On June 24, 1999, at 0730 central daylight time, an Air Tractor AT-301, N3159M, piloted by a commercial pilot, received substantial damage on impact with terrain near Veblen, South Dakota. Visual meteorological conditions prevailed at the time of the accident. The 14 CFR Part 137 aerial application flight was not on a flight plan. The pilot sustained minor injuries. The local flight originated from Britton Municipal Airport, Britton, South Dakota at approximately 0700. The pilot stated that he loaded the Air Tractor with 240-gallons of water and chemicals and that the fuel tanks had 70 to 80 gallons of fuel. He stated that winds were out of the southwest at 6 to 10 mph. He said he was spraying in some local hills. On his fourth pass, he said that he encountered air that did not seem "...to have much lift to it". He said that he started adding power to climb the airplane above trees. He said that he had applied full power as he got closer to the trees and the airplane started settling downward once over the tree line. He stated that the airplane settled and contacted the ground. The engine was torn off the fuselage, along with the landing gear and tail wheel. The pilot stated that the fuselage came to rest on a southeasterly direction. The pilot stated that "dumping the load" was his recommendation on how this accident could have been prevented.