Fertilizing Trees & Shrubs
Essential elements for plant nutrition include nitrogen, phosphorus, potassium, calcium, zinc, copper, molybdenum, magnesium, iron, sulfur, manganese and boron. They come from the soil and from applied fertilizer. Plants obtain carbon, hydrogen and oxygen from the air or through the soil.
Nutrient Needs of Shrubs
By Michigan State University - Extension
Essential elements for plant nutrition include nitrogen,
phosphorus, potassium, calcium, zinc, copper, molybdenum,
magnesium, iron, sulfur, manganese and boron. They come
from the soil and from applied fertilizer. Plants obtain
carbon, hydrogen and oxygen from the air or through the
Certain elements--such as boron, zinc, manganese, iron,
copper and molybdenum--are called micronutrients, because
plants require very small amounts of them. However, they
are just as essential for plant growth as the macronutrients -
nitrogen, phosphorus and potassium--which are required in larger amounts.
Objectives Of Fertilizer Application
Fertilizers may help improve the appearance and condition
of ornamental trees and shrubs. Increased vigor may make
the plants more resistant to attack by disease organisms
Many factors influence the fertilization program of plants
in the landscape. Unlike similar plants growing in the
nursery, landscape plants are often growing under stress.
Fertilization practices leading to satisfactory plant
growth must take into account these stresses.
Fertilizer response varies with the plant and the
environment. Soil fertility, aeration, drainage, exposure
to sun and wind, temperature of the site, and proximity to
buildings, walks and streets are but a few of the many
factors that influence plant growth.
Analysis or Fertilizer Grade
The analysis or grade refers to the minimum amounts of N,
P2O5 and K20 in the fertilizer. A 10-10-10 fertilizer
would contain 10 percent nitrogen (N), 10 percent P2O5
equivalent and 10 percent K2O equivalent. In 50 pounds of
10-10-10, there are 5 pounds of N, 5 pounds of P2O5
equivalent and 5 pounds of K2O equivalent.
In the future, fertilizers will most likely be expressed
entirely in the elemental form--N-P-K--rather than the
N-P2O5-K2O used today. Then today's conventional 10-10-10
fertilizer will be a 10-4-8 fertilizer. The percentage of
P in P2O5 is 43.6, so multiplying the pounds of P2O5 by
.436, gives the pounds of actual P in a fertilizer. The
percentage of K in K2O is 83, so multiplying the pounds of
K2O by .83 gives the actual K in a bag of fertilizer.
If any of these elements are not present in the
formulation, a zero would appear in the analysis. For
example, ammonium nitrate has no phosphorus or potassium,
and its analysis is 33-0-0.
To compute the number of pounds of nitrogen in a 100
pounds bag of ammonium nitrate (NH4NO3) multiply 100 x
.33, which equals 33 pounds of nitrogen. Dividing 33 by
the unit cost yields cost per pound of nitrogen.
Organic and Inorganic Sources
Fertilizers may be divided into two broad groups: organic
and inorganic, or chemical. An organic fertilizer is
derived from a living plant or animal source. Nitrogen
in an organic fertilizer is slow in becoming available for
plant use because the organic nitrogen (NH2) must be
reduced by micro-organisms to ammonium (NH4) or nitrate
(NO3). Generally, home gardeners tend to use organic
fertilizers more than commercial producers do because of
their high cost per pound of actual nutrient element.
Urea however, a synthetic organic fertilizer that is 45
percent N, is available at a low cost. In moist media at
a temperature above 60 degree F., it takes only about
three to five days for the complete conversion of urea to
Another organic fertilizer that may soon be used in
greater quantities is sewage sludge. Plants have been
shown to respond favorably when sewage sludge was applied
to the soil. Further research is needed before specific
recommendations will be made.
Chemical fertilizers are either mixed or manufactured and
have the advantage of low cost. Consequently, most
fertilizers used today are from chemical sources. High
analysis, water soluble, chemical fertilizers will injury
plants if not washed or brushed off the foliage.
Slow Release Fertilizers
Slow release fertilizers may be either inorganic or
organic. They are characterized by a slow rate of
release, long residual, low burn potential, low water
solubility and they cost more than water soluble
The most common element in a slow release fertilizer is
nitrogen. Several categories of slow release nitrogen
fertilizers are commercially available, including:
--Urea-formaldehyde (UF) (38-0-0). Released by
--Isobutylidene diurea (IBDU) (31-0-0). Released by
soil moisture and particle size.
--Sulfur coated urea (SCU) (36-0-0). Release rate
controlled by coating thickness.
--Plastic coated fertilizers (various formulations).
Release dependent on temperature and coating
--Natural organics--sewage sludge, process tankage and
Unlike most granular inorganic fertilizers, which contain
water soluble nitrogen (WSN), these slow release
fertilizers are primarily composed of water insoluble
nitrogen (WIN), which is released slowly. The majority of
the slow release fertilizers offer both rapid initial
release and long term release of nitrogen.
Soluble fertilizers have gained importance over the years
in landscape management and nursery production. They are
widely used to prevent and correct minor nutrient
deficiencies. Soluble fertilizers are applied either on
the foliage or on the soil.
Liquid fertilizers are important in production of nursery
stock, particularly as additives in spray operations.
Landscape and grounds personnel use liquid fertilizers
extensively for deep root feeding of trees and shrubs.
The purpose of fertilizing landscape plants during the
first year or two after transplanting is to increase
height, width and caliper. Once the plants are
established and growing well, however, the function of
fertilizing is to continue satisfactory growth and health
but not necessarily to produce maximum height or caliper.
Research has shown that about 3 lbs. of actual nitrogen,
the element most responsible for vegetative growth, per
1,000 square feet per year is all that is needed to
maintain the health of woody plants in most landscape
situations. If foliage color, annual growth or general
vigor is not normal, collect foliar samples, have them
analyzed and follow the recommendations that come back
with the results. Otherwise, use the suggested rate as a
To calculate the surface area under the branch spread of a
tree, multiply the radius times itself and then multiply
that by 3.14 (surface area = Radius2 x 3.14). (The radius
is the distance from the trunk to the edge of the branch
spread.) As an example, a 6-inch diameter trunk with a
total branch spread of 36 feet would have a radius of 18
feet. The area, according to the formula would equal 18 x
18 x 3.14, or 1,017 square feet. Following the
recommendation of 3 lbs. of actual nitrogen per 1,000
square feet, you would apply about 9 lbs. of 33-0-0
fertilizer (3 divided by .33 = 9 lbs.).
Woody plants respond well to fertilizers with a 3-1-2 or
3-1-1 ratio, such as 24-8-16, 18-6-12, 18-5-9, 15-5-5,
12-4-4 or similar formulations. An application of 3 lbs.
of actual nitrogen per 1,000 sq. ft. applies 1 lb. of P2O5
and 2 lbs. of K2O when using a 3-1-2 ratio.
The trend in recent years has been for fertilizer
formulators to use higher analyses in the fertilizer
package. Often the nitrogen content is 30 percent or more
and four or five times the phosphorus level. These
formulations, though promoted for turf, can be
satisfactorily used around woody plants. In fact, plants
with root zones beneath lawn areas that are fertilized at
least three times per year do not need additional
fertilizer applications. The use of fertilizer and
herbicide combinations around landscape ornamentals
increases the chance of herbicide injury on the
Timing Fertilizer Applications
In the landscape, plants are fertilized in spring and
fall. Fertilizing twice a year is preferable to the
common practice of fertilizing every two to three years.
The best time to fertilize is fall, generally after the
first hard freeze in September or October. The next best
time would be before growth begins in early spring,
usually between March and early May. If fertilizer is not
applied in the fall or the spring, it may be applied up to
July 1. Fertilizer applied after July 1 could promote a
late flush of growth that may not have time to mature
before freezing temperatures occur in the fall.
Methods Of Fertilizer Application
The various methods of fertilizer application include
injecting liquids into the soil, placing dry fertilizer in
holes drilled in the soil, applying fertilizer to the soil
surface and spraying it on the foliage. Which method you
choose should depend on the site and plant condition.
With most woody plant species, surface application is as
effective in provoking a positive plant response as other
methods. This method requires the least application time
and is the least expensive.
Liquid fertilizer injected into the soil is rapidly taken
into the plant by the roots, so injection is a good way to
apply necessary nutrients. Also, the addition of water to
dry soil is desirable during periods of drought.
Injection sites should be 2 to 3 feet apart, depending on
the injection pressure and 15 to 18 inches deep for established trees.
A major advantage to the drill hole method is the opening
of heavy (clay) or compacted soils, which allows air and
fertilizer to penetrate. With this technique and liquid
injection you avoid the excess grass growth that surface
applications cause in turf areas.
The drill holes should be placed in concentric circles in
the soil around the plant, beginning 3 feet from the main
stem and extending 3 feet beyond the dripline. Space
holes 2 feet apart and drill them 15 to 18 inches deep.
The recommended rate of fertilizer should be uniformly
distributed among the holes. Fill small holes with sand
following fertilization but only partially fill large
Liquid fertilizer sprayed on the foliage can not provide
all the necessary nutrients required by plants in the
amounts needed for satisfactory growth, but it can be very
effect for correcting minor nutrient deficiencies,
especially for treating iron deficiency using chelated
Micronutrient spray applications are most effective when
made just before or during a period of active growth,
usually from spring to early summer. Plant response--
greening of chlorotic foliage and normal growth coming
from buds on affected shoots--is usually observed from two
to eight weeks after treatment, but response time varies,
depending on species, age of the plant and its parts, the
time of year, the severity of the deficiency and the soil
conditions under plants are growing. One or two
applications during the year will prevent or control
deficiencies, but under some conditions it may be
necessary to make several treatments annually to continue
healthy growth. Using annual foliar sprays to correct a
chronic nutrient deficiency is usually not a practical
management practice for large trees.