The pilot was departing on the first agricultural application flight of the day. He said that as he taxiing, he noticed the engine temperature rising, so he leaned the mixture. He completed his engine run-up and departed. He said that as the airplane was accelerating down the runway, he noticed the airplane was not developing enough power, but he had used up most of the runway and could not abort the takeoff. During climb, the airplane experienced a degrade in performance 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's mixture control lever at the halfway leaned setting. The pilot reported that there were no mechanical anomalies with the airplane, and the accident could have been prevented if he had used a checklist.