Metal Contamination in Vegetables – Cadmium is Becoming a Concern

Gordon Johnson, Extension Vegetable & Fruit Specialist;

I recently had an interesting conversation with a vegetable processing company representative about Cadmium in vegetables.

The accumulation of toxic metals in vegetables has become an area of concern for consumers. The metals arsenic and lead have been extensively studied and the FDA has issued guidelines for allowable levels in certain foods. More recently cadmium (Cd) has been identified as a potential human concern and vegetables are a significant source of Cd in human diets. Chronic exposure to Cd (low level over an extended period of time) can result in kidney, bone and lung disease. Cd is also thought to be a potential carcinogen.

A natural metallic element, cadmium (Cd) is generally present in the earth’s crust at low levels. On However, cadmium is present at elevated levels in some soils, rocks, metal ores, fossil fuels (especially coal), human and animal waste, and as an impurity in phosphate rock. Fertilizer contains cadmium because phosphate rock is used to produce phosphorus fertilizers. Cadmium concentrations vary significantly by geography. Phosphate rock deposits in Florida used to produce fertilizer range from 3 to 20 mg Cd/kg rock, those in North Carolina range from 20 to 51 mg Cd/ kg rock.

Currently there are no standards in the US for Cd in vegetables. Risk assessment studies suggest that cadmium in food may be significant as low as 0.005 mg Cd/kg produce (5 μg/kg or 5 ppb). In Europe standards have been developed. The EU limits cadmium in vegetables and fruit to 0.05 mg/kg, with the exceptions of stem and root vegetables, which have a limit of 0.10 mg/kg, and leafy vegetables and herbs, which have a limit of 0.20 mg/kg.

Plants readily take up cadmium into their leaves, stems, roots and tubers, and to a lesser extent their seeds, grains, and fruits. Cadmium concentrations are typically higher in leaves than in other parts of the plant, but translocation varies by species and environment.

As Cd becomes more into focus and buyers include metals in risk management programs, vegetable packers and processors may be asked to monitor for Cd and limit Cd uptake in vegetable crops.

What can be done to limit Cd in vegetables?

1) Limit phosphorus fertilizer applications to rates that meet crop needs. Use low Cd source phosphorus fertilizers. Manures and composts may also contain cadmium. If these are used, check that cadmium concentrations are below acceptable limits.

2) Reduce chloride additions to soils with fertilizers (such as potassium chloride). Elevated soil chloride concentrations increase the plant availability of soil cadmium.

3) Choose low risk crops or varieties (if known). Certain crops are more susceptible to cadmium uptake and certain varieties accumulate more Cd. High Cd accumulators include lettuce, spinach, cabbage, some potato varieties, sweet potatoes, carrots, beets, eggplant, and peas. Low Cd accumulators include cucumbers, snap beans, and sweet corn.

4) Soil pH affects the uptake of cadmium by crops. Soil pH of less than 5.5 increased Cd uptake. The pH of soil should be above 6.0 to reduce Cd in crops.

On a positive note, plant breeders have been able to identify low Cd breeding lines in some vegetables and this may be an important tool in the future to limit Cd exposure in consuming vegetables.

Much of this information was taken from “FAQs about Cadmium in Fertilizer: Cadmium Contamination in Plants” from the Pacific Northwest Pollution Prevention Resource Center.