Young avocado trees should be fertilized very lightly. Apply one half teaspoon of Osmocote around each tree every spring. Mature avocado trees like a high nitrogen plant food and require 1 lb. of nitrogen per year but spaced so that they get half the nitrogen in spring and half in summer. One more tip. Your potted avocado trees may be susceptible to root rot if you overwater, let them stand in drainage water or put a mulch on top of the soil that is allowed to touch the stem.  Sunshine and good air circulation makes them happy. Keep growing, Marianne Binetti