The pilot reported that he rolled down the runway for takeoff a little over 60 knots. He stated that just after liftoff, the airplane was initially sluggish to climb so he lowered the nose to gain more speed in ground effect (about 70 knots). The airplane started to gain some altitude, so the pilot continued his departure. Shortly after clearing the end of the runway, there was a loss of power and climb performance. With trees and houses ahead and insufficient altitude to clear them, the pilot decided to land in a field with deep grass just north of the airport. Upon landing in the field the nose landing gear collapsed resulting in substantial damage to the front of the fuselage. The pilot and both passengers exited the airplane without significant injuries. After the accident the pilot said he believed that he may not have leaned the engine enough during run up resulting in less than 100 percent available power for the climbout. He also reported that the spark plugs may have been fowled due to the mixture setting resulting in the loss of available power immediately after takeoff. The pilot stated that the airplane was loaded to within 65 pounds of its maximum gross weight and with a calculated density altitude of 10,400 feet, the anticipated climb rate of 300 feet per minute required full power as outlined in the Pilot Operating Handbook. He stated that if there was not deep grass in the landing field, he could have landed the airplane without incident.