From reader Jim Borland:
Yes, nitrogen fertilizer can be used to make a bomb à la Oklahoma City bombing, though I doubt that the nitrogen fertilizer in Miracle Gro’s potting mix contributed much if anything to the fire that resulted from the insertion of a cigarette butt into the soil. What you missed in taking a picture of the package was the other part of the contents besides the fertilizer. All potting soils these days are made of synthetic products including those made by Miracle Gro. In the case of this particular potting mix it consists of forest products (shredded and chipped wood and bark), coir (shredded coconut husks), composts, peat, sphagnum peat moss, perlite and wetting agent.
All but the perlite and wetting agent are flammable, especially when dry. In this case the soil was undoubtedly not moistened as most soils are that have live plants in them. The nitrogen part of the fertilizer is contained within prills or small plastic coated spheres, here called Osmocote, that release nitrogen with each watering. After a couple of waterings the nitrogen is gone, leaving only the plastic capsule behind. Even with no fertilizer, this and other artificial soils are flammable, and care must be taken when located in a place convenient for snuffing out cigarettes.