It depends on the size of the farm, how they grow, how much they grow, what type of fertilizers they use, how much they use, how well your soil drains, and things like that. Right now, it's pretty unlikely that they're putting down fertilizer...later in the spring and summer, you might see phosphates but you also might not. It's hard to say either way. Phosphates surely aren't a bad thing to NOT have in your water though
I would test every once in a while though.