Saline, Sodic, and Saline-Sodic Soils
Sodic Soils
In contrast to saline soils, sodic soils have an EC less than 4 dS/m and levels of exchangeable sodium percentage greater than 15. An ESP of more than 15 percent is sometimes used to classify a soil as sodic. This means that sodium occupies more than 15 percent of the soil’s cation exchange capacity (CEC). The soil pH is often greater than 8.5, and plant nutritional imbalances may occur. It is the tendency of high pH soils to decrease the availability of essential nutrients, including calcium, magnesium, phosphorus, potassium, iron, manganese, and zinc, that is cause for concern. Sodium levels in soil are often reported as the sodium adsorption ratio (SAR).
Sodium Effects on Soil Physical Properties
Sodium has the opposite effect of salinity on soils. The combination of high levels of sodium and low total salts causes degradation of soil structure, which can be attributed to three underlying causes. First, exchangeable sodium increases the tendency of aggregates to break up or slake upon becoming wet. The clay and silt particles released by slaking aggregates clog soil pores as they are washed down the profile. Second, when expanding-type clays (e.g., montmorillonite) become highly saturated with sodium ions, their degree of swelling is increased. As these clays expand, the larger pores responsible for water drainage in the soil are squeezed shut. Third, and perhaps most important, sodic conditions—the combination of high sodium and low ionic strength (dissolved salt concentrations)—lead to soil dispersion.
Sodium Effects on Plant Growth
Just as crops differ in tolerance to high salt concentrations, they also differ in their ability to withstand high sodium concentrations. Sodic soils are detrimental to growth of most plants because most plants can’t tolerate the extremely poor soil physical conditions and slow permeability to water and air that accompany clay dispersion in sodic soils. High pH, low nutrient availability such as calcium, magnesium, phosphorus, potassium, iron, manganese, and zinc, and sodium toxicity are the constraints of their agricultural use.
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