The pilot was conducting an agricultural seeding operation into a cleared paddock which was heavily timbered along its western boundary. The swath runs were in an east/west direction, with a left hand procedure turn at the completion of each run. The pilot reported that during the procedure turn at the western end of the paddock, the stall warning sounded and the aircraft commenced to sink. The pilot attempted to dump the load but was unable to activate the hopper dump door. The aircraft stalled from a height of approximately 100 feet above ground level, impacting the ground in a nose and left wing low attitude. The pilot could not recall a loss of engine power during the procedure turn. The investigation did not reveal any airframe deficiency which could have contributed to a loss of performance. A subsequent bulk strip of the engine revealed that the number six cylinder exhaust valve pushrod was bent and the associated valve was sticking. It is considered likely that these defects would have contributed to a loss of power. However, the precise reason for the loss of performance could not be determined. The reason why the pilot was unable to dump the hopper load could not be determined.