[Gibbon1]

I stumbled on something also interesting. The oxygen from photosynthesis comes from water not CO2. So all the energy from food is essentially from splitting water.

Also I think I read in a book on aeronautics which was published in the 1950's that more energy is required from oil to grow food than the food itself. Trying to look that up again just now the internet spits out an efficiency as low as 10%.

[pfdietz]

You have to be a little careful about that figure because a lot of the energy comes from natural gas, not oil. Natural gas is a much bigger thing now than it was in the 1950s. Natural gas in the US is mostly "dry", that is not associated with oil production.