A self-calibrating metering system for dispensing chemical granules. The system incorporates a flow sensor configured to sense a greater flow rate at the inlet aperture than at the outlet aperture. Further included is an adjustment mechanism configured to adjust the flow rate of the meter so as to calibrate the meter according to a known flow rate through a calibrated orifice within the meter. An electromechanical solenoid may be energized by a subcontroller operably coupled to the metering apparatus to allow chemical granules to flow by gravity from the pesticide container that may be disposed above the metering apparatus. The metering apparatus may be disposed on a planter unit designed to deposit granular materials with seed during the planting process. The metering system further includes an in-meter diffuser that receives foreign material and lumps in order to prevent the metering apparatus from becoming clogged.