The same chemical that makes fertilizer so useful also makes it really cheap bomb fuel. Researchers at Sandia labs in Albuquerque wondered if they could render the explosive properties of fertilizer inert while still keeping the beneficial properties intact, and this week announced success in a test batch. Even better, they're sharing the innovation for free.
The problem with improvised explosives is that they're cheap, made from otherwise-harmless everyday materials, and the directions to make them aren't too hard to find. This is true of pressure cooker bombs, a terror weapon so ubiquitous that its been used by everyone from anarchists to religious radicals on at least three continents, and it's especially true of fertilizer bombs.
Ammonium nitrate is the culprit. The first recipe for ammonium nitrate is over 350 years old, and despite centuries of research into other fertilizers, ammonium nitrate remains one of the cheapest and best. As an added benefit for farmers, ammonium nitrate "improves both the quantity and quality of protein-containing crops," which is a tremendous benefit to humanity.
Except for that part where it explodes. Normally, of course, ammonium nitrate doesn't blow up; if that was a daily occurrence, the recipe would have been abandoned 349 years ago. Ammonium nitrate requires the addition of another reagent to go off. In modern fertilizer bombs, readily available fuel completes the process, turning the normally-stable fertilizer into an extremely volatile explosive.