It's worth taking a moment to think about how events might unfold if the process occurs without intelligent management, driven simply by oil and gas depletion.
Facing high fuel prices, family farms would declare bankruptcy in record numbers.
Older farmers (the majority, in other words) would probably choose simply to
retire, whether they could afford to or not. However, giant corporate farms
would also confront rising costs - which they would pass along to consumers by
way of dramatically higher food prices.
Yields would begin to decline - in fits and starts - as weather anomalies and
water shortages affected one crop after another.
Meanwhile, people in the cities would also feel the effects of skyrocketing
energy prices. Entire industries would falter, precipitating a general economic
collapse. Massive unemployment...
Many people would leave cities looking for places to live where they could grow
some food. Yet they might find all of the available land already owned by banks
or the government. Without experience of farming, even those who succeeded in
gaining access to acreage would fail to produce much food and would ruin large
tracts of land in the process.
Eventually these problems would sort themselves out; people and social systems
would adapt - but probably not before an immense human and environmental tragedy
had ensued.