The pilot had just performed three uneventful touch-and-go landings at a hard surface airport. He subsequently elected to depart for the local salt flats to perform an off-field landing. He overflew a prospective landing site, performing multiple flybys to assess the surface conditions. He observed an area with tire tracks, and concluded that the surface was therefore stable enough to perform a landing. He configured the airplane for a soft field landing, with the main landing gear touching the ground first. He reported that the ground felt firm as he lowered the nose; however, as the nose wheel touched the surface the airplane experienced a sudden deceleration, and nosed over. The airplane sustained substantial damage to the wings and vertical stabilizer during the accident sequence. Examination of the landing surface revealed that it consisted of soft wet mud, topped with a shallow dry crust. The airplane was equipped with conventional tires.