Contaminated soils can yield healthy harvest

Vegetables and grains grown in soils contaminated with toxic metals might be safe to eat if they're the right kind of plant, a Canadian researcher has found.

Conversely, somecrops, such ascarrots, might contain high levels of toxic metals even if they're grown in "uncontaminated soils," said the study by researchers at the University of Western Ontario, which was published in the July issue of Botany, a journal published by the National Research Council of Canada.

In fact, the level of toxic metals depended far more on the type of plant than the contaminant levels in the soil, and some plants took up certain types of metals more than others.

"The take-home message would be: Don't eat [just] one vegetable," said Sheila Macfie, one of the co-authors of the study.

She and her collaborators conducted the preliminary study because urban sprawl and growing demand for locally grown food may make it more attractive to grow crops on what was previously considered "marginal" farmland. In addition, many housing developments are being built on former industrial lands, where residents might plant gardens on contaminated soil.

The researchers looked at wheat, turnips, carrots, lettuce and soybeans grown in both:

• Soils considered a little too contaminated for farming, based on guidelines created by the Canadian Council of the Ministers of the Environment.
• Commercial potting soil.

Then they measured the levels of the metals cadmium, copper, lead and zinc in both the plants and the soil. They measured the levels in the edible parts of the plants and estimated the daily dose of metals that would be consumed by someone who ate no other vegetable in that food group and ate the number of servings recommended by the Canada Food Guide. They then compared that to the allowable limits set by the U.S. Environmental Protection Agency, the UN Food and Agriculture Organization and the World Health Organization.

"What we're presenting is the worst-case scenario," Macfie said, as most people do eat more than one type of vegetable.

The study found that root vegetables, especially carrots, took up the most metals, while wheat and soybeans never even came close to the limits. None of the crops took up enough lead to pose a human health hazard. However, carrots, radishes and lettuce sometimes took up levels of cadmium and zinc that could pose a risk.

Macfie now plans to study whether certain varieties of crops, such as carrots, are more prone to taking up metals than others.