An applicator assembly for applying an active substance to the root zone of a plurality of plants, the applicator assembly comprising: a subsoil applicator having a cutting element for generating an incision in the ground, an injector associated with the cutting element to allow the active substance to be injected into the incision; and a positioning device adapted to allow the position of the cutting element to change position such that an incision is generated in some material in the root zone while the cutting element moves over other material in the root zone.