Please ID This Movie

It would have been made before 1980, evidently featured an all/mostly
black cast, and starred (or at least featured) a young man who returns
home to his urban family after graduating from college. His degree is
in organic fertilizer production. (Literally; he has become a manure
expert) and various older members of the family repeatedly rag him
about this. The main thrust of the film, however, is about his
relationship with a young female acquaintance of the family, a black
sheep who is a prostitute or something equally unsavory. At a big
Thanksgiving(?) get-together, the family attempts and fails to keep
the two young people from ultimately hooking up.

Any ideas?

Thanks!