The pilot of the tailwheel-equipped airplane reported that during the takeoff roll, the airplane "pulled hard to the left." He recalled that he applied full right rudder pedal and realized that he was approaching the end of the runway. He reported that he applied both brakes, he applied reverse thrust, the airplane exited the runway to the left and ground looped. The pilot reported that he did not confirm that the tailwheel pin was locked, prior to the takeoff roll. He further reported that in the future, he will confirm that the tailwheel is locked by working the tail back and forth, and he will check the tailwheel for inhibiting buildup of fertilizer, grease and dirt that would prevent tailwheel pin actuation. The right wing sustained substantial damage. Per the Federal Aviation Administration (FAA) Safety Inspector that arrived shortly after the accident occurred, the airplane was dispersing fertilizer as part of a 49 Code of Federal Regulations Part 137 operation. The prolonged use of fertilizer coupled with dirt, will inhibit the tailwheel pin from locking in place. A photograph was taken shortly after the accident by the FAA Inspector revealing a canted tailwheel. The FAA Inspector reported that the mechanic that performed the inspection on the tailwheel assembly noted that the assembly was, "gummed up with fertilizer and the pin was not in place at the time of the ground loop.