I don’t mean for large agricultural farming, I just mean places that would work best for subsistance farming with minimal technology.
What areas in the U.S. have the most fertile soil for this kind of farming?
Eastern North Dakota to NW Minnesota and south to about Kansas and Missouri.
Pretty damn fertile.
You can grow quite a bit of stuff in Pike County, KY.
California’s San Joaquin Valley has been called “the world’s richest agricultural valley.” From a 1999 report to Gov. Gray Davis:
Market Value of Agricultural Products Sold (in $1,000):
Texas: $13,766,527
Iowa: $11,947,894
*San Joaquin Valley*: $11,645,271
This richness is due at least partially to the building of irrigation projects, so that doesn’t directly answer the OP’s question. However, one may guess that the soil was probaly pretty fertile already when people started farming it way back when.
The Connecticut River Valley has wonderful soil. A lot of New England got scraped down to bedrock by the glaciers, but the Connecticut River Valley was essentially Lake Hitchcock until the last Ice Age which resulted in deep, essentially rock free soil. Regrettably, it’s getting very built up and a lot of the land is being converted to housing.
Not to far away, in Monterey County, you can also find the Salinas Valley, aka The Salad Bowl of the World. My extended family has farmed there for a few generations.
Valleys often include what used to be riverbed or other drainage, and such soil can be extremely rich, as nutrients and sediment are washed in and deposited from surrounding areas and higher elevations. This accumilates on the valley floor over time, as the river changes course and snakes back and forth through the years.
The Imperial Valley in SE California is also quite fertile plus the growing season is nearly year-round (actually gets too hot in summer for some crops to grow), but the San Joaquin is probably the best overall. The growing season there is almost as long…
Starting in the Red River valley of North and South Dakota, picture a crescent going through eastern Nebraska and Kansas, includes southern Minnesota and Wisconsin, and all of Iowa, down through the northern half of Missouri, Illinois, Indiana and Ohio and winds up in upstate New York.
That’s the best blend of soil, climate and rainfall you’ll find in the U.S. Other areas may have one or two of those characteristics, but not all three.
The Willamette valley in Oregon beats the corn belt in 3 of the 3 categories kunilou posted. Methinks some local boosterism is going on here.
For the record: much milder climate, more reliable rainfall and the soil is rich from volcanic ash.
The region centered around Iowa does have nicer soil than most places (but not all), but that’s about it.
Places with fertile soil and other good farming properties are usually dense with “truck farms”. People can eke out a living on a small patch of land. Lot’s of truck farms in the Western valleys, not too many in Iowa. In the corn belt, it’s hard to make a living even on 160 acres.
Oh come now. I gave autz 12 states. How local is that?
I’ll concede the climate and soil of the Willamette. In my defense, the couple of farmers I’ve talked to up there are much more concerned about consistent, year-in, year-out average and timely rainfall than any Corn Belt farmers I know. But I’ll grant my sample base for that region is limited.
Contrary to popular belief, there is a lot of fruit/vegetable farming in the Midwest. If that’s a benchmark, I’d suggest the LeSeur valley in Minnesota and a rough circle about 50 miles around Toledo, Ohio.
Don’t have a site because frankly I’m too lazy but I recall that for years and years New Jersey had the most productive farms per acre (beleive it or not for those that assume its nothign but a couple of highways runnig between toxic waste dumps).
I dont know if this makes it the most fertile but it seems like a good arguement to me. I also remember reading that Michigan supplanted NJ in these stakes thus giving it a claim, but my assumption would be that NJ lost its claim because such a large percentage of its best farmland is being plowed under and paved over for more strip malls.
So there’s two more possibilities for you.
The coastal areas of Ventura County, CA have some of the richest farmland in the world, along with a very moderate climate: 55-75F year round, with NO sub-zero temps (well, maybe once per decade) which can destroy delicate crops like citrus trees. Tons of organic fruits & veggies are grown there. It’s the only part of metro L.A. which is still more profitable to farm than to build condos & strip malls on…for now.
It’s all irrigated, so I guess it doesn’t match up to the rainfall criteria – but hey, who needs rain when you can import all your water, eh? 
It’s been about eight months since I’ve read the book but Jared Diamond says in “Guns, Germs, and Steel” that the western coast of North America, from California to Washington, is one of the four most naturally fertile areas in the world due to climate and soil.
Anyone else that might have the book on hand and know which chapter I’m referring to might be able to give you a better answer though.