Fertilization of Trees and Shrubs: A PrimerFH003 (6/05)
Fertilization of Trees and Shrubs: A Primer
Dr. Jeffrey S. Ward
Department of Forestry and Horticulture
The Connecticut Agricultural Experiment Station
123 Huntington Street
P. O. Box 1106
New Haven, CT 06504-1106
Telephone: (203) 974-8495 Fax: (203) 974-8502
The Essential Nutrients
Trees and shrubs, like all other plants, require 16 essential elements for proper growth. Three, carbon (C), hydrogen (H) and oxygen (O), comprise more than 90 percent of plant tissue. The remaining 13 are divided into two groups, macro- and micronutrients. Three of the macronutrients, nitrogen (N), phosphorus (P) and potassium (K), are called primary nutrients because they are required in relatively large amounts. Calcium (Ca), magnesium (Mg) and sulfur (S) are called secondary nutrients because they are necessary in lesser amounts. The remaining seven, known as micronutrients or trace elements, are iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), boron (B), molybdenum (Mo) and chlorine (Cl). These elements are needed in very small quantities. In fact, in excess, they can be toxic to plants.
The first three elements, C, H and O, are derived primarily from air and water. Despite the fact that 70 percent of the atmosphere is gaseous N, microbial or chemical transformation is necessary to transform it into a form useful to plants. P and K are derived from decomposition of soil minerals. All three are easily supplied by fertilizer. Ca, Mg and S are also supplied by decomposing soil minerals. They, too, may be easily supplied by fertilizers. Micronutrients are also derived from decomposed soil minerals. All 13 elements are eventually recycled to plants when vegetation is allowed to return to the soil and decay.
Not every case of poor growth of trees and shrubs is due to shortage of nutrients. Soil that is too wet or too dry will inhibit growth. Restricted roots or a girdling root will affect growth on all, or part, of the tree. Insect or disease damage to roots or to the aboveground portion of the tree or shrub may also affect growth. If careful inspection rules out these causes, then look at the foliage and shoot growth. If the leaves are of normal size with good green color and shoot growth is 6-8 inches or greater, then no fertilizer is likely needed. Trees or shrubs growing in a lawn that is fertilized regularly and abundantly may never require deliberate fertilization. Nutrient needs will be supplied by the same materials added to the turf.
On the other hand, trees and shrubs whose fallen leaves are removed completely each year may eventually require fertilization as the nutrient pool in the soil is depleted. About half the nutrients absorbed by woody plants each year are found in the foliage. Unless soil mineral decomposition and atmospheric deposition equal the requirements, a shortage of nutrients will result from regular removal of leaves with consequent decrease in plant growth and vigor.
Application of N usually gives the most noticeable response in terms of foliage color and plant growth and vigor. Although the other macronutrients are necessary, the plant response to application is less visible. Therefore, a complete fertilizer containing N, P and K is usually used. Some fertilizers are completely soluble in water; their nutrients are immediately available to the plants. Other fertilizers are formulated to release their nutrients slowly. Usually, chemical or microbial activity is necessary to put the nutrients in available form. Micronutrients are rarely deliberately added in Northeastern United States. They are required only in very small amounts; excess may be toxic to the plants. Enough occur from soil mineral decomposition, atmospheric deposition or as impurities in chemical fertilizers to supply the need. One notable exception may be iron chlorosis (yellowing of foliage) of pin oak on sandy, infertile soils. Insufficient available Fe will interfere with chlorophyll production, hence yellow leaves with green veins.
However, if you are not certain whether your tree or shrub needs fertilizer, have your soil tested. See the section, How to sample your soil, for instructions on how to collect a soil sample. The soil analysis will provide you with a report of the amount of fertilizer needed to maintain healthy trees. The analysis will also provide you with additional information on soil fertility.
In the absence of a soil analysis, rule of thumb suggests that an annual addition of 10-10-10 fertilizer at the rate of 10 pounds per 1000 square feet of land surface occupied by the trees or shrubs would be an adequate maintenance dose. Ten pounds of 10-10-10 fertilizer supplies 1 pound each of N, P2O5 and K2O. Lawns frequently receive several times this amount of N. Deliberate fertilization of trees may call for 2-3 pounds of N per 1000 square feet, but not necessarily every year.
How, Where and When to Fertilize
There are several methods of applying fertilizers to trees and shrubs. Experiments have compared their effectiveness in stimulating tree growth. In order of most to least effective were broadcast application of granular fertilizer to the ground surface, subsurface injection of liquid fertilizer, punched or drilled holes with dry fertilizer, and foliar spray of soluble fertilizer.
The quickest and simplest is uniform broadcast application of an immediately available granular fertilizer to the ground surface. Rain or irrigation dissolves and leaches the nutrients into the root zone. Grass or other vegetation beneath the trees will also receive some of the benefit. At the higher rates of application grass will be burned unless the application, is made in early spring while the grass is still dormant.
Injection of liquid fertilizer into the root zone is also an effective method which provides uniform application, provided that the injection points are properly spaced. Carefully done, this method does not damage turf or other vegetation beneath the tree; therefore it may be done over a longer period. However, a high pressure pump and injection needle are required.
Placement of dry fertilizer in punched or drilled holes is a simple but less effective method unless the spacing of the holes is close. Lateral and upward movement of dissolved nutrients in the soil is not great. If the holes are too far apart the distribution of the nutrients will be irregular. Older literature called for holes to be 15-18 inches deep. For nearly all trees and shrubs, 80-90% of roots are in the upper 12 inches of soil and most are in the upper 6 inches. Deep holes place fertilizer below all but a few roots. The same criticisms could be made of plugs or rods of slowly available fertilizer inserted into the soil except that they are placed near the soil surface.
Foliar application of soluble fertilizer is least effective. Although distribution is uniform, the spray is very dilute to avoid damage to foliage and may require repeated application during the same growing season to be effective. Heavily cutinized or waxy leaf surfaces may limit or prevent absorption of the nutrients.
Think of a tree as a wineglass on a dinner plate. The wineglass stem represents the tree trunk, the cup represents the crown, and the plate represents the root system. It is very important to understand that tree roots often extend 2-3 times farther out than the dripline (tree crown edge). For example, if the tree crown extends to 10 feet from the trunk, then the root system may extend 20-30 feet from the trunk. The wineglass on a dinner plate analogy also emphasizes that tree roots, especially feeder roots, are shallow. Most feeder roots are found within the top foot of soil. Whenever possible, fertilizer should be spread over the extended root zone and not just within the dripline.
Experiments have shown that spring application of fertilizer gave better response in tree growth than similar treatments in autumn. Roots become active in early spring and remain active in the autumn even after shoots are dormant. Fall application should be delayed until after leaf fall to avoid stimulation of new growth which might not harden in time to withstand fall frost or winter freezing. Late summer application should also be avoided for the same reason.
How to sample your soil
Use a small shovel or trowel to collect thin slices of soil down to a depth of 6 inches from 10-12 places in the area to be sampled. Thoroughly mix the soil samples in a container and place one pint of the mixture in a sealed plastic bag. Print your name, address, and what you want to grow on a label and attach the label to the outside of the bag. Briefly describe any plant problems and whether you would like organic plant care suggestions. Send the soil samples to: Soil Test, The Connecticut Agricultural Experiment Station, 123 Huntington Street, New Haven, CT 06504-1106. In the Hartford area mail to: Soil Test, The Connecticut Agricultural Experiment Station, P.O. Box 248, Windsor, CT 06095. The results are normally returned to you within 10 business days.
For further information:
General tree care: (203) 974-8495 in Hartford: (860) 683-4977
Plant diseases: (203) 974-8601
Plant insects: (203) 974-8600
Soil testing: (203) 974-8521
Trees and shrubs acquire thirteen essential nutrients for proper growth from the soil. Too little, or too much, of any one nutrient can affect plant health and growth. This fact sheet discusses how to determine which nutrients are in short supply, and the proper application of fertilizers.