Injecting a liquid product or auguring holes for a granular product has been the traditional way of fertilizing trees. Holes or injectors going down the 12-15 inches is also pretty standard. I am unfamiliar with the technique of granular fertilizer applied o the surface for tree feeding. If you did not observe any adverse affects to the lawn, like excessive growth, then this method may work out ok in your yard. Trees can use more fertilizer than the lawn will tolerate as a surface application usually.