Cultivating tines, each having opposite cutting edges, are pivotably supported by tine mount members on a tine shaft reversibly rotatable about a horizontal lateral axis. One of the cutting edges of the tine is retainable in a position rearward with respect to the direction of forward rotation, while the other cutting edge is retainable in a position rearward with respect to the direction of reverse rotation. When held in each of these positions, the tine is restrained by resistant force imparting means from being pivotally moved by a centrifugal force. The resistant force to be imparted by this means is smaller than the reaction to be produced when the tine is driven into the soil.