The pilot reported that while approaching to land the airplane floated and he had to push forward on the control yoke to finish the descent. He stated that this used about 200 feet of runway. He said that the landing was normal, but after touchdown the braking effectiveness on the grass runway was such that the airplane was not slowing as expected. The pilot elected to turn off of the runway into a farm field adjacent the runway. He said that hidden in the tall grass along the runway was a 18 to 20 inch ridge. The airplane struck the ridge severing the nose landing gear and nosing over. The pilot reported no pre-impact mechanical difficulties.