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

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

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

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

21. 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’

25. 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%

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

27. 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

28. 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)

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

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

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

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

33. Saline Non-sodic Salinity v. Sodicity - Ca2+ Ca2+ K+ Ca2+ Mg2+ K+
Mg K Na+
Mg Ca2+
Ca Mg K+

34. Saline Sodic Salinity v. Sodicity - K+ Ca2+ Na+ Na+ Na+ Mg2+ K+
Mg K Na+
Na+ Na Ca2+
Ca Mg Na+

35. 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

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