through which the force f operates decreases at a faster rate than that of the moment arm through which the spring operates and as the spring 106 is compressed the trip force level increases from an initial trip force level which in the preferred embodiment is greater than eight hundred pounds and which preferably is approximately twelve hundred pounds this is best illustrated by the curve 130 on the trip force graph of fig1 at a tripped position between the soil working position and a final tripped position indicated generally at point 132 on the curve 130 the moment arm through which the spring 106 acts begins to decrease at a faster rate than the moment arm through which the force f operates and the trip force level begins to decrease as the shank 36 pivots upwardly fig2 and 11 the retaining struture 80 also pivots upwardly about the pin 92 to move the stop member 94 rearwardly and upwardly from between the sides of the main bracket 20 before the shank 36 trips upwardly to a position where the pins 42 92 and 64 are in alignment the stop member 94 protrudes out sufficiently to contact the upper portion of the shank 36 fig1 the stop member 94 prevents the shank from tripping to an over center position from which it would not return automatically to the soil working position of fig2 the spring bracket arrangement 60.