You lost me about 500 words in, but I have ADHD so take that with a grain of salt.

I don't know nuttin about birthin' no lawns neither. If lower organic matter is better, then so be it. However, from my perspective as a non-lawn-expert, it seems to be conventional wisdom that adding compost to (most) lawns is a good thing. I would certainly top dress mine if I had a good way to spread the stuff over a large area.

We compost nerds mostly use it for vegetable and flower beds which is a different thing.

"Isn't organic matter like %90 just a fertilizer source?"

Not at all!! In fact, compost and most other organic matter carries a pretty low nutrient load so a poor substitution for any commercial fertilizer.

The primary benefit of organic matter is its ability to encourage and nourish large populations of soil organisms. And it is the activities of these organisms that actually provide benefit to any garden soil used for growing any plant type. And that includes lawns as well! They help to create cation exchange sites, improving the soil's ability to reduce nutrient losses through leaching; they help to flocculate soils or encourage aggregation, which results in better water holding capacity in sandy soils and improved soil drainage in clay soils
by promoting larger sized pores. Improved soil structure and drainage will also decrease the erosion potential of the precious top soil by reducing
the runoff.

Higher organic matter levels also tend to favor soil temperatures that promote improved plant root growth,
healthy microbial populations and moderate pH ranges.
Soil microorganisms decompose organic material to obtain energy which leads to the formation of soil organic
matter. Not all organic matter disappears over time - some of it (SOM) is very stable and remains in the soil indefinitely. Microbial process, population size, diversity and health are dependent upon maintaining healthy levels
of soil organic matter. The sequestration of soil carbon that results from increasing soil organic matter levels can
also help in the reduction of greenhouse gases released to the atmosphere.

Have a look at this:

https://s3.wp.wsu.edu/uploads/sites/403/2015/03/soil-amendments-3.pdf

Organic fertilizers notwithstanding--which actually can be a pretty potent fertilizer in their own right, with soybean meal and cottonseed meal running around 7-1-2 or so in terms of equivalent fertilizer load and things like dried blood being 12-0-0 and fast enough to burn--soil OM is actually something completely different.

By the time it gets to that point, we're mostly talking about late-stage stuff that's been through the wringer. And the insect, worm, and/or other insect and creature. That kind of thing. Plus the bacteria. Lots of times.

It's great for holding water. It has lots of carbon. It hangs around for a long time. But that's about it. In terms of nitrogen, not so much. It's really tapped.

You usually can't tell by looking. I've seen soils that looked dead that were really quite nice. And seen soils that looked nice that had low OM. Only your soil testing lab can tell you for sure.

As for some of the claims you'll find on organic sales sites for organic products as per their abilities to stabilize everything under the sun...well, not so much. Peat bogs, for example, are gorgeously organic. Nearly a hundred percent. And so acidic you can preserve bodies in 'em for centuries. Organics tend to hang around best where bacteria don't flourish--anaerobic, highly acidic, highly compacted, highly awful soils. Aerobic, warm, sandy, neutral soils will tend to be low-OM and are destined to stay that way barring being a host to a nice tropical rain forest.

Don't spend on anything you can't get at the farmer's co-op, grain mill, or bulk aisle of the grocery store, in other words.

Still, it's nice to add what OM you can. Any soil can be made nicer by adding as much OM as you can manage. It won't really stabilize your pH in any realistic way for any realistic amount you can add in any realistic time period as a realistic person, but it'll tend to make your pH matter a tiny little bit less, chelate some resources for you, hold water, and seriously improve your lawn and/or gardens. It certainly worked for me.

It's nearly certain you have some worms. Even I did in a nasty tapped soil that used to be corn field with a pH of 4.5. If you feed them, they will....well, hang around, have families, and reproduce. Add plants and those plants probably have some baby worms now and again, or eggs. Which will add to the mix.

So mulch mow, mow in those leaves, and feed organically if you can.

https://thenutrientcompany.com/blogs/horticulture/npk-value-of-everything-organic-database

That's a great list of what has what. There are considerations; alfalfa is great, but really shouldn't be put down at over 20 pounds per thousand per year, for instance. Corn is wonderful but...at that nitrogen level? Enjoy hauling tons. I use soybean. At around 15 pounds per thousand square feet, it provides a very thorough feeding done four times yearly on an extremely demanding elite Kentucky Bluegrass lawn.

Most of the very high-nitrogen stuff has the least residue in terms of what's leftover in the soil afterward. Soybean is OK. Corn is, of course, much better. But again, enjoy hauling tons. Blood meal won't leave behind much at all, if anything.

If that's too much effort or simply annoying or you Just Don't Want To, a good synthetic is just fine and has much to recommend it. :-)

One comment: mostly it's not effective to purchase worms for your yard. If conditions aren't right for them, they won't magically multiply, and if conditions are right for them, they will. And usually there are some even if you're not seeing many. Mulch mow as suggested and the worms will have something to munch on.

@gardengal: You had me up until the part about sequestering greenhouse gases. Climate change is a political movement backed by junk science. Too many unsuspecting people don't understand that and get taken in, unfortunately.

Just my 2 cents...

"If that is your belief then your 2 cents isn't really worth anything..........."

My belief is a conclusion I've come to after studying the actual science since 1998, and is NOT based on what the scientists and politicians SAY about that science. What is your belief based on? Obviously this isn't the place to hash this out. My point is that there is more than one view of the necessity to sequester carbon.

thanks for the replies!

whether or not Organic Matter reduces greenhouse gasses sort of like trees do I guess, well, I sure don't know, but not as if I'd make that a deciding factor to add compost or not.

I'm still confused about if tilling in compost is sort of pointless, I will re read the replies and links, and sorry if my OP reads like a riddle.

Barplants, I was under impression you have already done the main work and tilling. Is that correct? If so, not worth it to do over. Or is this for future projects?

It's not worth tilling in compost as nature does not generally do it en masse with a big tiller and any future decay is simply going to tap soil nitrogen--so you better be durned sure that said compost is extremely well-decayed. Peat moss? Sure. Most commercial composts? Well...maybe not. Your own, aged two years? Again, sure.

If you want to start putting it up top, a quarter inch at a time, that's cool. Feed the worms you have, let 'em reproduce, and let them carry it down over time, eat it, and move the now extremely late-stage matter into the soil a bit at a time the way nature does do it.

Since I'm up early, a second thought. Tilling in, as it does decay (and it slowly will, regardless), is also a recipe for an uneven soil over the years. Not to mention that the tilling is probably going to be a bit uneven in and of itself.

That's why we don't recommend doing it that way. Applied up top, that doesn't happen. The worms are much more careful. :-)

Besides, we humans tend to be hasty creatures, and apply far more than the five or so percent of soil matter that the soil normally has. Granted, in my gardens, that's more like twelve percent, but four inches of bark mulch per year for a couple decades starts to add up.

Still, up top additions are definitely the way to go to be sure of complete decay and feeding everybody all the way down.

I personally agree that for a lawn tilling in compost doesn't make sense. And probably not tilling at all. (Maybe there are some circumstances where it would but generally not if soil is 'ideal' as was described in original post). Just trying to understand what current situation is, has tilling already been done? And agree also important to keep in mind that soil will settle - probably unevenly - after tilling even if no compost is added.

yes it's already tilled and yes tilling can easily mess up the grading which is usually ok and not too bumpy when mowing. I was kind of just wondering this topic but yes could still till in compost no problem other than was thinking it might be a waste of money.

Tilling elevates compaction much better and deeper than core aerating, and since there was no lawn to keep here I tilled it. But then grade it with a 36" landscape rake and lawn roller it because it'll be almost a swamp if not rolled not to mention rolling can show small uneven spots. Aerating would give decent compaction relief and could have done like 10 passes on it but it just doesn't get as deep down as tilling. Tilling DOES destroy soil microbes, with a garden it's best to just TURN the soil each year but since this soil hadn't been aerated or anything for probably decades I went the extra step to till it.

It sounds to me like you have a fair amount of experience and know how you want to proceed. Overall, I'd just add mulched leaves and grass as you plan to do and see/adjust as you go forward. Location and type of soil (warm/sandy for example) affect how much organic matter is retained in soil, as well. There are also practical considerations, like whether it makes sense to re-till after you've already done most of it, and how much 'compost' you have access to and its condition (how well aged, etc). If it's a large area, and you're tilling to ten inches, it'd actually have to be a large amount to make much of a difference. (Obviously your existing soil has at least some organic content - roots, stuff you've tilled in, etc., too, just not clear how much).

Probably my only specific comment is you mentioned aeration a lot, but not clear that's a big issue in sandy soil. Anyway, you've done it, and I'm not arguing with your reasons, specific/local conditions matter.

If you're feeling scientific, it might be interesting to till in compost to a smaller plot and see how that goes, just for comparison.

A few random thoughts in no particular order:

-I put compost in scare quotes because it really does depend. Compost from fibrous/woody materials will generally leave more stuff behind, "finished" or stable compost will reduce less in volume, and stuff that is 'raw' will settle and reduce in volume a lot more. Woody/fibrous inputs will (at least temporarily) tend to 'rob' nitrogen for the breakdown process, but also don't break down fully and can even become semi-permanent.

-You say you'll add milorganite, that's a type of compost-like organic material, but I have no experience with how it degrades over time in soil. I'm guessing it mostly disappears over time.

-A lot of the benefits of compost are for the structure of the soil and ability to support soil life (fungus, microbes, and yes, worms, etc). Also the partially-digested stuff and fungus hyphae and the like make the soil sticky and yet with air in them. Tilth and all that.

-Does the compost itself have a lot of nutrients? Not its most important role although mostly doesn't hurt. Plays a bigger role in storing/keeping/sharing some of the nutrients (fungi moving them around, etc).

-The roots of plants also do some aeration and affect drainage as they sprout and grow and die off etc., too.

-I think your points on worms are mostly accurate, some types of worms will do their own aeration, eat plants, bury some, etc. There are probably some exceptions, but mostly I've heard people say that adding / introducing worms usually not very effective, it's more a question of conditions for them to thrive than adding them. If you have some, can't hurt to spread them around, but just probably not worth spending money on them.

-Side note, compost worms - ones that live mostly on surface or close to it, or in piles of decaying matter - don't tend to dig down deep in soil, but it's amazing how they'll just show up. No need to go out of your way to introduce them, but if you spread some compost with compost worms in it, they won't hurt.

Anyway, let us know how it goes, always interesting to hear. I've written too much when all I wanted to say was 'mostly, spreading compost on top won't hurt.' In fact spreading uncomposted organic material on top - within reason, ie in somewhat thin layers - as mulch or whatever - mostly won't hurt either.

bar, earthworms don't generally feed on tender, living plant roots. They want the decaying stuff, the leaf litter, organic debris, and grass clippings that feed the soil microorganisms as well as the worms.

Are you an experienced expert in soil science and how to judge the physical and chemical properties of a soil sample by its appearance? If your soil is "light brown", it may have more OM than you think.

all I wanted to say was 'mostly, spreading compost on top won't hurt.' In fact spreading uncomposted organic material on top - within reason, ie in somewhat thin layers - as mulch or whatever - mostly won't hurt either.

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bar, earthworms don't generally feed on tender, living plant roots. They want the decaying stuff, the leaf litter, organic debris, and grass clippings that feed the soil microorganisms as well as the worms.

Are you an experienced expert in soil science and how to judge the physical and chemical properties of a soil sample by its appearance? If your soil is "light brown", it may have more OM than you think.

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true, light brown means lesser OM than dark soil but OM should only be about %5 of the soil. Too much compost/OM is actually not good for a lawn I've learned, it's also not recommended to start seed in pure compost i.e topdressing the lawn with compost and then seeding, although it should be fine but is said it holds too much moisture (or some say it doesn't hold moisture enough and needs clay/silt), could have too high of a salt mineral content. Then of course there is compost that isn't quite finished etc, there's a lot of different stages of compost or sources ie some animal products like manure, feather, blood, etc might be too high in nitrogen and kill plants, some might have an off pH at its stage.

A lot of this info comes up if googling stuff like "can you grow in pure compost".

To summarize though yes as also mentioned here, tilling in not-too-much compost will have some benefits in terms of thinks like microbes etc but I don't think it'll do much more than that as long as the soil composition is already a good sandy loam (for grass).

I say, if a lawn is being renovated, as long as there was some previous signs of life/plants, it should be fine to omit tilling in compost if the soil composition is good (full shade and no previous plants is understandable though) - but if it's strangely barren and nothing had been growing not even grassy weeds in full sun, then a lab soil test is warranted (not just NPK and pH test) and could be something wrong with the soil which compost might help, but in a more extreme case like that, I don't think compost will fully solve it (something like high salt levels etc [can add more examples if you want]), but this is usually rare thankfully, I've basically never added compost before and all lawns come in thick - if it dies it's usually much later the next summer died from brown patch or not enough watering etc - thing to be expected with low-ranking seed cultivars or no irrigation.

EDIT: yes even though a sandy loam is airy, I would still core aerate it to alleviate lawn compaction, even yearly for best results just for the sake of letting air water and nutrients down deeper to the roots and not counting how rhizomatous lawn species (kentucky blue, zoysia in the NorthEast) could benefit from coring to reduce excess thatch.