• Bert Quin

Cadmium and phosphate rock

Part 1

High P-using dairy farms in the Waikato and Taranaki have enriched soil cadmium (Cd) levels because of the decades-long use of high-Cd manufacturing phosphate rocks from Nauru and Christmas Island to make superphosphate. This shameful legacy is being 'managed' by MPI by restricting the Cd level in phosphate fertilisers that can be used on high-Cd farms.

All phosphate rocks contain some Cd. Low levels are not a problem. All life on earth, including Homo Sapiens, has evolved to deal with common levels of a very wide range of chemicals that are only deadly at excessive concentrations.

But too much is too much. This is why the WHO and individual countries set limits in soils, plants, feed and food of tens of of thousands of different naturally-occurring and chemically-manufactured chemicals. Organic farming organisations typically set much lower limits.

Similarly, some RPRs contain far more Cd than others. Algerian RPR has among the lowest Cd level (18 ppm) of internationally-recognised RPR deposits. That's why I import it.

So I was very pleased to hear that the NZ fertiliser industry has made the decision to stop importing RPR from Sechura, Peru. The mining company's own specs advertise it at 40-60 ppm Cd! In my view, 25ppm (195mgCd/kgP) is the absolute safe limit.

Part 2

Given that Sechura (Peru) RPR contains 40-60 ppm cadmium (Cd), why was it used as the 'reference' RPR in many RPR vs soluble P field trials around the world, including in NZ and Australia?

When the use of RPRs on acid soils started to create real interest in the 1970s, a small pilot plant (now gone) was set up to beneficiate the raw Sechura RPR (which contains a lot of clay) from one site in this deposit. Virtually all RPR tested around the world came from this one site.

The deposit as a whole is made up of 8 layers, laid down on the sea floor originally over vast periods of time, with non-phosphatic layers of clay laid between them.

Now here's the thing. Of the 8 layers, it is now known that 2 thin layers are quite low in Cd (21-23ppm). The other (thicker) layers are all much higher (36-60ppm). Where the the pilot plant was put, a low-Cd layer happened to be near the surface, so it was mined. In the rest of the deposit, they are at the bottom. So for commercial mining, it is not economic to selectively mine and beneficiate the low-Cd layers.

One small operator still mines at the original site, and exports the RPR/clay with no benefiation or quality control; the pilot plant has gone. The P ranges from 7.5-9.5% - so NZ value is say $230/t.

Part 3

Cadmium from high-Cd phosphate rocks - whether one used directly as a fertiliser like RPR or manufactured into superphosphate etc - is a poison and carcinogen if elevated levels get into the foodchain. Soil Cd in parts of the Waikato are so high that future applications have to be expensive ultra-low Cd.

The move to alternative grazing plants chicory and plantain, which reduce nitrate leaching, are not an option in these areas because they take up 10 times or so Cd from the soil than do ryegrasses and clover.

So I have mixed feelings to see the industry stopping importation of Sechura RPR. Sure it has an unacceptably-high Cd content, but it is a very good RPR from an agronomic perspective. It should simply be blended 50/50 - NOT with lower-Cd manufacturing phosphate rocks like the totally unproven Boucraa PB3 waste -but with low-Cd Algerian RPR.

Instead, they have switched to granulated products made from other low-Cd phosphate rocks. Are these recognised RPRs? To my knowledge, no. And no field data has been released. JUST HOW DOES THIS HAPPEN? Ah right, they pass the Fertmark test...