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Willows superb at sucking soil of toxic cadmium

Media Release 8/11/00

Willows superb at sucking soil of toxic cadmium

The enormous capacity of willow trees to suck the toxin cadmium from contaminated pastureland has astounded HortResearch scientists.

If one crop of the most efficient cadmium-sucking willow variety was planted on all of New Zealand's hill country, they could reverse 100 years of using super phosphate fertiliser, said head of HortResearch Environment and Risk Management Group Brent Clothier.

The scientists who recently completed the research were particularly surprised at the wide variation in the levels of cadmium individual willow varieties could absorb. The 16 different willow clones tested accumulated between nine and 50 times the cadmium concentration of the soil in which they were grown.

"The research has shown willows are genetically diverse and offer a great resource. We were not expecting to find such clonal variation within the willow species," Dr Clothier said.

Cadmium is found at low levels throughout most of New Zealand's pastureland. Although it occurs naturally, elevated levels are caused by the over-zealous application of cadmium-rich super phosphate fertilisers. Once ingested, cadmium accumulates in the kidneys of sheep. From there it may pass up the food chain to humans. For this reason there are regulations preventing the export of sheep kidneys from animals older than 30 months.

Initial research indicates those varieties which absorb high levels of cadmium also take up magnesium and zinc. There is a high possibility they will absorb other heavy metals as well.

Dr Clothier said further research is needed into practical ways farmers can use willow to rid soil of cadmium. However, he said 'coppicing' would be part of the end process. This is when the stem of the willow is taken off at the base every year and the cadmium-high cutting disposed of safely. The willow then grows another stem and the cycle continues.

Another advantage to individual willow varieties having different abilities to take up cadmium, is that clones which don't take up high levels of the metal can be planted in areas prone to drought and used as stock fodder when pasture levels are low.

The cadmium absorbency rate of willows is just one aspect of HortResearch Environment and Risk Management Group's phytoremediation programme, where the ability of poplar and willow to absorb toxic waste as a way of removing it from the soil is being researched.

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For further information contact: HortResearch Palmerston North, Tel: 06 356 8080, Fax: 06 354 6731. Dr Brent Clothier Ext 7733, Email: or Dr Tessa Mills Ext 7743, Email: or writer, Caleb Hulme-Moir Ext 7728, Email:

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