Pasture establishment in trial plots treated with aquatic weed compost. Establishment was poor in the untreated plot (foreground) compared to the treated plot (other side of divider) where public seminar participants are standing.

Quality compost can be produced from aquatic weeds, safe to use and beneficial for farmland.

That’s the finding of Australia’s first large-scale scientific trial of the risks associated with recycling noxious weed material.

The trial was undertaken on 35,000 cubic metres of aquatic weeds mechanically harvested from the Hawkesbury River after a major infestation occurred in the summer of 2004.

“The harvesting created a significant disposal problem and led to an 18- month research project to examine ways to recycle it,” NSW Department of Primary Industries (DPI) research scientist, Dr Chris Dorahy said.

“The weeds included Alligator Weed, which is invasive on land as well as water.

“One of the big questions was whether an environmental risk remained from using this material, even though it had been composted.”

DPI conducted the research in collaboration with the NSW Department of Environment and Conservation (DEC) and Hawkesbury- Nepean Catchment Management Authority to evaluate the feasibility of compost-recycling aquatic weeds harvested from the Hawkesbury.

Dr Dorahy said the trial found it was critical to ensure the material is heated for long enough at a sufficiently high temperature – and that it is mixed and turned sufficiently.

“The Australian Standard for composted soil conditioners and mulches states that the whole mass must be exposed to a minimum of 55 degrees Celsius for three consecutive days.

“To ensure Alligator and any other terrestrial weeds don’t survive, it is important to monitor compost windrow temperatures, as well as the site itself, to make sure none establish in or around the compost.”

Tests on the quality of the compost were also undertaken.

It was found to be comparable to compost made from organic garden material, though with lower concentrations of nitrogen, phosphorus and calcium, and higher amounts of inorganic material such as sand.

The concentrations of heavy metals and chemical residues in the composted material were very low, indicating a low risk from other contaminants.

Dr Dorahy said erosion control trials at NSW DPI’s Centre for Recycled Organics in Agriculture (CROA) have shown that the compost is effective in controlling soil erosion and improving water quality.

In addition, pasture establishment in the plots treated with the aquatic weed compost was very good.

“This indicates that the compost is likely to be a useful medium for re-establishing vegetation, particularly on sites denuded of topsoil.”

Dr Dorahy said that environmentally sensitive areas in the Sydney catchment can be affected by a high level of erosion, which impacts on the quality of waterways, and the productivity of agricultural land.

“Using good-quality compost to reduce erosion and reduce soil washing into waterways can help to improve water quality,” he said.

“There is potential to recycle aquatic weeds such as Salvinia and Alligator Weed, provided they are composted appropriately.”

Other contributors to the project were Hawkesbury City Council, who provided their site at South Windsor for the composting operations and Bettergrow Pty Ltd, which conducted the composting.

Results of the research trial were presented at a public seminar at CROA in July. Further information in Primefact 229 is available from the NSW DPI website, www.dpi.nsw.gov.au

Contact Chris Dorahy, Camden, (02) 4640 6443.