Africa United

So, I'm not sure if I'm doing something wrong, but there's a problem with one of my cities. It has 10 farms in it and my land area is 2300. if I did the math right(2300/500 * 1.70) I should be receiving 7.82 tons of food per turn. Farms don't need power and I have enough money for the operational costs, but my farms aren't producing any food at all. Why is this happening and how do I fix it?