I built a small fantasy kingdom with a New England-ish climate and it needs to be able to both sustain itself and trade a bit with the neighbors. You don't have to understand farming or economics in detail, but it's good to at least sketch these things out and think about them when you're world-building. It's going to affect what's on the table when people sit down to eat, what they're wearing, which craftsmen/guilds have more clout in society, and plenty of other things.
I'm not an economic expert, so I'm going to do this in very vague terms. I'll try to keep this "short."
Grains: mostly oats, due to climate, but wheat too. Hay.
Fruits, Vegetables: cabbage, potatoes, beets, peas, squash, spinach and lettuce. Rhubarb. Fruit will include apples and maybe pears, maybe cherries... raspberries and blueberries, yes. Concord grapes, or the equivalent.
Livestock: Sheep are a multipurpose critter: milk, meat, and wool. All of them very trade-able. Also, cows for milk, beef, and leather. Pigs, chickens, etc.
|Photo by Melissa R. Addison|
Salt: This can be tough (I recommend Mark Kurlansky's book for reasearch), but vital to the economy. My kingdom does have a northern coastline where one could produce sea salt via either evaporation or freezing. I doubt they'll make enough to export -- might need to import some, in fact.
Cheese: Very consumable and very exportable, both sheep and cow varieties.
Beer: Since I'm fond of beer, I give free rein to all kinds of beer and ale and the seasonal varieties thereof.
Lumber: You need to make beer barrels out of something, after all.
What does all this mean
The productivity of a land speaks directly to its ability to support a population of people who don't have to farm for a living and can instead spend all their time weaving cloth, carving wood, learning to wield a sword or, perhaps, reading books and thinking. Historically speaking, one can make a living off farming in a New England-ish climate. But the fact that all of New England has, since its colony days, had only one major city to speak of (Boston) gives one an idea that it's not that rich of a living.
My little kingdom may be short on the finer things in life, like education or high culture, but they can keep their children fed and warm, and they produce enough to sell raw materials and transportable food to that big empire to their south. Unfortunately, that was also what drew the empire's attention and made them worth trying to conquer. But that's a different story...