During the takeoff roll the single-engine tail-wheel equipped agricultural airplane exited the left side of the 20-foot wide gravel runway despite the pilot's correction of full right rudder and right brake. Once off the runway, the airplane crossed a creek and impacted an embankment, before coming to rest in an upright position. The pilot was able to exit the airplane unassisted. Moments later the airplane was engulfed in flames. The pilot reported that following the accident he observed that the tail-wheel was unlocked and that it may have contributed to the accident. He further stated that the accident may have been prevented by checking that the tail-wheel was locked prior to takeoff.