At agricultural co-ops across America, it's available by the truckload. Farmers spread more than two million tons of it across the U.S. every year.
It's ammonium nitrate, one of the world's cheapest fertilizers. But it can also be a powerful explosive when mixed with ordinary diesel fuel.
Timothy McVeigh used 4,000 pounds of that mixture to rip apart the Oklahoma City federal building. It's also a terror weapon of choice for making car and truck bombs — like the one in Bali, Indonesia in 2002, and in Turkey twice last November. In March 2004, British police found half a ton of it while rounding up suspected terrorists.
Now, with new warnings that al-Qaida planned to hit U.S. financial targets using car and truck bombs, some in Congress are calling for tighter restrictions on ammonium nitrate.
Among the proposals: Anyone buying it would have to show identification, with a background check to buy it in bulk.
"You can be on a terror watch list and walk up to any store that sells farm goods or other types of goods and buy as much ammonium nitrate as you want," says Sen. Charles Schumer, D-N.Y.
South Carolina now requires ammonium nitrate dealers to get a restricted fertilizer permit and record the driver's license number of every buyer. Nevada will soon do the same.
The fertilizer industry — already urging dealers to keep records of sales and report suspicious buyers — actually supports the state laws.
"We recognize that the security landscape has changed, and we want to be responsible," says Kathy Mathers, a spokeswoman for the industry. "We feel that support of these state regulations is part of being a responsible industry."
Meantime, research is underway to find a way to prevent ammonium nitrate from working as an explosive, while retaining its potency as a fertilizer. A Missouri company claims it has developed a coating that blocks fuel oil from bonding with the fertilizer, but dissolves on contact with water, releasing the fertilizer into the ground.
"The water soluble polymers prevent the infiltration of oil that a terrorist may apply to ammonium nitrate to make it more explosive," says Larry Sanders, CEO of Specialty Fertilizer.
The federal government has yet to take a position on any of these proposals, ideas that advocates believe could quickly make America safer.