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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.

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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. – Shule Feb 28 at 1:00
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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.

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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 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 at 15:45
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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 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 at 17:34
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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 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.

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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 at 8:35
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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 at 14:31
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    References for you: s3.wp.wsu.edu/uploads/sites/403/2015/03/soil-amendments.pdf, plus three additional links here (at bottom of the page): 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 at 14:34
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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. – Shule Feb 28 at 0:31
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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. – Shule Mar 12 at 7:53

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