Food value? ... how do I do this?

"... the new evidence that poundage or tonnage is no measure of the food-value of farm crops; the products of fertile soil may be qualitatively as well as quantitatively superior. We can bolster poundage from depleted soils by pouring on imported fertility, but we are not necessarily bolstering food-value. "

This raised a question in my mind: What is the food value comparison between a bushel of corn now as compared to prevvious years?  An interesting measure would be protien cotent.

Another question: compare corn that is GMO with non-GMO?

Same goes for soybeans, wheat, etc...

The core question for me is, can I sustain my farm by using methods which protect or enhance soil fertility without using artificial fertilizers?

The goal is to be economically viable while implementing sustainable farm practices from the perspective of an evolving land ethic.