Standard recommendation to plant tree saplings is to dig a hole deep and wide, expose it for many days, add manure & pesticides and fill it all back on planting.
I have however been reading up on the right way to plant trees and it appears that digging deeper than the sapling’s container bag is actually counterproductive since it disturbs the stability of the plant when soil gets compact. Essentially this means drilling a deep hole with an auger etc. is not required and actually not recommended. Instead a wider hole seems to be favoured in order to improve aeration to the laterally growing roots.
Second, many seem to suggest not using manure in the back filled soil since its benefits are only short term.
I am going to plant some trees soon and am favouring shallow pits with a spade or a shallow hole with an auger instead of deep holes. I welcome thoughts and ideas.