I have found out that most of the "good" soils are about level 45-50. Never bothered to look for any better soil.
Why? Because it doesn't really matter as much as other factors for settling a village - i.e. distance from water, some metals, the type of forest / fields around you, other villages, etc.
What I mean is, you will easily increase your crops quality to the point where it is equal to the soil quality (in about a month or so, depending on the plant growth speed - carrots are really fast, love carrots

). When you reach that point, you will start gaining +quality of the crops at the same speed, until your Farming skill becomes the limit.
So even if you have level 10 soil to plant the crops in, you will still be gaining +quality of your crops at the same speed as if you had i.e. level 50 soil (when you are already at level 50+ crops). So soil doesn't really matter for crops in the long term, just choose your settlement so that it is comfortable. Just imagine... level 1000 carrots... They were and are still somewhere around at world 7
I should note: trees do require more time to grow, so
only if you plan to grow your own trees should you look for high quality soil.
P.S. Just make sure your Farming skill is always higher than the quality of the crops you collect - so that the quality will keep increasing. Of course, just use the best seeds to plant new crops, and cook or eat the "worse" seeds.