Many kinds of maples are adapted to heavy soils and are planted in them by the thousands, as along streets for instance. It is the Japanese maple that is unusually touchy about drainage. If this planting site is big enough your best bet would be to excavate the clay soil and replace with lighter soil. If it is a narrow strip and the area becomes quite wet even that may not work, water coming in from the sides. Planting a tree or shrub over loosened soil is not a good idea because as the soils settles after planting the new specimen may sink to a level below the desired depth. Digging organic matter into a heavy soil will not have a lasting effect on the texture of the soil as the organic matter will gradually decompose and disappear. Even when non-decomposing amendments such as sand are used to alter the texture of backfill if the planting area is a small one there will still be the problem of how water moves into and out of the planting area being affected. Small areas of one soil texture surrounded by a much larger area of another are often a setup for problems. A large bed of soil amended with sand or similar material will also pose the problem of what to do when the dug over area rises up like a loaf of bread after the amendments are dug in - unless a soil already has a fairly high sand content a huge amount must be used to alter the texture of the finished bed significantly. If you can't excavate and replace a wide area of soil there or put a layer of more suitable soil over the top then it many just be that you can't safely plant Japanese maples in that particular spot. However, since you are in a fairly mild climate zone possibly it would be acceptable to plant them in suitably large tubs and place these on top of crushed rock or other covering in the same location.

Don't dispair Mari! Any well planned soil building is ABSOLUTELY WORTH THE EFFORT! You sure can use those materials to add organic matter to your soil. Sounds like you're considering bulding a compost pile: Great idea. I'd suggest chipping those materials you mentioned so that they "cook" faster. If you don't have a chipper, you can use a lawn mower on those leaves. And, really, for all things composting I'd recommend the composting forum. On a practical note, most gardeners cannot produce compost on a scale appropriate for soil conditioning--mostly on account of our joyful exuberance in hacking out new beds and ending up with many hundreds of square feet to care for. I feel your pain about the shopping spree, but maybe you can consider a small budget for this project during the next few years as a smart investment. Here's a link to a compost calculator, and just for comparison, I pick up compost in a truck by the cubic yard and pay $23-25/cu.yd. If that's not an option, you can get a good start with what you have and keep an eye out for good sources of free compost goodies! -Hannah Here is a link that might be useful: Compost calculator

Research suggests that shrubs adjust best if not planted in amended soil, that they should be just in native soil since then there aren't issues with texture changes causing problems with water movement in the soil. You can top dress or amend the entire planting bed if it seems needed, but just amending the planting hole isn't ideal. So I wouldn't worry about what the lady at the nursery said. Here are directions for planting trees, but the same things apply to shrubs.

"Clay has even smaller particles. I wonder if is even possible to amend clay soil in a container enough to keep the perched water at a low level." You can, but you seriously have to shift that soil across the triangle. If, for example, you're working with 100% clay, you'd be replacing AT LEAST half the soil. And that would be the minimum. You'd have to get it to the point where channels exist no matter how the clay tries to re-form. I'm not sure I'd even use 20% peat. When wet, it's awful stuff. When dry, it refuses to get wet. A little bit, sure. Five percent as stable OM, great. And keep in mind, it'll rot away over time in the container, so for longer-term mixes, you'll eventually have to add soil atop the pot later. Sand and silt would be the stable fractions that will hang around (pebbles can also be used, but those are gigantic and effectively hold no resources; they have an exchange capacity very near 0). Obviously, it does work--there are plenty of soils in nature with significant clay fractions that still drain quite well and in reasonable amounts of time. It's just a matter of not having the entire soil profile clogged with clay particles. There's some math involved here where diversity of particle size would come into play that I'm too durned lazy to do today. I've been up since 4:30. But it's why loam soils work quite well. There are also ways to force flocculation of soil particles, but it's not exactly something I'd bother playing with for temporary container storage. But it's something I regularly discuss over on the lawn forum and do in my lawn soil, garden soil, and containers (indoor and outdoor). All this is why of us simply state that it's not worth anybody's time for temporary containers. If it grew well outdoors in that soil, it should do OK in a container for a while in that soil. And for more permanent replanting, I'd knock the soil off (for plants that are tolerant of that, or remove as much as possible for plants that are not) and replace the soil with something better rather than try to re-use a soil I know to be suboptimal. But it's an interesting hypothetical discussion on soil amending for permanent container or "long-term" temporary that may be less temporary than one thinks. Like that poinsettia that I transplanted to see how long it would last and is now extremely healthy and likely to be a showpiece this fall and winter when it comes back indoors.