soil that looks like woodash

Mods - please do move if not in the right section 

When I have dug up some of my plants the soil looks like wood ash from a fire.  I know it didn't look like that when I planted the plants as I always use some form of compost.  What is making it like this???  Is it because sometimes the plants are near the privets??? and if that is so why isn't it all like that??

bit puzzling 

Whole root systems seem to be in it so I would say a lot.  Always happened since we moved in 1995.  I thought it was the hedges

I agree with JayG 

Fungus and dead do not always go together, healthy roots often have a symbiotic (my big word for this month) relationship with fungus.
Must be quite common on privet as i have seen it many times whilst putting in fencing posts.