DURING THE INITIAL CLIMB, JUST AFTER TAKEOFF, WITH TWENTY-FIVE DEGREES OF FLAPS EXTENDED, THE PILOT REPORTED THAT EVERYTHING WAS NORMAL. THE AIRPLANE REACHED AN ALTITUDE OF APPROXIMATELY 300 FEET AGL WHEN IT BEGAN TO DESCEND AND WOULD NOT MAINTAIN ALTITUDE. THE PILOT INITIATED A LANDING STRAIGHT AHEAD IN AN OPEN FIELD. DURING THE LANDING ROLL THE AIRPLANE COLLIDED WITH THE TERRAIN. POST CRASH INSPECTION REVEALED NO EVIDENCE OF A MECHANICAL FAILURE OR MALFUNCTION. WEIGHT AND BALANCE CALCULATIONS REVEALED THAT THE AIRPLANE EXCEEDED MAXIMUM GROSS WEIGHT.