With very dense root mass, I will usually cut away the greater portion with a
JR-0194 Repotting Sickle (also called a root scythe or sod cutter) and wash away remaining field soil with a high pressure sprayer on a garden hose. Then the remaining roots can usually be combed out with a
Root rake on smaller trees or a
Root Hook on larger ones.
There is an article
here on using a root sickle. Repotting with a sickle is so fast it's a pleasure. I used to use a serrated knife, but the curved edge of the sickle is furrowed, so it's self-cleaning, like a treaded tire, and doesn't bind up.
Regards,
Matt