THE ACFT COLLIDED WITH A FENCE DURING A TAKEOFF WITH A FULL LOAD OF CHEMICAL PESTICIDE. COLLISION WITH THE GROUND RESULTED IN A FIRE. THE PLT, WHO WAS SERIOUSLY BURNED AND LATER DIED, STATED ON HIS WAY TO THE HOSPITAL THAT HE ATTEMPTED TO DUMP THE ACFT LOAD BUT GRABBED THE FLAP HANDLE BY MISTAKE WHICH EXTENDED FULL FLAPS. THIS WAS THE PLTS THIRD AG OPERATION ACCIDENT, ONE PREVIOUSLY INVOLVED AN ABORTED TAKEOFF AND CRASH OFF THE END OF A RWY. THE RWY IN USE ON THE DAY OF THIS ACCIDENT WAS GRASS APRX ONE TO TWO FT DEEP OVER ALL BUT A 300 FT SECTION OF MOWED AREA IN MID FIELD THAT WAS USED BY LOCAL FLYERS OF RADIO CONTROLLED MODEL ACFT. THE DENSITY ALT WAS COMPUTED TO BE 2200 FT AND THERE WAS THUNDERSTORM ACTIVITY IN THE AREA NORTH OF THE ARPT AND ABOUT TWO MILES SOUTH OF THE RWY. THE PLTS FLT TIME WAS NOT REPORTED AND HIS LOG BOOKS WERE NOT LOCATED.