Plant roots can cause soil particles to release bound Cd, resulting in Cd entering the root and then the shoots of the plant. Grazing animals may consume the Cd-containing plant or, in some cases, ingest small amounts of soil directly. As with humans, the highest Cd concentrations in grazing animals are found in the kidneys and liver. Fortunately, there are no reports of muscle or milk products containing concerning Cd concentrations.
There is a large variation in the ability of plants to take up Cd from soil. Leafy greens such as lettuce and spinach have the highest Cd concentrations, while grains tend to have the lowest. That said, Cd concentrations in grains are of concern because they can represent a large proportion of the diet.
Managing Cd Concentrations in Food
Reducing or reversing the accumulation of Cd in soil requires the reduction of Cd applied with phosphate fertilizers. Removing Cd from phosphate fertilizers would halt further increases in the Cd concentrations of most agricultural soils. Unfortunately, there is no cheap way of removing Cd during fertilizer production.
In most cases, reducing the amount of phosphate fertilizer applied would result in an unacceptable drop in productivity. A characteristic of phosphate in soil is that it becomes immobilized and unavailable for plants, thus requiring additional fertilizer applications and further Cd accumulation. One line of research to reduce the reliance on phosphate fertilizers is investigating methods of liberating immobilized phosphate by using selected crop varieties or other soil amendments.
Sourcing phosphate rock with a lower Cd concentration reduces the amount of Cd added to soil. For example, most Cd in New Zealand’s pastoral soils come from the application of superphosphate made from Nauru phosphate rock, which contained ca. 550 mg of Cd for each kg of P. Subsequently, the fertilizer industry has reduced the Cd concentration in fertilizers to less than 280 mg Cd/kg P by using phosphate rock from other locations. To avoid the accumulation of Cd in soil, phosphate fertilizers would need to contain less than 50 mg Cd/kg P. Further exploration and innovation in mining may yield fertilizers with even lower Cd concentrations. An example is the potential source of low-Cd phosphate in the Chatham Rise, off the cost of New Zealand. Accessing this resource has the technical and environmental challenges associated with undersea mining.
Most Cd in agricultural soil is bound within the top 10 centimeters. Therefore, plowing the soil will dilute the Cd within the soil profile. Plowing can reduce plant Cd uptake by moving the Cd to a zone of lower root density. This dilution effect increases at greater plough depths. However, continued application of Cd-laden fertilizers will eventually increase the Cd concentration in the entire soil profile.
Cd-contaminated soil cannot easily be cleansed. There are no commercially available techniques to remove Cd from contaminated soil at a cost that is less than the value of the agricultural land. Extracting the Cd using fast-growing plants such as willow may work in principle, but it is unclear whether this will ever be a commercially viable technology for farmers.
The amount of Cd that is taken up by plants and subsequently enter food products is dependent not only on the total Cd in the soil, but also on a plethora of other soil factors and plant factors. Some of these factors can be managed to reduce Cd concentrations in food. There is considerable variation in the Cd uptake among plant varieties. Selective breeding or genetic manipulation can be used to obtain plant varieties that take up low concentrations of Cd.
Plants in acid soils, soils with low organic matter, or soils that are high in chloride more readily take up Cd. Liming to reduce soil acidity can effectively reduce Cd uptake in some soils, but not others. However, liming is a blunt instrument: Over-liming can induce deficiencies of essential nutrients.