holy hell..
But the fertilizer wasn't contained, so you can't analyze this like a typical explosive. There was just so much of it stacked up that it the pressure generated caused the explosion to go down–straight at the water which made the explosion far worse. It's like the potentiality of the Chernobyl disaster where they had to dig under the station to fill a space with concrete so the reaction couldn't reach more water (and cause a much larger explosion).