Should I add organic matter when filling holes made for planting trees? Charles Dowding answers
There is no need to do this – keep it for a surface mulch. I used to add lots of organic matter when planting trees and bushes in the 1980s, following the advice given at that time to enrich the soil around roots in holes made for planting. But the consensus of advice has now switched to applying organic matter on the surface, and this shows how myths can lead you to do the wrong thing, then suddenly disappear as our understandings improve. Here is some advice from North Carolina State University in 2000 (Planting Trees and Shrubs):
“Traditionally, the recommendation was to incorporate organic matter into the backfill (soil used to fill a planting hole). Some gardeners took the practice further and completely replaced the removed soil with purchased topsoil. Research has shown neither practice helps plants grow and in some cases can be detrimental. When water enters soil with one type of texture and later comes in contact with soil that has a very different texture, water movement (drainage) is impeded. Some researchers report that amended backfill can cause roots to remain in the planting hole instead of growing into the surrounding soil.”
I find trees grow well from being placed in a hole of native soil, just wide enough for their roots, then mulched with organic matter on top.