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.
How and When to Fertilize
Fertilizers are labeled to indicate proportions of available nutrients. For example, a label showing a 20-5-5 formulation indicates 20% nitrogen (N), 5% phosphorus (P) as phosphoric acid, and 5% potassium (K) as potash. Thus, a 50 pound bag of 20% nitrogen fertilizer contains 10 pounds of actual nitrogen (50 x .20 = 10).
The following general recommendations apply to trees and shrubs needing a fertilization program. Soil and foliage test results may indicate more specific nutrient requirements.
For all trees and shrubs. If needed, the best time to fertilize is late April or early May, or late fall once plants are dormant. The recommended fertilizer should be spread evenly across the soil surface. The amount of actual nitrogen applied should be 3 pounds per 1,000 square feet. Do not use fertilizer containing herbicides, such as those formulated for use on lawns. The nitrogen content of the fertilizer should be 12% to 30%, with phosphorus and potassium at 3% to 12%. Fertilizer application rates are based upon the area occupied by the roots. Roots spread well beyond the branches on established trees and shrubs; therefore, the area beneath the plant to be fertilized should be 1.5 times the diameter of the branch spread. For groups of plants, estimate the surface area underneath the entire planting to be fertilized.