Fertilizing Trees and Shrubs in Accordance with the New ANSI A300 Standard
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Thomas Hanzely
Research Assistant
Soil Characterization Laboratory
The purpose of the American National Standards Institute (ANSI) A300 standard is to provide a basis for developing specifications for fertilization of trees, shrubs, and other woody plants. This standard is a guide for the practice of fertilization that is most beneficial to the plants. Infertile soils are most common in new building areas, both commercial and residential, where the ‘natural' soil has been altered. Altered soils display poor physical and chemical properties for tree and shrub growth. Soil modifications, where required, along with proper fertilization may improve nutrient uptake of plants. Once the plant has been established and is showing poor growth, fertilization may improve plant performance. Use of fertilizers in combination with a 4- inch layer of hardwood mulch can stimulate plant growth and vigor.
Newly planted trees and shrubs, and those with severe root damage from recent trenching or construction, should not be fertilized. The root systems of these plants need to be established or re-established before any fertilizers can be applied. Plants having restricted root zones because of sidewalks, driveways, or building foundations should not be fertilized. Plants must maintain an equal ratio between root growths and shoot growth. Plants with a disease, insect infestation, herbicide damage, or other conditions, which could increase to damaging levels, should be fertilized only in conjunction with an appropriate treatment program (ANSI A300).
Determining the Need for Fertilization
Some symptoms that indicate the need for certain nutrients include leaves smaller than usual, light green or off-color foliage, ends of branches that contain dead twigs, short elongation of branches during the growing season, and a general lack of vigor. To determine the need for fertilizer, three analyses should be considered before selecting a fertilizer: soil and foliar nutrient analyses and a pH test. Analyzing of soil and foliar nutrients will indicate health status. When analyses indicate nutrient deficiencies, a pH test can help further pinpoint the problem. The pH analysis will determine if the soil is either acidic (less than 7.0) or alkaline (greater than 7.0), which is useful in determining treatment for micronutrient deficiencies but not nitrogen deficiencies.
When to Fertilize
Fertilizers should be applied so that nutrients are available when roots are growing. To have the greatest value to trees and shrubs fertilizers should be applied in late fall (October - November) after hardening or in early spring (mid-March to April) once the soil is free of frost. Trees and shrubs should not be fertilized in late summer because new foliage may be produced and it will not have sufficient time to harden-off before winter, resulting in dieback.
When using fertilizers as soil acidifiers, tree and shrubs should be treated in both early spring and late fall. By using an acidifying fertilizer, the pH of the soil will be lowered allowing micronutrients that are already in the soil to be available for the plant to use. This technique is commonly used when a plant is displaying symptoms of chlorosis, light, yellow-green color between the veins of leaves. This is most often associated with acid-loving plants growing on alkaline soil where iron and manganese are not available to plants at high pH levels.
Types and rates of fertilizer
The two types of general nitrogen fertilizers are slow-release and quick-release. Slow-release fertilizers are composed with at least 50% water-insoluble nitrogen and are the preferred over quick-release (ANSI A300). Quick-release fertilizers are composed with less than 50% water insoluble nitrogen (ANSI A300). Acidifying fertilizers are composed of elements that, when combined with soil pore water and microbial action, create an acidic matrix in the soil. Fertilizers with a salt index of less than 50 are preferred. Salt index is the osmotic potential ratio of a fertilizer compared to sodium nitrate. Fertilizers with higher than 50, are more likely to cause plant damage. Lawn fertilizers should not be used as tree and shrub fertilizer, since many contain herbicides for weed control that can damage or even kill trees and shrubs.
Slow-release general fertilizers should be applied at rates between 2 and 4 pounds of actual nitrogen per 1,000 ft2 per application and shall not exceed 6 pounds of actual nitrogen per 1,000 ft2, annually (ANSI A300). If the plant health objective is not met with slow-release general fertilizers, quick-release general fertilizers may be used. Rates for quick-release fertilizers should be between 1 and 3 pounds of actual nitrogen per 1,000 ft2 per application and shall not exceed 4 pounds actual nitrogen per 1,000 ft2 annually (ANSI A300). Preferred general fertilizers are fertilizers with nitrogen contents between 12% and 30%, with phosphorus and potassium contents between 3% and 12%.
Acidifying fertilizers should be applied at rates between 2 and 4 pounds of actual nitrogen per 1,000 ft2 per application and shall not exceed 6 pounds of actual nitrogen per 1,000 ft2 annually. Preferred fertilizers with acidifying characteristics are Ammonium Sulfate and Sulfur-coated Urea.
Application Methods of Fertilization (ANSI A300)
Surface Application. Surface application of pellet fertilizer shall be uniformly distributed within the defined area of fertilization. Using a hand-held spreader and walking in straight lines back and forth across the defined area of fertilization is the easiest way to accomplish a uniform distribution. Where turf or ground covers exist, subsurface fertilization shall be the preferred method. Surface application shall not be made where surface runoff is likely to occur or near a water source.
Sub-Surface Dry Fertilization. Fertilization holes shall be evenly spaced within the defined fertilization area. Holes shall be 2 to 4 inches in diameter, spaced 12 to 36 inches apart, and 4 to 12 inches deep. Fertilizer shall be evenly spaced among the holes and shall not be closer than 2 inches to the soil surface.
Sub-Surface Liquid Fertilizer Injection. Injection sites shall be evenly spaced with the fertilization area. Injection sites shall be 12 to 36 inches apart and 4 to 12 inches deep. Fertilizer shall be evenly distributed among the injection sites.
Alternative Fertilization Techniques. All fertilization products shall be used in accordance with manufactures' recommendations. Foliar applications, injection, or implants shall only be used when soil application of fertilizer is impractical or ineffective in achieving fertilization objectives. Fertilizer specified shall be formulated for the application method. When applying foliar fertilizer, the fertilizer shall be sprayed to thoroughly cover the affected foliage at the proper stage of growth to achieve fertilization objectives.
Injections and implants. Products shall be applied as low as practicable on the trunk or root flare. Holes shall be made as small and shallow as practicable with sharp brad point drill bits. Small diameter trees and drought-stressed trees shall not be treated with injections or implants. Application intervals shall be at least one year apart and timing of treatment should be at the proper growth stage to achieve fertilization objectives.
Where to obtain the A300 Standard
Copies can be obtained from the American National Standards Institute, Inc, 11 West 42nd Street, New York, NY 10036 or through the International Society of Arboriculture, P.O. Box 3129, Champaign, IL, 61826-3129 (1-800-ISA-TREES).