Soil is generally much worse at altitude, but you can grow a wide range of crops up to ~5,000 feet and some vegetables up to ~7,000 feet.  The lack of water is the largest limiting factor, followed by minimal needs.<p>Anyway, if you go by % of land under cultivation China and Japan are close but Japan has much higher population density which historically was supported largely through fishing where China was much more focused on grazing animals due to geography.