On April 28, 2000, at 0730 hours mountain standard time, an Ayres Corporation S2R-600, N4018P, sustained substantial damage when it ran off the runway while landing at a private agricultural airstrip near Buckeye, Arizona. The commercial pilot was not injured. The airplane was being operated as a 14 CFR Part 137 aerial application flight. The local area flight originated at an unknown time and was terminating at the time of the accident. Visual meteorological conditions prevailed and no flight plan had been filed. The pilot reported that he was landing with about 400 gallons of pesticide. He explained that the runway was half asphalt and half dirt. There was a bump in the runway where the two surfaces came together. The pilot reported that he hit the bump and the airplane veered to the left. He was unable to straighten out the airplane and added power to attempt to become airborne again, but did not have sufficient time. The airplane left the runway, hit a berm and flew over a concrete irrigation ditch that ran parallel to the runway, and then came to rest in an alfalfa field. The landing gear separated during the sequence.