A method for calibration of feed rate of a metering device for airflow-borne feeding of granular material in an agricultural implement, the metering device comprising: a first volumetric feeder (5, 50) with a first metering rotor arranged to feed a first granular material to an airflow channel (3, 31, 32), and a second volumetric feeder (7, 70) with a second metering rotor arranged to feed a second granular or powdered material to the airflow channel (3, 31, 32), the method comprising: to drive the agricultural implement during a first time period, to drive only the first metering rotor (5, 50) at a first rotational speed for feeding a first granular material 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 (3, 31, 32) 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 feeder (5, 50), to drive the agricultural implement during a second time period, to drive the first metering rotor (5, 50) during the second time period so that a first predetermined granular feed rate is provided, and to drive the second metering rotor (7, 70) during the second time period so that both the first and the second material are fed into the airflow channel (3, 31, 32). a first volumetric feeder (5, 50) with a first metering rotor arranged to feed a first granular material to an airflow channel (3, 31, 32), and a second volumetric feeder (7, 70) with a second metering rotor arranged to feed a second granular or powdered material to the airflow channel (3, 31, 32), the method comprising: to drive the agricultural implement during a first time period, to drive only the first metering rotor (5, 50) at a first rotational speed for feeding a first granular material 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 (3, 31, 32) 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 feeder (5, 50), to drive the agricultural implement during a second time period, to drive the first metering rotor (5, 50) during the second time period so that a first predetermined granular feed rate is provided, and to drive the second metering rotor (7, 70) during the second time period so that both the first and the second material are fed into the airflow channel (3, 31, 32). to drive the agricultural implement during a first time period, to drive only the first metering rotor (5, 50) at a first rotational speed for feeding a first granular material 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 (3, 31, 32) 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 feeder (5, 50), to drive the agricultural implement during a second time period, to drive the first metering rotor (5, 50) during the second time period so that a first predetermined granular feed rate is provided, and to drive the second metering rotor (7, 70) during the second time period so that both the first and the second material are fed into the airflow channel (3, 31, 32).