It's hard to say not knowing if the tree will eventually root into the subsoil. I'm growing fruit trees in 15 gallon pots. They don't get 10-14 ft tall and they need water every day in summer. If you plan to irrigate, then the soil volume mentioned will grow a 10 ft tree. But if not then that's probably not enough topsoil unless the subsoil kicks in.

I wouldn't dig a hole unless the bottom drains out the low side. Just mound the topsoil about 12 inches deep over a larger area and plant the tree in that. Then put on a heavy layer of mulch.

Filling the hole with top soil is a very bad idea. The hole then becomes a "pot" and prevents water from moving through the soil the way it should. The usual result is a water logged root ball, a tree that slumps in the hole and is then too deep, and a poor root system that is all twisted and balled in the hole instead of penetrating into the surrounding soil.
What you call sub soil is probably not as bad as you think and in nature, a true top soil is never more than 2 inches thick, even it the richest class A1 soils. Plant your tree and back fill with the same native soil. Use an organic mulch about 4 to 6 inches thick on top of the soil and water though the mulch. After about 6 months get some fishing worms, the red ones, and put them under the mulch. In a year, you will develop a good "A" horizon, also called "top" soil. Beneath that for about a foot, you will discover a second horizon, or "B" horizon that will be a little richer brown color that it was when you started.
These horizons are the mark of a good rich soil. The best way to get them is to mulch and water. The horizons will develop on their own. Trying to build one artificially almost always creates many more problems that it solves.

My yard was raised 2-3 feet with sub soil when the basement foundation was dug out. The native top soil was left compressed underneath. The front yard was given thick new top soil but the back was left sub soil and planted with Zoysa grass plugs. I've tilled much of the grass in as it browns in dry summers and winter. I added compost to grow grass from seed. I ran a sprinkler system.The first bare root trees I planted into the backyard subsoil did not grow. When I removed about 2 x 2 x 2 feet of subsoil and blended 1/2 with rich compost they all grew excellent. Use lots of wood chips all season to keep the soil healthy.

Okay, evidently I will need to clarify as my original post was not clear.

First, the hole is not going to turn into a mud hole. The slope in question is quite steep - 1 in 2.5 - which actually makes it a bit difficult to climb as it's very easy to slip and fall over. The tree will go into a hole carved into the slope. One side of the hole will actually be open to the air all the way to the bottom except for a retaining wall used to prevent the soil from washing away. (The retaining wall will be permeable so excess water in the hole will be able to easily drain away.)

Second, "subsoil" was the wrong term. My bad. The developer stripped away *much* more than two inches of soil of the top of the ground. I don't know the correct term for what's left, but I can tell you that this happened ~5 years ago when the house was built and large portions of the stripped areas literally have nothing growing on them, not even weeds. Although the tree might send roots down into this stuff in search of water, common sense tells me it's not going to be a major source of nutrients for the tree.

Perhaps a better way to ask my question would be this: If I were to grow a semi-dwarf fruit tree (e.g. an apple or peach tree) in a large container, how big should the container be in order to accommodate the tree when fully grown and what type of soil should I put in it?

Thanks!

The material below the subsoil is called the parent material. Parent material is essentially the unaltered material that the soil above developed from.

The parent material doesn't have to supply much or anything in the way of nutrients for good tree growth. It will supply some if the roots grow into it. What is much more important is water. Since you are in Knoxville, you may not be used to watering. A tree in a large container requires frequent watering. Your tree will also unless the tree gets most of it's water from the parent material. Being on a steep slope makes me even more concerned about water.

With frequent watering a 10-12 ft tree can be grown in the volume of soil you originally stated. As I stated before, a hole is OK if it drains out the low side. If the hole allows you to level out the area around the tree, this could help some by reducing runoff.

As I see it, water is your main problem and your questions so far don't seem to address that issue.

My advice would be to dig a posthole every place you want a tree, fill them with water and see how long it takes to drain. If most of the water's gone the next day you won't need to build those boxes. The tree roots will eventually reach out at least 6', but you can plant the trees without much soil ammendment then till in better dirt around them later.

A few different things...

Regarding water: Fruitnut, your point about water is well-taken. I had actually already been making plans for watering - there's a wet-weather creek that runs through my backyard, plus runoff from the roof when it rains. My thought was to use these sources to fill a water tank that I'll then use as a supply for watering the trees as needed.

I'm not actually intending to grow the trees in a container, although I may grow them in a raised bed containing good soil, with the ground underneath broken up and some amendments added to it to encourage the roots to grow down into it.

My reason for asking about growing the trees in a container was that it seemed my original question was not being answered in favor of related issues and the container growing idea is an alternate way both to get at the answer I need and to visualize what I'm getting at.

Thank you (you collectively, as several of you have contributed ideas and advice) for your comments / advice about water, drainage, etc. I appreciate it your desire and willingness to help me. Please understand that if I didn't comment on something you said about related issues, it wasn't that I was ignoring what you had to say, it's just that I've looked into this a decent bit, including already thinking through the related issues you raised.

Again, thanks.

I will answer your actual question, although I believe that some of the responses were dead-on, in theory. It sounds like you are smart enough to figure out the difficulties of your actual soil and without seeing it it is impossible to give reliable advice. However, the issue is how compacted it is and that's what my advice would be hinged on.

You didn't ask for advice on that, however, and my advice to the question of how much soil needed would be probably about a cubic yard if the tree was to rely exclusively on that. However, I'm not at all sure that if the roots can't establish at all in the native "soil" that the trees will be well anchored. They may need support.

I once established an orchard over a wierd grey clay subsoil that was a total plant killer. We used raised beds and mixed a combination of humus and sand- about a half yard each, with the native garbage (not actually native- it had been trucked in!) 50-50 or so.

By using a lot of what was there the hope was the trees would anchor and they did. Only the peaches had difficulty establishing and the trees, all 26 of them, have anchored well and are bearing productively. After about 5 years the peaches and one nectarine finally look pretty healthy. The other trees were apples, pears, aprictos, Euro and Jap plums and a cherry tree. The subsoil soil has naturally stunted the trees so I'm glad none were on fully dwarfing rootstock. Besides the pears and plums, they have about half the vigor of trees growing in a normal soil.

Using sand as an amendment is strongly recommended against in the literature when dealing with clay, but if you use enough of it I've seen it work- at least in some clays.

The reason I suggested testing percolation is because my orchard in south central Tennessee had no clay layer, rather thin sandy topsoil over deep red chert, which is very well-drained (and easy for roots to penetrate). KnoxGard should dig at least one 2' deep hole and see what the profile looks like, then decide how to proceed.

My question is how do you safely plan on pruning, harvesting, and caring for a 14' tree on a slope that is very easy to slip and fall over on?

I agree about testing the soil for drainage. If what's there now is permeable to water, then tree roots will grow into it and problem over, well mostly over.

You don't want to do anything that results in trees being planted in pockets of one material surrounded by a different material. Better to spread topsoil over the whole planting area, put the trees in that.

It does not have to be real deep.

all that deep raw mineral soil will develop into good soil in a very short time, well short by tree time. A year with mulch and it will look like top soil and in 2 years, you will have the o, a, b, c soil horizons I outlined above. wet mud and all that aside, it's does not help to back fill with top soil and it does not help to top dress with top soil.
since you have a sharp slope, probably the best thing you can do is grow a "green manure" crop like an annual grass mixed with clover or alfalfa for one year on the whole slope. use a light scattering of spoiled hay or commercial top dressing over the seeded area to help it hold and give it something to start with. That will help hold the slope and build the soil. You can rake what you cut or mow, around the tree for more mulch.
You original question is how much top soil. Well, the preponderance of testimony here is ... that's a bad idea, which is why no one answered it directly. You will almost always wind up with more problems that you solve doing that. Rebuilding a good soil from the parent soil is the best way to go in almost every case.
Also, the manor of planting on a slope that you mention, with the front that open, is not a stable planting for a tree on s slope. You should dig back into the slope making a flat terrace with a flat "cove" back into the hill. plant the tree so that the center of the tree at soil line is back in the cove and so that in a couple of years when the slope erodes back to the tree, the base of the tree will still be at soil line. Use the soil you cut for the "cove" on the down hill side but do not plant in that loose fill. Just let it add stability to the low side, but plant in soil that is a stable part of the hill still.
Well, good luck with all this. hope we helped.

Quoting the OP: "The area where there's space for them has no topsoil - quite literally. It's subsoil (the subdivision developer stripped off the topsoil and sold it) and almost nothing grows in it except some scattered weeds."

This is just EVIL!! People are just so greedy it makes me sick!!

People may be against bringing in topsoil but that's because they haven't had experience with situations where you just can't make do with what's there.

Of course you can't dig a pit into compacted or pure clay soil (I'm talking about the stuff you make pottery out of) and fill it with a sandy loam and expect good results and I'm pretty sure the questioner is well aware of this.

There are times when bringing in soil is essential and the questioner believes that his soil will simply not support healthy tree growth. A really bad "soil" simply cannot be repaired by some nice mulch- the roots will die before they ever get a chance to explore the aerated interface.

We haven't been told what the nature of the existing soil is specificly, so we really can't accurately advise on the best course of action. So what is it KG- what is the nature of this terrible soil aside from not supporting plant life? If you don't know, you should find out- take a sample to your cooperative extension and they can at least do a texture analysis of what you have.

From what I've seen, the soil pretty much anywhere around Knoxville TN is pretty good soil even at depth and supports wild Oak, Pecan, Hickory, and such, trees when left alone.

So Softmentor I assume you worked as a professional landscaper or related field in Knoxville? Can't see how you'd have such a grasp of the wide range of conditions in the area otherwise. I think most areas have a very wide range of soil conditions, especially when construction is involved.

Here in the NY area we have mostly soil that will support healthy forest but there are low areas where fine sediment settles (clay) and just as common we have soil that has been destroyed by heavy equipment (compacted beyond the help of anything but remedial subsoiling) and also sites where sub-sub soil has been trucked in for grading purposes.

I often try to give people advice over the phone and upon seeing the site later find the described conditions not quite matching the reality. On this page the soil has not even been described specifically, so I don't think any of us can predict what are the actual conditions.

Thanks for the replies!

I am not able to get on here very often, thus no reply from me until now.

Within the next few days I should be able to take photos to post here and respond to the questions and issues raised by various posters.

Thanks!

This is in reference to Softmentor's comments on making level spots on a slope for the trees. I had to do just that to plant my fruit trees. For one thing, otherwise any water you try to give them would just run down the hill and erode a little gully. I mixed in a 40# bag of topsoil with the pure sand I was planting each tree in. The trees are all thriving. The only problem came when it was time to expand my 6' high wire cages I had put around each tree against deer. Once one gets beyond the flat space, it gets tricky and you have to cut the wire in sections to accomodate the high and low sides of the slope in getting around the tree. When time comes to harvest, I don't see a problem--just use a fruit picker on a pole and reach from the high side. I'm still dreaming of that time! Northwoodswis