Beirut explosion: What is ammonium nitrate and how dangerous is it?

Farmers used to use it to blow stumps out of the ground, cheaper alternative to dynamite.

It's still used in fertilizer in the US. It is made inert by adding a small amount of calcium, phosphate or potash, which are also fertilizer.

It’s a standard cratering charge in the Junior Woodchuck Manual (FM 5-34).