You should sterilize old potting soil before you use it again. To do this, put the soil in a well draining container (not a thin nursery pot) and poor boiling water into it. Let the water drain, and poor some more boiling water into it.

This will kill any diseases that may have been living in the soil. Now you can replenish that soil with compost, or by mixing it with some new soil.

One more thing...

Remove the bigger pieces of old roots that are in the soil. You don't have to get every little piece, just get the bigger ones.