It's true in a no-true-Scotsman way. There aren't vast tracts of land that are fertile and have been prepared for farming, but are being neglected, because huma

It's true in a no-true-Scotsman way. There aren't vast tracts of land that are fertile and have been prepared for farming, but are being neglected, because human economic activity easily expands to fill the space available. So with the exception of some parks, sports facilities, etc, most fields in the world are being used for farming. On the other hand, there's plenty of land that is nearly suitable for growing sustenance crops but just needs a little preparation. We don't always favour converting that land to agricultural use, e.g. in the Amazon rain forest. However, the claim misses the point for a different reason. Even if all arable land is being farmed, it's very far from being exploited to its maximum capacity. In terms of calories per hectare per year, much of that land could be improved tenfold or more with a switch of crops or farming methods, if the economics demanded it and potatoes became as expensive as steak.