Systems, methods and apparatus are provided for determining the depth of a trench opened by an agricultural planter. Sensors are provided for detecting the position of a ground-engaging element of the planter such as a gauge wheel or seed firmer of a row unit of the planter. Apparatus and methods are provided for installing such sensors onto a row unit of the planter. Systems, methods and apparatus are provided for controlling downpressure on a row unit based on the trench depth. Methods are provided for mapping trench depth measured by the depth sensors.
