When a bomb exploded beneath a government building in Oklahoma City on April 19, killing 168 people and sending tremors of fear throughout the nation, one of the most disturbing aspects of the terrorist act was that it was in at least two respects homegrown. Not only were the perpetrators thought to be U.S. citizens, but their devastating weapon was fashioned from the most ordinary of materials: fertilizer.
The event triggered a call to have fertilizer chemically treated so that it cannot be used for bombs. At issue is a common fertilizer based on ammonium nitrate. Almost any neighborhood do-it-yourself store stocks the stuff because it delivers nitrogen, in the form of nitrates, directly to plants without the intercession of soil bacteria. It also happens to be what bomb specialists call an oxidizer. When combined with a flammable material such as diesel fuel, it can supply the oxygen needed for the fuel to burn even when no air is present. Confine the mixture in a closed space- -say, the back of a van--add a few sticks of dynamite as a detonator, and you’ve got the makings of a bomb.
Would-be bomb makers need to perform one more step, however. Fertilizer is actually much denser than bomb-grade ammonium nitrate, an industrial chemical that is regulated by the government and hence is more difficult to obtain. At such a high density, the material will explode only at terrifically high temperatures and pressures; even with TNT as a detonator, it melts benignly rather than exploding. Terrorists must first add chemicals--aluminum, zinc, or potassium sulfate--to lower the temperature threshold above which the compound will explode. Fortunately, this step requires a lot of explosives expertise, says Maurice Greiner, a safety engineer at the insurance firm Johnson & Higgins in Seattle. Add too much and the material is apt to blow up with routine handling; too little and the bomb is a dud.
An old method of making fertilizer detonation-proof by adding diammonium phosphate, a flame-retardant chemical, has not held up under recent tests--the chemical does not raise the temperature threshold enough to be out of reach of TNT detonators, says Greiner. There’s nothing else you can do except reduce the nitrogen content, he says, but then you have a product that’s not worth much as a fertilizer.