A Massey University scientist is calling for a reduction in the amount of fertiliser used on farms as a way of combatting the growing concentration of cadmium in the soil.
Cadmium - a carcinogen - accumulates in soil as a result of the prolonged use of superphosphate, and Mike Joy says areas that have been intensively farmed for a long time, such as Waikato and Taranaki, have high concentrations of it.
He says the only way to prevent the problem from becoming worse is to stop putting superphosphate on farms.
Dr Joy says part of the reason the rising level of cadmium has gone unnoticed by the industry is that it does not pass through into milk but stays in the livers and kidneys of cows.
He says it has been illegal for at least a decade in New Zealand to sell cows for human consumption once they're older than two, but the organs are used to make pet food so the cadmium shows up there.
Dr Joy says some work is being done to get cadmium out of pet food, and cats are being tested to see how it affects them, but the only way to get cadmium out of the soil is by plants taking it up very gradually over time.