The pilot stated that he took off into a 15-mph head wind from a dirt strip with a 1,800 pound load of fertilizer. He climbed the airplane to around 100 to 150 feet above ground level (agl) and started the downwind turn. The airplane began to descend as he turned downwind and he could not maintain altitude. He had insufficient altitude to release all the load before the airplane collided with the terrain. The pilot stated that the airplane and engine had no mechanical failures or malfunctions during the flight.