Soils that are most likely to show economic responses to gypsum application have the following features in the topsoil, subsoil or both:
- high clay content—greater than 30%. particularly greater than 40%,
- high sodicity level—ESP greater than 5. particularly greater than 10,
- low salinity level—electrical conductivity of a 1:5 soil: water suspension (EC I:5) less than about 0.4 decisiemens per metre dS/mi. particularly less than about 0.2 dSfm. depending on sodicity level.
Other soil properties that can affect gypsum response include:
- Ratio of exchangeable calcium to exchangeable magnesium Ca/Mg ratio). The lower the Ca/Mg ratio, the greater the likelihood of response,
- Organic matter content. Organic matter usually helps to prevent clay dispersion. particularly when present with calcium ions,
- Concentration of free lime (calcium carbonate) in alkaline soils. Naturally occurring free lime can assist in reducing clay dispersion,
- Clay type – different types swell and disperse to varying degrees. thereby affecting gypsum response,
- Degree of mechanical damage.
The application of gypsum to saline non-sodic clay soils is not recommended for two reasons:
- gypsum is unlikely to improve the structure of saline soil,
- gypsum is a salt, so it may have an adverse effect on salt-sensitive plants such as lettuce, particularly if applied at high rates to marginally saline soils.
However, gypsum is usually required for the reclamation of saline sodic clay soils because salinity levels are reduced by leaching, thereby resulting in swelling and clay dispersion when rain falls or low salinity irrigation water is applied.
Reference: NSW Agfacts AC.10.
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