Depth of penetration of a soil coulter is detected using a sensor being mounted on the side of the disk adjacent the edge such that the sensor as the disk rotates is located above the surface of the soil during a first part of its rotation and is located below the surface during a second part of its rotation. The sensor issues a signal which changes in response to whether the sensor is above or below the soil surface which is received by a controller which calculates from the signal a first time when the sensor enters below the soil surface and a second time when the sensor departs the soil surface and calculates from the first and second times the depth of penetration of the coulter in the soil. The system can also detect variations in depth indicative of a value of surface roughness.
