An agricultural tool for a tractor tool bar comprises a unit, such as a planter unit, that includes a ground contacting member for performing a working operation on the ground for changing the state of the soil, such as by opening a furrow to receive a seed from the planter unit. A downpressure system for the tool includes a two part telescopic air shock extending between the tool bar and a parallelogram linkage that attaches the unit to the tool bar. A sensor is operatively connected to the portion of the air shock that is attached to the parallelogram linkage for sending a signal representative of the length of the shock to an electronic controller. The controller operates through a feedback loop to attempt to maintain the air shock at a predetermined length which corresponds to a desired amount of downpressure.