COAL IN CROPS: Coal ash is used to boost soil fertility in some cases but is also adding a dose of arsenic and other toxic substances.
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Crops across the country are grown in soil amended with coal fly ash—the same substance that caused a massive environmental emergency in December when it gushed from a holding pond at a Tennessee power plant.

Tons of fly ash are routinely added to soil to nourish vegetables, peanuts and other crops, primarily in the Midwest and Southeast. But now the spill has raised questions about whether this longstanding agricultural practice is environmentally sound.

Fly ash is a fine powder recovered from gases created by the burning of coal. It is the largest component of coal combustion waste, totaling around 70 million tons annually in the United States.

Adding moderate amounts increases crop yields and stabilizes soils while reducing the need to throw huge quantities in landfills or holding ponds, said Yuncong Li, University of Florida professor of soil and water sciences.

However, fly ash contains various amounts of toxic metals. And studies have shown that food crops grown in large amounts can soak up hazardous concentrations of arsenic.

Because it is not classified as hazardous waste under Environmental Protection Agency standards, there is no federal supervision of its use in agriculture. Some states regulate it but their guidelines vary and often require no monitoring of how it is used, said Jeffrey Stant, director of the Coal Combustion Waste Initiative for the Environmental Integrity Project.

“For soil amendment, most cases are left to the industry itself to monitor where they put fly ash and how much they use of it,” said Stant.

For more than a decade, companies have mixed fly ash with other waste to produce soil and compost. About 50,000 tons are used annually for agriculture nationwide.

One example is N-Viro, an international corporation that specializes in turning waste material into products. The company uses 250 tons of fly ash per day to mix with bio-solids, said Raymond Mayo, Florida N-Viro plant manager. The mixture is then heated to kill bacteria and monitored before it is distributed to farms, added Mayo.

The volume of fly ash created by power plants is increasing, due to more coal burning coupled with more stringent air pollution rules. “Currently the U.S. produces 130 million tons of coal combustion waste every year. In another 10 years it will be 150 million,” said Stant.

As a result, researchers are studying whether larger amounts can be used safely in agriculture.

Fly ash mixtures provide phosphorus, calcium and other nutrients that crops need to grow while increasing soils’ capacity to hold water, said Li. “The material is practically free and coal companies will pay people to dispose of it,” he said.

But arsenic, lead and mercury in fly ash raise concerns about cancer or neurological damage.

Crops grown in quantities of fly ash ranging from 5 to 20 percent of soil weight absorbed toxic metals, according to a study by Indiana State University researchers.

When the amount of fly ash increased, the crops absorbed higher concentrations of arsenic and titanium. Basil and zucchini contained potentially toxic amounts of arsenic exceeding 6 parts per million. Concentrations of greater than 2 ppm had severe effects on vegetables, damaging the plants and decreasing production, wrote the scientists in a 2004 paper published in Environmental Geology.

Although the potential human health effects are unknown, fly ash fertilization can lead to possible toxic accumulation in crops if not monitored properly, concluded the scientists.

Plants grown with smaller amounts of fly ash have fared much better. In a three-year study, University of Florida researchers applied 22,000 pounds of fly ash per acre (1.1 percent of soil weight).

Mixed with yard waste compost, the fly ash increased tomato yields by up to 70 percent. The study found no groundwater contamination or soil-fertility decline after three years, while the presence of trace metals remained low.