Our 6-year old border collie is housebroken, but now we would like to
have her defecate in a sandlot at the side of our new house instead of
the back lawn. In our previous house she actually preferred to dump in
the sandlot of the swing-set area. (We previously had no children, but
happily, now we do.) She does not defecate during neighborhood walks.
Our new house has a 5x20' area filled with sand in the hopes that she
would adopt it as a dumping ground but she hasn't taken the hint. I
have hesitated locking her in this 15x50' concrete dog run at the side
of the house--seems to me she would just think she was being punished.

How would you train a dog to defecate in one specific area?