Actually it feels weird to me that we need a special building to spray the fertilizer. Isn't that something should be done by the farmer themselves? I think in maybe world war stage, we can have a new work mode for farms which consumes the manure/fertilizer to maintain high output while take less (if not non) impact from the surrounding environment

It would make more sense to model the Guano/Nitratine mining as the fertillizer mines that was a major industry during the Victorian times and have the industrial fertilizer production be based on the Haber-Bosch process where they pretty much create fertilizers out of thin air and a lot of electricity. From there, it can be made into Explosives with the aid of steel containers.