The aircraft was loaded with 100 gallons of 2-4-D and 40 gallons of fuel. During the preflight run-up check, the pilot noted a mag drop of 225 RPM. He stated that leaning the fuel mixture cleared the excessive decrease in RPM, and he observed 2500 RPM on the engine tachometer during the takeoff roll. The pilot said the aircraft did not seem to want to gain speed like it should, and he rolled about 2700 ft before lift-off. He reported that after lift-off, the aircraft did not gain speed as he climbed to about 75 ft AGL, and the engine popped and sputtered as the plane settled. During a landing in a soft field, the aircraft nosed over and the pilot was soaked with the 2-4-D chemical. The pilot reported that he would have dumped his load if he had a little more time. An examination and operational check of the engine revealed no precrash malfunction or failure. The takeoff performance data showed that the aircraft should have been able to clear a 50 ft obstacle in 1350 ft. The density altitude was about 6500 ft MSL.