Most deciduous hardwood trees such as oak or elm should be fertilized annually, but certainly not all with the same nutrient mix. Different trees have different needs depending on species, age and condition.  All deciduous trees are fertilized annually in the forest, when they drop their leaves.  A thick blanket of fallen leaves provides a constant supply of nutrients to a tree for the next several months.  In an urban setting, we remove fallen leaves, and we plant grass and shrubbery which compete with the tree for nutrition.  Proper fertilization for urban trees mimics the natural process by which forest trees receive their nutrition.