The cadmium (Cd) issue arose in New Zealand and Australia purely and simply because the manufacturing phosphate rocks both countries originally used to make superphosphate (Nauru and Christmas Islands) contained elevated levels (50-100 ppm) of Cd. All of this Cd ended up in the superphosphate, and therefore in the soil. Cd exists at low levels in all soils. But if too much is present in the fertiliser, the levels in the soil increase over decades to the point where elevated levels show up in pasture and then in animals, which concentrate it in their liver and kidneys, as happens to many poisons that cannot be excreted in the urine. The highest levels are found in soils which have had the highest application rates of superphosphate for the longest period of time; these are typically the dairy farms in the Waikato.
The highest levels of Cd are found in the liver and kidneys of young stock. To avoid the entry of excessive Cd into the human food chain, offal from young animals is not permitted to be sold for human consumption.
The reasons why some phosphate rocks are higher in Cd than others is not completely understood. Different hard ‘manufacturing’ phosphate rock deposits and different RPR deposits can contain very different levels of Cd. There can even be wide variation in levels within a particular deposit (more about this later). The concentration of Cd is thought to be largely due to the level of Cd in whatever organic matter is present in the phosphate rock as mined; that is, little is present in the phosphate rock crystal lattice itself. For this reason, the Cd can be removed by roasting the phosphate rock at high temperature. The organic matter in the phosphate rock is burnt off, along with the Cd in it. Ideally this is captured from the emissions. This roasting is expensive, and it also reduces the solubility of the phosphate rock, so it is not so suited for deposits that are used for direct application.
Internationally, most high Cd phosphate rocks (even those that otherwise are suitable for direct application) are used to make phosphoric acid. The reason for this is that most of the Cd precipitates out of the phosphoric acid and ends up in the gypsum. The phosphate fertiliser made from this phosphoric acid generally contains less than 20% of the amount of Cd originally present, but the enriched level in the gypsum by-product reduces its suitability for land application purposes, such as improving soil drainage and reducing salinity.
The science – and pseudo-science – of how Cd is managed by the industry in NZ
I first brought the issue of Cd accumulation to NZ farmers in the 1990s. Quinphos Fertilisers initiated a self-imposed limit of 2 ppm Cd for each % P in a product. This meant that an RPR containing 13% P could not exceed 2×13 = 26 ppm Cd. I based this limit on some excellent published independent scientific research conducted by Massey University, which showed that, at this level of Cd, there was no accumulation of Cd in the soil, and therefore in the food chain. At the time, we were importing Egyptian ‘Kossier’ RPR which contained only 12 ppm Cd, and Tunisian RPR, which ranges from 20-25ppm at the western end of the deposit to about 80ppm at the eastern end. We insisted that we receive RPR only from the low-Cd end; none of our shipments exceeded 25ppm. Averaged over the two RPRs, we had 19 ppm.
After years of ‘deny, deny’ that there was even an issue, the industry very slowly moved to adopting a self-imposed Cd limit of 280 micrograms of Cd per kg of P in any phosphate fertiliser. This equates to a 25 ppm Cd limit for a 9% P super, or 36 ppm for an RPR containing 13% P, nearly 40% higher than Quinphos’ maximum limit. This just isn’t good enough.
A decade ago the industry in effect admitted the seriousness of the Cd situation it had got NZ farmers into by introducing a ‘4-class’ categorisation of Cd in NZ soils. Farmers whose soils were in the highest Cd category could now only use fertiliser containing almost zero Cd.
What are the implications for RPR users? There are no RPRs with absolutely zero Cd (a cynic would say that the only motivation for having this pointless category at all was to prevent RPR being used). The main RPR now being imported by the industry (Sechura) is right at, or above, the industry’s own limit of 36 ppm. I have visited the Sechura deposit in Peru (as well as virtually all the other RPR deposits), as part of consultancy work for an international mining company. Unlike say the Algerian RPR deposit, which is essentially one thick homogeneous layer all containing 16-20 ppm Cd, Sechura has 8 different layers separated by clay; these layers range from 20-60 ppm Cd, with the lowest Cd layers being at the bottom. It is simply not possible to selectively mine the lowest Cd layers without enormous cost increase. The minimum weighted average achievable with current mining methods is about 38 ppm in my assessment. An importer may fluke one shipment at say 32 ppm, but the next is just as likely to be 45 ppm.
The very good news is that there are plentiful supplies of low Cd RPR. The Arad deposit in Israel is one. The Red Sea coast RPR deposits in Egypt are another. NZ’s underwater Chatham Rise phosphorite nodules – not mined as yet – are another. By far the biggest known to date, and already being mined, is the huge Algerian RPR deposit. All these RPRs are sufficiently low in Cd to lead to a slow but sure decline in the Cd levels in NZ and Australian soils if used as the source of phosphate.