Pollan claims, according to Tom Philpott's summary on an interview, that we won't have the fossil fuels to keep our current "industrialized agriculture" going in 30 years or so. I'm not clear what he means. If he's assuming "peak oil" so the price of diesel and inputs to fertilizer plants go up, that's likely. My impression, though, is that large diesels are at least as efficient as small diesels, so unless Pollan sees a reversion from tractors to horses/mules/oxen I don't see the problem. To the extent we replace fossil fuels in our transport, we'll also be able to replace them in agriculture for motive power. If we go to electric vehicles with the electricity supplied by nuclear, by sunlight, by wind power, by fuel cells, by whatever, we can go to electric tractors. (I'm not sure whether electric motors or diesels generate more torque.)
Yes, the phasing out of oil might raise the prices for fuel on the farm and possibly the cost of fuel, but I fail to understand how it would force a change in the mode of agriculture.