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I guess another way to ask the same question: Can compost be used as soil?
The question comes up as I have just removed a tree stump and would like to plant grass instead. So right now there is a hole I need to fill somehow. I have lots of ready compost.
So can I use the compost I have to fill the hole and then plant grass, and not run into the issue that the compost decompose over time and the grass will sink?
Your real problem will be that the compost isn’t the same as plain garden soil. A well-made compost is a nutrient-dense mix that can actually “burn” the roots of your grass, especially if you are using seeds instead of laying sod. You should always regard your compost more like a fertilizer than a kind of soil. Compost is usually applied in a thin layer or as a mix-in and not pure.
And yes, compost will sink, but the same is true for regular soil and large holes. It’s based on physics more than on organic processes. How much difference there is to be expected between soil and compost depends on the composition and particles of your compost as well, it’s safe to assume that compost will continue to decompose, i.e. larger pieces get smaller, for quite a long time.
Organic matter will surely decompose faster than mineral matter. Not only that bigger pieces will become smaller pieces, but the chemistry (chemical composition) will change too.
About the 100%, that is ambiguous. If you mean "what will happen in a few thousands of years", then 100% decomposition is highly possible. In the course of a few years, 100% is quite impossible.
So can I use the compost ... and not run into the issue that the compost decompose over time and the grass will sink?
Sinking is very likely. Compost is a quite fluffy matter. Even if you compact it, it will still be spongy. In time, the sponginess will go away, and the sinking will be obvious.
Sinking will have 2 big reasons:
As @Stephie mentioned previously, the "pure" compost has a high chance to actually chemically burn any living plant you might want grown there.
Maybe use some sand in the hole first, tamp it down good, top dress with some compost which you can mix with some regular top soil. You don’t need much for grass. Plugging grass will be a faster cover up. Remove large compost material by sifting through a grate. I use my large yard wagon and rake it back and forth, so mainly it’s just “fluffy dirt”. Just fill most of the hole, 2/3 maybe with heavy organic material. Sand fits the bill. Heavy. Compacts well, drains good. It’ll be fine for grass and sand drains well.water in your filled hole good, let the soil settle, add more dirt as needed and repeat. You can layer sand and compost too. Be sure to tamp it down, water it in to create it to settle, repeat as needed. Tree limbs and large materials take a very long time to completely break down and will sink a lot if you use a lot of it. Grass Clippings break down fast and will create a sink area. Fully composted material is best. Top soil and sand are relatively cheap, especially if the hole isn’t really big. Mulch the tree stump into saw dust for the compost heap. I chop up large twigs, limbs, and put some compost in my bins with them but it’s my “ take a long time” bin. Not my main. I usually just chunk big items at the edge of woods and let it rot.
