The U.S. government is encouraging farmers to spread a chalky waste from coal-fired power plants on their fields to loosen and fertilize soil.
The substance is a synthetic form of the mineral gypsum, and it also contains mercury, arsenic, lead and other heavy metals.
The material is produced by power plant "scrubbers" that remove acid-rain-causing sulfur dioxide from plant emissions.
The Environmental Protection Agency says those toxic metals occur in only tiny amounts. But some environmentalists say too little is known about how the material affects crops, and ultimately human health.