The airplane departed from a runway with a slight uphill grade, for the first agricultural application flight of the day. The pilot reported that his acceleration was less than expected, and he lifted off near the departure end of the runway with minimum airspeed. The airplane failed to accelerate or climb, and struck terrain about 600 feet from the airport. The impact collapsed the landing gear and caused substantial damage to the airframe and elevator. The pilot reported that he inadvertently departed with the engine mixture control leaned at approximately the halfway setting. The pilot added there was no preimpact mechanical malfunction or failure with the airplane. He also reported that the accident could have been prevented if he had used the checklist.