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Impressive results from soil amendment trials

It’s often said that there are no simple solutions to improving water quality in Geographe waterways, however results from research being carried out on farms in Yoongarillup (near Busselton) and Keysbrook (Peel) are showing impressive results on reducing phosphorus loss off farms.

The research is trialling applications of soil amendment products which bind phosphorus, keeping it in the farming system and stopping it running off into waterways. The trials are using Iron Man Gypsum (IMG) which is produced by Iluka Resources Ltd in Capel.

Department of Water and Environmental Regulation Senior Scientist Dr Brad Degens, who is leading the research, says results from the first three years of data collection show that applications of IMG can reduce phosphorus leaching and runoff by up to 80%.

The trials showed that either incorporating or topdressing IMG into P saturated pasture soils substantially reduced phosphorus loss.

“This is great news for farmers because both methods substantially increased the soil’s ability to hold onto phosphorus and keeping this a form that is available for plant growth,” said Brad.

Scientists measured the amount of phosphorus loss by using custom-made runoff simulators, buried collection devices (lysimeters) and shallow groundwater bores,  coupled with intensive sampling and soil analysis.

“Risk assessments for pasture nutrition, aquatic organisms, cattle health and exposure to humans via cattle products were also undertaken and confirmed the safety of these amendments,” said Brad. Fertiliser for grazing is the largest contributor of nutrients to Geographe waterways, with an estimated  80 % of nitrogen and 70 % of phosphorus coming from grazing properties.

Revitalising Geographe Waterways manager Dr Kath Lynch hopes that soil amendments will become a viable option for Geographe farms in the future.

“The research has shown that soil amendments such as IMG can significantly reduce phosphorus loss from soils with naturally low phosphorus holding capacity that have become saturated with P,” said Kath.

“Combining soil amendments in P saturated soils with improvements in fertiliser management are our best hope for improving water quality in this and other coastal catchments”.

The next steps are to trial ways of targeting IMG at areas on farms that are prone to losing phosphorus over the next four years as part of Royalties for Region’s Healthy Estuaries WA and Revitalising Geographe Waterways programs. These State Government initiatives aim to support the long-term health of our south-west estuaries.

To read the full Soil Amendment factsheet, visit

For further information on the Healthy Estuaries WA soil amendment program, visit


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