I dislike using "Fertility" for base food output capacity because of how very many different connotations that word has.
Frankly, if the words need to be changed (they could, I personally don't care) then we should look at the function the numbers have.
What is now "Grain" could be called "Food"
What is now "Food" should be called "Surplus Food" so that we know that the remaining number is the number of citizens the city can support under current food output.
You settle in a place because of the "Food" you upgrade your "Food" to get more "Surplus Food"
Clean, Simple, Understandable.