The material is produced by power plant "scrubbers" that remove acid rain causing sulfur dioxide from plant emissions. A synthetic form of the mineral gypsum, it also contains mercury, arsenic, lead and other heavy metals.
The U.S. Environmental Protection Agency says those toxic metals occur in only tiny amounts that pose no threat to crops, surface water or humans. But some environmentalists say too little is known about how the material affects crops, and ultimately human health, for the government to suggest that farmers use it on their land. Read more.