During an aerial application flight in a tailwheel-equipped airplane, the commercial pilot departed the airstrip with a full load of insecticide, but then realized the airplane was equipped with improper spray nozzles, and returned to the airstrip. Considering the fully loaded configuration, the pilot made one low approach and reported that everything felt normal. On the next approach the pilot touched down on the dirt portion of the runway, applied beta (reverse thrust), and immediately the airplane began yawing to the left. The pilot applied right rudder and right brake but the airplane continued to yaw left. According to the pilot, the tailwheel was not down when beta was applied, which contributed to the subsequent loss of directional control. The airplane exited the runway to the left and bounced over several crop rows causing the left main landing gear to collapse. The propeller dug into the ground and the airplane nosed over, resulting in substantial damage to the firewall and right wing. When the airplane came to rest on its back, the pilot remained strapped in, upside down with his helmet on and visor down. The insecticide chemicals immediately began pouring over his face and he had the sensation that he was drowning. The pilot was able to release his seat belt and crawl out of the airplane through the emergency door. The pilot reported no mechanical malfunctions or failures with the airplane that would have precluded normal operation.