How long will they be in the containers? If it’s just temporary I’d stick them in the pots in the soil they came out of. You’re getting rid of them so why spend time and money on the perfect mix? I’ve moved house with plants shoved cheek by jowl in plastic crates with just the soil on their roots. They're fine for a coupled of months until they’re replanted. You don’t need ‘ideal’.

So I am less interested in designing a container soil made from clay soil than I am in the learning opportunity to better understand perched water tables. Clay particles are incredibly small and would have very high cohesion forces. Given that fact, is there any way to amend clay soil that will keep the perched water table of the soil mix under control?

Gotcha. Sure--anything that breaks up the grain of the soil will do that. Perlite is a great choice there. The particle size of Perlite is roughly "Mount Everest" compared to the clay (really, an extremely coarse sand). If you can mix that in to beyond the depth of the clay intrusion, water will run through. If you can't when doing this in the ground, you just created a nice "cup" that will fill with water, but that wouldn't be an issue in a container.

Some people use sand. I find sand tends to settle a bit in some soils, but barring particles going into suspension, that's unlikely to happen here. You'll be making a dry or damp mix and putting it into the container, and changing this over to a sandier mix.

If what you're working with is a mostly-clay mix, seriously toward the upper part of the soil triangle, you may have to add a significant amount of silt or sand to move it across the triangle. Basically, you're going to have to break it up enough that the clay particles don't contact each other enough to form the sheets that limit water penetration. That's difficult if your native soil is very heavy clay.

http://lagardennotes.blogspot.com/2013/10/using-soil-triangle.html

@User @tapla (mid-Michigan, USDA z5b-6a) has said in his threads that if you have a soil mix that uses coarse particles, but you have one substrate with tiny particles that holds water tightly at over 30% of the mix, the entire soil mix will tend to preserve the high perched water table associated with the tiny particles. In the case of peat moss, he advises keeping the peat fraction at or under 20% because of this issue. 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.

Best thing to do is to burlap the root ball and keep it in good contact with the ground. That is exactly what nurseries do with their landscape stocks. Ground contact will eliminate the PWT.

If you insist on keeping it in a container then I am afraid you will need around 80+% of coarse material to start making a difference to the properties of clay,

If it is a temporary condition - a few weeks to a month or two before giving away or selling - just leaving it with its mostly clay rootball is perfectly fine. You are just unlikely to need to water it as frequently as something planted for a more permanent application in a proper container soil.

I have dug up all manner of plants from friends, my own and even client gardens that I wanted a chunk of or needed to be moved and just took the rootball - garden soil intact - and put it in a plastic grocery bag for transport with a couple of holes poked into the bottom for drainage. Some have been less temporary than others........I have a couple of bags sitting around now that are in their second season waiting patiently for me to assign them a permanent home and yet are still doing fine. Both flowered earlier this spring.

IMO, it is not worth trying to successfully amend a clay soil for a container. Either just go with the rootball with native soil intact and be cautious with the watering or use a proper container potting soil.

Just to add, I have done the burlapping and keeping it on ground many times with several large and small plants dug out of the ground. I have kept them like this for two seasons even before planting them in their new location. You can also keep it in a plastic container with lots of holes in the bottom to ensure ground contact. Often I add holes on the sides of the container too. In bonsai world this is standard practice when one acquires a ground grown plant that is dug out in non optimal time.

"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.

@tropicofcancer You are the master of connecting pots to the ground, and it is a nice trick. Do you think a thicker woven grow bag like this one would also eliminate a perched water table when set on the ground?

Westes: I am not the originator of the idea that connecting pots to the ground, if done right, will eliminate that PWT in a container. It has been known and practiced for long time. It may not be that well known technique for hobbyist like us. The trick is to properly understand the properties of the soil in the ground and the container and use them properly.

Since you are going to dig out a plant from its native soil it perfectly matches the native soil. The fabric bag you posted is a good wicking material and will work perfectly. Just dig a few inches of the ground to loosen it up and then press your container into the loose soil to ensure good contact. Then water well to let everything settle into position. Burlap is often used in nurseries since it is cheap and wicks well.

My soil here is pretty good. over years of mulching the top foot or so is nice well draining soil. Below that it starts getting more compact but still drains OK. I usually dig in my pots to be well within the top good soil. Then pile up some more mulch around the pot. Besides draining well it keeps the roots cooler. Cool roots means lot less stress on the plants.

If the ground is very compact and does not drain well then this method will not work as well. It can, in fact, create that dreaded bath-tub effect.

I have even dug my pots in pile of small gravel too. Does not help with PWT but does not make it any worse either, but keeps the pots cool.