Salts occur in the soil in one of the following three forms: salt ions dissolved in the soil water (the soil solution), cations adsorbed on the negatively charged surfaces of the soil particles (adsorption complex), and precipitated salts.
The principal salts are sodium, magnesium, and calcium chlorides; sodium and magnesium sulfates; sodium carbonates and bicarbonates; and nitrates and borates. These originate from rocks as they weather into soil. In this process the salts are carried downward (leached) with the percolating water. In the lower layers of the soil, they may either precipitate or continue to be transported in solution, eventually ending up in the sea. In certain cases, a high salt content in the soil may be related directly to the soil's parent material. This type of soil salinity is normally termed "primary" or "residual" salinity.
The most common cause of high soil salinity is salinization, that is, the accumulation of salts in the upper layers of the soil from some outside source. Salinization can be either a natural or an anthropogenic process.
The latter type is often the result of irrigation in regions with low precipitation, high rate of evaporation, and without artificial leaching or drainage. The result is a rather rapid accumulation of salts, which influence the growth of crops. The main effects are physiological drought, disturbance of the ion balance in the soil solution, degradation of the soil structure, and decrease of the biological activity of the soil (see Chapter 5, Sections 5.5 and 5.8).
The intensity of crop damage depends on the type and combination of salts in the soil solution, the soil management system, and the crop's tolerance to saline conditions. The United Nations Food and Agriculture Organization (FAO) gives the threshold values of the electrical conductivity of soil (ECe) and water (ECTO) at which potential yield of various crops normally is expected to be 100%, 90%, 75%, and 50% of the maximum yield .
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