I just started reading an interesting book called Farmers of Forty Centuries about an American's travels through China, Japan and Korea around the turn of the last century (around 1907 I want to say?) At that time, the author claims that the population of those nations number around 500 million - and they were sustaining themselves using essentially human labor and a very sophisticated farming system that made use of essentially every available patch of
land. Clearly organic farming can be made to produce
enough food to
feed us; the issue is finding the labor to do it.
We are going to have to transition our economy to sustainable energy, there really is no alternative in the long run, and so big ag will also have to adapt. With something like 900 million acres of farmland in the US, 400 million of which is arable land, the workforce needed to farm it with traditional methods would be mind boggling. Building an electric
tractor, or a
solar powered fertilizer plant sounds like it is going to be a lot more realistic than drumming up a few hundred million willing farmers.
Regardless of how you power it, modern agriculture does have a lot of other problems, though. I suspect that researchers will keep plugging away at the problems that mechanical mono-culture farming creates, and maybe in another hundred years they will be approaching a level of sophistication that will allow the system to be called "sustainable."
Maybe I am wrong, though. Perhaps I overestimate how happy people are with their modern lifestyles. Maybe if there was a viable way to return to an agrarian lifestyle while maintaining some of the refinements in medicine and technology that people have become so accustomed too, they would be willing to return to a simpler life.