Federated Farmers: Gypsum can reduce agricultural emissions

Rivers often get polluted by effluent discharge. Photo / File

Agricultural systems are leaky and losses of phosphorus, nitrogen, organic matter and suspended solids can impact on water quality.

While direct contamination of surface water can be prevented by avoiding livestock access and effluent discharge, it is less straightforward to prevent losses over and through soil that can eventually reach waterways.

These less direct losses are affected by complex hydrological and chemical factors.
Gypsum has long been used as a soil conditioner and fertiliser but it is only recently that gypsum's potential for reducing agricultural emissions to waterways has been researched.
Gypsum (CaSO4.2H2O) can improve soil aggregation through calcium-induced flocculation of particles and sulfate-induced leaching of excess sodium.

Such effects can reduce surface runoff volume by improving water infiltration into soil.
Improved stability of aggregates reduces the potential loss of soil particles to waterways both over and through soil.

The calcium ions can also increase precipitation of phosphate ions either directly as calcium phosphate or indirectly by increasing availability of aluminium ions.

Increased ionic strength of soil solutions due to dissolution of gypsum may also increase adsorption of phosphate ions and organic matter to soil particles.

These multiple modes of action can partially address both hydrological and chemical factors influencing nutrient losses. Gypsum application has been reported to at least halve phosphorus losses in some conditions but results have varied between experiments.

Variability may be partly due to experimental design (insufficient time for gypsum to take effect in the soil, or high simulated rainfall conditions) but could also be related to soil type and existing exchangeable calcium level in the soil.

An understanding of the causes of variability will assist in the choice of target areas for optimal economic use of gypsum to reduce phosphorus losses.

Surface runoff of nitrogen, organic matter and soil particulates, as well as drainage losses of of nutrients in organic form can also be reduced with gypsum.

The reduction in losses of organic forms of nutrients may be particularly important for mitigating effluent application losses.

Reducing nutrient loss

Gypsum is calcium sulfate (CaSO4.2H2O) and provides a readily available source of calcium and sulfate ions due to its partial solubility. It has been used for decades as a soil conditioner and fertiliser (Shainberg et al., 1989) but it is in comparatively recent years that gypsum's ability to reduce nutrient losses has been researched.

Soil structural improvement resulting from gypsum application can include reduced surface crusting and sealing, improved water infiltration (reducing potential for runoff), soil aggregation, drainage and aeration with subsequent benefits for plant growth.

Benefits to plant growth, including root condition, may also result from the calcium and sulphur nutrition provided by gypsum. In the case of sodium build up which can result from some effluent types (dairy factory effluent in particular), there is a clear benefit in assisting the leaching of sodium, benefiting soil structure particularly where there is dispersive clay.