Abstract: A towable seed drilling unit of the kind having a hopper-fed seed chamber whose base is constituted by a proportion of the lower run of an endless belt driven by a belt drive roller in a direction contrary to towing direction from a ground-contacting wheel and having regularly spaced holes therein for the containment of single seeds, a seed-repelling roller for agitating seed in the said chamber and a masking plate below the belt permitting release of seeds on a passage of holes therebeyond into a furrow formed ahead of temporarily unmasked holes by a coulter beneath the seed chamber and closed by a furrow closing member, wherein forward and rearward wheels supporting the unit are carried by arms pivotal about substantially horizontal axes and means are provided for adjusting the attitude of the arms relative to the remainder of the unit and thus the penetration depth of the coulter, and wherein a parallel linkage is provided by means of which the unit can be attached to a towing vehicle, said linkage being r