I have an apple tree that I've never fertilized, and has never been fertilized to my knowledge. Over the last 3-4 years the apples have been getting smaller and punier and pretty tiny last year. It was trimmed by a pro this last spring and a lot got whacked off (about 12-16 feet).

I have a feeling I should fertilize it... If I should, what should I use?

Don't laugh (too hard) but I don't have a clue to what kind it is, other than it's a "green apple tree."