The pilot stated that he was flying the airplane over a large open desert playa. He decided to land after flying low over the surface to determine if he could make a safe landing. The surface appeared to be flat and dry. Upon touching down, the landing gear dug into the about 8-inch deep soft tufa. The airplane cartwheeled, separating the left wing and bending and wrinkling the left horizontal stabilizer. The left side of the fuselage, the engine mount, and the firewall were also bent and wrinkled.