By Len Phillips
Research on trees transplanted from nurseries indicates that there is little benefit to fertilizing at planting. Also, tree fertilization is not recommended on native soils because it is usually not needed. And conifers rarely need fertilization at all since most are adapted to low-nutrient soils. That said, there are some situations — typically with regard to trees in stressed conditions — in which fertilizer application can be beneficial.
Determining the need for fertilizer
The best indicator of whether fertilization is necessary is a soil test. Ideally, a soil sample should be taken before trees are planted. In the absence of a soil test, the next best indicator of the need for additional fertilizer is shoot growth over the past three years. If current shoot growth is in excess of 6 inches, then fertilization is unnecessary. If shoot growth is between 2 and 6 inches, then fertilizer may be applied. And if shoot growth is less than 2 inches, then fertilizer applications are appropriate. Foliage color is another indicator of the need for fertilizer as yellow leaves typically occur on trees that are not taking up enough nutrients. A final indicator is the history of the location. Trees in locations that are fertilized for turf on a regular basis rarely need to have supplemental fertilizer applied. In such locations, supplemental fertilizer should only be considered if shoot growth is less than 2 inches, or if a soil test reveals a specific nutrient deficiency.
Low soil pH levels (5.5 and below) may negatively affect trees by reducing the availability of useful nutrients and increasing the availability of aluminum, a potentially toxic element. In low-pH soils, lime can be applied to raise the pH, and this is best done before a tree is planted.
Many trees are injured each year by improper fertilization. Mistakes due to calculation errors during mixing, poor technique, or incorporating the wrong chemical into the tank mix are all too common. Although improper mixing and application account for some tree deaths, researchers have also discovered that excess nitrogen and phosphorous (even at the recommended rates of 4 to 6 lbs N/1,000 sq. ft.) have been shown to cause the loss of beneficial mycorrhizal activity and to increase pathogenic activity in the soil.
When to fertilize
Most trees experience a single flush of growth during spring followed by slower growth throughout the summer and fall. Because it is desirable to have nutrients available to the tree before the spring flush begins, the most beneficial time to apply fertilize is late fall or winter. However, winter applications can extend from when the ground is workable in the spring until just before trees start growing a month later. Fertilizer should not be added during the spring growth period.
If a tree shows signs that might indicate a nutrient deficiency, fertilizer can be applied at any time during the growing season. If fertilizer is applied under hot and dry conditions, it is important to provide water for the tree soon after the fertilizer is applied so that salts from the fertilizer do not build up and damage the tree’s root system. One inch of water should be applied every week around the area where fertilizer was applied.
What to apply
A soil test provides the best indicator of elements that may need to be added to the soil to prevent nutrient deficiency problems. Unless a tree is deficient in some other element, increased nitrogen (N) — more than other plant nutrients — has the most pronounced effect on the growth. However, if an increase in N produces a visible increase in growth, other elements may still be required.
High rates of phosphorous (P) should not be applied unless a need is indicated by a soil test. High rates of P can negatively affect the environment, especially lakes and streams. Never use a fertilizer that includes any kind of herbicide around a tree. These fertilizers may be beneficial to turf but can damage broadleaf trees.
Trees typically go through three stages of nitrogen needs:
1. During the newly planted phase, quick-release N levels should not exceed 0.1 lb N/100 sq. ft. per year, or a higher rate of a slow release fertilizer, up to 0.2 lb N/100 sq. ft. per year can be used. Slow-release and natural organic fertilizers can be incorporated into the backfill soil. Quick-release fertilizers should be broadcast after planting and then watered in. Do not mix quick-release forms with the soil used to backfill the planting hole, because direct contact with fertilizer will burn the roots.
2. On young landscape trees and shrubs where rapid growth is desirable, use 0.2 to 0.4 lb N/100 sq. ft. per year. For trees in lawn areas, do not exceed 0.1 lb N/100 sq. ft. per application unless a slow-release or natural fertilizer is used. Higher rates will burn the grass. Research indicates that fertilizer at this stage results in only a slight enhancement of tree growth.
3. As trees and shrubs mature and the growth rate slows down, the need for N drops to the level needed to maintain landscape plants in a healthy condition without excessive vegetative growth.
Too much nitrogen fertilizer will push leaf development at a rate faster than the roots can support. Too much fertilizer will also cause fertilizer burn. This occurs because the high concentration of fertilizer draws water out of the root and into the soil, in turn causing a ‘drought condition’ to occur in the tree. Flooding the root zone with water should reverse the problem, provided the soil is well drained.
Surface fertilizer application
Plants respond best to surface applications of fertilizer broadcast over the area where the tree roots lie. A late fall or winter application will let the tree get the maximum value from the nutrients. Dry or granular fertilizers can be applied by hand or with the use of a mechanical spreader. Trees should always be watered around the area of fertilizer application soon after the fertilizer has been applied. This helps to ensure that the fertilizer will move down to the tree’s root system before it can be taken up by weeds or grass. A good rain (1 to 2 inches) will also be sufficient to move fertilizer to the tree’s root zone. However, environmentalists stress not fertilizing prior to a rain due to risk of groundwater contamination should too much fertilizer be applied or too much rain occur.
Drill-hole fertilizer application
For established trees requiring phosphorus or potassium, or to apply a higher rate of fertilizer than 0.1 lb N/100 sq. ft., fertilizer can be applied using the drill-hole method. This method is advantageous for supplying phosphorus and potassium to trees because these nutrients are relatively immobile in soils. Drilling holes will reduce soil compaction and increase aeration. The drill hole method is, however, extremely time consuming and is generally only used by professional arborists and landscapers for high value trees and trees under extreme nutrient stress.
The drill-hole application method involves digging holes in a grid pattern 2 feet apart using a soil auger, with the tree at the center of this pattern. Holes should be 1½ to 2 inches in diameter and 1-1½ feet deep. The holes should be drilled in a series of parallel lines beginning 3 feet from the trunk and extending 2 feet beyond the drip line. For columnar trees, holes should be drilled 4 to 6 feet beyond the drip line. To calculate the amount of fertilizer to place into each hole, use the following formula: (100/analysis of N in fertilizer) × 0.12 = amount of fertilizer to add to each hole in teaspoons. For example, if you are using a fertilizer with an analysis of 18-8-8, then use (100 /18) × 0.12 = 0.66 or 2/3 teaspoons of fertilizer per hole. After the holes are drilled, place the recommended quantity of fertilizer in each hole, water the fertilizer in, and refill the holes.
Hydraulic fertilizer application
Hydraulic injection of liquid fertilizers into the root zone of the tree is an acceptable way to provide nutrients. The use of specialized equipment and fertilizers for this method can be costly. However, when a large number of trees need to be fertilized, this system may be economical. Injections are applied in a grid pattern similar to the drill-hole method except that injection sites should be 3 feet from each other and should extend 15 feet from the base of the tree. Hydraulic injection allows nutrients, including immobile elements, to be available to the tree more rapidly than any other method of fertilizer applications.
Foliar fertilizer application
To rapidly correct particular micronutrient problems (iron, zinc and manganese), a foliar fertilization supplies nutrients directly to the leaves, where they are needed. Foliar applications, usually made to landscape trees with a handheld sprayer, are effective in correcting specific nutrient deficiencies for a short period of time. Soil pH adjustment and additional soil application of these nutrients are required to correct the problem long term. Pesticides can also be applied as a foliar spray. However, foliar sprays are difficult to apply to large trees and the materials will sometimes leave a whitish film or spots on the leaves.
Fertilizer injection application
Another rapid way to correct micronutrient problems is by injecting these nutrients directly into the trunk of the tree. The micro-injection technique requires a good understanding of how a tree system absorbs nutrients. To follow the basic guidelines of micro-injection, the arborist/technician must know proper placement, size, and depth of the injection hole, and the acceptable conditions under which proper injections should take place. When fertilizer is injected into the sap stream of the tree, the material moves quickly throughout the entire vascular system. The individual components go directly to where they are needed for optimum benefit to the tree.
In cases where the soil pH is above the optimum range for the growth of a particular plant species, interveinal chlorosis, or yellowing between veins, may occur on the foliage. This chlorosis is usually an indication of poor iron availability in the soil. Even though the soil may contain enough iron, the plant is not able to take up and use the iron efficiently. The solution is to decrease the pH of the soil and apply iron chelate to the leaves or the soil. Foliar applications of iron chelate need to be made several times during the growing season.
It is not common that trees suffer from soil nutrient deficiencies; trees usually obtain enough nutrients from the soil in which they are growing. More often, the cause of the suffering is from inadequate water, poor or damaged root systems, or a high soil pH. However, if a newly planted tree is in poor soil or if there is a need for more rapid growth, fertilization can be beneficial. Water is still the overriding factor that usually determines plant health, even for established, mature trees.
Len Phillips can be reached via e-mail at email@example.com
Clatterbuck. Wayne K., “Post-Planting Tree Care: Fallacies and Recommendations”, Agricultural Extension Service, The University of Tennessee, 2006.
Coder, Dr. Kim D., “Tree Root Growth Requirements”, The Journal of The Society of Municipal Arborists Vol. 38, Number 2 March/April 2002.
Elliott, Marianne and Robert L. Edmonds, “Soil Nitrogen and Disease Severity in Pacific Madrone Habitats”, Center for Urban Horticulture, University of Washington, Autumn 2001.
Gillman, Jeff and Carl Rosen, “Tree Fertilization”, FO-07410, University of Minnesota, Department of Horticulture, 2000.
Gilman, Edward E. et. al., “Fertilizer Impact In Sandy Soil”, “Journal of Arboriculture” 26(3): 177-182
“Nitrogen Relationships of Ornamental Trees in Urban Settings: A First Look”, 1998 Research & Technology Development Project.
Shinano T., M Osaki, and M. Kato, “Differences In Nitrogen Of Trees”, “Tree Physiology” 2001. 21:712-624
Trowbridge, Peter J. and Nina L. Bassuk, “Trees in the Urban Landscape”, John Wiley & Sons, Inc. 2004.
Watson, G. and Dan Neely, “The Landscape Below Ground”, International Society of Arboriculture, 1993.
Selected plants with the ability to withstand a pH of 7 to 8
Scientific Name Common Name
Acer saccharinum Silver Maple
Alnus glutinosa Common Alder
Betula papyrifera Paper Birch
Carpinus caroliniana American Hornbeam
Catalpa speciosa Northern Catalpa
Celtis occidentalis Hackberry
Cornus sericea Red Osier Dogwood
Juglans nigra Black Walnut
Pinus banksiana Jack Pine
Pinus nigra Austrian Pine
Quercus bicolor Swamp White Oak
Quercus macrocarpa Bur Oak
Salix alba White Willow
Tilia Americana American Linden
Tilia cordata Little Leaf Linden