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It sounds like the ammonium nitrate in the story is the porous, uncoated prills as it was made by an explosives manufacturer. This will absorb diesel fuel or nitromethane well. The denser and/or coated prills are used as fertilizer and do not detonate well due to the coating additives. However, it has nothing to do with making it more difficult to use as an explosive. Ammonium nitrate is deliquescent ie. will absorb moisture from the air until it actually forms a puddle. So the prills are coated to reduce their ability to absorb moisture for better storage life before being spread as fertilizer.
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Explosive grade AN is more porous for surface area contact with the additional liquid fuel.
Fertilizer grade can be coated (CAN-27) but most fertilizer grade AN is polished for surface durability.
Ammonia Nitrate has fuel and oxidizer on the molecule but it’s not balanced very well, hence adding a fuel to it.
Nitromethane is already a good explosive and just needs air gaps to create compression hot spots and it will propagate an explosive wave. Which is why pure nitromethane is not shipped by rail car any longer owing to an incident with someone slapping closed a valve creating a “water hammer” shock wave that ignited/detonated the rail car.