Your comment raised a thought in my mind that I often wondered about....

if you are relying on soil O.M. to supply a portion of your crop nitrogen needs, don't you eventually reduce your soil organic nitrogen, unless you replace it? So, if you need 300 lbs N - or 200 lbs N or what ever number - to grow a crop, don't you have to supply it somehow or somewhere, or you will eventually deplete your soil?