Sensor plugs are used to fill in sensor apertures formed in the front and rear walls of seed tubes used on row crop planters. The size of the plug and the depth of the plug is dictated by the diameter of the sensor aperture and the thickness of the walls of the seed tube. The interior surface of the plug is designed to follow the inside surface of the seed tube. The outside of the plug is provided with a cap assembly that is larger than the sensor aperture. This cap assembly has an inside surface that bears against the outside surface of the seed tube. A holding assembly comprising either a resilient C-clamp or plastic tie band are used to hold the cap assemblies against the seed tube.