In a project for the Fertiliser Association Industry of New Zealand, Edward mapped the cadmium content of 1980 soil samples from across New Zealand. He found clearly elevated concentrations in Taranaki and Waikato.
“This isn’t a surprise”, says Edward. “It’s come about from the historical use of phosphate fertiliser (from Nauru) that was naturally high in cadmium. These regions have been dairy farming areas for a long time, so have had more fertiliser applied to them in the past.”
Cadmium, a heavy metal, accumulates in the soil and can be toxic in food at high concentrations. A voluntary soil cadmium limit of 1.8 mg cadmium/kg of soil exists in New Zealand, but levels in most places are well below this value.
“We found that in four districts in the Waikato region, more than 5% of the farms had cadmium concentrations that need to be actively managed to stop it accumulating beyond this limit. Only four samples (0.37%) had soil cadmium concentrations that already exceeded the limit.”
The concentration of cadmium in the soil is only one part of the picture. Different foods take up the contaminant at different rates, so the trends were compared with foods tested in the 2009 New Zealand Total Diet Study, as well as international standards.
“People have looked at cadmium in soil around New Zealand before, but this is one of the largest datasets that’s available so it gives a really clear picture of what the issues are. We’ve now produced the information in a more detailed form than before, which we hope will enable the industry to get the most value from it.”
Edward says the fertiliser industry has interesting parallels with another large New Zealand industry.
“It’s a similar environment to the fisheries world. They both have industry bodies interacting with government, a responsibility to stakeholders, and a need for information to keep integrity through the process. There is also high public interest in issues in both sectors. It’s been interesting to make comparisons between them.”
Download the paper, which was published by the Fertiliser & Lime Research Centre at Massey University. The results of this research will be used to support the management of cadmium in New Zealand’s agriculture and food systems, and inform the planning for a review of the national Cadmium Management Strategy in 2017.