The pilot reported that he had made four previous agricultural aerial application runs prior to the accident flight, one of which had the same load as the accident flight. The grass airstrip was about 2,640 feet long and was soft due to recent rains. The weather was about 90 degrees F with high humidity. He put in 10 degrees of flaps, held the brakes, and ran the engine up to 1,700 lbs of torque. He departed to the north since the windsock was indicating a slight northeast wind. After he released the brakes and started the takeoff roll, the airplane encountered a soft spot in the runway which reduced his ground speed. Halfway down the runway the tail had still not come up, so the pilot “forced the tail up using the trim tab” and by pushing the stick full forward. The pilot eased the forward stick pressure and the tail went back to the ground. He immediately began to dump the load and he put in full flaps. About 100 yards before the end of the runway, the pilot “pulled the stick all the way back and became airborne, but stalled [the] airplane.” The left wing stalled and impacted the ground. The airplane came to rest in a corn field on the edge of the grass airstrip. The pilot reported that he “should have stopped the ground roll when I encountered the wet spot in the strip.” The airplane sustained substantial damage to the left wing, right wing, and empennage.