Adding some types of organic matter to soil can effectively reduce plant-Cd uptake. There is variation in the effectiveness of various types of organic matter in reducing plant-Cd uptake. Elucidating the critical factors for such Cd immobilization is an ongoing area of research. Plants take up more Cd from soils that are deficient in the essential micronutrient zinc (Zn). Alleviating Zn deficiency in soil may reduce plant Cd uptake and increase the Zn concentration in foods. This has the double benefit of alleviating Zn deficiency in people (which affects some two billion worldwide) and reducing the toxicity of Cd in Zn-deficient people.
Managing such plant and soil factors can reduce the Cd concentrations in food over the short term. While these measures do not stop the accumulation of Cd in the soil, they can extend the time that food production can safely occur on Cd-contaminated soils. This gives additional time for the development of low Cd fertilizers, or new soil cleansing techniques.
While Cd concentrations in agricultural soils are increasing and Cd concentrations in some foods are nearing food standards, there are no reports of widespread Cd intoxication in the general population. In many agricultural lands, food production can probably continue apace without a widespread health calamity. However, over the medium term, continued accumulation Cd of agricultural soils is unsustainable because the upper end of current dietary Cd intakes are already commensurate with tolerable intake limits, and regulators will be moving to ensure that Cd in foods stay as low as reasonably achievable. Modifications to soil and plant factors can soften the impact of Cd accumulation in soils, and potentially work to reduce Cd in both individual foods and the whole diets, but the benefits of this work will ultimately be lost if Cd continues to strongly accumulate in growing soils. Therefore, a primary goal for agriculturalists should be to move to a steady-state condition, where annual inputs of new Cd to soils are no larger than losses.
The immediate issues for food producing countries are that food exports may be blocked if food standards are exceeded and the image of the country as a safe food producer may be tarnished. For countries with protectionist governments, Cd concentrations in foods may be a useful means of circumventing the World Trade Organization and imposing non-tariff trade barriers to protect local producers, even if the local produce also contains high Cd concentrations.
Any management decisions or regulations that are designed to reduce Cd concentrations in food need to be balanced against the cost of food production and the need to feed a growing population.
In the meantime, the clandestine threat that Cd poses to food safety will inexorably increase.
Dr. Kim is an analytical environmental chemist who works at Massey University in Wellington, New Zealand. Reach him at N.Kim@massey.ac.nz. Dr. Robinson is a professor of soil and physical sciences at Lincoln University in New Zealand. Reach him at Brett.Robinson@lincoln.ac.nz.
References Furnished Upon Request
One serious case of environmental Cd poisoning first focused scientific and regulatory attention on the possibility that long-term accumulation of Cd could cause serious harm. Between 1910 and the late 1940s, several hundred people from villages on the banks of the Jinzu River, Toyama Prefecture, Japan suffered from chronic Cd poisoning. Among other sources, Cd fumes and particulate matter emitted from a nearby mining company caused an excessive accumulation of Cd in soils of a farming community. Rice and soybeans grown in these soils contained high concentrations of Cd (1 to 3 mg/kg). Cd was not recognized as the cause until the mid-1950s. It was typically 30 to 40 years before the onset of symptoms, the most prominent of which was that the victim’s bones fractured under slight pressure due to their decalcification and subsequent softening. The disease was extremely painful, and the sickness became known as Itai-itai disease, variously translated “it hurts-it hurts” or “ouch-ouch.” By the end of 1965, some 100 deaths had resulted from disease.—N.K. & B.R.