John,

Yes, of course we must harvest the grain to nourish livestock and people.  Trouble is, the poop (and carcasses) don't come back, as a rule -- they get heaped in piles, overapplied on small tracts, or shunted to the bottom of the ocean, etc.  So it is net exporting of many nutrients from the land.  Add in nutrients lost from runoff (and erosion if you're doing much tillage) and leaching, and it becomes a serious issue. 

And, yes, I'm saying apply your nutrients to the surface or in near-surface bands such as seed-furrow applied (using appropriate materials and rates) and/or 3x0 placement on planters, if equipped.  The advantage of getting the nutrients slightly below the surface (without doing lots of damage to the soil and residues) is the elimination of potential for losses in runoff if it occurs.