The pilot reported that during a soft field takeoff from a grass airstrip with 20º of flaps extended, about a third of the way down the airstrip, the left main landing gear wheel impacted mud as the airplane rotated and initiated the climb. The airplane drifted left of centerline and the pilot corrected with right aileron control. The airplane continued to drift left, and subsequently collided with a fence and a tree before it collided with terrain in an open field and came to rest in a nose down position. The wings and fuselage were substantially damaged. According to a witness, after the airplane became airborne, it attained an altitude of about 20 ft above the ground as it drifted to the left of the airstrip centerline. The pilot reported that there were no preimpact mechanical failures or malfunctions with the airplane that would have precluded normal operation.