Strategy Page has a small item about a recent development in this area:
The United States is trying to get fertilizer manufacturers to produce less explosive products. That’s because terrorists increasingly use ammonium nitrate (a commonly used agricultural fertilizer) for their bombs (by mixing it with fuel oil and setting it off with a detonator). There is now a new form of ammonium nitrate fertilizer that it will not function effectively as an explosive. The Honeywell Corporation found that by adding some modified ammonium sulfate to the ammonium nitrate, you actually improve the fertilizing ability of the mix (by making the treated soil less acidic), and also prevent the fertilizer from being used as an explosive. Actually, you can still use the ammonium sulfate nitrate mix as an explosive, but it requires some creative chemistry to do so, and serves as a technological barrier for most terrorist groups. Although not a fertilizer manufacturer, American conglomerate (it makes a lot of different stuff) Honeywell found this less explosive ammonium nitrate formula while developing fire retardants. New discoveries are often made that way, by accident.
This won’t be enough to stop the use of ammonium nitrate as bomb-making material, but it raises the bar sufficiently that the very lowest tier of would-be terrorists will be shut out of the game (until they find an alternative source of explosives or ingredients for other explosive mixtures).
