Trees and shrubs growing in their natural habitat survive very well without supplemental fertilization. Leaves, needles and fallen branches decompose on the ground and recycle nutrients.
In the urban environment, landscapes are kept neat, and leaves and branches are removed. Soil is packed by human and vehicular traffic. Buildings often reflect heat, creating additional stress on plants.
Woody plants, like trees and shrubs, in urban landscapes may need to be fertilized when the leaves become smaller and lighter green over a period of years. Often, in the early spring, the leaves are green, but as the season progresses, they change color and fall prematurely. The tree may not be as full as it once was and the annual growth of the individual twigs may be shorter.
In general, the best time to fertilize is after the new leaves have fully expanded in spring to early summer. Fall fertilizer applications are not generally recommended.
Over-fertilizing can severely damage or kill woody plants and fertilizing is not a cure-all. Good tree care, including proper pruning and watering, are necessary for fertilizing to be beneficial. If the tree is planted in a bad location or it is not adapted to the area, fertilizing may do more harm than good.
For more information, see the following Colorado State Extension fact sheet(s). http://www.ext.colostate.edu/ptlk/1720.html