At some point you will be obliged to add nitrogen to your soil, if you grow heavy consumers like tomatoes or corn, ect. Manure is the traditional choice, and a good one. I've been experimenting with legumes and rye. Legumes because they fix nitrogen in the soil (yes, I know that it is really bacteria), and rye because it puts so much organic material IN the soil.
Top soil is reduced by using chemical fertilizers, REGARDLESS of what Bill R. thinks (I'm giving him the benefit of the doubt here.). Chemferts work best with top soil but as the topsoil disappears, more and more chemferts must be added to maintain crop production. In the meantime, the chemferts flow with rain water into aquifers to poison the water (children are the most susceptible) or follow the water sheds to the sea where they create enormous blooms of algae that then die and decay, sucking up nearly all the oxygen from the water and create enormous dead zones. Don't tell Bill R. though. I want to surprise him ;O)