Fertilizing Shade and Ornamental Trees

Michigan State University Extension
MSU Extension Forestry Bulletins - FTE78601
01/03/01

Fertilizing Shade and Ornamental Trees

Melvin R. Koelling
J. James Kielbaso,
Department of Forestry
Michigan State University

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.

Need for Fertilization
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.

What are Fertilizers?
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.

When to apply
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.

How to apply
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.
ureaformaldehyde), 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 lb 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 of
nitrogen/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.

Application methods
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.

Other fertilizer needs
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 2
lb/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.

Commercial Materials
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.

Footnote:

(1) To calibrate a spreader, consult the manufacturer's
instructions or do it as follows. First, determine how much
fertilizer is needed to deliver the rate desired. (Refer to
page 5 for help in calculating the rate based on the
analysis you are using.) Next, convert this amount to that
needed to treat a 100 sq ft area. (Divide both numbers by
10.) If a 10-6-4 analysis fertilizer is to be applied at a

rate of 6 lb of actual nitrogen/1,000 sq ft, 60 lb of
fertilizer will be required per 1,000 sq ft, or 6 lb for
100 sq ft.

Weight out 6 lb of fertilizer and place in the spreader. On
a hard surface, such as the garage floor or driveway, mark
out a 10 x 10 ft square (100 sq ft or 1/10 of 1,000 sq ft).
Begin spreading the fertilizer within the marked area and
make the necessary adjustments until a setting is found
that will result in all the fertilizer being uniformly
spread over the area. This is a trial and error procedure
and will probably require readjustments of the spreader,
sweeping up the fertilizer and starting over. When the
correct setting has been obtained, multiply the amount
spread (6 lb in our example) by 10 to determine the
rate/1,000 sq ft. Follow the same procedure for other rates
or analyses, remembering to first convert to the amount
needed for 100 sq ft.

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