The pilot reported that she was practicing a low-level aerial application flight of applying water to a field. During the third pass, she pushed the spray lever forward and inadvertently pushed the dump lever with her knuckles, which opened the gate to the hopper. She closed the gate and continued spraying, then climbed out of the field. The airplane entered an aerodynamic stall during the climb and the pilot applied full power; however, the airplane continued to stall at a low altitude and landed hard in a field. The airplane came to rest upright and sustained substantial damage to the fuselage and both wings. The pilot reported there were no preimpact mechanical malfunctions or failures with the airframe or the engine that would have precluded normal operation. The pilot reported the airplane was equipped with a stall warning system. She further reported that she obtained her aerial application airplane operation logbook endorsement 36 days before the accident date.