True, but these are their 'tractors' as well. No Steiger or John Deere will yet help grow its own food.
As I recall, farming was pretty much a subsistence thing with the 'extras' traded for other goods the Amish, Mennonites, and similar folks could not supply for themselves. Now they grow produce for market as well, and the ones in Virginia made some of the best cheese around, back in the '70s when I lived there.
While the John Deere might be able to grow enough to feed itself (biodiesel), there would be a similar reduction in foodstuff output to the Amish growing their own fodder for the horses.
That would take millions of acres out of food production which would instead be used for growing fuel crops.
The agricultural surpluses we have under the current model would not be possible.
Gasoline reminds relatively cheap. At twice the price, people will still find it economical. The only thing that will change is the size of their cars.