PLT said he had 52 gals fuel on board when he departed El Paso, TX, en route to Roswell, NM on practice x-country flt. He made two practice ILS apchs at Roswell, then started climbing towards Roswell VORTAC, en route back to El Paso. As he reached 4500 feet MSL, eng lost pwr. PLT turned back towards Roswell, established glide, and declared emerg. He attempted to restart eng but had no success. Realizing he had insufficient alt to reach arpt, PLT made intentional wheels-up landing on soft, irregular terrain. Postaccident examination disclosed fuel selector was between off and right tank detents. Fuel supply line was disconnected. When master and boost pump switches were turned on, no fuel came out. When fuel selector was switched completely to right tank, still no fuel came out. When fuel selector was switched to left tank, fuel poured out of fuel supply line. Examination of intact fuel tanks revealed left tank contained fuel; right tank did not.