Abstract: A device and method are disclosed for treating compressible materials, such as hay or other fibrous materials, with a treating substance, preferably anhydrous ammonia, as a preservative for the treated compressible material. The treating substance is preferably introduced into a treating area as a gas, but may be introduced in aqueous or liquid form to establish a fog of treating substance in the treating area.The compressible material to be treated is introduced into the treating area which preferably has air locks, established by a pair of pressure rollers, at both the compressible material inlet and outlet to maintain the treating substance within the treating area.Within the treating area, a path is established for continuous movement of the compressible materials from the inlet to the outlet rollers.