The pilot stated that he and the passengers had just completed a local sightseeing flight and were returning back to the private airport from whence they departed. The approach was unremarkable. On landing the main gear wheels touched down onto the turf runway at an indicated speed of 90 mph. Upon the airplane settling, it encountered a soft spot on the turf runway. The right propeller made contact with the ground. The airplane then slid forward on its nose and ground looped toward the right incurring substantial damage to the wing and fuselage before coming to a stop 180 degrees from the direction of landing. An examination of the right main gear torque knee link by the responding Federal Aviation Administration inspector revealed the link fractured in overload; permitting the lower right gear, wheel assembly, to twist. The pilot did not report any mechanical failures or malfunctions to the airplane or any of its systems prior to the accident.