Your source does not say that. It expressly states that that mix is for containers, not in ground planting. In ground planting of trees and shrubs should be in the native soil.

From your source:

”Planting:

To plant your pomegranate tree in the ground, dig a hole deep enough to cover the root ball. We recommend digging a hole in a cone shape and planting the tree to the center of the cone. Plant so that the top of the root ball is flush with the native soil level. If your location has heavy clay soil create a mound about 12” above the native soil line with a better draining soil mix of compost and mulch.

For container growing, you will want to create a soil mix that is 5-parts coarse bark, 1-part coarse perlite, and 1-part premium potting soil. This soil mix is designed for maximum drainage which reduces the chances of root disease.”

if you dig a hole in mother earth.. and amend the soil in that hole.. in effect your are using mother earth as a pot filled with un-natural soil ... just sounds like a lot of work and cost.. and potentially leading to more problems than solutions...

my short rule is.. NEVER amend a planting hole ...as you are creating a condition in the hole only.. and if well tended.. the plant should grow roots right out to the hole planted.. so what was the point of amending it anyway ...

you shouldnt need different PLANTING rules for different plants... according to name...

though in this case.. dealing with the pH issue is not really a planting issue... its a cultural issue ... is you see the difference ...

ken

notice section one.. august is not really a planting time fro trees and shrubs .... so i hope you are talking now.. but not acting for another month or so ... by mid sept.. night temps are tending toward cold.. so the plant can recover from whatever stress a fall day throws at the transplant ....

https://sites.google.com/site/tnarboretum/Home/planting-a-tree-or-shrub

@floral_uk z.8/9 SW UK I left out important details so I edited the post to make this clearer. I am building a large circular stone wall that will act as a raised bed, So I have some opportunity to create a hybrid soil mix that is more organic in the raised sections.

In any case, even for a pure container situation, have others noticed that this genus prefers heavily-concentrated bark soils?

@ken_adrian Adrian MI cold Z5 I won't be planting until late September.

I am not a big fan of adding heavy organic content into a planting hole. That would result in a "bathtub effect" with a highly moisture-retentive organic mix potentially under the water table of the surrounding soil, exposing the roots to too much water.

I think a purist doing an in-ground planting would say to get a soil test, then amend with just the minimal amount of organic material needed to correct any organic matter deficiency in the native soil. Since extremely fertile soil normally does not have more than 10% organic matter, that would be absolutely minimal amendment in the worst case.

My situation is different, with an enormous mass of soil sitting above the ground level, and guaranteed to be above the water table, because I am using a very high raised bed behind a retaining wall. I, therefore, have the opportunity to create something more like container soil, rich in organic matter, that will never become a water trap and death sentence for the roots.

I know some people do not like to create huge soil layer changes from organic to clay, on the thought that the roots may not want to penetrate into the deeper soil layers. This particular species/hybrid is not a very large plant, if you even want to believe that trees do not deal with changing soil layers all the time in nature. If you go to a place like Texas, in many places like Austin the top two feet of soil are organic matter, followed by three or four feet of heavy clay, followed by rock very near the surface. I think nature deals with such situations all the time, and trees are smarter than people when navigating such issues.