A flow control system for controlling flow of materials such as seeds or fertilizer as the materials are fed into a manifold and meter unit of an air seeder. The flow control system generally includes a flow controller which is connected between a volume of materials, such as a tank, and a manifold of an air seeder. The flow controller is adapted to control flow of the materials into the manifold to be metered by a meter unit. The flow controller includes a plurality of movable gates; with each gate being adapted to selectively open or close a corresponding opening in the flow controller. The gates may be manually adjusted, such as by a handle, or may be automatically adjusted, such as by actuators. By selectively opening or closing various gates, the volume and speed of flow of materials into the manifold may be easily and efficiently adjusted.