There are right now farmers in France experimenting with burning plain unrefined sunflower oil to run their tractors. Search on 'HVP' or 'HVB' and 'tracteur', plenty of links.

A (very) rough approximation is that it takes about 1/20th of the farm surface to produce enough oil to cover the liquid fuel needs of the farm. But of course other energy 'intrants' are needed, such as nitrates, etc.

Is there anywhere a calculation showing how much energy overall is needed to produce a ton of wheat/whatever, everything considered ?

May I say sunflower oil is a 'renewable' if I don't know the net energy balance for its cultivation ?