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What is the harm of mixing common natural materials (wood, vines, roots, rocks) in with soil for landscaping?

On first thought, it seems compressible objects like wood will compress (and decompose) to maybe half of their volume in the long run, and low-density objects might wiggle their way up to the surface. Is there some other bigger effect (e.g., grass on the top will not keep sufficient water)?

To be more specific, I need to refill some dips which have formed on a hill due to erosion (for which water drainage has just now been improved, so assume that this erosion problem is now gone). The natural solution is to fill the dips with soil and plant grass on the top. But, I have lots of yard waste which I could use like a landfill, so I was thinking to mix the soil with tree branches/leaves (maybe even a ground-up tree stump) and compress it by repeatedly walking over it and watering it. That mixture might be 30 inches deep and, after all of my compression steps, I will add a top 6-inch layer of pure soil (and plant grass seed in the end). So, can I then just plan for a compression of roughly 10 inches over the next decade with no other side effects?

My hope is that I just need enough soil in the mixture to fill in all of the air gaps. It even seems like I could use an extreme mixture with almost no soil (causing maybe 40% air in the mixture) and then simply design for more compression - Are there really no other side effects in this extreme airy case?

In other words, why go buy a ton of soil when I already have a ton of yard waste?

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    Why go buy a ton of soil? Decay...and then settlement. – Lee Sam Sep 6 '20 at 5:46
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Just do it. You may find you need to adjust later, but then there's the question of "how flat does it really need to be, anyway."

If looking for information on doing this sort of thing intentionally, "hugelkultur" is a good search term.

Note that if termites are an issue where you live, buried wood is attractive habitat and may increase the population near your house, which might be a downside.

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The plant matter will decay to almost nothing. it's hard to predict how long that will take.

Fill the hole with concrete rubble , gravel, sand, or similar dry fill whatever's cheapest.

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  • Doesn't a ton of plant matter decay to about the same volume as a ton of soil? It still seems wasteful to go out and buy something. – bobuhito Sep 6 '20 at 11:22

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