To prevent a tractor pulling a plow (P) from digging itself in, if the plow position, with respect to soil or tractor level, is controlled by an automatic control system which causes the plow to be lowered if the pulling force of the tractor decreases, slip of the drive wheels of the tractor is sensed and an overriding command signal given to the plow positioning system (S) tending to raise the plow, or, alternatively, a visual indication is given to the operator that the plough should be raised manually; the slip condition is sensed by differentiating a signal representative of plow position, with respect to time, and comparing the so differentiated plow positioning signal (or, in other words, plow dropping speed signal) with a reference which, preferably, includes such factors as tractor engine speed, tractor and/or implement weight, ground wheel adhesion, tractor gearing, and the like.