You sound just like I do. I leave the little roots in. Seems to work pretty good. Can't afford new every year. I do not get carried away with fertilizer like some, but a bit of all purpose might be use full. Biggest problem is disease but not much we can do about that.

Linked a number of previous discussions about this below. As you will see there are several different approchaes folks use.

As I have posted in some of those discussions I dump all my containers out onto a large tarp and spread it out to dry well in the sun - that alone takes care of some of the resident bacteria. Then remove all the root clumps and other debris, mix in approx. 1/3 new quality container mix with it and also add a good dose of Osmocote or other slow release fertilizer and refill the containers.

dave

Here is a link that might be useful: Reusing container soil discussions

thanks!