What clay needs most is huge amounts of compost, like 50-50 clay and compost, mixed well.
You also have to KEEP feeding several more inches of compost every year to keep it from reverting back to clay. As the compost decomposes, the soil will subside and be tempted to go back to being soupy, pudding-like clay.
But who can afford an 8" layer of compost over their entire garden?
Plus another 2-4" every year?
My theory is that clay-plus-not-ENOUGH-compost can be greatly imp[roved by a little grit, coarse sand, bark chips, and fiber. They help the heavy soil stay "lofted" after some tilling every year or two.
I like to add coarse grit and fibers as well as compost, to help give some "structure" and help roots and worms prevent the clay from reverting. Grit can be crushed stone or very coarse sand , or coarse Perlite if you're rich. For "grit and fibers", I like shredded bark, for example ground-up bark mulch or nuggets. Coir would probably also be good.
Many people give clay-plus-sand thumbs-down. They say it makes concrete. Well, that's true! The problem is that clay needs COMPOST and lots of it. Without that, clay is hopeless. But if you can mix in SOME compost, ALSO adding some sand, fibers and grit help to keep the clay-and-compost from becoming pudding again.
I have another theory that fine roots growing deeply into this partly-amended clay help a lot in maintaining an open structure. Maybe, once you get enough openness and air into your clay, fungi grow mycelia which might also help to stabilize the clay and prevent it from slumping and filling all the tiny air channels.
I don't think that soft, gooey, flowing clay soil is suitable for no-till practices. You have to amend that clay and develop enough soil life and root penetration to support the clay and keep it open and aerated for "no-till" to work. (My opinion, anyway.)
While I am improving a bed over the first 2-4 years, I "fluff it up" once or twice the first year, once the second year, then again in a year or two more, depending on how many bags of compost I bought and spread over those years. Kind of like a very deep and through broad-forking, but usually with some mixing to get another inch or so of compost down into the root zone.
After "fluffing it up" like a souffle, to get as much open space and air as I can into the soil, I firm the surface back down to try to reduce the amount of slumping and "elluviation" that replaces the deeper air channels with anaerobic clay.
All that is just my quirky way of dealing with clay. I don't think it is widely believed to be effective. In fact, "sand-plus-clay" is heresy to many. Frequent deep tilling is considered barbaric by many, and destructive to soil structure by most. I just thnik that they are desperate measures designed to mitigate the effect of only buying 1/2 or 1/3 as much compost as the clay really needs, to turn into soil rapidly.
(Deep-rooted cover crops are probably the long-term inexpensive solution, if the roots go deep and leave lots of organic matter down there.)
Shredded bark fibers and small bark nuggets of all sizes below 0.2 inches would be better than wood chips or sawdust, for mixing into clay. Bark lasts longer, doesn't encourage nasty fungi, and doesn't cause as much nitrogen deficit (if any). I like pine, fir or hemlock bark!
"Heat-treated bark" sounds good to me, I just haven't ever seen it at the cheap places I shop.
Wood chips are great as top-dressing mulch. It will still break down over several years and release humus and soluble humus breakdown products to contribute organic matter to the bed.
I agree about raising a layer of amended soil ABOVE the clay. That way, you can garden successfully the first year, if you can afford enough soil. Water and soluble organics will leach down out of the raised bed, into the deeper clay layers, and soften the clay and attract worms. Over years, the root zone will grow deeper and deeper.
My subsoil is sub-clay, and SO impervious that I can't dig planting holes and amend them. If I do, they fill with water and make pure, anaerobic clay mud slurry. They take 2-3 days to drain, which means the roots have been drowned for almost that long.
I have to dig drainage trenches (slit trenches) to allow the "floor" of any below-grade hole to drain down to some lower spot in my yard, if I wnat to grow anything other than anaerobic batceria in those holes.
But if your subsoil drains ("perks") in an hour or so, you can probably do it in your yard.
