On January 12, 1995, at 1530 central standard time (cst), a Piper PA-28-180C, N8221W, sustained substantial damage when it exited the end of the runway and impacted a fence post during the landing roll on a private grass airstrip in West Liberty, Iowa. The private pilot and passenger reported no injuries. The 14 CFR Part 91 pleasure flight had originated from Green Castle, Iowa, at 1500 cst. Visual meteorological conditions existed at the time of the accident and no flight plan was on file. During a telephone interview, the pilot stated the airplane made a normal touchdown on the grass airstrip. He stated that when he applied the brakes to stop "it was just like being on ice." "The airplane hydroplaned," the pilot stated. In an attempt to try and avoid impacting a parked airplane and a fence which were located at the end of the runway, the pilot applied full power and full right rudder to turn the airplane. The airplane went into a bean field and its left wing impacted a fence post. The pilot reported that he has landed on this airstrip on many other occasions. He stated that the airstrip looked dry, but it was wet and muddy because the snow had recently melted leaving the ground very soft.