Nutrient discharge from 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 development and these can serve as a pathway for discharge of historic nutrients accumulated in groundwater on sites that were previously used for agriculture.
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 that has been found by CSIRO to be highly effective in reducing phosphate and dissolved organic nutrients in soil leachates and stream waters.
The Department of Water is investigating the performance of an IMG blend to treat groundwater discharging to subsoil drains in an urban development. Over 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 are also being conducted to refine the geotechnical behaviour of the soil amendments.
Soil amendments derived from mining by-products such as IMG are essential for agricultural soils on the Swan Coastal Plain, which are very poor at retaining phosphorus. In other words, they have very poor Phosphorous retention index. This means that phosphorus applied as fertiliser is not retained in the soil long enough for plants to use, and is lost to drains and streams, leading to nuisance algal growth in rivers and estuaries.
Trials have shown that IMG applied as little as 15 kg per hectare will retain phosphorus in agricultural soils and improve plant growth. Many agricultural soils have low pH which inhibits root growth and the ability of plants to use phosphorus that is in the soil. The opportunity exists to blend soil amendments with lime or gypsum or co-apply it to paddocks to improve overall soil structure and reduce phosphorus losses to streams and drains.
The use of soil amendments to improve phosphorus retention in combination with fertiliser management are the core components of the Fertiliser Partnership which in turn is critical to all Water Quality Improvement Plans.
For further information see:
Iron Man Gypsum amendment of subsoil drains to treat nutrients in urban groundwater discharge
Investigation of the mineral-based by-product NUA as a soil amendment: results from the Bullsbrook Turf Farm
Investigation of the mineral-based by-product NUA as a soil amendment: results from the Bullsbrook Turf Farm: trial extension 2008-2009
Characterisation of mining and industrial by-products with potential for use as environmental amendmen