On May 25, 1999, at approximately 1030 central daylight time, a Weatherly 620B agricultural airplane, N6305W, was substantially damaged following a loss of control during the takeoff roll from a private dirt airstrip near Whiteface, Texas. The instrument rated commercial pilot, sole occupant of the airplane, was seriously injured. The airplane was registered to and operated by Beseda Fertilizer and Chemical Inc., of Whiteface, Texas. Visual meteorological conditions prevailed for the Title 14 CFR Part 137 flight for which a flight plan was not filed. The flight was originating at the time of the accident. According to the operator, the airplane was loaded with 210 gallons of "Roundup" to be sprayed on a 70 acre field approximately 25 miles from his airstrip. The operator reported that the weather at the airstrip was clear with winds from the east at approximately 10 knots. The operator stated that the surface of his 4,200 foot long dirt airstrip was "heavily saturated due to heavy rains during the night and early morning hours." The operator added that this was the pilot's first flight of the day. The 5,000 hour pilot, who reported having accumulated 711 hours in the make and model, had been working for the operator for the last 10 years. The operator added that during the takeoff run to the east, the left main landing gear tire "bogged down in a muddy area" near a partially flooded area of the 50 foot wide airstrip, approximately 1,000 feet from the approach end of the airstrip. The airplane veered slightly to the left and nosed over coming to rest inverted on the airstrip, approximately 330 feet from the flooded area. There was no fire, and the fuel and chemicals were contained within the airplane. In the narrative portion of the NTSB Form 6120.1/2, the pilot stated that "during his takeoff to the east, the tail started to come up too early (soft spot on runway), and he shut engine down, but not fast enough, and the plane proceeded to flip over." Examination of the 1993 airplane by the FAA inspector, who responded to the accident site, revealed that the vertical stabilizer and the cockpit area of the airplane sustained structural damage. All 3 propeller blades, the engine cowling, and rudder assembly were also damaged. The 54-year old pilot, who was wearing a flight helmet at the time of the accident, was hospitalized for over 8 weeks with back and head injuries. The operator reported that he and several operators and pilots had the opportunity to examine the "mashed cockpit" of the airplane. They concurred that "the extensive damage sustained by the cockpit area and the seriousness of the pilot's injuries were not compatible with the speed and the severity of the accident."