As noted, there are tons of variables ranging from the granularity of the soil, wetness, average temperature, and so on. But as a general rule, about 40-60 pounds per thousand square feet per season seems to be the accepted amount to maintain OM levels. Give or take a lot, of course.

Existing OM moves to later and later stage (providing less and less feeding for any plant or organism) until it finally reaches the stable humate stage. Only a tiny percentage ever gets there, but what does, lasts centuries. Over time, most of the rest is lost as carbon dioxide, or other carbon compounds (and ends up deeper than we need to worry about).

I tilled in 4-6" of compost into my existing soil. After 25 years, based on hardscape added after the lawn was established, my lawn has shrunk (vertically) by 1-2".

Directional response deals with sound?

wow, 40-60# is a good bit. how much of that 40-60# comes from mulch mowing and fall leaves? trying to see how much "other stuff" i should put down through the season. I need to get mine up so that it better holds water.

Picking up a pallet of baystate today, kinda excited for this sh*t (pun intended).

Mulch mowing doesn't seem to count in that, but fall leaves would. And I think that was the target for a pretty highly organic soil already--the more you have, the faster it all burns off under normal circumstances (peat bogs and the like notwithstanding, of course). That number wasn't coming from me, so I'm giving it to you as best as I remember.

Certainly that level of OM addition would keep many pounds of bacteria, fungi, and worms happy per thousand square feet with no problems...

If you keep the microbes fed from the surface they will continually mine the dead root material for nutrition until there is nothing left but the stable product. That remaining stable product will help to hold moisture unless it gets tilled to the surface.

Jim Mat - I don't understand your question, but be that as it may, that's interesting that the 4-6 inches of compost translates to 1-2 inches of volume once mixed with soil. Have you added soil since then? And you based the hardscape height on the original soil with the compost tilled in, correct?

I'm starting to like compost less and less. It's not the fault of the compost, but somewhere someone is giving advice to 1) rototill and 2) to till compost into the soil. I believe that's a 1) mistake and 2) big mistake. Compost is a very expensive way to fertilize your lawn. If you want to use it the way to apply is to the surface, broom it in, and then leave it alone.

dchall, the OP asked for directional responses. My experience with directional response is in sound, microphones have directional response.

When I prepared my back yard for lawn, I followed Sunset magazine recommendations that included adding 4-6" of compost, then tilling that in, seeding, then covering with peat.

A year or two later, hardscape was added, I think level, now 25 years later, the lawn is 1-2" below the hardscape.

I have top dressed the edge with bagged compost and bagged planting mix/soil and my rotary mower picks it up.

I think the OP was asking if soil with compost added sinks, my experience is yes, but it may take 20-25 years.

edit:. The shrinkage is not 1-2" it is .5-1".

Does directional response equal stay on topic?

That 1 - 2 inches below the hardscape is interesting. While I have never lived on a piece of property for 25 years to actually measure it, I always felt that if you put all of the grass clipping back in the yard, that over time the soil level would come up.

This is based on observation of old side walks and the like that have been in place many years. To me it always seemed like the level of the sod got higher on the side walk with time. I have seen old side walks that are a couple of inches below the sod level.

Yeah I have also noted that. Even in not so old neighborhoods. Either ants or water or compaction is causing driveways/sidewalks to sink.... or soil is being added.

I don't think that many people are top dressing (heavily) regularly with soil or compost.... (?)

It's been awhile since I looked it up in the Sunset Western Garden book, but I'm almost certain they recommend against tilling in preparation for a lawn. When I get my books unpacked (recently moved) I'll check it. It would not surprise me that a Sunset magazine article would be different from the book.