Wind power

[Sibling Zono (anon1mat0)]

Calories require massive petroleum input

Unless the tractors, combines, trucks, plus the fertilizer are powered by... wind turbines!
 

One of the necessities of truly non-petroleum energy is decent electric construction equipment. There's a lot of diesel necessary to erect any sort of infrastructure.

Actually, they could use biodiesel... provided that the vegetable oil is a net producer of energy (something that doesn't happen with current ethanol production from corn, but happens when you use cane sugar), or that the algae processing becomes a practical reality outside the lab.

Practical electric heavy machinery will certainly take a while to take over but there are already a number of electric light machinery in use.