THE PILOT SAID THAT WHILE PRACTICING SLOW FLIGHT, THE LEFT ENGINE LOST POWER AND THE PROPELLER WENT INTO FEATHER. AFTER SEVERAL ATTEMPTS TO RESTART THE LEFT ENGINE, WHILE UNABLE TO MAINTAIN ALTITUDE, AN EMERGENCY LANDING WAS MADE IN A SOFT, FARM FIELD. THE GEAR DOWN LANDING RESULTED IN SUBSTANTIAL DAMAGE TO THE WINGS, LANDING GEAR, AND PROPELLER. POST-ACCIDENT INSPECTION OF THE AIRPLANE AND ENGINE FAILED TO REVEAL THE REASON FOR THE POWER LOSS.