A self-propelled soil stabilizer machine employs a heavy-duty single horizontal rotor for pulverizing and mixing soil it passes over. The rotor is driven by hydraulic motors which are mounted at the rotor ends and operated by a hydraulic pump (engine driven) which is hydraulically coupled to a hydraulic traction pump (also engine driven) which propels the machine. Overloads on the rotor system are automatically sensed by a closed-loop hydrostatic system and result in a decrease in the speed of machine travel until the load on the rotor diminishes. A hydraulic rotary servo-valve and mechanical feed back system automatically controls the rotor to maintain it at a preset depth and provides a visual read-out of depth.