John,

The 'root zone' includes everything from just below the duff layer to whatever depth the roots can go.  The big myth is that this area needs to have 'uniform' levels of all nutrients.  Roots will take up nutrients wherever they happen to occur, so long as the root tips are actively growing, which means adequate moisture and oxygen.  Tremendous uptake occurs in the surface few inches of soil, except in drought (in which case you're deficient in water, not nutrients).

In native ecosystems (prairies, forests), the plants keep bringing nutrients to the surface to build stems and leaves, which eventually die and decay.  (If the so-called stratification of nutrients was so bad, all these ecosystems would have collapsed a long time ago.)  Several mechanisms move nutrients in the opposite direction, such as percolating water, earthworms, etc.