[RedMonqey]

So long as we don't consume the entire harvest in the same growing year, in other words, we store some it, I am not too worried.

In a prefect world with responsible people and a responsible public that sounds great. Harvests are unpredictable. I doubt we would have enough bumper crop years to cover in insatiable demand of fuel in this world.
Charges of hoarding,and whispers of conspiracies to keep fuel costs high would doom any such responsible measures.
In this America we want what we want and we want it yesterday.

My preference would still be to have a system to can continue to operate on a petrol or a plant-based fuel source so we can use one if the other fails...or play one off against the other.

Dividing the crops used to feed the world in order to top off it's fuel tank is not a viable sustainable situation. IMHO Use the Tar sands and oil shale that North America has in abundance.

[P-40]

Harvests are unpredictable.

Yes they are...and there are always companies that sit and wait with their nut until it is a good time to sell it. This is the same in any market with a product that can be stored.