I think that there is already a city enchantment that gives one grain. 

But I see where you are going - you would like a spell that is extra useful on towns, the way that inspiration is great for conclaves, and the auras for fortresses.

I do not know whether it is possible with the existing engine, but this could work: Preservation: Increases the faction food bonus from this city by 20% per essence.

So when you build a butcher in a town with 3 essence, every city in the faction will get 32 extra food per grain, instead of 20.

Yeah, I do know that.  My primary purpose in suggesting some sort of Essence bonus for Towns and Fortresses is so that it is not an automatic decision to make a 2 or 3 primary essence city into a Conclave, which is pretty much a no-brainer right now.