The 450-hour private pilot reported that shortly after takeoff, the engine of his single-engine airplane developed an abnormal engine fluctuation while climbing through an altitude of 1,300 feet MSL. As a precaution, the pilot elected to return to the departure airport; however, while turning back to the airport, the engine lost power and he prepared for a forced landing to an open field. Prior to the forced landing, the pilot noticed freshly dug ditches on both side of the field, so he repositioned to an adjacent field located to the right of his initial landing spot. The pilot added that the engine began to surge when the airplane was about 300 feet above the ground, which extended his glide. The airplane touched down on soft ground, the tires sunk in the soft ground and the airplane nosed-over. Examination of the airplane revealed the fuel bladders in both wings were not breached and there was no evidence of fuel stains around the vented fuel caps. Approximately 1-gallon of light blue colored fuel was drained from the right tank, and less than 1-gallon of light blue colored fuel was drained from the left tank. The fuel was found to be free of water and debris. The engine mounted fuel strainer bowl was found empty and the fuel screen was also absent of debris. The pilot reported that he did a pre-flight inspection of the airplane, but did not visually inspect the fuel tanks. Instead he relied on a fuel flow meter installed in the cockpit, which indicated a total of 43.6 gallons. The pilot reported that the gauge was usually accurate and he was unaware of what happened to the fuel he thought was onboard. Weather at the departure airport was reported as calm winds, visibility 8 statute miles, and clear skies.