The pilot, in company with another aircraft, had been carrying out aerial top dressing operations from an airstrip located along the top of a ridge line. The conditions were calm, and the pilot, who had limited aerial agricultural experience, was relaxed and enjoying the flying. Returning to the airstrip at the completion of a sortie, the pilot made a glide approach and crossed the threshold just as the other aircraft was departing with a full load. Just before touch down the aircraft encountered turbulence and dropped the left wing, then veered sharply to the left over the side of the ridge. The pilot attempted to regain the strip without increasing power. However, the aircraft failed to respond and touched down on the sloping terrain at the side of the strip. During the landing roll he attempted to steer the aircraft up the slope, but it collided with a stack of timber which caused the right hand main landing gear leg to separate. The pilot subsequently advised that he had been complacent and had not given sufficient attention to the landing. He had then encountered wake turbulence, generated from behind the departing aircraft, which had not had time to dissipate in the calm conditions. It had not occurred to him to apply full power and go-around.