Fertilizing Shade and Ornamental Trees
by Melvin Koelling and James Kielbaso
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Fertilizing Shade and Ornamental Trees by Melvin Koelling and James Kielbaso |
Introduction
Need
for fertilization
What
are fertilizers
When to
apply
How to
apply
Application
scenarios: Open grown
trees, Trees in
shade or in clumps, Ornamental flowering trees or small fruit trees, Evergreens
Application
methods
Calculating
doses
Other
fertilizer needs
Commercial
materials
Calibrating
spreaders
Shade and ornamental trees can be greatly improved through regular tree care. Fertilizer application is one of a number of practices available to the homeowner.
Many of us are aware of the need for fertilizing our garden and lawn, but we frequently forget that trees also benefit from fertilization. Like all plants, trees depend on sunlight, water, air, and certain mineral nutrients present in the soil for normal growth. Under forest conditions, the annual fall of leaves and twigs, and their eventual decomposition, provides a fresh source of nutrient materials. However, trees in lawns or similar areas are usually denied this source of soil enrichment since most homeowners gather up these leaves. Over a prolonged period of time, this practice can lower the fertility of the soil.
Infertile soils are common around new houses where the soil removed for the basement has been spread during filling and leveling operations. Normally, this soil has poor physical properties and may be lacking in adequate amounts of the necessary minerals. Fertilizer will benefit such soils and improve the growth of trees planted on these areas.
Trees growing on good soils will also benefit from fertilization. This is especially true if the soil is compacted or if normal root growth is restricted by sidewalks, driveways or building foundations. Fertilizer also helps develop good leaf color, recovery from insect, disease or other injury, and stimulates general tree vigor.
There is no single indicator that trees need fertilization. However, some symptoms may indicate the need for certain fertilizers: leaves of smaller than usual size, light green or off-color foliage, ends of branches containing dead twigs, very short elongation of branches during the growing season and a general lack of thriftiness or vigor.
Some nutrient deficiencies cause specific discoloration in the foliage. Chlorosis is the most common condition of this type. It results in the development of a light, yellow-green color, especially in the areas between the veins. It is most often associated with a lack of available iron. Other essential nutrients, including nitrogen, phosphorus and potassium also exhibit deficiency symptoms; however, such symptoms are not as apparent as those of iron.
Many kinds of materials can be added to the soil to improve fertility, however, some are more effective than others. Organic residues such as peat moss, manure, wood chips, etc. are beneficial in improving the physical properties of the soil, but they will not greatly increase the nutrient content of the soil unless added in large amounts over a number of years.
Most commercial fertilizers consist of inorganic compounds blended together to provide one or more essential nutrients. The three most common ingredients are nitrogen, phosphorus and potassium. Plants, including trees, require larger amounts of these three nutrients than other nutrients. Nitrogen is obtained principally from the atmosphere, phosphorus and potassium by mining rock deposits high in these elements. These nutrients are then converted to an available form, combined with an organic or inorganic carrier and formed into the familiar fertilizer granule.
Manufactured fertilizers differ in the amount of nutrients they contain. This difference is shown in the analysis (grade) on the bag. Fertilizer analyses are commonly expressed with three numbers, e.g., 10-6-4. The first number (10), in this example refers to the percentage of elemental nitrogen (N) present, the second number (6), to the phosphorus (P) content expressed as percentage P2O5 (phosphate) and the third number (4), to the percent potassium (K) in the form of K2O (potash).Since many different analyses are available, it is important to understand that a fertilizer with a grade of 16-8-8 contains 60 percent more nitrogen, 33.3 percent more phosphorus (P2O5) and 100 percent more potassium (K2O) per pound of fertilizer than a pound of 10-6-4. This means that 37.5 lb. of 16-8-8 or 60 lb. of 10-6-4 would be required to apply nitrogen at the rate of 6 lb./1000 sq. ft. This explains why some fertilizers are more expensive than others.
To be of greatest value to the tree, fertilizer should be applied in the fall, after the growing season. In Michigan, this is usually after the middle of September. Since root growth can continue until early December or later, fertilizers will be available and beneficial to the tree. Any fertilizer not used at this time will be available when growth resumes in the spring.
Fertilizer can also be applied in the spring as soon as the soil is free of frost. In Michigan, this is usually late March in the southern portion of the state and early to mid-April in northern areas. Fertilizer applied in early spring is available for the tree to use as soon as growth resumes. Since root growth begins before leaf development, apply fertilizers as early as possible.
Trees are usually not fertilized in mid-summer, although some benefit from fertilizing is possible where injury or defoliation has occurred. Do not apply fertilizer in late summer or flushes of new growth may result. Such tissue may not harden-off sufficiently before fall, resulting in winter injury. Apply fertilizers after the middle of September to avoid this danger.
To be effective, nutrient materials in fertilizers must be transferred into the sap stream of the tree. While most transfer occurs through absorption by the roots, some uptake may take place by absorption through the leaves (fertilizer solutions sprayed on foliage) or as a result of direct injection of fertilizer materials into the trunk of the tree. Because of these several means of uptake by the tree, many different methods of applying fertilizer have been developed. These include (1) application directly to the soil surface, (2) application (dry or liquid form) in holes in the soil, (3) foliar sprays and (4) injections (dry or liquid) into the trunk of the tree. No one method is best, even though all are available and used by commercial arborists. Often a combination of methods is used.
The application methods suggested are based on current research and are designed to consider the requirements of a tree for nitrogen, phosphorus, and potassium and the size of the tree and the environment (lawn area) in which it is growing. A method for determining fertilizer needs which is based on the soil surface area around the tree in contrast to some traditional approaches which relate fertilizer needs to trunk diameter is recommended. Trunk diameter is not as good an indicator of fertilizer needs as soil surface area beneath the tree.
The frequency of fertilizer applications will depend on the material and methods used. For example, nitrogen is required in larger amounts than phosphorus or potassium. And, fertilizer compounds applied to the soil surface or in holes in the soil will provide a longer lasting effect than materials applied as foliar sprays. Due to the differences in tree requirements, apply nitrogen fertilizers annually, and phosphorus and potassium, at intervals of 3 to 5 years.
The recommended method of application will vary according to the amount and type of grass beneath the tree. This is related to differences in nitrogen requirements between bluegrass (common in sunny lawns) and fescue (frequently planted in heavily shaded areas).
We will consider application methods for nitrogen, phosphorus and potassium in several different situations.
SITUATION I - Small (less than 25 ft high) open-branched tree which casts light shade, or any tree with lowest branches 12 ft or more from ground; bluegrass growing satisfactorily beneath tree.
a) Annually apply nitrogen to the surface when grass is dry at 6 lb/1,000 sq. ft of soil surface beneath the tree. Make 3 applications of 2 lb. each at 2-week intervals to avoid damage to lawn. If non-burning fertilizers are used (i.e. urea formaldehyde), make a single application (6 lb/1,000 sq. ft). Or you can apply 3 lb. of nitrogen fertilizer in the fall and an additional 3 lbs. in early spring. Refer to page 5 for calculation methods. Do not apply fertilizers within 3 ft of the trunk.
b) At 3- to 5-year intervals, apply a complete fertilizer such as 10-6-4, 16-8-8, or 12-12-12 in holes at a rate equal to 6 lb. of nitrogen/1,000 sq. ft.
SITUATION II - Trees which cast heavy shade, or trees in clumps beneath which fescue or other shade tolerant grasses are growing.
a) Surface application at recommended rate of nitrogen would be harmful to grass. Apply nitrogen annually in soil holes (Figure 5) at 6 lb. of actual nitrogen/1,000 sq. ft (see page 5 for calculations.
b) For phosphorus and potassium, follow same procedures as in Situation I.
SITUATION III - For ornamental flowering trees and other small fruit trees.
Unless definite need of fertilization exists, do not fertilize. Heavy applications of nitrogen may tend to reduce flowering. If fertilization is necessary, apply complete fertilizer such as 10- 6-4 or 12-12-12 in soil holes in the spring at a rate of 3 lb. ofnitrogen/1,000 sq. ft.
SITUATION IV - Evergreen
For large trees, follow recommendations given in Situation I and II. For small trees and shrubs, use complete fertilizer (such as 12-12-12) at 3 lb. of actual nitrogen/1,000 sq. ft.
Surface application of nitrogen fertilizers to the soil is quick, practical and inexpensive. Nitrogen readily leaches into the root zone and is quickly available to the tree roots. In contrast, potassium, and particularly, phosphorus, do not move readily within the soil and should be placed in the root zone. Nitrogen-containing fertilizers can be conveniently spread on the soil surface with a lawn fertilizer spreader. These spreaders distribute the material evenly, and when properly calibrated, accurately deliver the amount required. (1) Apply fertilizer to the surface only when the grass is dry otherwise some burning may occur to the grass. It is usually advisable to water the area thoroughly following application.
When fertilizer is placed in the soil, make small holes by using a drill or soil auger. Holes should be 1-1/2 to 2 in. in diameter and about 12 to 18 in. deep. Diameters larger than this are not recommended since the fertilizer will be located at the bottom of the hole rather than distributed more evenly. Make the hole at a slight angle slanted towards the trunk for best distribution.
Make holes in concentric circles around the trunk of the tree. The first circle should be no closer than 3 ft from the trunk and successive circles at 2 ft. intervals. Distances between any two holes should be approximately 2 ft. Extend the circular pattern of holes a few feet beyond the drip line or edge of the crown. The amount of fertilizer placed in each hole will depend on the total amount being applied to the tree. Ideally, this is determined by dividing the number of holes into the pounds of fertilizer required for the rate being used.
To avoid uneven grass growth, do not place fertilizers within 4 in. of the soil surface. After the prescribed amount of fertilizer is placed in each hole, fill the hole with peat or other organic materials. Do not replace the original soil on top of the fertilizer unless it is of good quality.
There are a few simple calculations to assist in application of fertilizers. For ease of determination, the circular area beneath the tree may be considered to be a square or rectangle. The sides of this rectangle are the length and width of the tree's crown. Remember to extend the distances for 10 to 15 feet, where possible. If a part of this area is covered by sidewalks, driveways, streets, etc., reduce the total area and therefore the amount of fertilizer should be proportionally.
How many pounds of 10-6-4 fertilizer will be required to apply nitrogen at the rate of 6 lb/1,000 sq. ft to a tree whose crown dimensions are 40 x 50 ft (2,000 sq. ft)? We are aware that 10-6- 4 contains 10 percent available nitrogen. Since the recommendation is for 6 lb. of nitrogen per 1,000 sq. ft, we need 2 x 6 or 12 lb. of actual nitrogen.
Knowing that 100 lb. of 10-6-4 contains 10 lb. of actual nitrogen, we can use the proportion 10/100 = 12/X, and find that X = (100 x 12/10) or 120 lb. This relationship may be used for most fertilizers if we remember to change the percentage figures accordingly. (i.e., 12-12-12 should be 12/100 = 12/X; 16-8-8 would be 16/100 = 12/X, etc.) If we had made 150 holes in the area to be treated, the amount to be placed in each hole would be 120/150 or 0.8 lb. or about 12 oz. A 6-oz can (e.g. frozen fruit juice) makes a convenient measuring container.
Thus far, we have considered tree fertilization as related to nitrogen, phosphorus and potassium requirements. While these nutrients are required in the largest amounts, all green plant require several other mineral nutrients. These include: calcium, magnesium, sulfur, iron, manganese copper, boron, zinc, and molybdenum. Most soils contain adequate amounts of these nutrients, however, one or more is occasionally deficient.
Iron deficiency is perhaps the most common, especially in alkaline soils or where lime has recently been added. In such soils, iron may indeed be present, but unavailable for absorption by the roots due to the alkaline soil. Lack of iron can cause chlorosis - characterized by light yellow-green color in the foliage. Pin oak is particularly sensitive to iron deficiency.
Chlorotic conditions can corrected by applying acidic materials to increase the acidity of the soil solution. Powdered sulfur is effective at the rate of 1 3/4 to 2lb./100 sq. ft of soil surface. Successive treatments may be required.
Iron chelates, a form of iron fertilizer, work even quicker to correct this problem. These compounds provide an immediate supply of available iron which is unaffected by the soil reaction. They may be applied to the foliage, but soil applications give longer lasting benefits. When using iron chelates be sure to follow the manufacturer's recommendations.
In some parts of Michigan, manganese deficiency has been observed in maple trees growing on highly alkaline limestone soils. Manganese sulfate or manganese chelate foliar sprays have improved foliage color and general appearance. When using these or similar compounds, be sure to follow manufacturer directions.
Other nutrient deficiencies may be present in your locality. Soil tests and foliar analyses can help identify these conditions. For more information, contact your County Cooperative Extension Service Office.
A number of commercial materials (devices) are available for fertilizing shade and ornamental trees. These include root feeders, trunk implants, fertilizer stakes, and other similar products. Although they are convenient, their effectiveness has not been thoroughly documented. Concern exists about the reported variation in results of many of these products. Homeowners should realize that such products may not be as effective as more proven methods.
To calibrate a spreader, consult the manufacturer's instructions or do it as follows.
The information contained on these pages is for educational purposes only. Reference to commercial products or trade names does not imply endorsement by the MSU Extension or bias against those not mentioned. These bulletins are public property and may be printed verbatim with credit to MSU. Reprinting cannot be used to endorse or advertise a commercial product or company.
If you have questions or would like additional information,
please contact
Dr. Karen
Potter-Witter, Professor, MSU Department of Forestry