Soil sodicity - Soil Essentials: Managing Your Farm's Primary Asset - page 61

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Soil sodicity
What is soil sodicity?
Sodicity is an expression of both the physical and chemical problems in soils
resulting from an interaction between soil sodium content and the salt content of
soil water. Sodicity has no clearly defined limits so that its effect on soil properties
cannot be assessed based on the level of sodium alone, as the soluble salt level also
has to be specified. Even small amounts of sodium can produce adverse effects if
the soluble salt concentration in the soil solution falls below the level required
(electrolyte requirement) for soil stability. For example, a combination of high
sodium and low salt will produce extremely bad physical conditions in soils.
Higher salt levels could avoid these soil structural problems but might impact
adversely on plant productivity.
Some soils are more sensitive to sodium than others. This depends mainly on
soil type, mineralogy, pH and organic matter. Generally a soil is considered sodic if
the exchangeable sodium exceeds 5% of the cation exchange capacity.
How is sodicity different from salinity?
Sodicity is often confused with salinity. Salinity is an expression of the
concentration of all the salts (electrolytes) present in soil or water and is
commonly measured as electrical conductivity (EC). Salinity in Australia is
dominated by sodium chloride (NaCl) or common table salt. While sodicity and
salinity often occur together, the destructive effects of sodicity are only evident