The pilot reported that during an aerial application flight he lined up for a downwind spray pass at an approximate height of three feet above the crop. During the pass the engine sputtered and died. He entered an auto-rotation and attempted to slow the aircraft down from 50 MPH. The aircraft subsequently impacted terrain and came to rest on its side. The pilot noted that the helicopter was refueled for the second time during the job about an hour prior to the accident. An FAA Inspector's report noted that the helicopter was not refueled to its proper quantity, causing the pilot to miscalculate the fuel on-board. Post impact inspection of the aircraft revealed an estimated two gallons or less of fuel in the tank, and that there were no ruptures in the fuel tank. During the on-scene inspection there was no odor of fuel at the accident site. Post-recovery engine tests revealed that the engine operated without any significant issues and fuel contamination was ruled out.