The land would revert back to it's rich state if left
alone, because that is what it did after the last
ice age.  That only happens if the loop is closed.
 If you are
harvesting from a set plot of land, wether it
is a crop or cattle, you are removing atoms from the
land and depleting the chemistry.  The key element
is usualy phosphorus.  When left alone minirals
(atoms) are chelated from the native bedrock at
a slow rate, and fertility imporves unless
erosion is removing the material.  Thus the
Great Plains had accumulated in some places
tens of feet of rich topsoil since the ice age.

With a little intelligent management the land can
be quickly returned to a very fertile state.
One key to sustainable agriculture is to recycle
the human waste back to the land so the loop
is closed and then fertility improves.

Whether this will happen on a large scale in our
lifetimes remaines to be seen.  The only way this
could happen is if oil becomes more scarce and
agriculture is forced back to organics.  This is
just beginning to happen, but may be too little
too late.

R. Keene