The present inventors have recognized that a pressure gradient or differential in a product distribution line for conveying granular particulate material, such as seed or fertilizer, in an air flow to an agricultural field consistently decreases as air speed (velocity) in the product distribution line decreases until a critical air speed is reached. Below the critical air speed, the particulate material may become susceptible to falling out of the air flow to potentially cause a blockage in the system. Accordingly, pressure sensors arranged in the product distribution line can be used to generate an air flow at an optimum operating velocity above the critical air speed yet below a maximum air speed associated with inefficient operation. In one aspect, the air flow can be adjusted to maintain an optimum operating velocity despite changes in the type of particulate material and/or rate at which the particulate material is metered.
