>Sorry, but there are millions of acres that cannot be farmed; soil too
rocky, growing season too short, yadda yadda yadda.

It depends on what kind of "farming" you're talking about. I do extensive
research on plants, how they relate to their environment, and their uses for
the other plants and animals within that environment (including humans).
Only a few plant species are amenable to the "traditional" farming system,
but I can name at least 50 edible species that will grow where I live in
Minnesota, and I can name a good portion of those that will grow in
"adverse" conditions such as rocky soils in Minnesota.
Monocultural row-cropping is not the only system of agriculture ever
developed, and not all plants require long or even moderate growing seasons.
But, perhaps "traditional" farming systems (that strip the soil of it's
cover, exposing well-spaced and usually exotic plants) coupled with such
"adverse" conditions would make agriculture in those regions difficult
indeed.
As a horticulturalist and ethnobotanist by trade and passion, I would have
to agree with the statement that "There are *very* few landscapes not
suitable to plant foods."