Sodicity should not he confused with salinity.
Salinity refers to the total amount of salts dissolved in the water in soil, whereas sodicity refers to the exchangeable sodium cations bound to clay particles. Salinity can be reduced by leaching (draining rainwater or non-saline irrigation water through the soil but leaching has little effect on sodicity.
Soils with an excess of exchangeable sodium cations (positive ions) attached to clay particles are called sodic soils. Sodicity is expressed as the ESP (exchangeable sodium percentage), which is the amount of exchangeable sodium expressed as it percentage of the soil’s CEC cation exchange capacity.
The calcium in gypsum (Calcium sulphate) can replace exchangeable sodium when applied to sodic soils, whilst also raising the ionic strength of the soil solution helping flocculate otherwise dispersible clays. These soils then become more friable and crops grow better.
Many soils in NSW have sodic clay subsoils of low permeability. These soils waterlog easily after heavy rain or irrigation. Under these circumstances subsoil oxygen levels remain low for long periods during which root development is seriously restricted.
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Map of NSW highlighting sodic soils.
Map of VIC highlighting sodic soils.