Saline, Sodic, and Saline-Sodic Soils
Classification of Saline, Sodic, and Saline-Sodic Soils
Two types of salt problems exist which are very different: those associated with the total salinity and those associated with sodium. Soils may be affected only by salinity or by sodium or by a combination of both salinity and sodium. Thus, soils can either be classified as saline, sodic, or saline-sodic. Each of these three types of salt-affected soils has a different impact on soil and plant growth and requires different reclamation strategies, so the type of salt-affected soil must be determined prior to cultivation and/or reclamation. These categories are defined in soil classification literature by the Natural Resources Conservation Service (NRCS) (Table 15.1).
Measuring Salinity and Sodicity
The presence of salts in soil and water can be assessed by measuring salinity, the concentration of soluble salts in a soil, and sodicity, the relative concentration of sodium (Na⁺) compared to calcium (Ca2+) and magnesium (Mg2+).
Salinity
Salinity is a measure of the concentration of the soluble salts contained in the soil. Salinity is measured by passing an electrical current through a soil solution extracted from a saturated soil sample. The ability of the solution to carry a current is called electrical conductivity (EC). More commonly salinity is measured with an EC meter that estimates the concentration of soluble salts in a soil slurry or water solution. The ability of a solution to conduct electricity increases with increasing salt content; therefore, a high EC value corresponds with high amounts of soluble salts, and vice versa.
Sodicity
Sodicity is measured by calculating the exchangeable sodium percentage (ESP) and/or the sodium adsorption ratio (SAR). Exchangeable sodium percentage is the percentage of soil exchange sites occupied by sodium ions, and is calculated by dividing the concentration of sodium cations by the total cation exchange capacity (CEC). Units of concentration for ESP are milliequivalents per 100 grams (meq/100g).
Sources of Salinity and Sodicity
The vast majority of salts come from the natural weathering of parent material, which includes minerals from bedrock and ancient sea beds. Soils naturally high in soluble salts are usually found in arid or semi-arid regions, where salts often accumulate because there is not enough rainfall to dissolve them and leach them out of the root zone. The major source of salinity problems is usually irrigation water. This is a gradual process—the salts must accumulate over time before any effects are seen. The salinity level of the irrigation water depends on the geology of the aquifer and the land through which it passes. Some irrigation waters, typically those pumped from deep wells, contain high concentrations of bicarbonate and have a high SAR (high concentration of sodium relative to calcium + magnesium).
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