The pilot reported that, prior to the accident flight, water and a neutralizing agent were added to the hopper tank for cleaning purposes. Several hours later, the pilot conducted a preflight inspection for a night flight. After fueling the airplane, the pilot taxied onto the 2,500 ft. dirt runway, not using the entire length of the runway as he was under the assumption that the hopper was empty. He slowly advanced the power lever to a partial power setting for takeoff. The pilot stated that around the half-way point on the runway, he noticed the airplane was not obtaining the speed needed for takeoff and applied full power. As the airplane was close to the departure end of the runway, the pilot aborted the takeoff and attempted to stop the airplane using the brakes. Subsequently, the airplane overran the departure end of the runway and impacted vegetation. A post-crash fire ensued which destroyed the airplane. The pilot further reported that, after the accident he discussed the events with the company manager and determined the hopper had not been drained of the water prior to the flight. The pilot said that he believed the hopper to be empty and did not apply full power for takeoff. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation.