"He is going to a farm"
Well, kwenye shamba for sure. I do recall reading somewhere that one of the organisations that dictates how Swahili is to be spoken (maybe BaKiTa?) says that kwa for direction/location should only be used with animate nouns (people etc., Naenda kwa Rehema.) and kwenye with inanimate nouns. If lots of people use kwa in this case and it's not badly stigmatised, then it should be accepted though, because Swahili doesn't belong to any organisation - it belongs to its speakers!