I have a tree that the soil has washed out around some of the roots. Should I replace the soil?
Yes, you should replace the soil to the same level it was at before the soil washed away. Don't bury the roots deeper that they originally were, though, because this can damage the roots. You can also place mulch over the new soil to prevent more erosion, or even plant a shade-tolerant grass, ground cover, or moss around it.