Soils containing enough soluble salts to interfere with the ability of plants to take up water. The conventional measurement that determines the salinity of soil is a deciSiemens meter ; soils are considered saline if the conductivity of their saturation extract solution exceeds 4 deciSiemens meter-1. This unit of measure closely approximates the ionic salt concentration, and it is relatively easy to evaluate. The most common salts are composed of mixtures of sodium, calcium, and magnesium with chlorides, sulfates, and bicarbonates. Other less soluble salts of calcium sulfate and calcium and magnesium carbonate may be present as well. The pH is commonly less than 8.5.
In saline soils, there is often what is known as a perched water table—water close to the surface of the land. This phenomenon can be caused by restricting layers of fine clay within the soil, or by the application of waters at a rate greater than the natural permeability of the ground. When the water table is so close to the top of the soil, water is often transmitted to the surface where evaporation occurs, leaving the ions in the water to precipitate as salts. The resulting complex of salts can be seen on the surface of the soil as a white, crust-like layer.
The surface layers of a saline soil commonly have very good soil structure, and this is important to understand if the soil is to be reclaimed or leached of the high salt concentrations. This kind of structure has relatively large pores; water can flow quickly through them, which aids in carrying away salty water, thus making it easier to flush out the soil. Artificial drains can also be installed beneath the surface to provide an outlet for water trapped within the soil, and once these drains are in place, salt concentrations can be further reduced by the application of good quality water.
Excessive irrigation and the use of fertilizers and animal wastes can all increase the salinity of soil. The yields of common crops and the level of agricultural production are severely reduced in areas where salts have been allowed to accumulate in the soil. Salinization can be so severe in some cases that only salt-tolerant crops can be grown. Leaching is often necessary to reduce the levels of salt and keep the soil suitable for crop production. But this process can remove other soluble components from the soil and carry them into the waste stream , polluting both groundwater and surface water. The environmental degradation from this form of agricultural pollution can be extensive, and a method needs to be developed for leaching saline soils without these consequences. If this problem cannot be solved, it will no longer be possible to use some saline soils for agriculture. As population growth continues and the global demand for food increases, another approach might be the development of more salt-tolerant plant species .
[Royce Lambert and Douglas Smith ]
Brady, N. C. The Nature and Properties of Soils. 13th ed. New York: Macmillan, 2001.
Millar, R. W., and R. L. Danahue. Soils: An Introduction to Soils and Plant Growth. 6th ed. Englewood Cliffs, NJ: Prentice-Hall, 1990.