Does the world have enough for food production? Should we be using it more sparingly? Could we recover phosphorus from waste?
The answer is almost certainly ‘yes’ to all questions – but how much are we prepared to pay?
New Zealand uses about one million tonnes of phosphate each year to meet plant growth demands. In September this year, the proposal to mine rock phosphate from the Chatham Rise received media coverage, with a debate on environment versus economics.
The arguments are based on what might happen to the ecology of the sea floor if mining is allowed to proceed. The Chatham rise is under protection from bottom dredging (it is a Benthic protection area) but not from mining. Mining could affect a very wide area by increasing sediment which could then affect corals and sponges that act as nurseries for fish.
The contra argument is estimates of 15 years mining, delivering $900 million benefit to the New Zealand economy and offsetting the need to import phosphate rock. Of further importance, explained by the developers, is that the Chatham Rise phosphate has lower cadmium concentrations than overseas sources. As nobody wants cadmium concentration to increase in New Zealand soils, the low cadmium concentration could be attractive.
What hasn’t been mentioned in the debate is uranium.
Chatham Rise phosphate rock has relatively high uranium concentration and relatively low phosphorus concentration. This was shown in a paper from the Fertilizer and Lime Research Centre, Massey University, published in 1986. Of the ten sources of rock phosphate analysed from around the world, Chatham Rise phosphate rock had the worst ratio – the most uranium per unit of the required phosphorus.
The researchers concluded that “higher concentrations of arsenic, cadmium and uranium in a number of phosphate rock materials highlight the need for a more thorough understanding of the significance of the amounts of these elements in assessing the potential of a phosphate rock for direct application”.
The research indicated that Chatham Rise phosphate rock had low concentration of cadmium – but that Moroccan phosphate rock, the source of New Zealand’s phosphate rock imports, was also low. Arsenic was not determined in Chatham Rise rock and was low in Moroccan rock.
Cadmium and uranium in soils has been analysed under long-term superphosphate trials by a team lead by Professor Louis Schipper at the University of Waikato. The researchers concluded that the small amounts of uranium present in New Zealand soils were tightly bound in the organic matter. They also concluded that at the “moderate and realistic” quantities of superphosphate used in hill country, cadmium is unlikely to become a concern.
Naturally this prediction presupposes no major increases in the source material.
Using Chatham Rise phosphate rock would constitute a major change.
New Zealand fertiliser companies take great care and, indeed, pride to ensure the phosphate rock they source for New Zealand farms meets high quality standards. They are aware of the importance of analysing source material and of considering the effects once the superphosphate is applied at common rates.
This step doesn’t seem to have been done in the current proposal for mining Chatham Rise phosphate rock. Confirmation that the uranium concentration is as high as reported in the 1980s would save a lot of time in the environment versus economics debate, and prevent a considerable amount of money being wasted in investment.
Certainly the developers have said “it will come down to the science”, but the right science is vital. It always is.
• Jacqueline Rowarth is professor of agribusiness, The University of Waikato.
