Trees require certain essential elements to function and grow. An essential element, or nutrient, is a chemical constituent that is involved in the metabolism of the tree or that is necessary for the tree to complete its life cycle. For trees growing in forested sites, these elements are usually present in sufficient quantities in the soil and are recycled from fallen foliage.
Landscape trees or urban trees, however, may be growing in modified soils or environments that do not contain a sufficient amount of available essential elements required for satisfactory growth and development. Nutrient-rich topsoil may have been removed during construction or may deteriorate over time.

Nutrient cycling can be limited due to the removal of fallen foliage, which in turn will limit the population and activity of earthworms and soil organisms that help to break down those fallen materials.

Therefore, it may be necessary to fertilize or to adjust the soil structure, biology, and chemistry to increase available nutrients in urban soils. Fertilizing trees can increase growth, reduce susceptibility to certain diseases and insect pests, and, under certain circumstances, help reverse declining health.

However, if the fertilizer is neither needed nor applied wisely, it may not benefit the tree at all—and may even adversely affect the tree and the environment.