The operator of the aircraft had contracted to spray 160 hectares of barley with weed killer. On arrival over the property, the pilot flew an inspection orbit and commenced spraying the first paddock towards some tall trees and a powerline. After this first pass the aircraft was pulled up into a procedure turn before diving steeply over the same obstacles, in the reciprocal direction. When the aircraft was clear of the obstructions, the pilot attempted to level the aircraft for the next spray run. However, the aircraft continued along its established flight path until striking the ground in a pronounced nose high attitude. The impact damaged the propeller, spray booms and landing gear. Chemical sprayed over the windscreen depriving the pilot of forward vision, and he was unable to control the subsequent landing run sufficiently to prevent the aircraft from broadsiding. The aircraft was being operated at a weight in excess of the Agricultural Gross Weight and at a relatively slow airspeed. During the pullout from the dive, the load factor ("G" loading) applied to the aircraft caused an aerodynamic stall. The pilot was unable to effect a recovery at such a low altitude.