If you are going to buy truckloads of something buy replacement topsoil and put that on top of the subsoil, plant in that. Unless that would cause grading problems (make your beds too high). In which case the most direct approach would be to dig out beds of some size and refill them with replacement topsoil, remove the excavated subsoil from the site.

Amending the subsoil is the least desirable approach to the situation. In particular you do not want to amend individual planting holes or other small areas.

I can add up to 6" without grading too high which is enough for lawn and possibly garden beds, but I surely want tree roots to be deeper than that.

Let's address #5 first. This is an incorrect assumption. In fact, it is an incorrect assumption to conclude this subsoil is devoid of nutrients and is - by your words -'uninhabitable'. The depth of the root systems of the native trees may just as easily be influenced by the amount of water that was available in the soil. The primary quest for tree roots is going to be a source of moisture. That's why for almost any kind of tree, the bulk of the root system, including all the fine feeder roots that take up that moisture, is located in the top 15-18" of the soil profile, regardless of how deep the enriched/amended soil may be. The only way to know if the soil is infertile or nutrient poor is to have it tested.

Any amending will be influenced by drainage. Does the subsoil currently drain well? You can check online for various perk tests you can do yourself to determine drainage. To build on the previous comment, among other reasons it's not advised, amending individual planting holes in slow or poorly draining soil just creates pockets of overly wet soil around plant roots - the 'bucket effect'. These can lead to root rots and other disease issues.

Increasing the easily plantable depth by bringing in a sufficient layer of soil can help but the 'topsoil' or a sandy loam garden mix is preferable to just compost because of more closely similar textural characteristics to the native soil and so avoid creating interface issues between the two different soils. It also never an problem to prepare wide but quite shallow planting holes and plant high, so the top of the rootball is above grade. You then bring in the better soil to mound up to the proper level

If you feel you need to add organic matter, use some compost as a mulch. And this is something you will need repeat annually to replenish OM, which is always degrading and decomposing.

#6 Don't assume the roots of the plants you are planning on adding will remain concentrated in the original planting hole (your 'quantity' factor in determining size of planting hole). Tree and shrub roots can be very pervasive and penetrate through some pretty nasty, compacted soils. They are actually more likely to stay concentrated in an enriched planting hole, as there is less incentive to seek out soil moisture and nutrients.

There's a couple of other misconceptions in your plans as well but this should at least get you started in creating a desired soil planting situation for the immediate future. Probably the most important take-away from this - and the one most folks find to be counterintuitive - is that whenever possible, planting trees and shrubs in the unamended indigenous soil is the preferred approach. Any amendments should be applied as a mulch or top dressing.

Well, it looks like you have some good evidence that the subsoil layer wasn't plant-friendly. There s something down there that roots don't like. But to the extent that tree roots are all located in near-surface strata, I guess it really doesn't matter what's down there, does it? Though it's true that roots can permeate the worst soils. Now, this may be a survival-versus-thrive question. Your trees may survive if limited to the hole you improve, but will they thrive?

I have to say that your case seems an argument against the no-till strategy. You've got a foot of rich humus sitting on top of compacted sand, silt, and clay. Probably been that way for centuries. But you still have a compacted layer of sand, silt, and clay.

gardengal48: Thank you for your elaborate comment

"almost any kind of tree, the bulk of the root system, including all the fine feeder roots that take up that moisture, is located in the top 15-18" of the soil profile,"

It is in the area where I am now, but not where I come from. I consider it is not very healthy in terms of trees uprooting when they grow too high or creating to much competition for lower trees and shrubs. I would rather try to garden my old ways where I could expect the tree roots to go as much as 6' down.

"Does the subsoil currently drain well? You can check online for various perk tests"

The soil drains well, the perk test was done for septic system. Unless there is an accidental layer of clay under, it drains to bedrock (which is about 6') than along the bedrock. The average grade on the site is 14%. So creating a swamp by digging and prepping a hole for a tree is not a concern. As a matter of fact we have a pond dug that is about 6ft deep and it drains within 3 weeks with no rains in the summer.

"bringing in a sufficient layer of soil can help but the 'topsoil' or a sandy loam garden mix is preferable to just compost because of more closely similar textural characteristics to the native soil and so avoid creating interface issues between the two different soils."

I have had contractor bringing "topsoil" to finish the septic field. It looked like pure clay to me, less sandy that the onsite subsoil mix, but not something what I would put my money in for planting the garden. I totally understand the interface issues, and that's why I would mix the compost and native subsoil together with a FEL of a tractor before dumping them in the planting whole. What I don't understand why should I be bringing clayish soil when I have already clayish soil on site instead of bringing the most missing black humus part (as long as I want to spend time mixing it in)?

The last 2 paragraphs of your comment are very encouraging as to there is no reason to spend money on poor soil amendment during the planting, plants would seek nutrients deep in the soil regardless how poor/compacted it is, however my onsite observations show otherwise - shallow roots of native trees up to 50' high (maple, birch, fir, spruce). Why shall I expect that apple trees would be more resilient in sending roots down instead of spreading them on the surface where all organic mass going to be accumulated?

To further elaborate on:

"almost any kind of tree, the bulk of the root system, including all the fine feeder roots that take up that moisture, is located in the top 15-18" of the soil profile,"

RE: It is in the area where I am now, but not where I come from.

Our apple trees in (relatively poor but fertile) clay soil used to spread 40' in crown, yet we had perennial flower beds within 5' of the trunks, and annual veggies beds thereafter, never encountering problems with roots when turning them.

How can I put shrubs and veggies under or around trees when the tree roots are spreading 2-3 times beyond the crown (at least that it is what is happening with native trees now in 1' of top soil)?

A thought more than specific advice - are you set on planting your trees right away, or willing to take some time? The longer you can wait, the more options you have in terms of amending what soil is there now. You could put a lot more compost, mulch, wood chips in, have some tilling done, grow some cover crops, etc. There would be more time for this stuff to settle, compost, etc., and perhaps this would get that compacted soil moving on the path to a more root-friendly mix. Maybe even encourage local earthworms and other members of the soil food web to get busy. Plus you'd be more certain of what's in there, compared to buying 'topsoil.' Time is your friend in amending soil.

"It is in the area where I am now, but not where I come from. I consider it is not very healthy in terms of trees uprooting when they grow too high or creating to much competition for lower trees and shrubs. I would rather try to garden my old ways where I could expect the tree roots to go as much as 6' down."

You may think that deeply growing roots are a better situation but that is simply NOT the way tree roots grow! This has been confirmed via scientific studies and is reported by many arboriculture-related websites. Initially, trees may send some roots down vertically for anchorage but as they develop and grow, the roots spread out laterally, often to great distances. Tree roots - regardless of species - grow predominately at the soil surface. As much as 90% of the root system is located within the top 60cm of the soil profile with the majority just below the soil surface. The reason for this is that this upper layer of the soil is where the oxygen, soil moisture and nutrients the tree needs for healthy growth are located - not deeper.

Deep root anchorage is really rather incidental. Even those tree species that produce tap roots (and there are not all that many) do so only as young specimens with the tap roots generally disappearing as the trees mature. There are some exceptions with trees growing in very arid areas that will maintain tap roots to access underground acquifers but deep roots are the exception, not the rule. Anchorage against windstorms - really all necessary structural support - is provided by the widely spreading lateral root system. And this lateral spread can be quite significant. Many assume it stops at the tree's dripline or a spread equal to the canopy width. This is incorrect. The spread is more closely aligned with the height of the tree but depending on growing conditions, can even exceed the height significantly, by as much as 3 times!!

garden web photos · More Info

The above diagram illustrates the way tree roots grow - not down but out!! It is very important to to understand and accept this concept and is one of the primary reasons why you are encouraged to plant in the indigenous soils. Altering the soil structure by copious amending can also alter - adversely - the way the roots form and grow and often does lead to structural failures or blow downs.

"Roots require three things: water, oxygen, and soil compaction levels low enough (or with void spaces sufficiently large enough) to allow root penetration. If all these conditions are met, roots can grow to great depths. Under ideal soil and moisture conditions, roots have been observed to grow to more than 20 feet (6 meters) deep."

http://www.deeproot.com/blog/blog-entries/how-deep-do-tree-roots-really-grow

The other problem with shallow roots I see in a forest garden is too much root competition near the surface, thus not being able to plant anything under or near the tree without cutting its roots (and when you cut enough roots I have seen some branches die). Have you ever tried to plant or transplant anything in the shallow root forest? Whatever spot you try with the shovel, it is a web of roots ranging from nearby shrubs to trees 60ft away that are impossible to separate or plant into. That is not my idea of easy gardening, thus my interest in changing soil condition under the future trees to allow deeper root system (by digging wider&deeper holes and prepping the soil within those holes).

So I think roots spread out wide out of necessity (the soil under is too compacted, too wet, or too dry), and not because this is the most efficient way they could grow had the conditions been better.

I skimmed a lot of this thread so I may have missed things or be repeating. A few thoughts:

First, I am horrified that builders are allowed to do this in your area or that they do it at all even if it's allowed. Do their mothers know they act like this?

I like the idea of bringing in some topsoil, but obviously you've seen how that can go, so you need to go INSPECT it yourself before purchasing. Same with compost.

The compacted subsoil should be broken up first to at least relieve the compaction, even if you don't amend it. If it's a large bare area, consider having it plowed and maybe even plowing in some compost. Then level it, and bring in that topsoil.

I am sure someone in this thread would have said to avoid highly amended tree planting holes, particularly deep ones. Dig more of a dish shaped hole, wider than deep, and let your tree and shrub root balls stick up a few inches (about 1/3 the total height of the root ball) ABOVE the surrounding soil. Mound up amended soil around so there is a gentle slope down to ground level. Cover with mulch to prevent erosion. This is a good technique for clay soils as it doesn't create as much of a bathtub for water to sit in, and it lets the roots spread OUT gradually into amended, then partially amended, then unamended soil. Combined with plowing the compacted subsoil, I think this will improve things.

I'm not going to argue with you. Reread what you posted and the blog you linked to carefully and in detail. Notice the use of "can" rather than "will". Deeper penetrating roots will not necessarily increase the health or longevity of the tree nor will they alter the lateral spread of root system to any significant degree. The fact of the matter is that regardless of how deep any anchorage roots may penetrate, it is the surface oriented feeder roots that comprise the bulk of any tree's root system and these will be present just under the soil surface regardless of how deeply prepared and enriched the subsoil is. That is where the highest concentrations of oxygen, soil moisture and nutrients are located - within 12-18 inches of the soil surface, NOT deeper!!

FWIW, the difficulty in successfully underplanting established trees is a well known horticultural fact, some species presenting a much more difficult situation than others.

If you still have a conflicting opinion and difficulty in accepting these widely acknowledged and scientifically substantiated concepts, then feel free to post your questions in the Trees forum. There are many foresters, tree experts and certified arborists that hang out there and they will tell you exactly the same things that I have been trying to communicate.