Tiller for preparing forest soils for tree planting, comprising: • an excavator-mounted cutter with - a mounting bracket for attachment to a moveable boom of an excavator, - an engine, - two rotationally driven cross-cutting heads, which are mounted on both sides of the longitudinal axis of the mounting bracket and carry numerous cutting tools, wherein the cutting tools of each cross-cutting head describe a cylindrical cutting surface during its rotation, - a driven rotating cutting chain with further cutting tools, the running direction of which extends parallel to the longitudinal axis of the mounting bracket and which extends between the two cross-cutting heads, wherein a front reversal line of the cutting chain lies substantially in a plane tangential to the cutting surfaces of the two cross-cutting heads; characterized by • a ground guide housing, which at least partially engages around the mounting bracket of the excavator-mounted cutter, with - attachment points for attachment to the mounting bracket and/or to the boom, - a cover plate, which closes the ground guide housing upwards, - a housing shell extending downwards from the cover plate substantially parallel to the longitudinal axis of the mounting bracket, - a lower housing opening opposite the cover plate, wherein the housing shell has an axial extension such that the two cross-cutting heads protrude substantially completely and the section of the cutting chain extending between them protrudes from the housing opening, - two runners, which are attached to the housing opening in a feed direction and can slide on the forest floor, - a discharge plate, which forms a section of the housing shell and opens a discharge opening between the runners. • an excavator-mounted cutter with - a mounting bracket for attachment to a moveable boom of an excavator, - an engine, - two rotationally driven cross-cutting heads, which are mounted on both sides of the longitudinal axis of the mounting bracket and carry numerous cutting tools, wherein the cutting tools of each cross-cutting head describe a cylindrical cutting surface during its rotation, - a driven rotating cutting chain with further cutting tools, the running direction of which extends parallel to the longitudinal axis of the mounting bracket and which extends between the two cross-cutting heads, wherein a front reversal line of the cutting chain lies substantially in a plane tangential to the cutting surfaces of the two cross-cutting heads; - a mounting bracket for attachment to a moveable boom of an excavator, - an engine, - two rotationally driven cross-cutting heads, which are mounted on both sides of the longitudinal axis of the mounting bracket and carry numerous cutting tools, wherein the cutting tools of each cross-cutting head describe a cylindrical cutting surface during its rotation, - a driven rotating cutting chain with further cutting tools, the running direction of which extends parallel to the longitudinal axis of the mounting bracket and which extends between the two cross-cutting heads, wherein a front reversal line of the cutting chain lies substantially in a plane tangential to the cutting surfaces of the two cross-cutting heads; • a ground guide housing, which at least partially engages around the mounting bracket of the excavator-mounted cutter, with - attachment points for attachment to the mounting bracket and/or to the boom, - a cover plate, which closes the ground guide housing upwards, - a housing shell extending downwards from the cover plate substantially parallel to the longitudinal axis of the mounting bracket, - a lower housing opening opposite the cover plate, wherein the housing shell has an axial extension such that the two cross-cutting heads protrude substantially completely and the section of the cutting chain extending between them protrudes from the housing opening, - two runners, which are attached to the housing opening in a feed direction and can slide on the forest floor, - a discharge plate, which forms a section of the housing shell and opens a discharge opening between the runners. - attachment points for attachment to the mounting bracket and/or to the boom, - a cover plate, which closes the ground guide housing upwards, - a housing shell extending downwards from the cover plate substantially parallel to the longitudinal axis of the mounting bracket, - a lower housing opening opposite the cover plate, wherein the housing shell has an axial extension such that the two cross-cutting heads protrude substantially completely and the section of the cutting chain extending between them protrudes from the housing opening, - two runners, which are attached to the housing opening in a feed direction and can slide on the forest floor, - a discharge plate, which forms a section of the housing shell and opens a discharge opening between the runners.