I have built a town with 70 people and Im going to have it be a mainly agriculture town with massive fields and I have tried several different farms sizes all the way from 22 x 18 to 10 x 6 and at most I have gotten a yield of 14 percent. Im growing spinach on Verdant plains with arid climate. I have an abundance of food but I will need those farms soon. Maybe there is some game mechanic and Im being stupid but I dont know why their yield is so low.