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TREES & Plants

Fertilizing

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. However, 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 (e.g. dead branch tips, dieback) resulting from insect attacks or disease problems. Trees and shrubs which 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 will need to re-establish before fertilizers are applied. 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 for the Arboretum’s Plant Clinic.

Fertilizing trees and shrubs
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 (lbs) 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 11/2 times the diameter of the branch spread. For groups of plants, estimate the surface area underneath the entire planting to be fertilized. The example on the back page demonstrates how to determine the area to be fertilized, the amount of nitrogen needed, and the corresponding amount of fertilizer to use.

Example: Determining Nitrogen Application and Amounts

  1. Determine the dimensions of the area to be fertilized. To do this, estimate the diameter (D) of the tree’s branch spread and add one-half (D) to determine the area’s dimensions (see illustration). If (D) is 24 feet, then 11/2 times 24 feet equals 36 feet on each side. The area to fertilize is 36 feet by 36 feet or 1,296 ft2 (round to 1,300 ft2).
  2. Next, determine the amount of actual nitrogen (N) to apply based upon the treatment area. At a rate of 3 lbs actual nitrogen per 1,000 ft2, 3.9 lbs of N is needed for 1,300 ft2. (See table below.)
  3. Finally, determine the pounds of fertilizer to apply. To do this, divide the amount of actual nitrogen (N) to apply by the percentage of nitrogen in the fertilizer.

Treatment Area (ft2)

  1. 700
  2. 1,000
  3. 1,300
  4. 1,600

Amount of Actual Nitrogen (N) to Apply at 3 ils/1000

  1. 2.1 lbs.
  2. 3.0 lbs.
  3. 3.9 lbs.
  4. 4.8 lbs.

Using 20-5-5

  1. Apply  10.5 pounds of fertilizer
  2. Apply 15.0 pounds of fertilizer
  3. Apply 19.5 pounds of fertilizer
  4. Apply  24 pounds of fertilizer

Using 12-12-12

  1. Apply 17.5 pounds of fertilizer
  2. Apply 25.0 pounds of fertilizer
  3. Apply 32.5 pounds of fertilizer
  4. Apply 40.0 pounds of fertilizer