I use subsoil here as soil, but I amend it or just watch and fertilize as the plants grow, expecting some lower yeilds. I have a healthy crop of hardneck garlic on some I placed last year. I topped it with an inch of garden soil and threw out some organic fertilizer on it from time to time. Other spots I just have perennials and expect to fertilize like they are a bit hydroponic until the fertility naturally picks up. (We tend to have ambitious worms here, and I put raw
wood chips between plants, so it shouldn't take too long) You could do a soil test to see what it's missing (besides structure) and then amend based on that. Our dirt here is heavy clay, so when it doesn't have structure, it dries to a nice cement-like hardness. So, if I want things to grow well, I have to keep it moist (the hardneck garlic didn't have issue with this because they were on melting snow, but the other things that will grow through the summer will need some extra TLC.
As for actual cover crops, it depends on your region, soil texture, soil pH, nutrient content, etc. Cooperative extension in your area might be familiar
enough with the soils in your area to give you a good guess as to what usually works in your area. Otherwise, a lot of people see what seed will likely go bad before they can plant it and throw that out with some mulch and spare compost/fertilize/innoculants. If you are surface broadcasting on bare soil, I highly recommend mulch. Mulch for your wormmies to eat and build structure and to keep the seed from floating away so much.
If you are going to buy seed, see what you can get real cheap. If you want to go with a specialized mix, people usually put a fibers rooter (usu. grain), then a tap rooter (usu. a brassica), then a nitrogen fixer.
Good luck!
