THE PLT DEPARTED WITH FULL FUEL IN THE ACFT TO PRACTICE AERIAL SPRAYING TECHNIQUES. AFTER 45 MINS OF PRACTICE HE RETURNED TO THE ARPT FOR LANDING. HE SAID THAT PRIOR TO ENTERING THE TRAFFIC PATTERN HE SWITCHED FROM THE HALF-FULL RIGHT TANK TO THE FULL LEFT TANK. THREE MINS LATER, WHILE ON THE DOWNWIND LEG AT 300-400 FT AGL, THE ENG LOST PWR. THE PLT SAID HE WAS TOO LOW TO LOOSEN HIS SHOULDER HARNESS AND FEEL THE FUEL SELECTOR BUT HE VISUALLY CHECKED IT AND IT APPEARED TO BE POSITIONED CORRECTLY. HE SAID HE APPLIED CARB HEAT AND TURNED THE BOOST PUMP ON BUT PWR WAS NOT RESTORED. THE PLT ATTEMPTED TO LAND IN AN OPEN FIELD ADJOINING THE ARPT BUT INADVERTENTLY STALLED THE ACFT ABOUT 20 FT AGL AND CRASHED. INVESTIGATION DISCLOSED NO EVIDENCE OF MECHANICAL FAILURE/MALFUNCTION OF ACFT OR ENG. FUEL WAS PRESENT IN TANKS AND LINES. ENG WAS LATER TEST RUN WITHOUT DIFFICULTY.