The aerial application pilot stated that he was making a low pass over a field, dispensing smoke, to determine wind direction and velocity. The airplane struck a 20-foot-high irrigation vent pipe, which was located in the field that he was working. He proceeded straight ahead and attempted a forced landing on a dirt road immediately next to the field because the engine was losing power. Upon touching down, the right main wheel got into some soft dirt and the airplane nosed over inverted. The engine mount was bent, and the fuselage and vertical stabilizer were wrinkled.