The fuel required to cultivate and fertilize corn can be obtained from the stalks and leaves, with a considerable surplus.  However, this doesn't deal with the rest of the chains to and from the farm, and the demands of society at large could never be supplied this way.  They are just too inefficient to be supplied with what we can grow.

On the other hand, we have technologies on the shelf or in the pipeline which are efficient enough to replace fossil fuels with biofuels and maintain our current standard of living (very important for political acceptance).  I wrote this up last November in Sustainability, energy independence and agricultural policy.