An agricultural baler has a bale chamber for the compression of crop into bales with a floor, ceiling, and two walls. A plunger forces crop into the bale chamber. An actuator system presses the ceiling and walls of the bale chamber inward. At least one retractable friction block in the ceiling or walls is used to increase the compression and density of the crop. The friction blocks extend inwards as an increasing function of either decreased pressure being exerted against the ceiling or walls by the crop being baled, or inward displacement of the ceiling or walls by the actuator system.