I know this topic has been discussed before in this forum, but I was casually perusing Southern Living's gardening articles online, and came across one titled "Good Soil is Job #1." It walks you through a southern homeowner's clay soil improvement journey, but IMO, I wouldn't touch this strategy with a 10 ft. pole! Sand & peat tilled into clay beds to improve drainage?? I made the mistake of adding sand (COARSE sand, btw) to my compacted clay - per the advice of various gardening sources in magazines, online, etc - and the resulting aeration/drainage was far worse! I pasted a few paragraphs from the SL article below... soil gurus, why do so many gardening resources talk about adding sand to clay soil? Am I missing something?
(from "Good Soil...")
Topsoil isn't enough by itself. It usually lacks the organic matter that most plants crave. So Chris added sphagnum peat moss--about 1 (4-cubic-foot) bale for every 30 square feet of bed. Composted manure is an acceptable alternative. Some folks substitute pine bark, but I don't like using it up against the house. Wood attracts termites, and I'd just as soon they schedule their picnic at somebody else's place.
After using a hard rake to spread the peat moss, Chris added the final ingredient--sand. He spread 3 to 4 (40-pound) bags on each side of the steps. Though sand is inert, it loosens soil and aids aeration and drainage, as does organic matter. And when dealing with clay soil, good aeration and drainage are just what the gardener ordered.
Now it was time to mix together the topsoil, peat moss, and sand. Chris rented a heavy-duty power tiller for the job. These machines can be a handful to control, but Chris is a big, very macho guy. (Just ask him.) However, smaller tillers, weighing as little as 25 pounds, are readily available and more easily operated.
I've learned that lesson the hard way too (clay+sand=concrete), that's why I don't understand why people keep recommending adding sand to clay... I agree with the organic matter comment - pine bark works very well, I think!
I added sand to heavy clay in one of my beds without problems, but I also added alot of compost at the same time. Maybe that's why it worked, I don't know.
If I remember this correctly, sand comes from the breakdown of rock/minerals. Sand breaks down into silt, which further breaks down into clay. If one's soil is heavy clay, this is because it has broken down too far. The way to return it to balance and bring it more towards a loam is by adding sand. I'm not sure why some people have the concrete problem and others don't. Adding organic matter is important, but won't replace adding sand, though I suspect that rock dusts do work in a similar way to sand, but it's important to make sure your adding the right ones for your soil mineral content.
Somewhere there is a thread on DG which talks about all this in more detail.
From what I remember from college, sand and clay are formed from different types of minerals. Clay is colloidial -- the individual particulate is oval and much tinier than sand particulate. That's why they stick together and form a solid barrier. Sand has edges and corners and is larger and therefore liquid can get through it. I'm sure the aeration thing has a lot to do with proportions, but the solution, yes, yes, yes, is compost. Anyone poured cement? Major ingredient -- sand!
I have heard enough stories about people having good luck amending clay soil with sand to believe that it can work. I also believe that in order for it to work you need to add a LOT of sand...on the order of 40%. That is a lot more than a few bags from the home improvement store.
Me? I am a believer in compost. The soil in my veggie garden turned from "so hard that I could barely break it up with a pick axe" type soil to "dark and crumbly and light enough to dig with my bare hands" type soil in just a few years. Some of this was from the addition of compost, but a lot of it was from just piling shredded leaves, wood mulch and other organic matter and letting it compost in place.