Cadmium sits immediately below zinc in the periodic table, explaining their chemical similarity.
Cadmium
Cadmium (Cd), element number 48, is a heavy metal. Although Cd ores are rare, Cd is widespread and is a common contaminant of zinc ores and phosphorus (P) fertilisers. Because it is highly toxic and carcinogenic, Cd is banned around the world in electronics applications. Workers in smelters and factories that produce Cd products must take special precautions to avoid inhalation.
Because Cd is widespread, it is a natural component of many soils, even without the application of P fertilisers. The use of superphosphate will add Cd, although fertiliser laws now restrict the content of Cd to a manageable level. (It is expensive to treat fertiliser to remove the Cd, and it is easier to limit the Cd content by starting with low-Cd rock phosphate.) A study in New Zealand found that the natural average background level of Cd in soil is 0.16 mg/kg, and that levels in agricultural soils are up to 5 times this, most notably on dairy farms, in association with high use of superphosphate.
Reducing the risk
The risk of Cd ingestion from food grown on contaminated soils depends on a wide range of variables:
- Soil pH: Cd availability for uptake by plant roots increases as acidity increases (that is, pH goes down below 6).
- Crop species: Some plants are more able to block the uptake of Cd into the roots than others; some plants better restrict the movement of Cd into grain or fruits than others. For example, wheat is better than flax at keeping Cd out of the seeds.
- Absolute Cd content in the soil: The more Cd is present in the soil, the more plants will usually take up.
- Chemical form of Cd: CdSO4 (cadmium sulphate) is more soluble than CdS (cadmium sulphide), and so the Cd is more easily taken up. Phosphate reduces the availability of cadmium to plants.
- Soil type: It is easier for Cd to leach down through a sandy soil than through a clayey soil, offering a solution for soil remediation if enough water is available. On the other hand, a high clay content allows a soil to adsorb (hold on to) Cd, effectively making it unavailable to plants.
- Soil zinc (Zn) content: Because Cd and Zn are chemically alike, adding Zn fertiliser will increase plant uptake of Zn and therefore reduce plant uptake of Cd. A zinc-to-cadmium ratio of >60:1 in the soil will generally protect animal life, as plants will not take up the Cd.
- Soil organic matter content: Increasing the organic matter content increases the soil’s cation exchange capacity (CEC), which holds onto more Cd. A low CEC makes the Cd more available to plant uptake. So tillage practices that reduce the organic matter content of soil will make Cd more available to plants.
- Soil salinity: The chloride component of saline soil makes the Cd more soluble, so reducing soil salinity will reduce Cd uptake.
- Irrigation water source: Irrigation water drawn from mine runoff will have a much higher Cd content than “clean” water. This has historically been a problem in parts of Japan.
What is the risk to humans?
The risk to humans from the ingestion of Cd in food is low. It is kept low by food standards laws and fertiliser legislation, but Cd is nearly always accompanied by Zn, which offers the main form of protection for higher animals. Smokers have a much greater risk of Cd intake than non-smokers on account of the high Cd content of tobacco.
Further reading
Wikipedia: http://en.wikipedia.org/wiki/Cadmium
Cadmium Working Group. 2008. Report One: Cadmium in New Zealand Agriculture. Ministry of Agriculture and Forestry, Wellington.
Jiao Y et al. 2004. Effects of phosphorus and zinc fertilizer on cadmium uptake and distribution in flax and durum wheat. J Science of Food and Agriculture 84: 777–785.
Senn A, Milham P. 2007. Managing cadmium in vegetables. Agfact. NSW DPI, Orange.