Because it is not classified as hazardous waste under Environmental Protection Agency standards, there is no federal supervision of its use in agriculture. However, fly ash contains various amounts of toxic metals. 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.

And studies have shown that food crops grown in large amounts can soak up hazardous concentrations of arsenic. 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.

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.

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.