Fertility.

[jasonwright4838]

I have tried leaving a field fallow, as well as using the perk for sheep, with 40+ sheep on a fallow field as pasture for a few years. Nothing raises a field above its starting fertility value. If this is not a bug, I feel it needs to change.
I can understand leaving it fallow not bringing it up above base value, but spending the point and having the sheep fertilize it should improve it above starting value at least. Not being able to make improvements to the land is disappointing, and frustrating if your starting layout is bad. It forces the player to reroll a map until they get good fertility, which is immersion breaking.

[thmathgeek21]

I think it is designed in such a way as this. In each region you have two rich deposits. If you only have one, that means you have good fertility. Having poor fertility is normal for a region, and you can quite easily work around it. I'm currently in year 4 of a game and I still haven't started farming (because fertility is so poor).

If you aren't farming you can enact the development points that boost meat production, and the policy that doubles the breeding of deer. Clothing and food you can work around easily as there are plenty of other options. The only bottleneck at this point seems like the barley production chain into ale, so for now I just import a few a month just so I can get to level 3 housing.