The pilot stated he was performing an aerial application on a celery crop with the skids about 24-inches above the tops of the crop. While in straight and level flight, the landing skids contacted the crop and the helicopter pitched forward. The front of the skids dug into terrain and the helicopter tumbled before it came to rest on its side. Examination of the helicopter revealed substantial damage to the fuselage and the tail boom was separated. No mechanical anomalies were noted with the engine or flight control system.