Farming itself, is an unsustainable practice. Modern, intensive farming is unsustainable because it completely denudes a given section of land of its nutrients in very short order due to the methods employed and the only reason this denuding is masked is via the annual application of hydrocarbon-derived fertilizers. Smaller scale, low-intensive farming is equally bad, however, because it requires far more land to be turned over to production for a given population size because of it's lower yields. And, in the end, it will denude the land just the same, it just takes longer, that's all. just about every human civilization in the entire history of man has fallen due to the destruction of it's environment from over-farming.
If we really want to be sustainable with the land, we need to reduce our global population down to about 1 billion at the very most. That way, it would not matter if our farming was intensive or non intensive. The rate at which we robbed the land of nutrients would be equally offset by its capacity to heal itself. In the absence of the above, everything else requires massive energy inputs from outside the agricultural/living system to keep it going. Everything else is unsustainable and, in the absence of a reduction in our global population's size and concomitant demands, communism is no more sustainable than capitalism is. It's merely a bit less unsustainable.
Communism, capitalism and a global human population of 7 billion are all children of the industrial age and the industrial age is drawing to a close. Don't misunderstand me, to the extent that this fall from our current industrial perch can be managed at all, communism, or some variant of it, is our only hope of managing that transition. But, make no mistake, the old socialist verities need to adapt to what is to come no less than anything other form of hitherto perpetual-growth based human system of organization.