The pilot was conducting superphosphate spreading operations from an agricultural strip. Shortly after the aircraft became airborne on about the fifth or sixth takeoff for the day, the engine failed. The pilot immediately changed tanks and placed the fuel-pump switch in the high/prime position, but the engine did not respond. The pilot dumped the load and attempted a forced landing on steeply rising terrain. Touchdown was heavy, the nosegear collapsed and the aircraft overturned. The engine failure was determined to be caused by fuel starvation owing to fuel system mismanagement. It was established that the pilot had been pre-occupied with a personal problem, which distracted him from the task at hand. As a result, one tank had been allowed to run dry, and although the other tank had been full, a defective microswitch had prevented the fuel pump from operating in the high/prime position.