MAF to manage heavy metal build-up in NZ farmland

Ministry of Agriculture and Forestry (MAF) officials plan to ``manage'' the gradual build-up of cadmium contaminating New Zealand's agricultural soils.
New Zealand farmers are pouring about 30 tonnes of the toxic heavy metal in total onto farmland annually, but MAF today called on farmers and growers work closely with fertiliser companies on the most appropriate fertiliser application and land management options.
The cadmium occurs naturally in rock used to make phosphate fertilisers, which are applied at rates as high as two million tonnes a year, and companies have been trying since 1996 to blend rock from different sources to reduce the rate at which they contaminate farms.
A senior veterinary scientist, John Hellstrom, warned four years ago that a national cadmium strategy was needed to ``to mitigate future risks'' - including the fact that some farmland would be classed as contaminated if it were subdivided for homes.
In 2003, a trans-Tasman food standards authority revised levels for cadmium allowed in meats such as liver and kidney, based on a World Health Organisation recommendation for a maximum exposure of seven micrograms of cadmium per kilogram of bodyweight per week.
At the time, New Zealanders were estimated to already be exposed to cadmium levels in food up to 41 percent of this.
The main concerns raised by cadmium were that chronic exposure can lead to kidney damage and bone disease. But more recently, cadmium has been recognised as a likely risk factor for breast cancer, because rat tests have shown it has a strong ability to functionally mimic oestrogen.
MAF has known about the problem for nearly 20 years and decades of reliance on superphosphate built up cadmium in sheep to such a degree in the mid-1990s that it banned export of some offal from animals older than 2-1/2 years.
Testing revealed up to 28 percent of sheep kidneys and 20 percent of cattle kidneys sampled between 1989 and 1991 exceeded the maximum residue levels allowed in NZ meat, 1mg/kg.
Health guidelines for soil contamination at the time had a maximum level of 3mg/kg of soil.
The natural average level of cadmium in NZ soils is 0.16mg/kg, but when farmland is taken into account, the average is over double that, 0.35mg/kg, and soils on farms which have had a lot of superphosphate, such as dairy farms, can have as much as 2.52mg/kg.
Dairying areas with high fertiliser use tend to have the highest average contamination, including Taranaki (0.66mgkg), Waikato (0.60mg/kg) and the Bay of Plenty (0.52mg/kg).
MAF's director of natural resources policy, Mike Jebson, said today that monitoring indicated that levels had slowly increased, over decades. Excessive levels in soils increased the risk of the heavy metal entering the food chain and harming human health.
MAF's cadmium management strategy was outlined yesterday Massey University's annual fertiliser workshop.