If corn were the only usable source for ethanol and soy beans the only source for biodiesel, then there would be a supply crisis caused by competition with food crops over land use. We are still in the early stages of biofuel use, so what I am saying is partially speculative:

Cellulosic fiber is the most abundant substance made by living things. It can be hydrolyzed into fermentable sugars to make ethanol. So we are not necessarily constrained by corn, wheat straw, or sugar cane as feedstocks. Other feedstocks would include grasses and forestry residues. It is also theoretically possible to bioengineer a floating mat of biomass that can be grown on the open ocean. Seventy percent of the earth's surface is covered in water, and that's alot of sunlight that could be soaked up. And of course then the availability of water would be a non-issue.