigated areas are threatened in the long term!

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For example, the development of cotton production and unsuitable management of irrigation in the Aral Sea has lead to extreme salinization of the soils. It is estimated that about 20% of the yield of cotton production is lost, corresponding to more than US$200 million per annum. Salt-affected soils also affect food production, with some dramatic consequences for rural populations. According to the World Bank nearly US$2 billion (approximately 5% of the GNP of Central Asia) are lost because of the salinity of the soils (Mashali, 2002).

Drainage prevents the build-up of salts which reduce the productive potential of irrigated land, but it generates effluents which have to be dealt with. The signs of salinization are not immediate. In general it takes at least a decade for them to appear (reduction of yields, etc.).

"Cure": reclamation of salt-affected land.

Reclamation of salt-affected land is expensive. It can represent from 65% to 100% of the investment costs. It is sometimes technically impossible.

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