To add to that, many forget.....or don't realize, that only a % of our applied N actually goes into the crop in the year it is applied. It is not a 1:1 ratio, or for every 1 pound of N, I get 1 pound into the crop.

Nitrogen use = (N removed by grain(or plant) - {soil N + atmospheric N}) / fertilizer N

Going through my notes, I have several researchers (Josh McGrath, Jim Hoorman, Joel Gruver, others) who have all given the same  basic numbers.

A corn crop only uses 30-60% of fertilizer N in the year it is applied

The rest, (70-40%), is supplied by the soil.

We get set in the idea that adding N fertilizer means we are feeding the crop.......when the larger function of our N applications, are in fact, feeding the soil. For instance, I think Jim Hoorman indicated that, 50% of Soil N in a no-till system comes from nematodes.

Basically, we are applying N to feed both the crop AND the soil. The soil then provides the major portion on N used by the plant from previous years of N applications.