Where to build markets - only in fertile lands?

[MCM]

Having 60 food surplus AND functioning samurai army is impossible. I leave about 50% of all castles without upgrades, just the 1 market, sake den or what ever caters to the province specialty. I too have +15 food surplus at around 20 provinces, but after turns pass you capture provinces that have negative food balance and you have to upgrade castles to get experienced/upgraded samurai fast.
My point is, there is a tipping point after certain amount of provinces. After that point, preparing for realm divide and ensuing madness, you lose all your food surplus. I've been to negative food with less than 30 provinces, with less than 4 upgraded markets. I'm not saying that I've perfected this part of the game, I'm just saying that food surplus isn't there at start, and it won't be there at the end.

What does this have to do with upgrading Markets? Upgrading castles is a question of spending money/food on military. Upgrading markets is a question of money/food for money. This sounds like an argument against upgrading Markets, that you think you need to food for Castle upgrades. Fine, that's all the less reason to upgrade Markets. You have better things to use the food and gold on.

But this is not an argument in favor of upgrading Markets. The Global Town Growth factor is the same whether you are at 0 food or 60 food. It is a marginal change every time you decide to upgrade a Market.

Just looking at your example, if you had "less than 4 upgraded markets" and were at -2 food... you would have been at +1 food if you had upgraded ZERO markets!

If I am at TWO FOOD, whether or not I upgrade a Market is the same maths question if I am at two hundred food. It is spending X gold to get Y wealth and lose Z town growth. How much you have had to upgrade your castles is irrelevant.