Abstract: An in-bin, controlled atmosphere, grain drying system has a storage chamber for holding a supply of grain or the like with a recycling duct leading from the upper end of the chamber back to a conduit for introducing air to the lower end of the chamber. A humidistat mechanism sensitive to the humidity of the air flow is provided. Also, a proportioning conduit portion has side-by-side inlets lying in substantially a common plane, one inlet connecting with the recycling duct and the other being open to atmosphere. A valve controlled by the humidistat mechanism is movable crosswisely relative to said inlets such as to admit more atmospheric air and less recycled air, or less atmospheric air and more recycled air, dependent on the humidity sensed by said humidistat mechanism.