First always have a soil test on the area before planting any tree. Newly planted and younger trees should probably not be fertilized at planting because it can damage new roots, unless soil test shows need. This is an issue that has yet to be resolved. Some research says fertilize new trees and some does not. Seems if you do so carefully though you will come out ahead Established trees in yards should not need extra feeding. They receive adequate fertilization as lawns are fertilized using a 4-1-2 or 3-1-2 ratio fertilizer. (Note from Master Gardener – I make an extra pass around the drip line of trees at the time I fertilize the lawn and this seems to be enough fertilizer). Any heavy feeding should be in late winter for spring and summer growth; no special summer or fall feeding. Do not apply high-nitrogen fertilizers until the tree has developed good root system that can support vigorous top growth.
Answer courtesy of:
Courtney Blevins, CF
Regional Urban Forester
Texas Forest Service, Ft Worth