The agricultural airplane was substantially damaged when it impacted terrain during takeoff from runway 18 (2,200 feet long, asphalt/gravel). The pilot stated that prior to the aerial application flight, the airplane was loaded with about 500 lbs of fuel and 2,700 lbs of urea. The takeoff weight was about 7,600 lbs. The maximum demonstrated gross weight for the restricted category airplane was 8,300 lbs. The pilot reported that the wind was approximately "10-12 mph" out of the north and that he performed a "straight downwind takeoff" using full engine power. Halfway down the runway, the pilot "knew [he] was in trouble" and decided to dump the agricultural load. He stated that when the airplane got to the end of the runway, it hit a levee and pitched to a nose high attitude. When the pilot lowered the nose, the airplane "mushed down, losing airspeed." The airplane then impacted terrain tail first, nosed over, and came to rest upside down. Wind conditions reported near the time of the accident were 010 degrees at 8 knots.