Eric
As someone has already suggested you will want an Equipment Grounding
Conductor (EGC) in that feeder unless it is supplied from a separate
service with it's own meter directly from the local power company. If
that feeder will share a service with any other building then you really
want an EGC in that feeder. The reason for the concern is that if a
neutral connection ever failed the current it carries would try to
return to the source via any other grounded conductive pathway. Those
can include telephone lines, cable jackets, intercom lines, security
wiring, metallic piping systems, and the local earth itself. Such stray
currents can cause electric shock, erratic behavior in pets, pour
production in dairy cattle, destruction of metallic systems, and under
some circumstances fire. The practice of running feeders without EGCs
will no longer be allowed under the next edition of the US National
Electric Code.

On a separate subject if the trench has not been fully back filled you
should take this opportunity to install a one inch low voltage raceway
so that you can run communications, alarm, and video wiring between the
two buildings at a later date.
--
Tom Horne

Thanks for that info!! I wondered why as the third big wire was after
all a ground, why one would need a dedicated ground wire besides. How
big does it need to be? If it needs to be as big as the others, it
would add significantly to the cost of the project. The conduit has
been in the ground for a little over 2 years, but I did run a 2
conductor direct burial communications wire when the trench was open.