Amending soil for trees is actually fairly often recognized as bad practice. It can prevent the roots from stretching out and reaching down and thus can lead to a weak root system.
Here is some info:
....Amending soil impacts drainage. Amended soil often dries out faster in dry times and stays wetter in moist times, compared to native soil. Also, some studies indicate the possibility of a more congested root system being formed in amended soils due to multiple factors. Amending backfill soil has not been shown to be beneficial in any modern scientific study I have ever seen. Multiple studies have shown it to be of no benefit or to be detrimental in various soil types from pure sand to pure clay.
If you want to amend the soil, do so sparingly.
The Myth of Soil Amendments (from sustainablehorticulture.com) The Myth of Soil Amendments (by Linda Chalker-Scott)
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