Sounds like you have a good handle on fertility, then, perhaps with the exception of being a bit low in N.  (and possibly K.)

If it's 'just' a single field, perhaps there's something in its history that explains the droughtiness.  Maybe decades ago it was eroded severely.  Or compacted by a wet harvest.  Maybe it's grown more soybeans and less corn in recent years.  Perhaps some nutritional differences haven't been manifest in the soil tests . . . .