Ammonium nitrate is highly explosive if a large volume of it is contained within a strong container such as a ocean freighter.
Through the 1940's there were several ammonium nitrate explosions in bulk cargo ocean freighters.
Texas City is our worst example. Changes in the manner of loading and shipping prevented further disasters like Texas City.
Look it up. Fuel oil leaked into the ammonium ntrite.
According to our site explosion expert at the time of the Ok city bombing, the purity of the Amonium nitrite is important also. If they had sold a less pure product (95% or less), then the use as an explosive would have been negligible.