At about 300 feet agl on climb out, the pilot heard a loud bang and saw that the propeller had separated from the aircraft. He lowered the nose to regain airspeed but the aircraft descended steeply and landed heavily in a rice paddy. The engine crankshaft counterweights were found to be worn and detuned. The resulting vibrations were transferred to the propeller hub, which failed and released one of the blades. This type of detuning usually occurs as a result of large and rapid power variations, and as this aircraft was used for agricultural operations it is likely that the work requirements precipitated the failure.