The current horticultural convention is to NOT amend planting holes, for many of the same reasons laag suggests as well as a few others. However, that is quite different from amending an entire planting area, which is perfectly acceptable and often, in the cases of poor soils, recommended. What you are attempting to do is improve the soil texture and fertility over a widespread area rather than focus on individual plants or planting holes, thus avoiding pockets of enriched soil that may not drain properly or do not promote root development into the indigenous soil.

As others have noted, adding quantities of organic matter - compost or composted manures - is the best way to achieve improved soil conditions. Ideally this should be done before any planting occurs, but with care it can be done after the fact. And if the area is already heavily planted, similar soil improvement can occur, given enough time, if one consistantly mulches with a good quality organic mulch, Eventually, with the help of rainwater percolation, soil organisms, and routine tilling or cultivating, the organics make their way down into the soil structure.

There are some hardliners that object to altering indigenous soil at all, believing that plantings will succeed better if they become adapted to local native soils, but I have yet to see the disadvantage of the routine amendment of large planting areas. Overall improvement of the soil will result in increased fertility and drainage, stimulation of beneficial soil organisms, even the introduction of micorrhizae with certain products and a reduced need for chemical fertilizers. As an old professor drilled into us as students, if you feed the soil, you feed the plants.

I also plant in an area with heavy clay soil.I have found three things that definitely help in these situations
#1.Plant high
#2.amend the heck out of the top 1'or more of soil in a 2' or 3' radius of your hole and in the bottom of the hole.
#3.If you can't break up the bottom hardpan,then drive large holes in the bottom and sides of bottom of the hole and fill with drainage material.
This gives the xtra water a place to go that would
otherwise sit in the root area and rot the roots.
Planting high allows the water to drain away from the planting hole and amending helps the clay handle the water and allows room for air too.
Useing these methods I have had 99% survival rate with all the plants,trees,and shrubs In the years of planting I have done in clay.YOU almost have to ammend clay soils.

You're pretty new to the GW, MountainLandscaper. Welcome aboard.

I use the same procedure you describe, and am in full agreement with you.

Many thanks for the responses. I have in the past amended clay locations that held water like a bucket, thinking this will create a better initial environment for the plant to establish itself, but will no longer due so, now understanding the ground water hydrolics involved.

To me it boils down to this:
For shrubs, amend WIDE so that the root system will always be in amended soil. Mulch or add compost around the plant on an annual basis around the drip-line of the plant.

For trees, do not amend native soil. Tree roots do not want to cross from amended soil into unamended soil. Amend soil around trees just as nature does it, from the top down. Plant trees in a wide hole, and replace the native soil (clay or sand) - spread a layer of compost, mulch, or better yet, composted leaves around tree under the dripline.

I recently read a study which showed that oak trees grow best when only mulched with oak leaves. (As my son would say, "well, DUH...") As gardeners, we are often best off when we emulate nature.
clfo

One must prepare lodging for the Rrhizae Brother,s both Endo and Ecto, Notho

The most completely misunderstood portion of what makes things grow is the soil. I have found surprisingly few people who have understood soils...all of them have been university professors specializing not in horticulte, not in landscape design or architecture, but in the study of soil science.

I myself do not profess to know anything more or less about soils than anyone else. I collect information, process it through my filters of bs, ego and bluster, compare it to that which I have learned before, and await what I can learn next.

I do know that the single subject about which there is more contradictory information than any other is how to plant. There is no contemporary horticultural wisdom, one way or the other. I know numerous different ways that people deal with tacky clay, from berming high and planting shallow, to drilling a drain hole from the bottom of the planting hole and filling it with a variety of materials (styrofoam peanuts, corn cobs, rice hulls, peanut shells among them)to mildly amending, to absolutely eschewing anything remotely close to amending. What works? Depends of who you ask.

There are people who believe that nurseries are owned by large chemical companies, which in turn are owned by the same people shot Kennedy, or may be the same people who shot Kenndy, who are the same people that engineered teh whole UFO cover-up in Roswell, NM.

"I do know that the single subject about which there is more contradictory information than any other is how to plant"

Ehh .. I don't agree ... the basics are the same just visit a site like ISA .."International Society Of Arborist" and you can get standard planting routines and guidlines.

There are ofcourse many different environments and even more diversity of plants and each gardener may have different problems to solve so given a basic routine it makes sense to see variation ... I would not expect someone planting in Florida to follow the same procedure as someone in the Mohave desert but the same principles apply ... in otherwords same equation different numbers.

Any gardening plan should be backed by basic science and research in the industry. There will always be judgement calls but research seperates voodo from good gardening.

Good Day ...

I think gardengal48 gave one of the best of insights here, that is do the whole bed and not just the planting hole. One overlooked condition you may be faced with is that of poor soil around new construction. Lord knows what's buried there, contractors usually just push dirt over stuff and figure the landscapers will deal. To all that, they often "grade" the site after construction with a tracked vehicle (like a small bulldozer) thus compacting it severly. I can't tell you how many times I've seen them do this while the soil is still very damp just to close the property on time. Given the properties of clay, this practices absolutely ruins the existing soil. Consequently, what we're often left with, then, is a landfill for a site and soil not fit for growing.

That said, amending is your best approach. Depending on the size of the area, you can use bagged material or bulk. Bagged works for small areas. Bulk is better for any large undertaking, even if you have to truck it back to the site via wheel barrel. For the homewowner, you can buy a generic Earth Mix or soil amender by the cubic yard for far less than you could get bagged material. If you have left over, cover it and use later (I always have a half-pile around for odd projects). You can usually find bulk material through local landscape nurseries or bulk dirt haulers.

Hope that helps.

Sand is more useful than you would think.

Organic matter decays to nothing in notime.

Because of the religous aura surrounding organic, do deep discounting of what is spoken.

Unless there is continuous commitment to topdressing, go with sand.

Amen.

Unless you are bringing in truckloads of the stuff, sand isn't going to do you one bit of good and could very easily exacerbate the problems with heavy clay soils rather than improve it. Organic matter decomposes into humus, one of the building blocks of good loamy soil - sand doesn't decompose at all. Organic matter creates an environment suitable to the development of healthy populations of beneficial soil organisms - sand does not.

I think you are confusing the "religious aura" sometimes associated with the phrase 'organic gardening' (gardening without chemical intervention) with the common soil science term of 'organic matter'. The benefits of amending soils with organic matter has been recognized for centuries and is certainly not some new gardening fad. Although helpful, a continuous committment to topdressing is not required - in a cultivated garden as in the natural environment, OM is continually being added to the soil, although not necessarily in the quantities we deem sufficient.

"Organic matter decomposes into humus, one of the building blocks of good loamy soil - sand doesn't decompose at all."

Well .. sand is certainly a ingredient of a loamy soil ?? The mineral component of soils do breakdown and release soil nutirents. Like Potassium for example.

"Organic matter creates an environment suitable to the development of healthy populations of beneficial soil organisms - sand does not. "

The sand component of a soils texture helps lighten a soil allowing the soil to have higher porosity .. hence more air ( Oxygen , water ) can enter the soil which in turn affects microbes. Right ??

Sand can indeed be a important addition to dealing with clay soils. Simply dumping it on the ground though is not the solution.

Mechanical aeration ... or other cultivation techniques are needed to incorporate sand into soil.

Good Day ...

>Well .. sand is certainly a ingredient of a loamy soil ?? The mineral component of soils do breakdown and release soil nutirents. Like Potassium for example.True, sand is a component of loam, but its breakdown into usable plant nutrients is extremely slow and of very low consequence.

>The sand component of a soils texture helps lighten a soil allowing the soil to have higher porosity .. hence more air ( Oxygen , water ) can enter the soil which in turn affects microbes. Right ??Not exactly. First, sand does not lighten a heavy soil. Clay soils have small pore spaces, sandy soils large. When mixing sand into a clay soil, the larger pore spaces of the sand allow the smaller clay particles to fill in, resulting in a heavier, denser soil with with less total pore space than that of either the sand or the clay soil alone. To truly lighten a clay soil with sand you must add 50% total volume with sand and till deeply, a process that will be prohibitively expensive and overly time-consuming to most consumers. Anything less and you run the risk of creating a concrete-like substance.

And in addition to required oxygen and water, soil microbes need a constant food source which is only supplied by organic matter. Compost or other organic matter is relatively inexpensive - free if you make your own - and provides immediate benefit to soils. It is not necessary to work it into the soil although if the opportunity presents itself, as in the preparation of a new planting area, so much the better. OM is also recommended to improve the nutrient content, the biomass and the water holding capacities of sandy soils. It's hard to go very wrong with the incorporation of OM into garden soils.

amend or not to amend.
Always amend, in some way shape or form. I have only encountered one or two yards that wouldn't benefit from a little something.
the manner in which it is done will depend on a million variables (thus why this thread is still in existance).

within 100 miles of here, I've planted in pure clay, in which case I just built up beds of high quality dirt, even added gravel, and I've planted in pure sand, in which case I actually removed sand to fill with organic material....
In pure clay I've even run my trencher around the planting area to assure water wouldn't sit in the planted enviroment.

it all depends.
of course I live 80miles away from a mushroom farm, and can get fully composted mushroom compost for $450 per semi load ($150 if I bring my own semi).

so there
dp

Sand repels water. Doesn't take much sand to move water out.

Though sand doesn't bond to nutrients at all, clay does. The ideal soil is loam because it has both clay and sand.

Sand derived from granite is actually a very mild fertilizer except for nitrogen. If you can see different colors in sand grains, rather than pure white, you probably have fertile granitic sand.
Though the breakdown products of granite that don't wash out or form a concretion at greater depth are sand plus clay.

Sand is cheap.

Myself on lawns I use a local "lawn mix" product .. it contains sand and organics ... kill's two birds with one stone ..

Sand .. silt .. clay .. organics can't have one without the other ...

"It's hard to go very wrong with the incorporation of OM into garden soils."

Unless ofcourse you are growing plants that do not require and are damaged by organic soils. Some plants require a heavy mineral soil.

Organic material can kill some plants. Believe it or not.

Good Day ..

Loam, by definition, contains organic material. No organics = no loam.

Sand is a particle size not a specific composition of minerals. Gravel, silt, and clay are also particle sizes (although clay structures make funky things happen). Each of these can have the same mineral structure as the next. Sand is not necessarily going to bring in any nutrient or mineral that is not found in othe particle sizes. The smalle the particle, the more easily it is solubalized or absorbed by a plant. Clay and silt are smaller than sand.

Mostly true but there are many kinds of clays .. so there are more ways to describe clay then simply particle size .. The mineral component of soils does indeed supply some nutrients .. as ex: where I live the breakdown of limestone which makes up much of our parent rock material plays a big role in our soils pH and Calcium levels.

Good Day ...

I mentioned clay structures making funky things happen. That was the short way of saying that there are many diffeent structures to clay. Limestone being calcium carbonate has nothing to do with particle size. I don't see the controversy between what you said and what I said.

Hello Laag ..

Oh .. no controversy ..LOL.. but "clay" can be used to describe particle size ( when dealing with sedimentary rock weathering ) or to impoly mineral composition when dealing with the products of weathering silicate materials.

I did not clearly understand the distinction myself when I replied to your post ...so I did not intend to attack your post .. clays fromweathered silicate materials contain many important plant nutrients. The chemical weathering of these minerals is at leat in part regulated by microbes.

Just a long winded way of saying soils are not simply organics and microbes ... the mineral component is important and a plausable amendment in the long run.

Good Day ..

As for ph, organic tends to be too high a ph. Leading to chlorosis and other problems with assimilating iron.

Sandy soils tend to be too acid, competing with root tip generated carbolic acid.

And clay soil neutral to sweet.

A lot to think about when planting Rhododendrons, conifers, and a lot of others.

superphosphate, I simply don't think you can make such broad generalities and be accurate. Soils tend to vary considerably from area to area, even within a relatively small geographical location, and the soil composition - sandy versus clay - really does not have any bearing on pH. You can have very sandy but quite alkaline soils in many parts of the southwest, yet clay soils common to areas of the PNW or even the southeast tend to be slightly to moderately acidic.

And organic matter can be all over the board depending on what it is comprised of. FWIW, finished compost tends to be nearly neutral in pH.

To answer the original question, I tell my customers that when planting trees, do not attempt to ammend the soil. You dig the hole at least twice as wide, but no deeper than the root ball. Make the hole an ugly hole, not smooth. When you plant the tree, fill the hole back in with the soil you dug out. Once you have finished planting the tree, you top dress with compost and mulch, but don't make a mulch volcano, rather just smooth the mulch around the root area of the tree, without pushing the compost and mulch up against the trunk. I also tell them to water deeply, once when they've filled the hole half way, and again when the dirt is completely filled in. The reason for this planting method is that it is nearly impossible to ammend the soil for the entire future root area of a tree, and other people in previous posts have explained the bowl effect of using ammended soil in a hole. The problem with ammending or loosening the soil deeper than the root ball is that the tree could possibly sink in the hole, causing it to then be too deep. This will cause the death of the tree. Most shrubs have a much smaller root zone than trees, so I do recommend ammending the soil for shrub beds.

As for organic matter being bad for plants - if that were the case, then we wouldn't exist. Think of nature. If organic matter killed plants, then the world would be an uninhabitable desert. Think about it.

Sally

Much of what has been hashed over here is addressed by this book, which should be tracked down at a library or purchased by anyone seriously interested in this subject.

Here is a link that might be useful: Establishment and Maintenance of Landscape Plants

"As for organic matter being bad for plants - if that were the case, then we wouldn't exist. Think of nature. If organic matter killed plants, then the world would be an uninhabitable desert. Think about it. "

Ohhh .. in fact you should think about it ?? .. the desert is not a dead zone !! I know I live their ... organic matter is a rare commodity in the desert ... important here and there the little that is available BUT there are many plants that do fine with very very little organic materials ... some Cactii are more then happy to grow on a block of limestone and little more !!

Perhaps you need to travel more ???

"You dig the hole at least twice as wide, but no deeper than the root ball."

Great advice !!! Out here it is a pleasure to dig only as deep as the root ball !! .. in fact hard to dig a hole without a jack hammer ??

Good Day ...

Okay, now we are attempting to split hairs. Where there are plants there is organic matter, perhaps not a lot but it exists in fallen foliage and flowers, dead plant matter and countless feeder roots that die off in times of drought only to be replaced with the next rainy cycle. Birds and animals will also contribute to organic matter. Unless something has changed since my last visit, Nevada is hardly an "uninhabitable desert" but there are many different types of desert and certainly parts of the world where that description applies.

Recommendations from another investigator of planting practices, like Whitcomb (ESTABLISHMENT AND MAINTENANCE Etc.)

Here is a link that might be useful: Horticultural techniques for successful plant establishment

I know you have posted this info before and it is very interesting and though it goes against what my gut tells me I think it is probably best as it has been tested with planting studies. But isn't there a point where the native soils suck so bad, and I am not referring to the ability of the soils to soak up water, but rather a true lack of organic matter. Sandy shale and rocks with a bit of clay mixed in. This is what much of my place has and I just have to add amended soil when I plant stuff or I can't sleep. I know, it's a personal problem.

jb

Yes, putting soil that is better for the plants you want to grow on top of the existing soil works well. Whitcomb (1991) says

"Use no soil amendments except in very specific conditions of raised or amended beds for plants with very limited root systems. If the existing soil is very poor, remove and replace with good field soil or place at least six inches of good field soil on the surface. However, you should match soil types as backfilling with a good sandy loam in a heavy clay will serve as a collection point for water and the roots will suffocate. Soil amendments in a small planting hole DO NOT assist plant establishment and growth. It is better to use the amendments as mulch. The only exception is where the entire plant root zone for many years can be amended."

Jb ...

"But isn't there a point where the native soils suck so bad"

Tell me about it !!!!!

I think there have been many cases where people have mindlessly amended the soil and this has caused a opposite reaction to not amend the soil. Just my observations.

There are so many plants and different geologies ect..

Out here we have a native tree .. "joshua tree" .. Yucca brevifolia .. the worst thing you could do when planting this in our landscape would be to add copius amounts of organic material. The moisture rentention alone would spell doom for the tree.

On the other hand if you are looking to grow roses in our native desert soil you better amend the soil ... or as we used to say in the old days "It will be Roses for ya".

So it is really very general to say "amend the soil" .. with what .. and how is not implied by the statement.

Do your reseach .. know the plant your environment .. and have reasons for how you plant.

Put two and together and keep records.

Good Day ...

>I think there have been many cases where people have mindlessly amended the soil and this has caused a opposite reaction to not amend the soil.Recommendations made be researchers based on comparative studies. These have been done for quite awhile:

"In 1968, a study was begun to determine the 'optimum' amount of soil amendments to use in the planting hole since recommendations varied from 5% to 50% by volume. The optimum amount turned out to be none. The findings were such a contrast to tradition, the question arose, 'If tradition was in error regarding soil amendments, what about other practices?' This led to numerous studies relative to reducing stress and accelerating growth of landscape plants." (Whitcomb)

The question Ron is what amendments .. what kind of plants and what types of soil ?? I don't think researchers could of covered everything ..

Out here you won't grow a thing without adding simple low salt organic material to the soil .. unless ofcourse the plants require a mineral soil.

The interface between nursery container mix and the native soil is very extreme .. It can be diffused by adding some organic material to the surrounding native soil. Makes sense to me .. about the only thing I add at planting time .. except water ... and with all the rain in the desert I skipped that too recently ...

I have to think someone was out there watering in the rain this week because some research said to water at planting time !!! LOL

Thanks for your post ... good to hear from you.

Good Day ...

>The interface between nursery container mix and the native soil is very extreme .. It can be diffused by adding some organic material to the surrounding native soil."If the soil is amended around the container growth medium, two moisture/water relationship barriers are established as opposed to one. When the soil around the plant is amended, the roots are forced to grow out into an amended zone and then later grow from that amended zone into the surrounding soil. The principles of water movement and the capillary attraction of the surrounding soil on the amended soil is the same as the soil has on the container growth medium directly when no soil amendments are used. In short, amending a limited area around a newly transplanted tree or shrub is nearly always detrimental or at best of no benefit. On the other hand, in some situations, amending a very large area relative to the size of the plant and soil conditions may be beneficial." (Whitcomb)

Okay, sorry to step on any desert dwelling toes. Maybe desert was a bad choice of words. I do know that deserts are vastly interesting places, full of life. Uninhabitable wasteland may have been better.

The advice I gave pertained to those with clay soil, which is what the OP was asking about.

Sally

Ron ...

Thats the point ... an interface is a sharp change .. in this case texture ... break down the sharp gradient and by definition you break down the interface. Like white paint next to black is a sharp lined interface .. some shades of diminishing grey and the sharp interface is gone .. roots can handle that .. add one heavy medium grey and you created two problems as explained by Whitecomb.

"On the other hand, in some situations, amending a very large area relative to the size of the plant and soil conditions may be beneficial." (Whitcomb)"

Not may be benaficial ARE benaficial in desert environments when planting with plants in rich nursery mix AND making a good sized hole to create a fuzzy interface.

Keep in mind in some areas of the world simply planting a tree in a hole causes only minor interface problems ?? So Why create one ?? In the desert though there is a serious interface problem from the start ... Not trying to reduce it would be a misapplication of Whitecomb's statement ... and I agree with what he wrote.

Again a pleasure ...

Good Day ...

Amended material inside the planting hole and how water enters and exits it, as a result of the amending, is the problem, much more than the soil interface at the edge of the hole. Roots of plants that were healthy at planting time (a critical factor), and not badly circled or heavily lopped often readily penetrate the walls of the planting hole.

When you look at all of the posts and combine all of the experience, one thing is clear. It depends on the situation.

Sometimes we are simply not putting native plants into native soils or even native climates. We are starting with an artificial situation that needs to be dealt with. Reasoning has to come into play no matter what the current philosophy of the day is. Part of that reasoning is to read about current trends and research, but ultimately we have to look at what works for our own desired results. Sometimes these results are not the same results that satisfies the researchers who put out these reports.

It is, as I always say, a diverse field with diverse goals and objectives within it.

"Reasoning has to come into play no matter what the current philosophy of the day is."

Indeed true .. research is narrow almost always by design .. we have to infer from research and apply it to the situation.

"Roots of plants that were healthy at planting time (a critical factor), and not badly circled or heavily lopped often readily penetrate the walls of the planting hole."

Indeed they do but it helps if they can get started first in a low interface environment.

I'll post some images later on the subject... there is quite a difference between Washington and the Mohave.

Good Day ...

This has been a very interesting discussion to follow. If I may add to the many good points that have been made. (At least I hope to make a good point) Someone said that native plants should be paired with native soils. Yes indeed; I have so many clients who wish to plant Bradford pears, Mountain ash, and weeping cherries ad nauseum. I can hardly interest anyone in oaks, hickories, sugar maples (not in new developement of course) and beech, yet those are the trees that are meant to be planted here in central IL.
But apart from choosing the correct plant, I think it mostly a game of understanding the drainage in a given spot. But as we have witnessed, there is no prescription for any given spot. I have clients on pure sand, heavy clay, bean field hardpan and river bottom loam...all within a mile of one another. One of my most complicated properties is a former oak savannah that has sand hills, loam, limestone shelves and underground springs. Add to that, construction compaction, each home built having changed the drainage patterns and each new home owner having installed irrigation systems, (against my advice)and you have certain death for all of the trees in the former savannah.
So what to do? Open discussions such as this one up to the average homeowner, via news print, park programs etc.. If we just get to talking about the whole connection between what was here, is here and will be here, and how the soil, plant, water matrix works, people will begin to think.

These are discussions that should be running on non professional forums. Deb

You match the plants to the soil type or you replace the soil with one they can grow in. This is accomplished through excavation of a large area or with berming/raised beds. Modifying individual planting holes is not adequate, unless the plants have tiny root systems and the backfill is not so different in texture that the amended hole receives water from the surrounded soil or has it attracted away by it.

Variables like differing soil types, differing plant varieties, and differing treatments (varying combinations of amendments) have failed to produce significant variations in results. The question is too basic, the phenomenon too fundamental: make the backfill more coarse than the soil outside the hole with amendments and how moisture enters and leaves the hole will be affected, often to the detriment of the newly planted stock.

Glazed/compacted planting hole walls are dealt with by loosening them back up with a fork or spade, not by amending the backfill.

"This is accomplished through excavation of a large area or with berming/raised beds. "

Not really practical in many cases ... landscapers from out of town are in horror when they see what we plant trees in ... nevertheless there are scores of trees that do well all over the valley ... somewhere in the middle Ron is what is practiced out here ...

Good Day ...

"Open discussions such as this one up to the average homeowner, via news print, park programs etc.. If we just get to talking about the whole connection between what was here, is here and will be here, and how the soil, plant, water matrix works, people will begin to think. These are discussions that should be running on non professional forums."

Excellent point. This is something that Master Gardeners do.

Unfortunately, Extension continues to recommend amending of planting holes in some states. Discussion in Chapter 1 of Whitcomb's book (at link, above) opens with

"Research findings and new technology are slow to be incorporated into nursery and landscape practices."

Be that as it may, the mission of MGs is to disseminate accurate, research-based information to the garderning public - although not nurseries or landscapers.

I am aware that MGs have been discussed on this forum in the past, and don't really want to open that whole can of worms. I thought that Deb had touched on a broader point that there needed to be a mechanism for educating gardeners, and thought I would point out that MGs is meant to be that.

Non-profit botany doesn't talk to for-profit nurseries.

The Christmas Tree industry doesn't talk to nurserymen.

There are some great forage grasses and groundcovers in farming that are underutilized in landscaping.

In my opinion, Master Gardeners are overrated.

There are great plants missing from commerce, as well as good opportunites for nurserymen and landscapers who dig for the info.
A good place to start is online.

Ron .. your very stuck on Whitecomb ... do you also take into account and give credit to your own professional experience ...

I do ..

Look at it this way .. is it better to have two weak interfaces or one VERY large maybe insurmountable interface ?? Do the math ?? What does Whitecomb have to say about that one ??? LOL

I also read the primary literature ... but also work with plants 40 plus hours a week for almost two decades ... I don't bow down to research but apply it in the real world.

"In my opinion, Master Gardeners are overrated."

In my book anyone that wants to give to this industry for free is all right in my book. Master gardeners do quite a bit in my community.

Good Day ..

As the original poster of this topic, I am shocked and sincerely grateful for the vast amounts of information given by members of this forum. I am a landscape contractor who has moved my business from the California to Georgia, and have thus needed to learn completely new plant materials, soil/structures, and climate. I still get to work out west occasionally, but maintain my business in Georgia - so it has been great to hear people weigh in their opinions from their wide and varying backgrounds. I had my own pre-conceived notions about soil amendment (learning gardening on a large, nutrient deficient, arid tract of land), but this has changed since moving to the south. It seems indeed the best idea is to use a good heaping of Reason, with tricky plants, tricky soils AND tricky clients.

My recent schooling on soils taught us to avoid soil interfaces wherever possible. Water moves more evenly through uniform soils structures. When you ammend planting holes you creat an interface or two wildly different soil structures butting up against each other. Water has a hard time moving across this border and the roots of a plant would much rather stay in the planting hole where the soils is nice and rich instead of spreading out into the surrounding soil. So, backfilling planting holes with native soil and then ammending from the top down seems the best plan to me.

Just a point to keep in mind:

A rootball whether b&b or out of a container is already different from the soil you are planting in. The question then becomes whether it is best to be in that very small isolated soil root ball or a larger one you create by ammending the hole. I'm not telling you it is, just thinking about it.

It makes perfect sense that two soils of different structures is not going to have the same type of exchange as if it were all consistent. That does not make it inherently wrong. There is enough situations and experiences listed in the 50 posts above to make us all realize one thing. That is that this soil interface is one situation in the dynamics of what it is to make one individual plant survive in its individual circumstance. Factors outside of this soil interface sometimes overide it. There is no definitive across the board answer. It is case by case.

Roots don't "choose" to stay in nice, soft amended holes. Roots grow away from the center of the plant, unless obstructed by an uncrossable barrier, like the wall of a pot, or the plant isn't in good enough condition to produce a normal amount of root growth. And amended holes aren't nice, that's the problem. They are often soggy or droughty, due to having a coarse texture and how this affects the movement of water into and out of the amended area.

Healthy, vigorous plants soon root across the amended zone and into the unamended soil beyond, unless so badly affected by the conditions in the amended hole that their vigor fall off to much for them to manage this. Amended, large beds or farm fields only have this water movement problem around the edge. Raised beds/berms don't have walls of different-textured soil to shed water into them or wick it away.