The commercial pilot departed a private airstrip on an aerial application flight after refueling and being loaded with a full load of fertilizer. The pilot was flying his first application pass shortly after takeoff when he said the engine stopped developing power and the airplane started descending. The airplane hit a levy, impacted in a field and came to rest against a second levy. Temperature at the time of the accident was reported by the pilot as 103 degrees Fahrenheit. Examination of the airplane showed the left wing spar was bent down and aft. A compression check of the engine performed by FAA personnel revealed the number three cylinder producing 24 pounds of compression, the number nine cylinder producing 50 pounds of compression, and the remaining seven cylinders producing 68 pounds of compression. The number three exhaust valve was found stuck open. The engine was not disassembled to determine the cause of the sticking exhaust valve. The engine had accumulated 943 hours since the last overhaul and 129 hours since its last annual inspection on March 3, 2008.