Nitrogen, one of three common additives to agricultural fertilizer, is often in the form of ammonium nitrate, a potentially explosive chemical compound.
Thirty-four production plants across America produce nitrogen fertilizer. The facility in West, Texas that exploded on April 17, 2013, was a retail facility, not a production facility. There are around 6,000 retail facilities around the country that sell fertilizer directly to farmers.
Ammonium nitrate can explode under conditions of heat or shock. The chemical was a component of the bomb used by Timothy McVeigh in the Oklahoma City bombing of April 19, 1995.