The universal unconscious that lays behind modern farming

That would be oil.

And here is a link to Jeremy Grantham's latest missive on the markets. His comment on farming is, as usual, trenchant:

Modern agriculture has been described as a way of turning hydrocarbons into food. Without cheap energy – a single gallon of gas is the energy equivalent of 100 hours of old-fashioned labor – the world would certainly have trouble producing half of the current food supply, and that fraction could be substantially less. Hydrocarbons are not only critical to farm equipment and food distribution over very large distances, but also play a dominant role in fertilizer production. With sparse hydrocarbon usage, American agriculture would have to be totally and painfully restructured away from very large scale monoculture. Hydrocarbons are very efficient in the use of manpower but surprisingly inefficient with everything else, including output per acre and output per unit of energy.

Can anyone tell me what the last sentence meant? In what way, for example, are hydrocarbons 'inefficient with respect to output per acre?'