Build them on your open spots
FARMs by themselves only have a limited amount of storage capacity. For additional storage capacity of food you need to build a separate new building called FOOD STORAGE in the other open spots.
You can read all about those buildings and more other basic stuff in this wiki which will help you understand the fundamentals of the game better. Happy playing and Good luck
Thank you very much! It seems I can only build food storage in some open slots.
That’s correct. The game only allows a certain number of buildings at any time depending on what your stronghold level is. As well, the maximum level of your buildings can never exceed the level of your stronghold.
The wiki Ronin posted has lots of good information. If you are a new player you might also find great benefit from reading the unofficial game guide at http://tinyurl.com/CopperskyCompendium.