An air flow system for an air seeding machine includes a tank configured to store an agricultural commodity, a plenum, conduits, and a blower configured to push air flow through the plenum and the conduits to the tank. The plenum includes a uniformly-sloped side wall and geometric first and second ends bounding the side wall. The plenum further includes bleed ports coupled to bleed conduits and configured to conduct air flow to an interior volume of the tanks. The plenum further includes primary ports coupled to primary conduits and configured to conduct air flow to the tanks. The primary conduits receive commodity from the tanks to be deposited in the soil. The geometry of the plenum facilitates an adequate balance of air flow between the primary and bleed conduits so that the commodity most-effectively enters the air stream conducted by the primary conduits.