The pilot of the aerial application flight stated that he landed on a company grass strip and loaded the airplane with liquid fertilizer. He then departed to spray the fertilizer and returned to the airstrip about 30-40 minutes later. He loaded the airplane with the same quantity of fertilizer for a second flight. He said that during the second takeoff, he was turning the airplane and powering up the engine, when he hit the wing flap rocker switch to extend flaps. During the takeoff roll, which was to the east, the sun was in his eyes and when the airplane tail lifted off the ground, he saw that the airplane had veered a little to the left into an area of taller grass. He returned the airplane along the intended takeoff path down the runway, and toward the runway end, he rotated the airplane, but the airplane settled into a soybean field. The airplane impacted the field and sustained substantial damage to the left wing. The pilot was not wearing tinted glasses, and the helmet visor that he was wearing was also not tinted at the time of the accident takeoff. The pilot stated that after the accident he observed that the wing flaps were retracted. The pilot stated there was no mechanical malfunction or failure of the airplane that would have precluded normal operation. The pilot believed that during the accident takeoff, the airplane settled into the soybean field because the airplane lost takeoff speed when it went into the area of taller grass on the runway.