Thank you! Yes in hindsight the food system is a bit too unclear, I should have made some of the tooltips more explicit. I didn't want it to be too clear with visible hard numbers as it wouldn't fit the experience, and forcing the player to guess a bit and then eyeball it, but I've swung a bit too far.
The system is that [SPOILERS then I suppose] each turn, each soldier eats (the food of) one granary, food is produced in variable amount by the farms (a bit over one on average), and each farm eats one as well from what is left. Any soldier or farm that didn't eat has a high risk of dying of starvation. If there is lots of food, some can be consumed to create an extra farm. until max population is reached for the type of region, then extra food can be turned into a bit of extra silver. That means that a low-pop region can't support lots of soldiers, especially if reserves are low. And an army will stunt the population growth of a region, and on a bad harvest will cause a famine and potentially kill many of them.