During flight, the pilot used fuel from 2 of 4 propane tanks, then switched to the 2nd 2 tanks and started a descent. At approximately 300 feet above ground level, he tried to start the burner, but could not get a light because the pilot light was out. He attempted to manually relight the burner with a flint sparker, but the burner would not relight. Subsequently, the balloon struck a metal fence, bounced, and hit a tractor. Five of the passengers fell out. The pilot and one other passenger were carried another 200 yards into a swampy area. An examination of the fuel system showed that the pilot light valve was a toggle type switch near the 'FIRE 2' valve. The pilot reported that he could have inadvertently struck and shut off the pilot light valve as he rotated the 'FIRE 2' valve. When checked, the pilot light and burner operated normally. The 'FIRE 2' would only light with the flint sparker when the valve was only slightly opened.