>> They tell us that mixing clay and sand makes concrete.
Stone, that is what almost everyone says, but I think it is too simplistic and puts the emphasis in the wrong place. This is just my opinion, I don't even claim that I read this "somewhere on the Internet".
"Adding sand to clay makes concrete"
First, reword it: "That clay is already like concrete. Adding sand makes it concrete with fine aggregate. Neither better nor worse for growing plants." And the problem is not with the sand, it is with the clay that was the starting point.
If we made that detailed enough to be more accurate, it would be:
Clay without sufficient compost is hopelessly dense.
It slumps and flows and elluviates / illuviates to fill any air channels.
It also turns rock-hard and shrinks when dry.
Adding sand or grit to clay without sufficient compost doesn't help much.
Once clay DOES have enough compost added to be half-decent, or even almost-enough compost to be half-decent, adding a lot of coarse sand and medium grit DOES improve it enough to be pretty-decent.
First and foremost, pure clay or very-heavy-clay-soil needs LOTS of compost added, like 30-50% compost.
If you only added 20-30% compost, then adding sand and grit DOES help a lot.
I've had that change make my "clay-compost-pudding" gritty enough and clod-forming-enough to let me fluff it and then compact it until it "crunches", leaving lots of air spaces between gritty clods. Those might slump and collapse over 12-18 months of rain, but I can get one season of pretty well-aerated soil until I have to add more compost and "fluff" it again. After a few years of growing in it and adding compost every year, roots seems to provide the rest of the needed soil structure.
Looking at the soil triangle, it's clear that any soil composition that has too much clay is improved by adding sand or silt, unless the clay is such a high % to start that adding insufficient sand or silt fails to move it OUT of the "clay" region.
I think THAT is the truth underlying the misleading claim that sand only tuns clay to concrete.
The clay was ALREADY like mortar.
Adding INSUFFICIENT compost and sand is insufficient to make the clay usable.
You HAVE TO add ENOUGH compost! That's the only way to change "unusable" to "usable".
Once you've done that much, then it also helps some to add more sand.
If your amendments don't improve the clay ENOUGH to make it loamy instead of clayey, then the mortar is still mortar. You can add sand, aggregate, grit, bark, or even rebar, but they don't convert the mortar into loam.
"Sand" is irrelevant until you add enough compost (or maybe silt) to amend the clay ("mortar") so it isn't as sticky and free-flowing as pudding or silicon sealant.
But once you add almost enough compost, adding sand and/or grit and/or screened bark becomes relevant. Those can improve "heavy clayey soil" or "very-heavy-clay-loam" to be sandy-clay-loam or even loam.
In my opinion. And I've seen this specific scenario often in my own yard.
Very heavy clay + not enough compost = really lame, marginal soil like runny pudding when wet and mortar when dry.
Add 15-20% coarse sand and grit (crushed rock) to that pudding, and suddenly you can make it sit up, clump and drain.
