What is the best way to fertilize a tree?

Root or trunk injections tend to be the best ways in which to fertilize trees.

Specifically, injections get the fertilizer immediately inside the tree without competition from other plants and without obstructions in the soil. Either the roots or the trunk give access to the tree's intake channel, the xylem. Otherwise, above ground fertilizers need to be spread from a couple of inches away from the trunk out to the drip line.