Soil amendments to keep nutrients out of waterways

Soil amendments are used to increase the soil's ability to retain nutrients and reduce leaching into rivers and estuaries.

Last updated: 14 December 2023

Nutrient discharge from agricultural soils and new urban developments is a major source of nutrient input to waterways that feed estuaries across the south of the state.

The sandy soils of the Swan Coastal Plain have a very poor ability to retain nutrients, especially phosphorus. Additionally, in high groundwater areas subsoil drains are installed to enable urban development. These can serve as a pathway for discharge of historic nutrients accumulated in groundwater on sites that were previously used for agriculture.

Iron man gypsum

Soil amendments are useful in these areas to increase the soil's ability to retain nutrients and reduce leaching into rivers and estuaries. For example, Iron man gypsum (IMG) is a brown loamy soil material high in iron minerals that has been found by CSIRO to be highly effective in reducing phosphate and dissolved organic nutrients in soil leachates and stream waters.

We have investigated the performance of an IMG blend to treat groundwater discharging to subsoil drains in an urban development.

More than 0.5 km in length of subsoil drains have been installed with a blend of the IMG and sand adjoining similar non-amended subsoil drains in a new urban development in Perth's south-east. These have been coupled with a network of monitoring bores sampled regularly for a wide range of water-quality components and equipped with water-level instruments.

Parallel investigations have also been conducted to refine the geotechnical behaviour of the soil amendments.

Soil amendments derived from mineral processing by-products such as IMG are currently being investigated for use in sandy agricultural soils on the Swan Coastal Plain as part of the Healthy Estuaries WA program.

Many sands used for farming are very poor at retaining phosphorus built up from applied fertiliser. These soils annually lose large amounts of phosphorus to drains and streams, leading to nuisance algal growth in rivers and estuaries.