Due to a bunch of construction projects I now have a huge pile of subsoil. I can't get rid of it, and I don't have too many projects where subsoil would come in handy, so I think I will spread it out and plant some sort of cover crop to start turning it into topsoil. What summer cover crops would do this best?
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.
Whenever I'm planting bare subsoil with a cover crop, I try as mixed of a variety as possible. The goal is to get roots into the dirt and biomass to chop & drop — organic material. You never know what's going to grow well on bare dirt, and whatever grows well is going to be your best bet. If you really want to spend time researching, I'd suggest looking into:
- How long will you be growing cover crops? Some like Sweet Clover are biennials, meaning they require two years to produce optimal biomass.
- What's considered invasive in your area? Some like Sweet Clover are invasive to native grasslands in California. So if you live next to a meadow, choose something else.
- What are your winter temperatures like? Are there cover crops that will winter-kill without you needing to chop & drop?
Otherwise, I'd just get a mix of cheap seeds and any inoculants they require. Most nurseries have their own cover crop mixes, as do most online seed vendors.