A bit loose with the facts here:
” The one here stored as much as 270 tons of ammonium nitrate, a common fertilizer that has been used as an explosive”
Ammonium nitrate mixed with fuel oil (ANFO) is what is used as the explosive. Ammonium nitrate by itself is not used as an explosive.
ANFO is used because it is cheap and more stable than other explosives.
Still no word from any source on how the plant handled and stored the ammonium nitrate.
NW corner of the building appears to be the source of the blast. The side photos appear to show a crater. The top photos show an area of fresh dirt.
But, ammonium nitrate by itself can be explosive, under certain circumstances. It requires heating or a source of ignition, and at least partial confinement.
The technical term is transition from deflagration to detonation.