This very useful tip comes courtesy of from Michael Hagedorn of Crataegus Bonsai, author of Post Dated: The Schooling of an Irreverent Bonsai Monk.
For those of you who like growing maples and other deciduous trees that might develop a marvelous fused nebari, when that tree is young, fertilize right next to the trunk of the tree. Fine fusable roots will grow right under your fertilizer cakes, the kind that develop into the solid nebari pancakes that we see on really old bonsai and trees in the wild. If we fertilize only at the outside of the root system, the roots will simply try to run to the pot wall… We want them to stay closer to home in the early years.