DURING A TURN-AROUND WHILE SPRAYING A COTTON FIELD, THE AIRCRAFT STARTED ROLLING TO THE LEFT AND THE PILOT COULD NOT STOP IT. HE STATED THE LEFT SPOILER REMAINED DEPLOYED AND WOULD NOT CLOSE. HE FOUGHT THE ROLL UNTIL IT IMPACTED WITH THE GROUND. THE LEFT SPOILER WAS FOUND IN THE VERTICAL POSITION AND THE RIGHT SPOILER WAS FLUSH WITH THE WING SURFACE. DUE TO POOR ROLL RATE AND HIGH ASPECT RATIO, SPOILERS HAD BEEN ADDED TO THE LOWER WING. THEY CAN BE INDEPENDENTLY OPERATED FROM THE COCKPIT OR WORK IN CONJUNCTION WITH THE AILERONS. ACCORDING TO AN FAA ENGINEERING TEST PLT, THE SPOILERS HAVE LIMITED AUTHORITY, AND IF ONE IS MANUALLY DEPLOYED TO THE FULL UP POSITION IT CAN BE OVER- RIDDEN WITH OPPOSITE AILERON. THE PLT STATED THAT THIS WAS THE THIRD OR FOURTH TIME A SPOILER HAD STUCK OPEN. THE WING WAS CUT OPEN AND NO REASON FOR THE SPOILERS STICKING COULD BE FOUND.