An agricultural tool used to form furrows includes a generally cylindrical support and a plurality of tires. Each tire includes a sole by way of which the tire is mounted on the support, a tread opposite the sole (3) and two sidewalls that connect the tread to the sole. The sole, the tread (5) and the sidewalls jointly form a deformable casing. The tread of at least one of the plurality of tires has a generally domed shape. The sidewalls of this tire are generally inclined radially. The domed shape of the tread and the inclination of the sidewalls are such that the deformation of the casing urges the sole axially outward.