Hi Sue
>> The problem with adding sand is that you have to add it in such large amounts, something like 50:50 sand:clay soil or even higher to actually make a difference.
I agree - if the soil has only a very small amount of organic matter. I think of that as "not enough OM to work", because buying OR wheelbarrowing 50% crushed stone, grit, or very coarse sand is just impossible for me.
If they did a test like "start with very heavy clay soil containing almost no sand or grit or OM", and then added 5% grit/sand, 10% grit/sand, 15% grit/sand and 20% grit/sand, then still found that the soil had worse drainage and aeration with increasing amo8unts of sand, I would be very surprised (based on my observations of MY clay).
I understand why a materials scientist would assume that clay+sand was harder and stiffer than clay - the idea of a plastic matrix making a composite material with solid grains stiffer than the plastic material would have been on its own is a basic assumption and usually true with engineered composite materials. And, probably, if you packed and pounded the sand and grit and clay down as tight as you could, maybe it would make a fine concrete until it got damp and oozed away. (Is that what adobe is?)
And a real soil scientist is probably assuming "soil for field crops", with tractors and cultivators and ten-ton-harvesters rolling over it repeatedly. That situation is different from my situation: raised beds 2-3 feet wide fluffed up like an omelet every few years while I talk to it lovingly to encourage it not to slump back down next year, either.
Clay grains are sticky, and flow together to exclude air. Composite materials engineers don't have that to worry about, but if they thought about it, they would cheer that the material was making itself denser and harder.
What matters most to the soil in my yard is not anything that concrete designers worry about: the material slumping and flowing together: voids filling with clay instead of either air or water. If someone designing a composite material sees voids, he or she promptly changes the processing method to eliminate the voids. Those voids, pores and tortuous crevices are what I am trying to encourage to sustain themselves for more than a few weeks. That's where the snad and grithave been helpful to me.
That said, if someone offered me 6 cubic feet of compost OR 6 cubic feet of crushed stone delivered to the sides of my beds, I would probably take the compost, even if the crushed stone cost 2-3 times as much and weighed ten times as much, and lasted 1,000 times as long.
Because I agree with the most important factor: you need a LOT of compost to redeem clay.
Unfortunately, 6 cubic feet of compost, at 50% compost, would only treat 12 square feet of bed (a 3' x 4' bed) to amend the clay 12" deep with 50% compost. Maybe I should have asked for the grit instead: at 10% grit, I could amend 60 square feet 12" deep with 10% grit. Then I could buy and carry more compost and have a 3'x 20' bed.
>> like stiff chewing gum when wet.
When I first started amending my current yard, I had forgotten the trick about wetting clay before trying to dig it. I was CHIPPING away at the clay and rocks with a big pick! Duhh. Yes, it gets even heavier and sticky when wet, but you don;t have to chip it like flint.
