*Cadmium (Cd) is a non-essential element for humans, animals and plants. Excessive cadmium uptake by humans and animals leads to serious health problems.
Human uptake of cadmium takes place mainly through food. Foodstuffs that are rich in cadmium can greatly increase the cadmium concentration in the human body. Liver, mushrooms, shellfish, mussels, cocoa powder and dried seaweed are all examples.
Cadmium accumulates in kidneys, where it damages filtering mechanisms, causing diarrhoea, stomach pain, severe vomiting, bone fracture, reproductive failure and possibly even infertility. In severe cases, damage can be caused to the central nervous system, the immune system, DNA and cadmium is implicated in the onset of psychological disorders.
In New Zealand the major source of cadmium on agricultural soils, has been from the overuse of phosphate fertilizers derived from various phosphate rock imports. Superphosphate (SSP), triple superphosphate (TSP), DAP, or partially acidulated phosphate rock (PAPR), are manufactured by acidulation of the phosphate rock. Reactive phosphate rock (RPR) can also contain varying levels of cadmium.
As you can see in this chart, historical sources of phosphate rock have been very high in both cadmium and flourine. Numerous studies have shown that long term use of phosphate fertilisers has increased cadmium and flourine concentrations in NZ topsoil. A national survey done in 1994 (by Roberts et al) of cadmium in pasture and non-agricultural topsoil showed that pasture topsoil that had received regular phosphate fertiliser inputs contained a cadmium concentration ranging from 0.04 – 1.53mg cadmium/kg. In comparison, non-agricultural soil receiving no fertiliser had cadmium concentration of 0.02 – 0.77mg Cd/kg. This gap between the two represents an increase of 120%.
In a study of irrigated pasture grazing sheep that had been annually top-dressed with different rates of SSP for 44 years, it was noticed that the topsoil cadmium increased from 0.05 to 0.45 as a result of SSP application. An increase of 800%
The rates of cadmium accumulation in New Zealand topsoil as a result of phosphate fertiliser applications appears to be higher than elsewhere, probably due to the higher rates of phosphate fertiliser used on our volcanic soils. Total cadmium and flourine concentrations in regularly fertilised topsoil show a positive correlation with total phosphate concentrations. Various charts show the cadmium and flourine levels have increased with the increased application of phosphate fertilisers.
What does this mean?
One can only guess at the long term effects on human health from the continued use of phosphate imported from those countries lised above . New Zealand already has one of the highest levels of cadmium in agricultural soils. Today, the majority of phosphate rock imported into NZ, comes from Morocco.
Before you buy phosphate in any of its various forms (Superphosphate, Triple superphosphate, DAP, RPR, dicalcicphosphate) ask your supplier to show you an analysis of the cadmium levels in their product and disclose the country of origin. It is advisable to avoid the use of Dicalcic super. It is made from SSP and TSP which has some of the highest levels of cadmium. It is also made using limestone rock. Research shows that increasing the soil pH by too much increases the cadmium uptake from the soil to the plants and from there into the food chain.
We should be aware of the potential future trade implications. If our trading partners decide that the cadmium levels in our soils are too high, import restrictions on our primary produce could follow.
The Answer?
The obvious one is to stop importing and using phosphate rock from those countries that have high levels of cadmium. There is research available that shows certain mycorrhiza fungi absorb cadmium from the soil and store it in their mycelium. This prevents plant uptake of cadmium.
Instead of spending more money on research into use of phosphate on NZ soils, we should be researching how to repair the damage already done to the soils by overuse of phosphate fertilisers.
*Cadmium is a shiny, silver-white, ductile, metal. Its surface has a bluish tinge and the metal is soft enough to be cut with a knife, but it ‘dusts’ into air. 75% of cadmium is used in the manufacture of batteries, the remaining 25% is used in the production of pigments, coatings, plating and as stabilizers for plastics. Cadmium also has the ability to absorb neutrons, so it is used as a barrier to control nuclear fission.