If the plants are already established, then a thin layer of compost (1/2"-1") will be sufficient as a top dressing. If you are preparing planting holes, then dig the hole twice the diameter of the root ball but only as deep as the root ball. If the container is six inches in diameter and 6" deep, then dig a hole that is 5.5 inches deep an twelve inches in diameter. Mix about 30-40% compost with the native soil and use that to backfill the hole. What you don't want to do is create a 'pot' in your existing landscape.