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Gypsum

What, why, when, and issues to look out for

What

Gypsum is a naturally occurring soft crystalline material which is a hydrated form of calcium sulphate. It is mined from deposits, and is organic and non-toxic. You can get it in different textures and densities, according to how you intend to use it.

Why

Gypsum acts as a coagulant. It gives a slow-release source of sulphur and calcium, which is especially useful for heavy clay soils and also for fine (dispersive) soils such as loess. It improves soil structure, aeration, water retention and drainage in these soils and reduces waterlogging.

When

Superfine gypsum is mainly used in the following situations:

  • Spread on the surface of alkaline soils and/or soils affected by salinity (eg, loess, estuarial berms, dairy effluent)
  • During bench testing before flocculation
  • To add to a pond if the pH of a batch-dosed pond is below the effective range for the flocculent.

Golf ball-sized gypsum rock is mainly used in the following situations:

  • To line pre-stabilised drainage channels, before sediment treatment tools like floc socks and decanting earth bunds, floc-dosing unit outlets and sediment retention ponds.

Issues to look out for

  • Gypsum improves the structural stability of dispersive top soils quite quickly but may take several years to reach the subsoil. So it is not a long-term solution – you will need to put longer-term management strategies in place to maintain and increase organic matter in soils.
  • The rate at which gypsum moves down the soil profile can be affected by matters outside your control such as the amount of rainfall and the hydraulic conductivity of the soil layers.
  • Before applying gypsum, determine what effect it will have on your other chemical treatment systems.
  • Don’t spread gypsum onto road surfaces because it makes them slippery and potentially hazardous.
  • Monitor water pH carefully when using gypsum, because if overly-alkaline (beyond pH 8) conditions develop in waterways, aquatic life will be affected. If this happens, you may need to buffer treated water with a weak acid, for example from CO₂ bubbled into the water before discharge.
  • Get advice from a suitably experienced person before using gypsum.
Design essentials

When applying gypsum to surface soil

The rate for applying gypsum depends on how strong the sodicity/salinity is and also on the soil’s pH. There are guidelines available like the table below, but the best way to be sure is to test soils from your site.

Rates of gypsum application per soil type
Dispersive behaviour or sodicity rating Exchangeable sodium percentage (ESP%) Gypsum application rate (t/ha)
Neutral-to-acid soils Alkaline soils
Slight 6–10 0–1.5 1.0–2.5
Moderate 10–15 2.5 5
Severe >15 5 5.0 or more
Source:
Department of Agriculture and Food: “Managing dispersive soils” (PDF 594 kB)

When applying gypsum to exposed, sodic subsoil

  • Minimise disturbance.
  • Avoid concentrating water flow over the area. Fill in any trenches or holes immediately, so that water doesn’t collect and pond. Don’t create soakage pits in the area.
  • Apply as soon after exposure of the subsoil.
  • Compact any dispersive subsoils that have been disturbed or excavated, more strongly than usual. Use a ‘whacker packer’ for small areas or a sheep’s foot roller for larger areas.
  • Apply gypsum while infilling, then cover with topsoil and revegetate.

When bench testing before using flocculants

  • Test samples of water during bench testing with gypsum to determine whether it enhances the performance of flocculants, and the effects of different dosages. Check the pH of treated water to know whether it will be suitable to discharge.
  • Before flocculating a batch-dosed pond, take a sample in a container to test whether pre-treating the pond’s water with gypsum powder will enhance the performance of the flocculant. Monitor the pH to ensure it is suitable to discharge.

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