A fertilization program is used to maintain trees and shrubs in a vigorous condition and to increase their resistance to injury from diseases and insects. But the addition of any soil nutrient is recommended only if soil or plant foliage tests indicate a deficiency. Trees and shrubs that need fertilization to stimulate more robust and vigorous growth include those exhibiting pale green, undersized leaves and reduced growth rates and those in declining condition (for example, dead branch tips, called dieback) resulting from insect attacks or disease problems. Trees and shrubs that should not be fertilized include newly planted specimens and those with severe root damage from recent trenching or construction. The root systems of these plants need to re-establish before fertilizers are applied. Older, established trees do not need to be fertilized every year.
For trees and shrubs in northern Illinois, the two most common causes of nutrient problems are high pH (alkaline) soils, which can lead to chronic deficiencies of nutrients in some tree species, such as red maple and pin oak, and nitrogen-deficient soils. Nitrogen (N), phosphorus (P), and potassium (K) are essential plant nutrients and these are most commonly applied. A list of soil testing services is available.