While searching the Albion forums this morning, I learned that it takes 81 farming plots to keep a territory fed. As best I understand that just means the territory itself, not the buildings within it.
Also, all food has the same effective nutrition/day. So carrot soup is just as good as the top-tier stuff when feeding buildings. The difference is that some food is lighter than others, so you can carry more. And the higher tiers provide various buffs that our characters might want to use. But for the purpose of feeding territories/buildings, we should use whatever is cheapest to produce or lightest to carry in mass quantities (high-end food has a 0.8x weight multiplier and low-end a 1.2x multiplier) depending on the need at the time.
I haven’t witnessed this myself, but it’s possible that buildings and territories have a favorite food which will give a bonus when fed to them.
I placed my first 9-plot farm yesterday. I hope to upgrade my personal island today but I might use that for herbs and try alchemy for a bit.
I hope other guild members are placing their own farming plots and working to produce food.