I have done a little reading on ammoniun nitrate bombs, and, they're not as easy to make as you might think. First, and most importantly, you have to extract the pure ammonium nitrate from the fertilizer. Skip this step, and you're not going to take down 1/3 of the Murrah Bldg, that's for sure. If you're talking about a van-load of the stuff, this is quite a project, and in such a case, using another type of explosive might be more realistic.