You need only nitrogen for explosives, but nitrogen, phosphorus, and potassium for fertilizer. Therefore guano (which contains all three) is overkill for explosives but great as a fertilizer, while synthetic nitrogen compounds are insufficient as a "complete" fertilizer and have to be supplemented with the two other elements from other sources.
"As a manure, guano is a highly effective fertilizer . . . Guano was also, to a lesser extent, sought for the production of gunpowder and other explosive materials. The 19th-century guano trade played a pivotal role in the development of modern input-intensive farming."
Chicago should build new designs of these around Millennium Park, and maybe reduce their strife with what is both native wildlife and a productive resource. They really can't stand pigeons.
