TREES & SHRUBS
When it comes to trees and shrubs in your landscape, fertilizing is
not necessarily an annual ritual. Many gardeners have the false
impression that the more fertilizer they apply the more the plant
will grow. Fertilizer is not plant food . Plants use water, carbon
dioxide, elements from fertilizer, and energy from the sun to
produce their own food. Synthetic (manufactured) and natural (some
times incorrectly called organic) fertilizers provide nutrients for
Adding the correct amount
of fertilizer can promote healthy flower production and foliage
growth while an excessive fertilizer application can decrease plant
health and can lead to decline and death. Over
application or incorrect application of fertilizer can contribute to
polluting our rivers, streams, lakes, and estuaries. Excess
fertilizer can increase the likelihood of some plant diseases.
Fertilizing plants that have already outgrown their allotted space
can only lead to more pruning. A moderate rate of growth and good,
green color is desired for most woody plants. Excessive vigor, which
is evident by lush, green leaves and long shoot growth is often
undesirable. Such plants are more susceptible to injury by cold in
winter, are more likely to be broken during wind and ice storms, and
usually will require more pruning than plants making moderate
All too often gardeners
assume that if a plant is not doing well they should fertilize to
correct the situation. Fertilization may be helpful but only after
the problem causing poor growth has been corrected. Plants which are
growing poorly will exhibit one or more of the following symptoms:
- light green or
- leaves with dead
- leaves smaller
- fewer leaves
and/or flowers than normal
- short, annual
- dying back of
branches at the tips
- wilting of
These symptoms of poor
growth may be caused by inadequate soil aeration, moisture, or
nutrients; adverse climatic conditions; incorrect pH; or disease.
Recently transplanted trees and shrubs often will not resume a
normal growth rate until the original root system is reestablished.
Plants disturbed by construction within the past five to ten years
may be in shock and exhibit limited new foliage growth. Do not
assume that an application of fertilizer will quickly remedy any
problem which is encountered, in many cases it can make existing
problems worse. You should attempt to determine the specific cause
in each situation and apply corrective measures.
Plants require 17 elements for normal growth. Carbon, hydrogen,
and oxygen are found in air and water. Nitrogen, potassium,
magnesium, calcium, phosphorous, and sulfur are absorbed from the
soil. The latter six elements are used in relatively large amounts
by the plant and are called macronutrients.
There are eight other elements that are used in much smaller
amounts; these are called micronutrients or
trace elements. The micronutrients include iron, zinc, molybdenum,
manganese, boron, copper, cobalt, and chlorine. The nutrients that
are most likely to limit plant growth are nitrogen, phosphorus, and
a primary component of proteins and is a part of every living cell.
This nutrient is usually more responsible for increasing plant
growth than any other nutrient. Shortages can cause slow growth,
reduced leaf size, yellowing, short branches, premature fall color
and leaf drop, and increases the likelihood of some diseases. An
over abundance can cause excessive shoot and foliage growth, reduced
root growth, low plant food reserves, and increased susceptibility
to environmental stresses and some plant diseases.
Nitrogen is a mobile nutrient it is in constant motion. Nitrogen
applied to the soil can be used by plants, washed off the soil
surface, lost to the air as a gas, or leached through the soil).
Nitrogen from granular fertilizer can enter streams from surface
runoff. Nitrogen loss is higher when a heavy rain immediately
follows a surface application of fertilizer, especially on sloped
areas. Incorporating fertilizer into the soil or lightly watering
(1/4 to 1/2 inch) after making a surface application will reduce the
amount of nitrogen loss.
Phosphorus (P) plays
a role in photosynthesis, respiration, energy storage and transfer,
cell division, and cell enlargement. It promotes early root
formation and growth, and the production of flowers, fruits, and
seeds. Many of our urban soils are low in phosphorus. Cultivated
farm land often has a high phosphorus level from years of
fertilization. In these cases, the addition of more phosphorus is
not going to increase yields and can potentially harm the
When applied as
fertilizer, phosphorus is quickly bound by soil particles.
Phosphorus is extremely immobile in soils (except sand); it moves
about 1 inch from its original placement. Unless phosphorus is
incorporated into the soil, watered in, or applied as a band, plants
may not be able to use it.
involved in many plant growth processes; it is vital to
photosynthesis and helps regulate water in plants. Potassium
fertilization helps plants overcome drought stress, increases
disease resistance, and improves winter hardiness. Potassium can be
leached through the soil by water, but not as quickly as nitrogen.
Nutrient uptake by
Nutrients in the soil can be in a solid form (granular fertilizer,
organic matter), attached to the soil particles or dissolved in soil
water. For an element to be absorbed by plants it must be in a form
the plant can use, and present (dissolved) in soil water. Water and
oxygen are required for nutrient movement into plant roots. Without
adequate oxygen in the soil, there is limited nutrient absorption.
Anything that lowers or
prevents the production of sugars in the leaves can lower nutrient
absorption. If the plant is under stress due to low light or
extremes in temperature, nutrient deficiency problems may develop
even though adequate nutrients are available in the soil solution.
Diseased or damaged roots, improper soil pH, water logged sites, and
plantings that are too deep can result in inefficient nutrient
absorption. Adding fertilizer under these conditions will not
enhance plant growth, and may actually cause plant damage.
The stage of growth or
how actively the plant is growing may also affect the amount of
nutrients absorbed. Many plants go into a rest period, or dormancy,
during part of the year. During this dormancy few nutrients are
Fertilizer: Any material used to supply one or more of the
essential plant nutrient.
Fertilizer ratio: The
relative proportion of N, P, and K. 16-4-8 fertilizer has a ration
of 4:1:2 or 4 parts nitrogen to 1 part phosphorus to 2 parts
Balanced fertilizer: A
fertilizer containing equal parts of each major element, i. e.
Complete fertilizer: A
nitrogen, phosphorus, and potassium. Examples of commonly used
complete fertilizers are 10-10-10, 16-4-8, and 12-4-8.
Incomplete fertilizer: A
fertilizer missing one or two of the major elements, i. e. 0-20-0.
Fertilizer analysis: The
minimum amount of each element in a fertilizer as stated on the
Weed and feed fertilizers: A
combination of fertilizer and herbicide. They are often used on
lawns to prevent certain weeds from germinating, or to kill existing
Chelate: Chemical compounds that help hold metal ions (such
as iron) in solution so the plant can adsorb them more readily.
High analysis: A
fertilizers containing 30 percent or more active nutrients, i. e.
ammonium nitrate 33-0-0. The cost per bag is usually more but the
cost per pound of nutrient is less (therefore the cost for
fertilizing a given area is less).
Types of Fertilizers
All fertilizers are labeled with three numbers, giving the
percentage (by weight) of nitrogen (N), phosphorus (P2O5),
and potassium (K2O). A 100 pound bag of fertilizer
labeled 0-20-10 has 0 pounds of nitrogen, 20 pounds of phosphorus,
10 pounds of potassium, and 70 pounds of filler. Filler is added to
make the fertilizer easier to spread and to reduce the likelihood of
burning plants with too much fertilizer. A fertilizer may contain
secondary nutrients or micronutrients not listed on the label.
The diversity of
fertilizer types and brands can be overwhelming when you go to make
a purchase. Generally speaking, the timing and rate of application
are more critical than which fertilizer you purchase. In regards to
water quality, the most important issue is the type of nitrogen in
the fertilizer you purchase. Most fertilizers contain nitrogen in a
quick release form (10-10-10 and 8-8-8), others are in a slow
release form (16-4-8, 12-4-8) some are a combination of both. Quick
release forms of fertilizer are immediately available, but do not
last as long and can cause plant damage if a large application is
ammonium sulfate, calcium nitrate, and potassium nitrate are all
water soluble, quick release forms of nitrogen. The nitrogen becomes
available as soon as it come in contact with soil water. Urea is an
organic form of nitrogen but it is also quickly converted to nitrate
nitrogen. High application rates combined with high irrigation or
rain fall can result in large amounts of nitrogen being leached
below the root zone.
nutrients at a rate that makes them available to plants over a long
period. Slow-release fertilizers need not be applied as frequently
as other fertilizers and there is less potential of leaching into
The initials W.I.N. and W.S.N. on fertilizer labels stand for water
insoluble nitrogen and water soluble nitrogen, respectively. The
water soluble nitrogen dissolves readily and is usually in a very
simple form, such as ammonia nitrogen or nitrate nitrogen. Water
insoluble nitrogen is referred to as a slow release nitrogen source.
Nitrogen which will not dissolve readily is usually in an organic
form of nitrogen (with the exception of urea) that must be broken
down into simpler forms by soil microorganisms before it can be
Sulfur-coated urea is a
slow-release fertilizer with a covering of sulfur around each urea
particle. Different thicknesses of sulfur control the rate of
nitrogen release. Watering does not affect its release rate.
Sulfur-coated urea applied to the soil surface releases nitrogen
more slowly than if incorporated into the soil. This material
generally costs less than other slow-release fertilizers, and it
supplies the essential element sulfur.
Some fertilizer products
are coated with multiple layers of resin. When they come into
contact with water, the layers swell and increase the pore size in
the resin so that the dissolved fertilizer can move into the soil.
Release rate depends on the coating thickness, temperature, and
water content of the soil. There is often a large release of
fertilizer during the first 2 or 3 days after application. Release
timing can be from 0 to 12 months, depending on the coating.
Natural fertilizer materials can serve as effective fertilizers, but
only if their nutrient contents are known and their mineralization
rates are estimated closely.
The advantages and disadvantages of natural and synthetic
fertilizers relate to the consumer, not to the plant.
Natural fertilizers are
mostly complex chemical substances containing carbon. In general,
natural fertilizers release nutrients at a slow rate over a fairly
long period. This can be advantageous for perennial crops, since
only one large application may be needed.
Most natural materials
are far less predictable in nutrient content, nutrient release, and
nutrient efficiency than commercial grade fertilizers. Before most
organic nutrients can be absorbed by plants, they have to be broken
down to an inorganic form by soil microorganisms through a decaying
process called mineralization. This process is affected by moisture,
temperature, and the microbial species and populations in the soil.
Most organic materials are effective only when the soil is moist and
soil temperature is warm enough for the soil organisms to be active.
They may not release enough of their principal nutrient at a time to
give optimum plant growth.
When packaged as fertilizers, natural fertilizers will have the
nutrient analysis stated on the labels. Some organic materials,
particularly composted manures and sludges, are sold as soil
conditioners and do not have a nutrient guarantee (small amounts of
nutrients may be available). Most natural fertilizers are high in
only one of the three major nutrients; many are low in all three.
Natural fertilizers can be expensive if applied in amounts adequate
to supply nutrients for good plant growth; however the value for
improved soil structure should be considered. The nutrient content
may need to be supplemented with other organic or inorganic
materials to achieve a balanced ration of nutrients.
How much to use varies
with the nutrient content of the material. The age of the material
is also a factor; when organic material decays and is rained on,
they lose nutrients, especially potassium and to some extent
nitrogen. Fresh manures may cause plant injury, especially when
large quantities are used.
Examples of Natural
Dried blood is a
byproduct from beef processors. As blood is drained from recently
killed animals, it is dried and ground into a powder. It is a rich
source of nitrogen and supplies certain essential trace elements
including iron. The gardener must be careful not to use more than
the amount recommended on the label.
Fish emulsion is a
partially decomposed blend of finely pulverized fish. The odor is
intense, but dissipates within a day or two. Fish emulsion contains
up to 5 percent readily available nitrogen and is a source of
several trace elements. Contrary to popular belief, too strong a
solution of fish emulsion can burn plants, particularly those in
Animal manure is
also a complete fertilizer but is low in nutrients. Manures vary in
nutrient content according to the animal source and what the animal
has been eating. A nutrient ratio of 1-1-1 is typical. The amount of
nitrogen in manure varies but is normally around 0.5 percent
(poultry litter is higher in nitrogen). This is 1/20 of the nitrogen
found in an equal amount of 10-10-10 fertilizer. Commonly available
manures include horse, cow, chicken, and sometimes sheep. The actual
nutrient content varies widely: the highest concentration of
nutrients is found when manures are fresh. As it is aged, leached,
or composted, nutrient content is reduced. Fresh manure should not
be used where it will contact tender plant roots. Manures are good
soil conditioners. Manures should be incorporated into the soil to
conserve nitrogen. If left on the soil surface, up to 25 percent of
the ammonia nitrogen can be lost within 2 days and 75 percent or
more can be lost within 1 month after application.
Rock phosphate is
ground rock that contains as much as 30 percent phosphate. It is not
in a readily available form and is released very slowly.
Granite dust contains
about 5 percent potassium but little of it is available for plant
6 percent potassium. It is also available very slowly.
Wood ashes are
often used as a soil amendment. They contain potash (potassium),
phosphate, boron, and other elements. Wood ashes can be used to
raise soil pH; use twice as much wood ash as limestone for the same
effect as lime. Ashes should not come into contact with germinating
seedlings or plant roots as they may cause root damage. Spread a
thin layer during the winter and incorporate into the soil in the
spring. Check pH yearly if you use wood ashes. Never use coal ashes
or large amounts of wood ash (no more than 20 pounds per 1,000
square feet), as toxicity problems may occur.
Wait until spring to fertilize fall planted trees and shrubs.
Wait six to eight weeks to fertilize plants installed in the spring.
Apply a slow release fertilizer in a light band along the perimeter
of the planting hole. Remember that newly installed plants are under
stress and should receive only a light application of fertilizer.
For 1 gallon container plants, apply 1 teaspoon of a nitrogen
fertilizer or 1 tablespoon of 10-10-10. For larger plants apply 2 to
Fertilization rates should be based on plant age, current and
desired growth rate, plant type, or by using general guidelines. The
rate should also be influenced by rainfall and soil type. A wet
season will normally increase the need to fertilize especially in
sandy soils. During periods of dry weather, reduce the amount of
fertilizer. Fertilizer encourages water-demanding new growth and can
injure roots of ornamentals under drought stress.
Growth rate -
The amount of new growth can be used as a guide to determine
fertilizer needs. When new shoots are more than 6 inches long in one
season, fertilizer is normally not required. When new shoot growth
is between 2 and 6 inches long, fertilizer is optional. When plants
exhibit poorly colored leaves, smaller than normal leaf size, or
premature fall color or leaf drop the plants may need additional
fertilizer. These signs may also indicate some type of root problem.
Deficiency symptoms do not give an indication of how much fertilizer
is needed --- only that fertilizer is needed.
Age - Newly
installed plants should be given time to reestablish their root
system before trying to push new growth with high nitrogen
fertilizer. If you are trying to push the growth of a young hedge
you may wish to make several light applications of fertilizer per
year (March, May, July). As woody plants mature, the need for
nitrogen decreases; rapid growth is no longer needed or desired.
Most established woody plants perform well with just one application
Type of plant/location
- Plants growing in a
restricted root zone will need less nitrogen. Plants with a fibrous
root system, such as azalea, rhododendron, and blueberry, are very
easily damaged by fertilizer. Light applications are recommended.
Plant roots normally grow three times as far as their branches.
Ornamentals located near a lawn that is fertilized regularly may not
need additional fertilizer since many of their roots extend into the
lawn area where they will absorb nutrients. It might be all that an
established plant needs.
General guidelines - Normally
2 to 4 pounds of a complete fertilizer per 1,000 square feet per
year is recommended for optimum growth. If your are trying to push
new growth use the higher rate. To determine how much of a
particular fertilizer to apply, divide the percent nitrogen into 100
and multiply times the amount of nitrogen recommended. For example,
the amount of 12-4-8 needed to apply 1 pound of actual nitrogen per
1,000 square feet is obtained by dividing 100 by 12 and multiplying
by 1. Thus 8.3 pounds of 12-4-8 should be applied per 1,000 square
For small trees and
shrubs use 1/2 to 1 cup of 10-10-10 fertilizer. The amount of
fertilizer should not exceed 1 tablespoon per foot of height for
fertilizers containing 10 or more percent nitrogen. Do not apply
more than 3 pounds of actual nitrogen per 1,000 square feet. For
large trees measure the diameter 4 feet from the ground and apply
0.1 pound actual nitrogen (0.1 pound of actual nitrogen equals 1
pound of 10-10-10, 0.3 pound of ammonium nitrate (33-0-0), or 0.2
pounds of 5-10-10) for each inch of trunk diameter. If the area
under the tree is known, simply broadcast 0.1 pound of actual
nitrogen per 100 square feet.
Unless a soil test
indicates otherwise use a fertilizer containing 10 to 16 percent
nitrogen. Fertilizers such as 16-4-8 and 12-4-8 have the ideal ratio
for woody plants, however, fertilizers such as 10-10-10 or 8-8-8 can
be used. At least 30 percent of the nitrogen should be in the
ammoniacal or urea form. They are slower to release nitrogen than
those in a nitrate form.
Time and method of
A nitrogen fertilizer application will have its greatest effect
three to four weeks after application. Woody plants can absorb
nutrients as long as the soil temperature is above 40°F. Root growth
occurs during cool weather even when the foliage appears dormant.
Root growth of woody ornamentals is most active in fall and late
winter/early spring but slows during hot, summer weather.
Fertilize trees and
shrubs in the spring or fall. Spring fertilizer application should
be made before new growth starts. Fall fertilization should be made
approximately one month after the first killing frost. Many
gardeners are reluctant to fertilize in late fall for fear it will
stimulate new growth if a period of unseasonably warm weather
occurs. Fertilizer applied in late fall is more effective in
promoting plant growth than spring applied fertilizer.
Fertilization in late
summer (mid August) should be avoided since it may stimulating late
growth that will not harden off before frost. Remember when applying
slow-release fertilizers around trees or shrubs to not apply late in
the season (after July 15) because they may keep the plant growing
rapidly late in the summer. The late season growth may not "harden
off" completely, and winter damage may occur.
The fertilizer should be
spread evenly over the entire root zone which can extend two to
three times the width of the branches. Remember that some of the
root zone may have already been fertilized when fertilizer was
applied to the lawn or flower bed. Sprinkle the fertilizer on top of
the soil or mulch and water lightly. Since the fertilizer will
quickly move through the mulch there is no need to remove it or to
place the fertilizer below it. Spread the fertilizer evenly under
the branches. Dumping fertilizer in one spot can caused the roots
below the fertilizer to be burned and die. Apply fertilizer when the
foliage is dry so the fertilizer does not stick to plant foliage and
cause burning. If fertilizer lodges in the whorls of plant foliage,
use a broom to brush the fertilizer off.
The practice of placing
fertilizer in holes around mature trees has been tested and research
indicates that surface application of fertilizer is sufficient since
most of the feeder roots are in the top foot of soil. If the soil is
compacted you can bore holes 4 to 6 inches deep, 2 to 3 feet apart
using a punchbar or a drill with a 2-inch auger. Start 2 feet from
the trunk and continue to 2 feet beyond the branches. Divide the
fertilizer into as many equal parts as there are holes and place in
the holes. Boring holes in soil can increase soil aeration and water
penetration into the root zone.
Fertilizer spikes and
stakes that are driven into the ground contain satisfactory amounts
of nutrients. Unfortunately, the spacing is such that very little
fertilizer comes into contact with most of the root system. Lateral
fertilizer movement in the soil is very limited.
Foliar sprays of a liquid
or water soluble fertilizer on the foliage can be used for
correcting deficiencies of minor elements such as iron or manganese.
This method should not be used to provide all of a plant's
fertilizer needs. The benefits of foliar sprays are short lived.
Since nutrients deficiency are often caused by a disease or improper
soil condition (pH, drainage, soil compaction) foliar sprays give
only temporary relief and do not correct the main problem.
Tree injections of
micronutrients is another method of fertilization that should be
used only as a means of last resort. Trees can be permanently
injured by drilling holes and the decay that could develop may out
weight any benefit the fertilizer might provide. Any benefit from
the fertilizer will be temporary at best.
Do not use weed-and-feed
fertilizers under trees or shrubs unless the label says it is safe.
Some plants, such as dogwoods, are very sensitive to dicamba
herbicide that is contained in many weed-and-feed lawn fertilizers.
VEGETABLE and FLOWER GARDENS
Plants grow using energy from the sun combined with nutrients taken
from the soil. Because the organic matter in soil holds nutrients
like a sponge until they are needed by plants, soil that is fertile,
well drained, and regularly enriched with compost often holds a
reasonable supply of plant nutrients. Unimproved, newly cultivated
soil is usually low in organic matter, so it is also low in
All edible plants remove some nutrients from the soil, and some have
such huge appetites that they quickly exhaust the soil (and then
produce a poor crop) without the help of fertilizer. Fertilizer is
especially helpful early on, when plants are making fast new growth.
You can mix fertilizer into individual planting holes, work it into
furrows, or use a turning fork to mix it into beds. You also can use
liquid fertilizers, but granular products last longer in vegetable
Always follow the rates given on the fertilizer label when deciding
how much to use. Too much fertilizer can be worse than too little!
Overfed plants often grow huge, yet bear a light crop late in the
With experience, you will learn how to match fertilizer amounts with
plants' needs for your climate and soil. Onions, tomatoes, sweet
corn, and vegetables grown in containers respond to special
fertilizing techniques, but most crops grow well if you simply mix a
balanced fertilizer into the soil as you set out the plants. Use the
lists below to help estimate the fertilizer needs of your favorite
Light feeders often benefit from a small amount of starter
fertilizer but require no additional feeding when grown in soil that
has been enriched with compost:
Moderate feeders often need good drainage and moisture-holding mulch
more than they need fertilizer. Avoid using organic fertilizers made
primarily from processed manure when preparing the soil for beets,
carrots, and other root crops. Manure can contribute to scabby
patches on potato skins and forked roots in carrots and parsnips.
Heavy feeders are often highly productive plants, so a few minutes
spent mixing in fertilizer before you set out plants is time well
spent. Just don't go overboard by applying too much! Plants often
grow slowly in cool spring weather, so wait until the weather warms
to decide that the application rate given on a fertilizer's label
was not enough. Some heavy feeders also respond to second helpings
later in the season.