Sorry, guys, I am not getting these post notifications in my email, so I didn't know what has been added.
NJNative, well, I didn't forget about the rhizosphere, it's probably the main focus of everything we do. Me, personally, I amend soil for the sake of the roots. The fact that the plant has leaves is just an indication that I am correctly feeding the roots. Once I learned to feed the soil, not the plant, it really helped how I go about it.
I saw what Elaine Ingraham said, that a high level of actinobacteria (the white stuff) is detrimental to levels of mycorrhizal fungi, and that something like a tree doesn't want high levels of actinobacteria, but brassicas do want high levels of actinobacteria. A few points about her lecture.
1. She's describing Unfinished compost in her lecture. That pile she's got there isn't done, and when it's completely broken down those levels will not be so high in one thing and low in another. I think she just needed an obvious example. When compost is finished the while stuff will not be there in high levels, there are many more components that are created in the making of compost that make it a balanced mix.
Although putting deep mulch on top of the soil might create the same high levels of white actinobacteria in the process of breaking down, it will be what annual plant roots grow into. I don't know who would plant perennials in just mulch. We know they live longer, they get big, they need to get down into the soil to get water and micronutrients and just to hold themselves up in the wind. In that case, mulch with high levels of actinobacteria will stay on top and go very slowly into the subsoil.
I've never seen where Permaculture has ever suggested we plant a woody perennial in a mound of uncomposted mulch. Even hugelkulture is not about planting perennials in mulch. Mulching after planting, yes, but there's soil between the mulch and the roots.
2. A big difference between her example of brassicas and a tree, is that tree roots are huge and go down extremely deeply and extremely wide, way, way below even a foot of compost on the top where potential high levels of actinobacteria could be that they don't want. There are so many tree roots that they can take from many other places for many years than the small roots of annual brassicas. I think, again, she just needed a really obvious example.
It is one reason why it is recommended not to amend the planting hole of bareroot fruit trees or perennials, and always keep the mulch on top, that way the roots will not be overwhelmed by high levels.
3. Worms are a huge part of taking all of that compost and passing it through their systems, yet again helping the final product. In healthy soil that has plenty of organic matter in it the worms will take care of just about anything. There are dozens, probably hundreds of different kinds of soil critters, depending on where you live, that are also further breaking down organic matter in the soil, and making the environment the way the plant roots like it.
Soil biology is a huge, huge complicated field of study, and we gardeners touch on various parts of it. I'm sure that I'll never understand every possible thing, but learning what I have has changed how I grow things and has made a world of difference.
"Life flows on within you and without you"...George Harrison
Coastal California, USA, Mediterranean climate - no summer rain, a little frost mid-winter