Low cadmium in Chatham Rise phosphate

The rock, located on the Chatham Rise seabed showed an average of 2.2 parts per million (expressed as mg/kg of P) from a range of samples gathered by CRP in 2012 from 11 separate locations, he says. The lowest value was 1.3 parts per million with a high of 5.3 parts per million.

This compares with the voluntary limit of 280 parts per million that New Zealand fertiliser companies observe at present for manufactured superphosphate, a limit that is often approached by rock imported from overseas.

"The tests we have conducted show our rock phosphate has among the lowest cadmium levels known," Castle says. "This will be good news for farmers who choose to use our product (either as superphosphate or as direct application rock) when we start production in 2015 and ultimately it will be good for New Zealand food consumers."

Cadmium is a naturally occurring heavy metal found in New Zealand soils. Excessive levels of cadmium in food can have implications for human health and excessive levels of cadmium in soils can restrict land-use.

The Ministry for Primary Industries is managing the gradual build up of cadmium in New Zealand soils through the cadmium contained in imported phosphate. The cadmium control programme follows research that shows cadmium levels have gradually increased over decades.

The programme recommends farmers and growers work closely with their advisers to determine the most cost effective, efficient and appropriate fertiliser application and land management options. Since the mid-1990s New Zealand fertiliser manufacturers have blended their high-cadmium phosphate rock with sources lower in cadmium.

Castle says low cadmium levels are one of the environmental pluses of developing a local phosphate resource. Providing CRP rock phosphate for New Zealand would also reduce New Zealand's carbon footprint through lower transport costs, and benefit the country's balance of payments and foreign exchange exposure. When used as a direct application source of fertiliser, rock phosphate also dramatically reduces soil leaching into waterways.

Cadmium can cause kidney failure and has been statistically associated with an increased risk of cancer and can also cause bone damage. Food is the dominant source of human exposure in the non-smoking population.

A Stuff website report says the build-up of cadmium levels in sheep made MPI ban the export of some offal from animals older than 2-1/2 years. Testing showed up to 28% of sheep kidneys and 20% of cattle kidneys sampled between 1989 and 1991 exceeded the maximum residue levels allowed in New Zealand meat of 1 mg per 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 more than double that, 0.35mg/kg, and soils on farms which have had a lot of super phosphate, 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 (0.66mg/kg), Waikato (0.60mg/kg) and the Bay of Plenty (0.52mg/kg).