After taking off from runway 5, the pilot of a Mooney M18-L remained in the traffic pattern to practice landings. The pilot lined up on the final approach to the runway with 10 degrees of wing flaps extended, and utilized the visual approach slope indicator to adjust his glide path. After passing over the runway end identifier lights, he then turned to line up with the grass between the runway and taxiway, to practice a soft field landing. His intent was to touchdown just past a taxiway that ran perpendicular to his flight path. As the airplane approached the perpendicular taxiway, "the plane dropped suddenly" with no change of "engine power or tone." The airplane then touched down hard, crossed the edge of the perpendicular taxiway, and the landing gear collapsed, substantially damaging the airplane. Winds about the time of the accident were 045 degrees at 8 knots gusting to 15 knots.