I had problems with this as well. You HAVE to have a building touching on the horizontal or vertical of the fertile land to build the farm (or it seems a mine, etc.) unlike previous builds. Personally, I am not a fan of this. I think you should be able to build these improvements, however, they can be razed by a monster or enemy unit that stumbles across is unless there it touches the city, in which case it is in the enclosed barricade wall.
