On the day before the accident, the pilot installed dry fertilizer equipment on the airplane. Later that afternoon, the pilot removed the dry fertilizer equipment and installed spray booms and a spray pump in preparation for dispensing a liquid chemical the next day. The following day, during the takeoff roll, chemical started covering the windshield. The windshield washer had no effect. The pilot lost all outside visual references and was unable to maintain level flight. The airplane impacted the ground and slid to a stop. Postaccident examination revealed the hopper vent had not been turned around. The hopper was still pressurized and chemical blew out of the hopper lid onto the windshield.