I have heard that same 15% figure. Not sure if it was, as you say, ALL the corn and soybeans or just the "surplus" but either way it is a limited option that will create as many (or more) problems as it solves.

I saw on the news the other day that the drought in the southeast US is causing a new problem. Many farmers planted corn instead of cotton this year but corn needs more water so they are facing the possibility of not getting much of a crop. I doubt if they irrigation as an option because they haven't needed it.