Heavy Metals in Fertilisers

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Heavy Metals in Fertilisers

These days most people realise that using too many chemicals on the garden could be damaging to the environment. We received a letter from Jo Erkelens, a very concerned viewer who purchased a packet of Yates Gro Plus Complete Plant Food, and was dismayed to read this product warning on the back of the packet:

“Use of this product may result in cadmium and mercury residues in excess of the maximum permissible concentrations in plant and animal products and may also result in accumulation of these residues in soils.”

Jo wrote to Yates complaining about the heavy metals in the fertiliser. She also said she would not buy Yates products again until they changed the way their products were marketed. We decided to examine jo’s concerns.

Legal requirements

Regulations vary from state to state, but packaged fertiliser products manufactured in New South Wales and Victoria, and manufactured or sold in South Australia must carry a warning statement on the pack if concentrations of heavy metals such as cadmium and mercury are higher than 1mg per kg in the case of cadmium, and 0.2mg per kg for mercury.

What the experts say

To find out if the levels of heavy metals in fertilisers should be of concern to gardeners, we spoke to a number of technical experts including leading soil scientist Kevin Handreck. Kevin has spent many years researching soils, potting mixes and fertilisers and he also chaired the committee which set the Australian Standard for potting mixes. Kevin said that rather than being at fault in any way, Yates had acted responsibly by complying with the law. However, many fertiliser and manure producers are ignoring the regulations, even though some of their products appear to contain levels of heavy metals over the prescribed limits.

The sort of information the law requires on bags of fertiliser doesn’t help the consumer at all. It would be more helpful if they said how much of a product you’d need to apply before heavy metals in the soil built up to harmful levels. In the case of Gro Plus, Kevin said that if it was applied heavily or at the recommended rate, it would take 100 to 200 years to raise the level of cadmium to any significant level. Over that time the cadmium would probably be locked away into insoluble forms, so any danger to the environment would either be non-existent or minimal. He goes on to say that most farmers in this country have added more cadmium than this to their soils via applications of superphosphate over the past century or so.