משנה על בבא בתרא 2:1

What constitutes an orchard? Any field in which there are at least three trees for every bet seah. If each tree is capable of yielding a cake of pressed figs, the size of sixty in the Italian maneh, then the entire bet seah may be plowed for their sake. If less than this amount, they may only plow the area that is occupied by the gatherer when his basket is placed behind him.