A control system for controlling the depth of an opener device in an agricultural planter comprises a gauge wheel on a pivotably mounted support arm, a mechanical element coupled to the support arm to move in response to changes in the angle of the support arm, a sensor adapted to measure changes in the relative elevations of the opener device and the gauge wheel to produce an output signal representing the current relative elevations of the opener device and the gauge wheel, and a control device receiving the output signal from the sensor and producing a second output signal for maintaining the opener device at a selected elevation relative to the gauge wheel.