TERROR IN OKLAHOMA CITY : Ingredient a Staple in Farming, Mining : Explosive: Ammonium nitrate, used in producing green crops, turns powerful when blended with fuel oil.

They call it ANFO--ammonium nitrate and fuel oil.

For homeowners demanding lush, verdant lawns and farmers producing bountiful crops, ammonium nitrate is a common product. For large-scale mining firms and industrial demolition operations, that chemical blended with ordinary fuel oil produces a relatively inexpensive and effective explosive.

And, for terrorists--including those who blew up the federal building in Oklahoma City or New York’s World Trade Center--ANFO is the substance of choice and a vital ingredient for making car bombs. The deadly chemical mixture produces a stinking, gooey substance that is cheap, legal, abundant and powerful.

“A lot of our everyday needs are dependent on the availability of ammonium nitrate,” said Per-Anders Persson, director of the Research Center for Energetic Materials at the New Mexico Institute for Mining and Technology in Socorro, N.M. “For example, a tremendous amount of ammonium nitrate is used in coal mining and about 90% of all rock-blasting explosives sold or used in this country consists of ammonium nitrate mixed with a small amount of fuel oil.”

Federal authorities theorize that someone parked a truck loaded with about 1,000 to 1,200 pounds of ANFO outside the Alfred P. Murrah Federal Building last Wednesday and detonated it to produce the horrific explosion in Oklahoma City.

But given the wide variety of fertilizers and explosives on the market, do we really need this stuff? Should just anyone--from back yard botanists to international terrorists--be allowed to purchase it? And, can anything be done to make it safer?

Yes and no, say experts.

“A great part of the mining industry would not exist today if they couldn’t use ammonium nitrate,” said George Griffith, of Sun City West, Ariz., a retired executive with an explosive manufacturing firm and currently a researcher and consultant on the use of explosives.

But, he added, not everyone should be allowed to purchase it. “Ammonium nitrate should only be sold to people who are well known and have been licensed or fingerprinted to buy it. There should be very tight, rigid controls over the sale of ammonium nitrate.”

Others, however, say that even if the chemical were regulated, terrorists almost certainly could acquire it through underground channels--or they would turn to one of numerous other unregulated substances that can be used to make bombs.

In fact, commercial grade ammonium nitrate, used almost exclusively in mining, is far more dangerous than the agricultural variety and is heavily controlled by federal, state and local government agencies. For example, the Transportation Department restricts how the high-grade substance can be shipped from manufacturers to wholesalers. The Bureau of Alcohol, Tobacco and Firearms regulates how all commercial explosives are stored or warehoused.

But laws do little to prevent determined criminals from stealing large quantities or from using the lesser grade product for terrorist activities, Persson said.

“Increased restriction on ammonium nitrate would not be a total stop to all terrorist activity because there are many other explosives that can be made given . . . ingredients, such as urea and nitric acid,” he said. “But ammonium nitrate is such a convenient material . . . that I think its sales should be restricted.”

Concerned by the threat of car bombings in Britain, Europe, South Africa and the Middle East, governments around the world have taken a variety of steps to make agricultural-grade ammonium nitrate less fearsome. In Germany, Sweden and Northern Ireland, for instance, officials restrict who can buy the chemical. While in South Africa and Britain, ammonium nitrate is laced with a 25% solution of ground limestone that improves it as a fertilizer and makes it less practical as an explosive.

Griffith, who once served as president of the Institute of Makers of Explosives, an industry organization, said that the explosive properties of agricultural ammonium nitrate has been a longstanding concern to U.S. government and industry executives, going back to the time before World War I.

For a while, there was a government-led movement to require ammonium nitrate manufacturers to tag their batches with some sort of chemical or ingredient that could be uniquely identified after an explosion, Griffith said. But that idea fell away in the face of industry rejection to the increased costs and difficulty posed by tracking the added ingredients.

“As bad as the (Oklahoma City and World Trade Center) bombings are, this is nothing new to us,” Griffith said, referring to the danger of criminals using ammonium nitrate for illegal or terrorist purposes. “The industry has been faced with this problem for years.”