The pilot stated that he was taking off to the north on a highway for a crop spraying operation. The aircraft was loaded with 720 pounds of chemical and 36 gallons of fuel. He perceived a breeze from the south but took off north because of a hill to the south. The aircraft was rotated after about 1500 feet with vehicular traffic in sight. To avoid flying over the traffic, the pilot turned left over a wheat field bordered by 20-30 foot tall trees. He realized he was not maintaining altitude and became concerned with clearing the trees, so he lowered the flaps to 20 degrees and dumped his chemical load. While continuing the turn to avoid the trees, the left wing contacted the ground causing the aircraft to cartwheel to a stop, upright. After the accident, the pilot realized that the southerly wind had increased to about 10 mph. Aircraft weight at the time of the accident was approximately 2229 pounds. According to the aircraft log, maximum gross weight at takeoff is 2070 pounds.