Let's say that you change drainage, or do deep tillage on a part of the field, and the yield on those parts of the field comes up. Then the yield monitor will catch that and fertilizer applications to that area will increase. 

You can always make a rec to apply more fertilizer. On a lot of fields we're still doing builds, for instance. 

Unless you're RTK and applying in the exact same spot over and over, it kind of becomes a moot point, anyway. Fertilizer moves in the soil.