Method for calibration of feed rate of a metering device (1) for airflow- borne feeding of granular material in an agricultural implement. The metering device (1) comprises a first volumetric metering rotor (5, 50) to feed a first granular material (Ml) to an airflow channel (3, 31, 32), and a second volumetric metering rotor (7, 70) to feed a second granular or powdered material (M2) to the airflow channel (3, 31, 32). The method comprises to drive the agricultural implement during a first time period, to drive the first metering rotor at a first rotational speed during the first time period, by means of a granule- counting device (10a 10b), to count a number of fed granules in the airflow channel of the first granular material, based on the number of granules and the first rotational speed, to calculate a first feed rate of the first metering rotor, to drive the agricultural implement during a second time period, to drive the first metering rotor during the second time period with a first predetermined granular feed rate, and to drive the second metering rotor during the second time period so that both the first and second materials are fed into the airflow channel.
