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I have a large area of miserable clay soil which I'm looking to start rehabilitating. For the past 15 years it's been neglected and had nothing but some unwanted grass. It's very compacted due to driving machinery over it this summer to build a retaining wall.

At the moment I am (slowly) going through and loosening the soil with a combination of shovel and broadfork. At the same time I'm turning in the fall's crop of fallen leaves and apples from our yard to start adding something more than clay.

After it's loosened, I'm thinking of buying night crawlers and adding them to the soil to help continuing to loosen it up. Is it worth it (e.g. will they make a difference)? If so, is there any guidance as to how many worms (it's about 1,000 square feet)?

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    I would recommend dumping a lot of leaves on top of it, to feed and shelter the worms. Nov 15 '21 at 23:40
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There's little point in adding worms to soil which seems to have none, they'll just go elsewhere - the thing to add is as much organic material as possible, so if you can get hold of things like leaf mould, spent mushroom compost, composted manure, composted materials generally, add those as you turn over or break up the compaction, and the worms will come all on their own. It doesn't need to be dug in, it can just be laid on top like a mulch, but if you are digging, then adding as you go works well.

If there genuinely are no worms in the soil as it is, buying some in won't solve the problem because it's inhospitable for them. Breaking up the compaction and amending the soil should make it a place they'd like to be.

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    Absolutely. The "garden" of my first house was solid grey clay that had literally never been dug in 30 years. It stank when you dug into it, because the only things in there were anaerobic bacteria. I double-dug it and mixed in several tonnes of spent mushroom compost, and the worms just arrived. A couple of years later it was beautiful sticky black soil. You do literally need tonnes though.
    – Graham
    Nov 16 '21 at 10:52
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As mentioned in Bamboo's answer, maybe worms would not be useful, but it would probably be worth seeding your added organic material with beneficial mycorrhizae and other microbes. There may also be cover crops that would be useful, certain plants (daikon is sometimes used for this) make a long taproot and can break up clay, and then winterkill (assuming a cold enough winter climate) or be mechanically terminated and then decompose at the surface.

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