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• Salinity is the presence of excess salt (influences the growth of plants)
• Sodicity is the presence of excess sodium in the soil (influences how soil behaves)
Impacts of Sodicity
• Sodicity affects soil by degrading its structure, creating poor conditions for plant growth, and making the soil susceptible to surface crusting and erosion
• Sodium bound to clay particles displaces more useful cations such as calcium
• The sodium weakens the bonds between the soil particles when wetted, resulting in clay swelling and dispersion (particles becoming detached)
• Dispersed clay particles can move through the soil, clogging pores
• Both swelling and dispersion decrease permeability of soils, thus reducing infiltration, aeration and drainage
Dispersible layer Entry ofrunoff
Rainfall
Oldrootline
Impermeable layer
Surface runoff
Dispersible clay becomes mobile
Subsurface collapsesforming ‘pipes’
Sediment fanappears on surface
Sodicity is often indicated by crusting in the soil surface and hardsetting of subsoils
• Soil sodicity is a natural feature of many Australian soils, both in surface soils and subsoils
• Soil sodicity can be measured by doing simple field tests to see what happens to the soil when water is added
• If the clay particles break apart, the soil is said to be ‘dispersive’
• Soil sodicity can also be measured in a laboratory by determining the Exchangeable Sodium Percentage (ESP) which measures how much sodium is in the soil, compared to other cations like calcium and magnesium
• Soils are considered sodic once the ESP is above 6%
• Water sodicity can be measured by determining the Sodium Absorption Ratio (SAR)
• To understand how sodicity impacts on soils, remember that soils consist of sheets of clay arranged on top of each other
• The clay attracts and holds onto water and nutrients because clay surfaces are charged
• Different clay types hold different amounts of water and nutrients
• Sodicity is often confused with salinity, because both salinity and sodicity are associated with sodium
Exchangeable cationsExchangeable cations- implications for physical characteristics of soil- implications for physical characteristics of soil
Ca2+ is a desirable cation (promoting aggregation – clay particles are attracted to each other)
Na+ is undesirable (causes dispersion – clay particles are repelled by each other)
Flocculated clay (lots of calcium on the clay surfaces)
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Clay particle
Negative charge on clay surface
Ca2+
K+
K+
Mg2+
Nutrients (e.g. cations, fertilisers etc)
Nutrients (e.g. cations) in the soil water
Ca2+
Mg2+
Solid soil Soil solution (water)
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Ca2+
K+
K+
Mg2+
Ca2+
Mg2+
Non-saline
Non-sodic
Salinity v. SodicitySalinity v. Sodicity
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Ca2+
Mg2+
Salinity v. SodicitySalinity v. Sodicity
Non-saline
SodicNa+
Na+
Na+
K+
Mg2+
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Ca2+
K+
K+
Mg2+
Ca2+
Salinity v. SodicitySalinity v. Sodicity
Ca2+
K+
Mg2+ K+ Na+
Mg2+ Ca2+
Ca2+ Mg2+ K+
Saline
Non-sodic
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Ca2+
Salinity v. SodicitySalinity v. Sodicity
K+
Na+
Na+
Na+
Mg2+
Na+ Na+
K+
Mg2+ K+ Na+
Na+ Na+ Ca2+
Ca2+ Mg2+ Na+
Saline
Sodic
Managing Sodic/Dispersive Soils• The best way to manage these soils is not
to disturb them• Gypsum and/or applying organic matter is
often used in agricultural situations• Managing water flows over and through
these soils is very important• During construction, the dispersive soils
can be buried beneath a stable soil cover • Maintaining vegetation on the surface is
also important
Application of gypsum can control surface sodicity,
it is more difficult to treat subsoil sodicity
