The pilot reported that he had contacted the fixed base operator the day prior to the accident to check on the condition of the grass airstrip, and was told that airplanes had departed and landed on the grass airstrip during the day. The pilot called the fixed base operator on the morning of the accident but no one answered the phone. The pilot was unaware that it had rained at the airstrip during the night. He over flew the grass airstrip prior to landing and did not observe any standing water or signs of ground softness, so he landed on the grass strip. During landing roll, the main landing gear sank into a wet, soft area of the airstrip, and the airplane nosed over.