WHILE CONDUCTING HIS FIRST SPRAY RUN ON AN AERIAL APPLICATION FLIGHT, THE PILOT WAS UNABLE TO ARREST HIS DESCENT AND THE AIRCRAFT STRUCK THE GROUND. HE ADDED FULL POWER AND WAS ABLE TO GET THE AIRCRAFT AIRBORNE, BUT IT WOULD NEITHER ACCELERATE NOR CLIMB. AFTER TRANSITING ABOUT ONE HALF MILE ACROSS THE FIELD BEING WORKED, THE AIRCRAFT STRUCK POWER LINES AND CARTWHEELED. CALCULATIONS PROVIDED INFORMATION THAT THE AIRCRAFT WAS APPROXIMATELY 400 POUNDS OVER MAXIMUM ALLOWABLE GROSS WEIGHT AT TAKEOFF WHICH WAS ABOUT 15 MINUTES PRIOR TO THE ACCIDENT.