An agricultural baler for compacting material into square bales, in particular for compacting straw bales, comprising - a preliminary channel, - a main channel, - at least one pre-compressor, - at least one main compressor, as well as - at least one driving device, wherein the preliminary channel and the main channel are connected together by means of at least one transfer section, so that the material originating from the preliminary channel can be transferred into the main channel, wherein the material inside the preliminary channel can be pre-compressed by means of the pre-compressor and then can be moved into the main channel by means of the pre-compressor, wherein the main compressor at least indirectly interacts with the driving device, so that the main compressor is movable inside the main channel in a bidirectional oscillation and in this manner, material located in the main channel can be compacted into a square bale, the main compressor can be repeatedly and alternately uncoupled from the driving device or coupled to the driving device, wherein, when the main compressor is in an uncoupled state in which it is uncoupled from the driving device, a transmission of force between the main compressor and the driving device is interrupted in a manner such that the oscillating movement of the main compressor inside the main channel is suspended, wherein the main compressor comprises at least one transfer device which has at least one piston and cylinder unit, wherein the piston and cylinder unit comprises two working chambers (11, 12) in the form of a piston chamber as well as an annular chamber and the two working chambers (11, 12) can be connected together in a fluid communicating manner by means of a fluid line, characterized in that the fluid line interacts with at least one valve device which, when in an open position, enables a flow of fluid between the working chambers (11, 12), so that a piston of the piston and cylinder unit can be moved relative to the associated cylinder, and when in a closed position, blocks the flow of fluid between the working chambers (11, 12), so that a relative movement between the piston and the cylinder is blocked. - a preliminary channel, - a main channel, - at least one pre-compressor, - at least one main compressor, as well as - at least one driving device, wherein the preliminary channel and the main channel are connected together by means of at least one transfer section, so that the material originating from the preliminary channel can be transferred into the main channel, wherein the material inside the preliminary channel can be pre-compressed by means of the pre-compressor and then can be moved into the main channel by means of the pre-compressor, wherein the main compressor at least indirectly interacts with the driving device, so that the main compressor is movable inside the main channel in a bidirectional oscillation and in this manner, material located in the main channel can be compacted into a square bale, the main compressor can be repeatedly and alternately uncoupled from the driving device or coupled to the driving device, wherein, when the main compressor is in an uncoupled state in which it is uncoupled from the driving device, a transmission of force between the main compressor and the driving device is interrupted in a manner such that the oscillating movement of the main compressor inside the main channel is suspended, wherein the main compressor comprises at least one transfer device which has at least one piston and cylinder unit, wherein the piston and cylinder unit comprises two working chambers (11, 12) in the form of a piston chamber as well as an annular chamber and the two working chambers (11, 12) can be connected together in a fluid communicating manner by means of a fluid line, characterized in that the fluid line interacts with at least one valve device which, when in an open position, enables a flow of fluid between the working chambers (11, 12), so that a piston of the piston and cylinder unit can be moved relative to the associated cylinder, and when in a closed position, blocks the flow of fluid between the working chambers (11, 12), so that a relative movement between the piston and the cylinder is blocked.