Does anybody know?
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The terms "tenant" and "landlord" are good candidates, but can also be applied for other kind of real estates than farms. Hence the "agricultural" qualifier.
"Agricultural landowner" is your best bet.
"Agricultural tenant" implies that he rents the land FROM someone else (and is thus not the owner). "Agricultural landlord" implies that he rents it TO someone else, which may not be the case you are looking for. A landlord is a landowner but a landowner is not necessarily a landlord.
I like Ross's suggestions, and I think some answers are getting hung up on a secondary issue, land ownership, when the answer should be focussing on the primary thing, that this person does agricultural work.
I would suggest the following in additon to Ross's answer:
producer, grazier, pastoralist, orchardist