They had to win their final match against Hampshire yesterday to progress, but a close run two-wicket defeat left them adrift of Cornwall for the last 16 qualifying place.
Alex Allum (353) and Jim Ryall (301) were leading batsmen, while skipper Joey Wilson took 15 wickets.
Dorset 50s
Standing (from left): David Humphreys, Gary French, Mike Stonier, Mike Collinge, Steve Gerety, Jim Ryall, Krish Subramanian. Front: Mike Harper, Joe Wilson, Ian Paulley, Alex Allum, Martin Miller.