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Sodicity and Salinity

Learn about the impacts of sodicity on soil structure, plant growth, and erosion. Discover how to measure and manage sodic soils to improve conditions for agriculture.

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Sodicity and Salinity

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  1. Sodicity and Salinity Kristie Watling NRW, Toowoomba Ph: 4688 1092 E-mail: kristie.watling@nrw.qld.gov.au

  2. 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)

  3. 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

  4. 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

  5. Sodicity is often indicated by crusting in the soil surface and hardsetting of subsoils

  6. Soil sodicity is a natural feature of many Australian soils, both in surface soils and subsoils

  7. 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’

  8. 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%

  9. Water sodicity can be measured by determining the Sodium Absorption Ratio (SAR)

  10. 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

  11. Exchangeable cations - 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)

  12. Flocculated clay (lots of calcium on the clay surfaces)

  13. Solid soil Soil solution (water) Clay particle Nutrients (e.g. cations, fertilisers etc) Ca2+ - - - - - - Ca2+ K+ K+ Mg2+ Mg2+ Negative charge on clay surface Nutrients (e.g. cations) in the soil water

  14. Salinity v. Sodicity Non-saline Non-sodic Ca2+ - - - - - - Ca2+ K+ K+ Mg2+ Mg2+

  15. Salinity v. Sodicity Non-saline Sodic Ca2+ Na+ Na+ Na+ K+ Mg2+ - - - - - - Mg2+

  16. Salinity v. Sodicity Saline Non-sodic Ca2+ - - - - - - Ca2+ K+ K+ Mg2+ Ca2+ K+ Mg2+ K+ Na+ Mg2+ Ca2+ Ca2+ Mg2+ K+

  17. Salinity v. Sodicity Saline Sodic K+ Na+ Na+ Na+ Mg2+ Ca2+ - - - - - - Na+ Na+ K+ Mg2+ K+ Na+ Na+ Na+ Ca2+ Ca2+ Mg2+ Na+

  18. 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

  19. Application of gypsum can control surface sodicity, it is more difficult to treat subsoil sodicity

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