Fertility

Fertility is what determines how many non-agricultural workers a specific area of land can support. Fertility is not static and can change due to events in game. It is an abstract concept so it may not necessarily reflect the direct fertility of the soil, but also other factors such as how steep the hills are etc. Anything that could make the land harder to work.

Barren
The land produces no food and attempting to farm it at all is pointless

Extremely Infertile
Farms on extremely infertile land can only produce enough to support the people working it, and even that only at bare survival levels. 1 agricultural worker can support 0 non-agricultural worker

Infertile
Farms on infertile land are capable of producing a slight excess over the requirements of its workers. 10 agricultural workers can support 1 non-agricultural worker.

Average
Farms on average land are capable of producing enough food. 5 agricultural workers can support 1 non-agricultural worker.

Fertile
Farms on fertile land are capable of producing plenty of food over the requirements of its workers. 3 agricultural workers can support 1 non-agricultural worker

Extremely Fertile
Farms on extremely fertile land are capable of producing enormous quantities of food. 1 agricultural worker can support 1 non-agricultural worker.