The solo student pilot was applying power during a practice soft field touch and go landing when the aircraft veered to the left, departed the paved surface of the runway and struck an airfield sign. The student pilot stated he had accomplished three soft field practice takeoff and landings in the accident airplane the day of the accident (15 to 20 total), prior to being cleared by his flight instructor to perform the maneuver solo. The student pilot stated he was uncomfortable performing the maneuver due to the airplane's high engine power and tendency to pull hard to the left during the maneuver. The weather was clear with winds 230 degrees at 4 knots. Examination of the aircraft revealed damage to the right aileron. The student pilot stated there was no mechanical malfunction or failure of the airplane prior to the accident.