The African Languages online Swahili dictionary says that the FIRST meaning for 'shamba' is 'field' or 'plantation'. The SECOND meaning (in the ji/ma class) is 'farm'. I remember it mainly meaning 'field' years ago in western Kenya. But in the same dictionary, if you look up "farm" the only noun it lists is 'shamba'.
(In another class of nouns (9/10) 'shamba' means (in the) countryside.)