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I came across this article and I find that it goes against everything that's said about soil amendment before planting. I've copied bits of it down below.

You are planting a new tree, and you want to do everything that you can to make it grow well. You dig a hole and examine your soil. It might be very sandy, or it might contain a lot of clay. You decide to add organic matter to ‘condition the soil’. You complete the planting process, and water well. What happens?

You have created a big hole in the ground. Around the outside of the hole you have your normal native soil. Inside the hole you now have a different kind of soil – it contains more organic matter and is your amended soil. You have created the same condition we talked about in the previous post, namely two types of soil in contact with each other. We know that water has difficulty moving between two types of soils. You have created one of two problems depending on the type of native soil you have:

1) you have created a hole that retains water. Excess water sits in the hole and does not move away, drowning the tree roots.

2) you water the area but water tends to stay in the native soil and does not enter the hole. Your tree roots are dry.

Neither is good for your tree. So just replace the soil. Don't amend it.

Brōtsyorfuzthrāx
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Hamid Sabir
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    FYI: I personally don't think this would be an issue unless you amended pretty heavily, such that the tree hardly got a taste of the native soil, wherein the amended soil was a lot easier for the roots to enter. I doubt adding a few handfuls of worm castings or some powdered soil amendment would hurt (but I think it would likely help). In fact, if it's mixed with the native soil much at all, I'm not 100% sure that this would even happen. But yes, if it's pure compost, I could see it happening easily. It probably depends on how much of what you add, how mixed it is, and your soil type. – Brōtsyorfuzthrāx Feb 28 '19 at 01:00

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It probably does mean putting back into the hole the soil you just dug out,rather than replacing all the soil with other soil. But the most telling phrase is 'you dig a hole...'. If you're going to plant a tree into soil which is solid clay or generally poor,then proper preparation of the whole area is called for. That means you dig over a much wider area than the single hole you require to plant your tree and incorporate organic material across the whole area, leave it to settle for a week or so, then dig a hole and plant the tree straight into it, without adding anything else, other than possibly a mulch over the top.

I'm currently in the midst of preparing an area of soil 6 feet wide by 22 feet long, but all I want to plant in it initially is 2 (eventually) large shrubs and a single,small tree. The soil is heavy clay and full of rocks, hence the preparation of digging and amendment with organic material (in this case,composted manure). This has meant extracting 3 or 4 smaller plants I want to keep while I do the preparation, which will eventually be replanted in the same area. I could just have dug 3 separate holes, but whether you add organic material to the hole, or simply replace the soil you've removed when planting, you still end up with a 'sink' where rainwater will immediately collect, because the surrounding soil is undisturbed and heavy clay, and water will take the route of least resistance. Which then means your plants are sitting in a wet hole, as described in the extract you included in your question.

The lesson is, yes amend the soil,especially if its poor, but do it over a much larger area than just the single hole you need for a particular plant. If that is not possible because of the presence of large shrubs in the area, then digging a hole and planting straight into it and replacing the original soil (known as backfilling in the UK) is better, followed by a 2 to 3 inch layer of organic material as a mulch, preferably over the whole area if the soil is particularly heavy or light.

Bamboo
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    You're more ambitious than most gardeners I've met. Here, we're lucky to get people to dig a hole twice as wide as the plant and as deep as the soil level in the pot. Of course, there's no substitute for doing it right - I bet your garden is amazing :) – Jurp Feb 26 '19 at 14:42
  • Bamboo, you mentioned that your working on an area thats 22ft long by 6ft wide but how much soil do you plan on digging out in terms of depth? – Hamid Sabir Feb 26 '19 at 15:45
  • None at all - I'm simply breaking it up and removing any old roots and lumps of rock, bricks and any other rubbish I find in there. I'm not intending to take any soil out of it at all, merely to emend it with organic materials once I've turned it over.Actually, I finished digging it today (using a large garden fork, not a spade...) organic materials are going down tomorrow, then it will sit and wait for a week or so to settle before I plant. – Bamboo Feb 26 '19 at 17:25
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    @Jurp - I will admit to being a bit of a perfectionist,but I also hold the belief that if you're going to do a job, it should be done properly or its a waste of time and effort! Plus, once its all been prepared, I shall never dig it again, other than removing weeds and planting. – Bamboo Feb 26 '19 at 17:27
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    @HamidSabir - I forgot to say,although I'm not removing any soil, in terms of how deep I'm digging, about one to one and a half spits (a spit is the depth of my fork, so about 7 inches) unless I come across something that won't be shifted without going deeper. Which I did - turned out to be an old fashioned glass milk bottle buried about 18 inches down, along with plenty of old bricks. – Bamboo Feb 26 '19 at 17:34
  • @bamboo, im working on an area that quite similar to the length and width your working but the problem's a bit more severe. I had to dig down 2ft to find soil clear of stones and bricks. Initially i had planned to just replace the soil with new soil. Now being convinced otherwise, the only problem is that i have the old soil pilled up with tons of rocks and all sorts of garbage in it and removing it by hand would take forever. What would be the fastest way to rid that soil of all garbage before placing it back in the ground? Do you take out even the smaller stones or are they a problem aswell? – Hamid Sabir Feb 26 '19 at 18:27
  • Stones under about an inch across I ignore, everything else goes. In the circumstances you describe, rather than sort through what you've dug out (which would mean using a sieve or screen to sift out the soil), look at buying in good replacement topsoil instead, though it can work out expensive for a large area. It usually comes in different grades; some are screened for both weeds/stones, but some are just motorway spoil, or dug up from meadows and transported to whoever wants it with no screening, and its always worth paying for the most expensive one to avoid getting stony, heavy clay soil. – Bamboo Feb 26 '19 at 19:38
  • @Shule - I can't see anything extra there that is not covered by what I've already said – Bamboo Feb 28 '19 at 10:46
  • @Shule - I never assumed you did have an agenda... it never occurred to me. – Bamboo Feb 28 '19 at 11:08
  • Sorry for misreading your interpretation of what I said, @Bamboo. I'll delete my comments. – Brōtsyorfuzthrāx Feb 28 '19 at 11:12
  • @bamboo, in the comment section, one individual brought up the idea/concept of washing nursery bought plants roots before planting it in the ground into your native soil which a lot of people found to be very fascinating including the author himself. Btw, did your organic matter arrive for the area your working on? How much of it do you plan to use per square ft? – Hamid Sabir Feb 28 '19 at 12:24
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    Organic matter went down this morning - 4 x 80 litre bags of it. That'll do for now, but more will be added once or twice a year as a mulch to keep the soil in good condition ongoing, and to reduce or remove the need for additional chemical fertilizers. As for washing off soil from plant roots, the only time I'd do that is in the presence of some root pest or other in order to reduce the chance of any recurrence - not sure I'd want to take the risk of damaging roots by attempting to do it otherwise. – Bamboo Feb 28 '19 at 14:26
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I was confused by the "replace the soil" phrase at first, but then realized that the author meant "...just put the soil that came from the hole back into the hole. Don't amend it." This is the exact same advice I learned in my degree program and which I and my coworkers have given plant purchasers for years. Where I live, clay soils are very common - if someone plants a shrub or tree and puts "good black dirt" (as it's known here) into the hole instead of the clay that came from it, they're very likely to create an underground flowerpot and drown the plant. And then they complain that we sold them a "lousy" plant and want their money back!

I have known some people who've amended the soil for their plants, but they've done so over a large area and only concentrated on surface amendments and organic mulch.

Jurp
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    Your "don't amend it" is very specific on your situation (soil, climate). I do amend the soil (also clay) because summer is very dry, It will help to keep some more water (and it don't require me to water every few days). Amend mean also mixing, so not having layers, and tree roots should go further quickly (avoid too much watering in first years). In any case "amend" mean to improve soil, so it should be done rationally (and you are right, not with layers). – Giacomo Catenazzi Feb 26 '19 at 08:35
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    Actually, Giacomo, the "don't amend soil in a planting hole" philosophy applies to ALL soils. If you want to avoid watering often, use a good mulch: from the manual "Soil Science and Management" - "Mulched soil absorbs water much more readily than bare soil, improving soil-water content..." and "mulches limit water evaporation from the soil surface..." The best way for tree roots to go quicker into the native soil is to backfill the hole with (non-compacted) native soils. – Jurp Feb 26 '19 at 14:31
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    References for you: https://s3.wp.wsu.edu/uploads/sites/403/2015/03/soil-amendments.pdf, plus three additional links here (at bottom of the page): https://puyallup.wsu.edu/lcs/ (article three deals with the concept of "healthy soil"). Compaction is far worse for trees and shrubs than the native soil. – Jurp Feb 26 '19 at 14:34
  • If your dealing with rocky soil that calls for amendment over a large area, how deep would you have to dig down? Would that vary depending on the type of tree roots your dealing with (Taproots or lateral roots)? – Hamid Sabir Feb 26 '19 at 16:25
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    This reminds me of when I grew a plant in a container that had outdoor clay loam soil on the bottom, and potting soil on the top. The roots hardly entered the clay loam soil at all, but they completely filled the portion of potting soil. They needed more root room, but they wouldn't go down and fill the rest, whereas plants planted in only the clay loam soil grew roots in it. – Brōtsyorfuzthrāx Feb 28 '19 at 00:31
  • @shule, interesting comment. It makes wanna think, what if i buy a plant from the nursery and take it out of it's pot or bag and place it into my native soil, does that mean that it would not outgrow the soil in which it came in? So what then? Do you wash all the roots individually before planting it into the ground? – Hamid Sabir Feb 28 '19 at 12:29
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    @HamidSabir Your plant should be fine. People transplant plants grown in store-bought potting mix into the garden all the time and it works. If there's a lot of store-bought potting mix, and you have hard, clay soil, I might be more concerned if you don't loosen up the garden soil and let the roots of your plant get a taste of it. Put some native soil on top and water it in (that should get some of the native minerals into the potting mix, which should help the plant get used to using them some). I think it's probably more of an issue for indoor plants and things that aren't transplants. – Brōtsyorfuzthrāx Mar 12 '19 at 07:53
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    Some people recommend loosening up the roots in a certain fashion at transplant time (at least for some kinds of plants). I'm not sure what to say about that, whether it would be better or worse, but I think it would help with the particular concern, whether or not it would be beneficial overall. A lot of the other gardeners can probably tell you more about the right way to transplant with regard to what to do with the roots (although they probably won't mention my reason for mentioning it). – Brōtsyorfuzthrāx Mar 12 '19 at 07:56