A midrow levelling attachment for a seeder includes a set of leveller gangs each of which is mounted on a respective one of the packer wheel gangs of the seeder by a pair of mounting members one at each end of the common shaft of the leveller gang. A depletion layer is generated in a vicinity of a junction interface between the substrate and the well region. The compensation value is determined by measuring the response of the new path to a set of tones that are applied to the conductive path, the response to which is measured against a table of responses previously recorded. The two housings of the upper part do not or essentially do not extend outside the area on the upper part.