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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.

Contributors: Michigan State University

  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 
soil. 

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 
and insects. 

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 
ammonium. 

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 
fertilizer. 

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 
microbial degradation. 
--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 
thickness. 
--Natural organics--sewage sludge, process tankage and 
fish scrap. 

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. 

Liquid Fertilizers 

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. 

Fertilizer Rates 

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. 

After Planting 

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 
guide. 

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 
ornamentals. 

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 
holes. 

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 
iron. 

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. 

 

 

 

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