I just stumbled onto the website of the Expanded Shale, Clay and Slate Institute. I assume that "Turface" is one example of the kinds of materials they promote, "lightweight aggregate". In their words:
"Produced by firing shale, clay, or slate in a rotary kiln at temperatures in excess of 2000°F, this fully calcined, ceramic material ..."
They talk mainly about large landscaping projects, like lawns, golf courses, ballfields, roof gardens, and elevated parks or plazas. I found a map of producers, but not distributers (or distributers of small quantities).
This will take me many paragraphs to get the whole idea out, so please bear with me. First there are some analogies, modeling soil as a bucket of balls, or room full of tangled ladders. Then there a Science-Fiction-like "why not?" section.
The point will eventually be: "I wish that when they made fired clay or shale products (like Turface, Perlite or rock wool), they would also make some shapes other than balls or pellets or simple straight limp fibers. Why can't they make tiny pretzles, I-beams, short tubes, irregular zig-zags or tight spirals?"
I've been thinking about how large particles create pores and channels, or fail to. The big pieces must "support" each other by resting atop each other. If they are regular in shape, they can probably pack pretty densely.
Balls (spheres) will always pack pretty densely - there is no "interlocking" or "catching" or stacking into an interlocked, more-open network.. So the gaps are minimal.
In soil or container mixes or raised bed mixes, there is a range of sizes and shapes of solid soil components.
Now I'm approaching the point of this post - any soil mix with a goodly % of particles in several size ranges must be like "basketballs mixed with sand" - the fine stuff inserts itself into the gaps between the big pieces, so that the big pieces may no longer even rest upon each other, but instead rest on some small pieces and some big pieces. It can all pack together with almost no void space
In any event, anyrange of sizes of round soil particles destroys the "openness" of the soil. Less air and less aeration and less drainage. Even if you "fluffed it up", watering and vibration and compression would pack it back down together, because the mostly-round particles can roll over and around each other pretty freely. They don't lock into a more-open mesh.
As Al might say, "the small particles fill the pores" between large particles.
Hence he advises against adding more than 15% fines (1 part in 7) to container soil.
Fines would fill the few, small gaps provided by 1/16th inch to 1/8th inch bark chunks.
Since bark chunks are roughly spherical or cubical, they pack so closely all by themselves that there is little pore space avaible to "soak up" any fines.
Do I have that roughly correct?
When the only pores are the gaps between roundish, packed particles like ball bearings or bark chunks (or sand or silt or clay), these gaps are tiny, twisty, and perhaps not connected with each other.
Too bad we can't buy Perlite or Turface, shaped like pretzels, with loops. Better yet, twisty irregular spirals - too irregular to nest together compactly. Or small, twisted I-beams. Or short, small tubes like toilet-paper rolls, but with a diameter of 1/32 inch to 1/8th inch. Or "foamy rock yarn" composed of some fibers and some foamed rock, maybe 1/32nd inch in diameter, thick enough to have a little stiffness. Then that yarn could be twisted into larger spirals or irregularly-kinked lengths.
(Yarn or spirals would be stiffer than single fibers like rock wool. If you shredded a lot or a little rock wool into soil or even gravel, it would pack tighly against itself and even against gravel, because each fiber is pretty limp, and straight thin strands can pack near 100% density. If it were stiffer, and rasndomly oriented to start with, it could "hold up" a greater weight of soil above it, maintaining void space.)
NOW, when fines were added, it would be like pouring objects into a room filled with a tangled Jungle Gym made of twisted, kinked ladders. The structure might start with 90% open space, and be too interlocked to compact further. It helps if the ladders have hooks or notches on the rails, so that they would ionterlock, instead of sliding around they all lay parallel and flat (densely packed).
Pores and interstices in the foamed rock pieces would be like pierced buckets in the ladder-Jungle-Gym - they would hold some fines, and keep it from clogging the pores lower in the soil.
I think you could even tumble a mix of such ladders and smaller objects, and still wind up with a lot of open space, because the ladders can't be made to compress beyond a certain point, especially if they interlock at all. If the ladders started with 80-90% open space, you could add 65-75% fines without using up too many of the voids and channels. (It would be great to have s oil amendment where adding 10-20% by volume would mechanically force a lot of void space into the soil.)
The only danger would be that the fines could "wash out" of the upper layers and pack the lower layers solid, if the weave were too open. That's why I want to see them in spirals and tubes and twisted I-beams. With rough surfaces, notchs, or small fibers protruding to cause stable interlocking. Maybe something like Velcro (tm) hooks and loops! Short rock wool fibers sticking out of foamed rock, creating furry surfaces that catch and grab.
Fines would be caught and held inside spirals and tubes, in corners and on top of I-beams. Also in the open bubbles on the surface of foamed rock. Between the fibers of rock yarn. Wherever two big pieces touch and create a valley.
In chemical engineering, when you don't want things to pack together in a distilling coumn, you use irregular "saddles" with two curvatures and, ideally, wrinkled edges, so they not only can't nest densely, they "catch at" each other and tend to lock into whatever open distribution they first had when they first settled.
I know that some synthetic fibers like polyester have some twist or "natural kinkiness" from the way they are extruded , I think with two phases of differing composiiton. As the fibre cools, it spontaneously twists and kinks which gives thread and fabric some desirable qualities..
Maybe the same kind of equipment that now extrudes Perlite, Turface or rock wool could be fed in such a way that it puts out thinner, twisty foamed rock pellets (sticks) , or rock wool fibers that combine into some spiraling, open yarn. Or a twisty, kinky I-beam or irregularly-peforated box beam. Even flat, thin saddles would be an improvement, if they could not stack tighly.
Doing it might be science fiction (and/or expensive). But I wonder how well it would work, or what the most-desirable kind of micro-structure would be? For tomatoes in 5-gallon buckets? Small trees in big containers? Seedlings in trays of inserts? Mail-order plants? Orchids?
My most-desired application would be something fairly large and stiff, not at all fragile, where roto-tilling 10-20% by volume into heavy clay soil in a raised bed would provide a "soil support lattice" that would fight compression of the remaining soil, and hold open some structure for many years, while the organic content built up from roots and migrating worms.
I guess there aren't many science-fcition fans in the Container forum!
After thinking about it, fine fibers twisted into threads, and threads twisted into open yarn, stiffened with coarser threads sounded more-nearly practical to manufacture. Like rock wool, but with more structure and more stiffness.
It could in effect "suck" some of the fine clay out of poor soil, and open it up. It ought to be almost as permanent as "Turface" or Perlite as long as you didn't roto-till it vigorously.
I just found "Orchid Bark" and bought 8 quarts for what ought to buy 2-4 cubic feet of pine bark mulch.