The federal government is encouraging farmers to spread a chalky waste from coal-fired power plants on their fields to loosen and fertilize soil even as it considers regulating coal wastes for the first time.

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 Environmental Protection Agency says those toxic metals occur in only tiny amounts that pose no threat to crops, surface water or people. 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.

“This is a leap into the unknown,” said Jeff Ruch, executive director of Public Employees for Environmental Responsibility. “This stuff has materials in it that we’re trying to prevent entering the environment from coal-fired power plants, and then to turn around and smear it across ag lands raises some real questions.”

With wastes piling up around the coal-fired plants that produce half the nation’s power, the EPA and U.S. Department of Agriculture began promoting what they call the wastes’ “beneficial uses” during the Bush administration.

Part of that push is to expand the use of synthetic gypsum — a whitish, calcium-rich material known as flue gas desulfurization gypsum, or FGD gypsum. The Obama administration has continued promoting FGD gypsum’s use in farming.