Get your soil tested by a professional lab and go from there. Don't just go throwing down fertillizers here and there. (Organic mulches are always good in the interim)
Nitrogen is often NOT tested because it fluctuates so much. In any case it's also easy to apply with ordinary/free products (grass clippings for example...)
Do any plants that you have growing SHOW deficienciesor stunted growth? If not, I wouldn't worry too much.
