The commercial pilot was conducting an aerial application flight to a field that had a small hill on the north side. After the completion of the second pass, the pilot maneuvered the airplane out of the field to turn away from the hill and to get turned around for another pass. Upon starting the turn back towards the field, the pilot realized that he had not allowed adequate space to perform the turn given the airplane's "current load and environmental conditions." The pilot leveled the wings and attempted to fly over the hill; however, the airplane encountered an aerodynamic stall about 30 ft above ground level and subsequently impacted trees and rocky terrain. The airplane sustained substantial damage to the fuselage, the left wing, and the empennage. The pilot reported there were no preimpact mechanical malfunctions or failures with the airplane that would have precluded normal operation.