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NATURAL HIST. SURVEY

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Fertilizing and Watering Trees

Dan Neely and E. B. Himelick
Section of Botany and Plant Pathology
Illinois Natural History Survey

Hlinois Natural History Survey Circular 56

Printed by authority of the State of Illinois

Circular 561s a revised edition of Circular 52, which bears the same title. Circular 52
was originally printed in 1966 and reprinted in 1968 and again in 1972.

Cover illustration: Lloyd Le Mere
Design: Molly Hardin Scott
Composition: Patty L. Duzan and Eva Steger

No charge is made for most publications of the Illinois Natural History Survey, anda

list of these publications is available upon request. Single copies of most publications
are available to anyone requesting them. Requests for multiple copies should be made

in writing and should explain the use to be made of the publication. Address
correspondence to the Office of the Chief at the address below.

Illinois Natural History Survey
Natural Resources Building
607 East Peabody Drive
Champaign, Illinois 61820

NP10-4M-6-87

Citation:

Neely, D., and E.B. Himelick. 1987. Fertilizing and watering trees. Circular 56.
Illinois Natural History Survey, Champaign. 24 p.

hed Contents

Why fertilize? 1

What is a fertilizer? = 2

Should you fertilize? 6

Where should you place the fertilizer? 10
When should you fertilize and with what? — 12
How should you fertilize? —-13

Why should you water trees? 18

How and when should you water trees? 19
Summary of recommendations = _21

Selected references 23

REES are an indispensable part of a pleasing
landscape. Their establishment and maintenance concern
homeowners, arborists, municipal foresters, and those
responsible for the grounds in parks and around public and
private institutions and commercial buildings. The care of
trees involves several cultural practices, including fertiliza-
tion, about which this circular gives basic information.

A section on watering is also included because the soil
solution is the major, if not the only, vehicle for moving
nutrients from soil colloids and organic matter to the roots
of trees.

Why fertilize?

The correct and timely application of fertilizer benefits most
trees. Newly established trees grow more rapidly following
fertilization with a nutrient or a combination of nutrients
that occur naturally in limited amounts in soil: increased leaf
size, increased twig growth, and more rapid increase in
height. Slow-growing tree species, many of which have
desirable characteristics, can be stimulated to grow faster by
fertilization and can thereby be used in situations where slow
growth is undesirable.

Stunted leaves and the early loss of leaves often indicate
nutrient deficiencies in the soil. Leaf color, especially pale
green or yellow, also indicates deficiencies as do leaves with
mottled patterns between the veins or leaves with dead spots.
The leaves of many trees become a darker green following
fertilization, and this change of color makes them more
attractive.

Fertilizing also helps to maintain mature trees in a vigorous
growing condition. A vigorously growing tree is less
susceptible to certain diseases and to insect pests than is a
less vigorous tree. Canker-causing fungi occur more
commonly on weakened trees, and many noninfectious tree
diseases develop when soil nutrients and moisture are

Illinois Natural History Survey Circular 56

inadequate. Healthy, vigorous trees tend to resist borers,
whereas those growing under unfavorable moisture or
nutrient conditions are more susceptible to attack.

Established trees weakened by leaf diseases, insect defolia-
tion, mechanical injury, soil compaction, or drought often
show poor growth or the dying of branch ends. Fertilization
may stimulate additional growth so that the plant can
compensate for the conditions that caused the decline.

What is a fertilizer?

A fertilizer is a supplement, usually added to the soil,
composed of elements beneficial to plant growth. The
essential elements present in plant tissue in relatively large
quantities are called macronutrients. They are nitrogen,
potassium, phosphorus, calcium, magnesium, sulfur,
oxygen, carbon, and hydrogen. The essential elements
present in plant tissue in relatively small quantities—the
micronutrients—are iron, manganese, copper, zinc, boron,
molybdenum, and chlorine.

Magnesium, sulfur, and the micronutrients are adequate in
most soils and rarely limit plant growth. The carbon,
hydrogen, and oxygen used by plants are components of the
atmosphere and of soil water; under normal conditions these
three nutrients are never deficient. Calcium in soils is a plant
nutrient that primarily serves to neutralize soil acidity and
to increase the availability of other nutrients. It can be
required as a fertilizer in rare instances when soil pH 1s 4.0
and below. Nitrogen, phosphorus, and potassium are of
primary concern as soil supplements.

Nitrogen

Plant growth is more often limited by a deficiency of nitrogen
than by a deficiency of any other element. Nitrogen
compoundsare rare in the rocks from which soil is formed.
Although nitrogen comprises 78 percent by volume of the
earth’s atmosphere, that is a form not available to plants.
Certain bacteria in the soil use atmospheric nitrogen and

Fertilizing and Watering Trees

change it into a form that can be used by plants. In addition,
some atmospheric nitrogen is added to the soil during
electrical rainstorms. Most soil nitrogen available to trees,
however, is derived from decomposed plant material
returned to the soil. Microorganisms in the soil must break
down this complex plant material into simple inorganic
compounds before the nitrogen can be used by trees.

Nitrogen in plants occurs in proteins, which are the primary
components of protoplasm, the living material in plant cells.
Nitrogen is a component of chlorophyll pigments and
therefore is important in the production of food in plant
leaves by photosynthesis. Nitrogen is also found in some
plant vitamins and enzymes and is consequently essential in
metabolism.

An abundance of available nitrogen in the soil promotes
plant growth, particularly of the above-ground portions as
compared with the roots. When nitrogen is deficient, stunted
top growth, pale green to yellow foliage, and the yellowing
or drying of older leaves are common, especially during
drought.

Materials commonly used to supplement nitrogen in the soil
are ammonium nitrate, ammonium sulfate, and urea. These
materials are readily soluble in water. When they are applied
to the soil surface and followed by adequate rainfall or
supplemental watering, nitrogen is carried down into the
soil and made available to roots. Since nitrogen is also carried
away by water, nitrogen must be added to the soil at regular
intervals to maintain an ample supply.

Phosphorus

Most phosphorus in soil came from the rock material from
which the soil was derived. This form of phosphorus is
abundant but not readily available to plants. Soils with the
greatest amount of readily available phosphorus contain
abundant organic matter and a high percentage of clay. Most
soils have sufficient phosphorus for adequate plant growth,

Illinois Natural History Survey Circular 56

but additional quantities supplied as fertilizers may be
needed for maximum growth.

Plants use from one-tenth to one-fifth as much phosphorus
as nitrogen. Phosphorus is found in all living plant tissues
and is essential for good root growth, proper tissue
development, and the production of flower buds. It 1s
abundant in seeds and other storage organs. Phosphorus
has a direct role as acarrier of energy throughout the plant
and is also involved in photosynthesis. When the soil is
deficient in phosphorus, plants fail to get a good start at the
beginning of the growing season, have poor root growth,
and experience delayed flower production.

Phosphorus 1s available in commercial fertilizers as super-
phosphate, double superphosphate, and with nitrogen as
ammonium phosphate. The available phosphate in these
fertilizers reacts rapidly with the soil and remains in the area
of application. For this reason fertilizers containing
phosphorus must be placed in the soil near the tree roots.
Surface-applied phosphorus remains near the soil surface
and is available only to plants with roots in this region. Almost
no phosphorus is carried away by water.

Potassium

Most of the rocks from which soils were formed contained
potassium. Soils, therefore, usually contain more potassium
than nitrogen or phosphorus. The salts of potassium are
readily soluble in water and may be carried away in areas of
heavy rainfall, especially in sandy soils. Soil containing clay
or organic matter has a large amount of potassium ina form
unavailable to plants; however, this potassium is slowly
released in a form that plants can use.

Potassium is present in woody plants in quantities larger
than those of all mineral nutrients except calcium. It is not
found in any permanent structure but is involved in
changing a plant’s food into forms that can be used for
growth and other functions. It acts as a balancing agent

Fertilizing and Watering Trees

between root growth and top growth and between nitrogen
and phosphorus utilization. A deficiency of potassium is not
readily apparent in most trees and shrubs.

The most important commercial fertilizer containing
potassium is potassium chloride, commonly called muriate
of potash. Potassium is distributed from the point of
application in the soil somewhat faster than phosphorus but
not nearly as rapidly as nitrogen. Fertilizers containing
potassium should therefore be placed in the soil and not
applied to the soil surface. Only roots quite near the point
of application are able to absorb ample quantities of
potassium.

Formulations

Most granular or crystalline commercial fertilizers contain
nitrogen, phosphorus, and potassium in specified amounts.
Some calcium, magnesium, sulfur, and micronutrients are
also included either as impurities or in combination with
nitrogen, phosphorus, or potassium. The guaranteed
analysis of most fertilizers is shown on the bag as three
numbers, for example, 12—12—12. The first number gives
the percentage of nitrogen (N); the second gives the
percentage of phosphorus as phosphoric acid (P2O;);

the third gives the percentage of potassium as potash (K,O).
In many areas the application of all three primary nutrients
is not desirable, beneficial, or economical. Each of the three
can be purchased separately.

The value of organic versus inorganic fertilizers is often
debated. Organic sources contain a much lower percentage
of nutrients, are slower to release those nutrients, are more
difficult to obtain and apply, and are more expensive per
pound of nutrient received. Organic fertilizers containing
humus, such as manure or composts, improve soil aeration,
soil structure, and the capacity of soil to hold water. When
plant nutrients are of primary interest, the economics of
fertilizing definitely favor inorganic fertilizers.

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Illinois Natural History Survey Circular 56

Should you fertilize?

A number of factors should be considered before fertilizing
trees. Annual growth and leaf color can indicate nutritional
deficiencies. If trees have poor growth or pale green leaves,
fertilizer may make them grow faster and give their leaves
a darker green color. If trees are subject to attack by
canker-causing fungi or borers, fertilizer makes them more
vigorous and less subject to these attacks. The condition of
the soil also affects the fertilizer needs of trees.

Rate of annual growth

The annual shoot growth of a tree can be easily determined
on species with terminal bud scale scars. Bud scales enclose
and protect buds on the ends of twigs during the winter and
leave scars that encircle the twig after the scales fall in the
spring. These scars remain evident for several years on many
tree species.

The distance from the tip of the branch to the ring of bud
scale scars nearest the tip is the current season’s growth. The
growth of previous years can be determined by observing
the distance from bud scale scars to bud scale scars as they
occur down the twig (Figure 1). By observing the length of
growth for the preceding 3 or 4 years on several twigs, you
can estimate whether the growth rate was satisfactory or
unsatisfactory, increasing, decreasing, or stable, and suitable
for the species.

Growth rates among tree species vary and the growth rate
of a given tree is in turn affected by its age. Soil types, the
spacing of trees, and other environmental conditions also
affect rate of growth. As a general guide, terminal twig
growth on most trees should be 9—1 2 inches or more a year.
Trees approaching mature size may grow only 6—9 inches
a year.

A second method of determining growth rate in many tree
species is to measure the width of annual wood rings
produced in the trunk. This measurement is taken with most

Fertilizing and Watering Trees 7

ease and least damage to the tree with an increment borer or
increment hammer (Figure 2), which are available at most
garden supply stores by special order. Both are commonly
used by arborists and foresters, who compare cores of wood
from trees to determine their growth-rate characteristics.

Condition of the soil

In addition to the condition of the plants, the condition of
the soil must be known. In most instances, the best tool is a
soil-profile tube (Figure 3), but a spade or trowel can also
be used for taking samples of the soil. Several factors
affecting the condition of the soil should be considered.

1. How deepis the topsoil? Depth is important because topsoil
contains the organic matter and the microorganisms
essential to the recycling of mineral elements in plant debris.
It is the volume of soil with the physical, chemical, and
biological properties favorable for root growth. The greater
the depth of topsoil, the greater the water storage capacity
and the greater the depth of root penetration.

FIGURE 1. The upper two twigs are from a vigorous white ash. The current
season’s growth—the part between the tips of the twigs and the bud scale scars
(indicated by arrows) nearest the tips—is long and thick and has plump buds.
The previous season’s growth (partially shown to the right of the arrows) is also
long and thick. The lower two twigs are from a less vigorous white ash. The
current and previous seasons’ growths are shorter and more spindly and have
smaller buds.

Illinois Natural History Survey Circular 56

2. What is the color of the soil? Soil color often indicates
organic matter content (dark or black soils usually have
abundant organic matter) and the degree of weathering that
the soil has undergone.

3. Whatis the texture of the soul? ‘Texture reflects the relative
proportions of sand, silt, and clay in the soil. A loam soil
contains all three soil-grain sizes, has optimum water-
percolation rate and water-holding ability, and is highly
resistant to soil compaction.

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FiGURE 2. The increment borer (left) and the increment hammer (right) are tools
used by the arborist or forester to obtain cores from the wood of standing trees.

The cores obtained with each have been marked with ink to make the widths of
the annual rings more evident. In each instance, the upper core’s most recent
annual rings are closely spaced, indicating a slow growth rate; the rings of the
lower cores are widely spaced, indicating the rapid growth of healthy trees.

Fertilizing and Watering Trees

4. What is the structure of the soil? Soil structure reflects
organic matter content and results from the aggregation of
the various soil particles. Soil that remains in clusters or is
crumbly when sifted through the fingers even when moist
is most desirable.

5. Is the subsoil compacted, a tight clay, or does water move into it
readily? When water remains in a soil hole for 24 hours
following a rain, the soil is poorly drained. Much of the pore
space in such a soil is saturated with water for extended
periods during the growing season; in effect, that saturation
reduces the depth of topsoil suitable for tree growth.

6. Has the soil been disturbed? Soil compaction, a change in
drainage, the removal of a layer of topsoil, or a fill of clay
above the original topsoil often reduces plant vigor and
growth.

Soil with deep topsoil, silty loam texture, aggregate structure,
high organic matter content, good aeration, moderately high
water-holding capacity, and a subsoil that allows internal
draining is ideal for growing trees and seldom, if ever, will
fertilization of trees be required. Soil with a clay subsoil and
less than 8 inches of topsoil that is light brown or gray in
color, sandy in texture, and sticky when wet, is much less
satisfactory for optimum tree growth. Under these soil

FIGURE 3. The soil-profile tube is a handy tool for removing cores from the
upper 10—14 inches of soil. Many properties of the soil can be observed by
examining such cores.

10

Illinois Natural History Survey Circular 56

conditions, trees often benefit from fertilization, and annual
applications may be required for several years. An
agronomist, farm adviser, or extension agent should be
contacted for assistance with local soil problems.

Two chemical tests are used to determine soil deficiencies —
soil tests and plant tissue analyses. Although no method of
determining deficiencies in soil nutrients 1s applicable to all
plants under all conditions, soil tests reveal general soil
deficiencies and help to determine if the phosphorus or
potassium content is low in the soil around shade trees.
Diagnosing soil deficiencies by analyzing plant tissue is a
useful research tool but is impractical for determining the
fertilizer needs of trees.

Disadvantages of fertilizing

Although the advantages of fertilizing usually far outweigh
the disadvantages, certain outcomes should be kept in mind.
Fertilizing trees or shrubs in lawns also stimulates grass
growth, and frequent mowing may be necessary. Unless
regularly pruned, small ornamental shrubs that have been
fertilized may become too large for their locations in a few
years. Heavy nitrogen applications tend to increase twig
growth and to reduce flowering in some ornamental shrubs.
Prolonged fertilizing may cause some woody species to
become tall, spindly, or succulent and to develop a weeping
appearance.

American beech, white oak, and some varieties of crab apple
are reported to have been injured by fertilizer formulations
containing nitrogen, phosphorus, and potassium. All
fertilized plants should be observed critically each year to
determine the effect of fertilization.

Where should you place the fertilizer?

Roots grow where the soil environment is favorable. They
do not grow where oxygen is unavailable or where the soil
is compact and difficult to penetrate. Since soil pores and

Fertilizing and Watering Trees

oxygen decrease with depth, most active, absorbing roots are
near the soil surface.

The placement of fertilizer for the most efficient uptake of
the mineral elements by tree roots cannot be determined
with precision because the location of the absorbing roots is
usually unknown. Between four and ten major woody roots
originate from the root collar of most trees. These grow
horizontally through the soil and are most often limited to
the topsoil. They decrease in diameter rapidly within a
distance of 3 to 15 feet from the trunk and form an extensive
network of long, ropelike roots 4 to 1 inch in diameter. At
the ends of these roots, a network of smaller roots branch
to form fans or mats composed of thousands of fine, short,
nonwoody tips. These fine roots, with accompanying fungi,
are the primary sites for the absorption of water and
minerals.

These fans of fine, absorbing roots are not uniformly
distributed around the tree. Ina mature tree, a circular area
four to seven times the area covered by the branches will
encompass these scattered fans. In newly established trees,
in specimen trees in open lawns, and in trees in parkways,
the distance and direction of root spread can be estimated
with reasonable judgments. Determining the root area for
a particular mature tree in a grove or wood lot is difficult,
if not impossible, and the efficiency of fertilization is greatly
reduced.

For ease in calculation and application, we recommend that
fertilizer be applied in square or rectangular areas and that
the soil area available, not the size of the tree, be the
determining factor in placing the fertilizer. The minimum
area should have its four corners positioned so that all the
surface area beneath the drip line of the tree is treated
(Figure 4). Increasing the size of the treatment area may
encourage additional tree growth but with decreasing cost
effectiveness.

12

Illinois Natural History Survey Circular 56

When should you fertilize and with what?

Tree branch and trunk growth occurs primarily during May,
June, and July, but roots grow and actively absorb nutrients
throughout the year whenever the soil temperature is above
40°F. Scheduling of the time and rate of application will
depend on the types of fertilizers to be used.

Time of application

Nitrogen fertilizers should be applied annually. Little
available nitrogen remains in the soil from year to year, since
most of it is used by plants or carried away by water. Nitrogen
fertilizers are most efficiently utilized by trees when applied
in April or May; however, applications in October or
November will stimulate growth the following year.

Phosphorus and potassium fertilizers are chemically bound
in the soil and become available slowly throughout several
growing seasons. They should be added to the soil every 3—5
years in spring or in fall, whichever is more convenient.

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FiGURE4. Stakes have been placed at the four corners of the area to be fertilized.
They are far enough from the tree so that its entire branch spread is included.

Fertilizing and Watering Trees 13

Rate of application

Nitrogen is the nutrient most often lacking in the soil and,
therefore, the first to limit plant growth. Nitrogen fertilizers
can safely be added to the soil annually at the rate of 6
pounds of nitrogen per 1,000 square feet of area.

The amounts of phosphorus and potassium in soils vary
greatly. In some areas additions of either are unnecessary;
in others, an occasional application may be required to
provide optimum supplies. Applications of phosphorus and
potassium are of little or no benefit when sufficient quantities
are already present. The need for phosphorus and potassium
and the frequency of application should be determined by
chemical tests of soil. Contact your cooperative extension
agent for the locations of experiment station or private
soil-testing laboratories.

To prevent the soil from becoming deficient in phosphorus
or potassium following increased tree growth from annual
nitrogen applications, add these nutrients at intervals of 3—5
years and at these rates: phosphorus at 3.6 pounds

of phosphoric acid (P;O0;) per 1,000 square feet and
potassium at 6 pounds of potash (K,QO) per 1,000 square feet.

How should you fertilize?
Three successful methods of fertilization are surface

application, the placement of dry fertilizers in holes in the
soil, and the injection of liquid fertilizers into the soil.

Surface application

Nitrogen fertilizers applied directly to the soil surface are as
effective as or more effective than nitrogen fertilizers
applied by other methods. With rainfall or supplemental
watering, inorganic nitrogen fertilizers readily move down
into the soil. These fertilizers can be uniformly distributed
over the root area with one of two types of spreaders used
to fertilize lawns (Figure 5). Lawn spreaders are the easiest,
simplest, and most economical means of applying fertilizers
containing only nitrogen.

Illinois Natural History Survey Circular 56

Fertilizer should be applied when grass blades are dry.
Immediately after the fertilizer has been distributed, it
should be washed from the grass blades with a lawn sprinkler
or a spray nozzle on a hose. Fertilizer remaining on grass
blades that become wet following a light rain or the
formation of dew occasionally causes burning.

The amounts of fertilizer by source materials that will supply
the required 6 pounds of nitrogen per 1,000 square feet
and can be safely used in surface applications are listed below
(select only one):

Material Pounds per 1,000 sq. ft.
Urea 45—0-—0 13
Ammonium nitrate 33.5—0—0 18
Ammonium sulfate 21—0—0 29

Fertilizers containing phosphorus and potassium should not
be broadcast or spread on the surface except at rates
recommended for lawn fertilization. Applying such fertil-
izers as 10—10—10 at the recommended rate for nitrogen of
6 pounds per 1,000 square feet may cause severe damage
to grass. Do not use nitrogen fertilizers that have gotten wet
and become lumpy or caked for surface application.

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FIGURE 5. Two types of spreaders used to apply fertilizer to lawns can also be
used to apply nitrogen fertilizers to the soil around trees. A kitchen scale and a
bucket help to ensure accurate application rates.

Fertilizing and Watering Trees 15

Dry fertilizers in holes

Another method of fertilizing trees is to place dry fertilizers
in holes in the soil. Because phosphorus and potassium
fertilizers applied to the soil surface are not readily available
to the nutrient-absorbing roots of trees, these materials
should be placed in the soil occupied by plant roots. Contrary
to popular belief, approximately 90 percent of the nutrient-
absorbing roots of trees are not deep but are located within
the top 12 inches of soil. Here moisture, aeration, and
nutrient conditions are favorable for root growth.

Holes can be punched in the soil with a punch bar or drilled
with an auger attached to an electric drill (Figure 6). Holes
may be drilled if the soil is dry and punched if it is wet.

FiGuRE 6. A punch bar (top) or an electric drill with a soil auger (bottom) can
be used to prepare holes for the application of dry phosphorus and potassium
fertilizers.

16

Illinois Natural History Survey Circular 56

Holes should be 12—15 inches deep and placed at 2-foot
intervals in a series of parallel lines 2 feet apart throughout
the area to be fertilized (Figure 7). Holes should not be made
within 21% feet of the tree trunk. Approximately 250 holes
are required in each 1,000 square feet of area to be fertilized.

If holes are properly spaced, the following quantities of
fertilizers by source materials should be placed in each hole
(select one P and one K source or an NPK source):

Material Amount per hole

Phosphorus (P)
Superphosphate 0—20—0 2 level tablespoons
Double superphosphate 0—40—0_ 1 level tablespoon
Potassium (K)
Muriate of potash 0—0—60 1 level tablespoon
Nitrogen, phosphorus, and
potassium (NPK)

10-10-10 ¥2 cup
12-12-12 slightly less than
¥2 cup

Preparing and filling holes is time consuming and labor
intensive, but merely drilling holes in a circle around the
drip line of a tree is unsatisfactory because the fertilizer is
inadequately distributed. In addition, root injury may occur
if too much fertilizer is placed in too few holes.

Injection of liquid fertilizers

Nitrogen, phosphorus, and potassium fertilizers in solution
may also be injected into the soil with a hydraulic pump and
a soil needle (Figure 8). Relatively expensive equipment is
required for this method of fertilizing, and fertilizer
materials must be completely soluble in water. Water-soluble
fertilizers containing both phosphorus and potassium are
much more expensive per pound of nutrient than are farm
and lawn fertilizers not soluble in water. Potassium chloride
and potassium nitrate are water-soluble sources of potassium.
Ammonium phosphate and potassium phosphate are
water-soluble sources of phosphorus. These materials can
be purchased from chemical supply stores.

Fertilizing and Watering Trees 17

SPREET

FiGuRE 7. Sites for placing phosphorus and potassium fertilizers in the soil should
be uniformly spaced in parallel lines throughout the area to be fertilized. If dry
fertilizer is to be placed in holes, the holes should be at 2-foot intervals. If liquid
fertilizer is to be injected, sites should be 22 feet apart.

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FiGuRE 8. A soil needle that is fed by a hydraulic pump may be used to inject
water-soluble fertilizers into the soil. It may also be attached to a hose and used
to water trees.

Illinois Natural History Survey Circular 56

The readily available, commercial, water-soluble fertilizers
are mixtures containing nitrogen, phosphorus, and
potassium. A satisfactory NPK ratio is approximately 1:1:1
or 2:1:2. Suggested formulations of water-soluble nitrogen,
phosphorus, and potassium fertilizers are listed below with
the number of pounds that should be dissolved in 200 gallons
of water and injected into 1,000 square feet of soil (select
one):

NPK formulations Pounds per 200 gallons

20—20-—20 30
23-19-17 26
25—10—20 24

Fertilizer solutions are injected into the soil at a depth of
approximately 18 inches. Injection sites are placed at
intervals of 2 2 feet in a series of parallel lines 2 /2 feet apart
throughout the area to be fertilized. Approximately 160
injections should be made for 1,000 square feet. Each
injection site should receive 1.2 gallons of solution. About
150—200 pounds of pressure is required to force the liquid
into the soil. Experience is required to distribute the material
uniformly.

Why should you water trees?

Water in plants has three vital functions. The hydrogen in
water is a true nutrient and is indispensable to photosyn-
thesis. Water also serves as the sustaining liquid in plant cells,
filling them and keeping them turgid. This turgidity keeps
stems upright and leaves fully extended. In addition, water
serves as a carrier. Nutrients can enter plants and be used
only in their ionized state, which requires an aqueous
solution.

Water in the soil is classified into four groups: water bound
chemically to mineral salts, water bound hygroscopically to
solid soil particles as a very thin film, water held in the soil
by capillary action, and water moving due to the influence
of gravity.

Fertilizing and Watering Trees

Water that is chemically or hygroscopically bound is not
available to plants. Gravitational water rapidly seeks a lower
level in the soil or runs off on the surface and is of limited
importance. The amount of suspended capillary water in
soil depends on the texture and structure of the soil. The
maximum amount of capillary water a soil can hold, after
the gravitational water has percolated through, is called field
capacity. Water available to plants is at its maximum when
field capacity has been reached.

How and when should you water trees?

Plant roots require both moisture and air for normal
development, but trees can be overwatered. Adding large
quantities of water too frequently to heavy clay soils may
bring about a water-logged condition. With the exclusion
of air, roots decline and die, and trees and shrubs may be
killed. Such losses occur most frequently in disturbed soils
when plants are located in clay fill or in potholes in clay
subsoil following construction work. The soil around plants
in such sites should be tile drained.

Recently planted trees

Trees or shrubs that have been transplanted may need to
be watered for 2—3 years to provide an adequate water supply
while their root systems are becoming established. Some
trees are not fully established for 3—6 years. Trees and
shrubs planted with bare roots normally require longer to
develop adequate root systems than do plants moved with
balls of soil. Older and larger plants require more time to
become established than do younger and smaller plants.

A newly planted tree or shrub is most easily watered if a
circular mound of earth 3—4 inches high is prepared around
the plant at the edge of the planting hole (Figure 9). This
mound serves as the dike of a reservoir that should be filled
with water at 7- to 10-day intervals during the growing
season. The reservoir holds a supply of water adequate to
soak the soil of the backfill and the soil in the ball around
the plant roots.

19

20

Illinois Natural History Survey Circular 56

Established trees

When rainfall is normal, established trees obtain an adequate
supply of water from the soil. During a drought or during
extended dry periods in the summer, all trees benefit from
watering, but trees weakened by injury, disease, or insect
pests are especially benefited. The relative moistness or
dryness of the soil can be determined by inspecting a soil
core removed with a spade or soil-profile tube. Soil taken
from different depths should be examined as it is crumbled
between the fingers. Dry soil will be powdery, but moist soil
will retain its structure.

Water applied to the soil surface fills the capillary spaces
from the top down. A daily surface sprinkling that wets the
soil to a depth of | inch or so 1s of little value to trees or
grass because most plant roots are at greater depths and
remain in dry soil. Instead, water should be applied less
frequently and in larger quantities.

Water should not be applied more rapidly than the soil can
absorb it. When water is applied too rapidly, it is lost through
runoff and erodes the soil surface. Heavy clay soils are

FIGURE 9. A mound of earth 3—4 inches high around a newly planted tree serves
as the dike of a reservoir that holds sufficient water to soak the soil of the backfill
and the soil in the ball about the plant roots.

Fertilizing and Watering Trees

difficult to wet and slow to dry out. They require more water
per application and applications at less frequent intervals
than do sandy soils. Sandy or light soils are easy to wet but
must be watered more frequently than heavier soils because
their water-holding capacity is less.

The most satisfactory means of supplying and uniformly
distributing adequate water to an established tree is with a
garden hose and an oscillating lawn sprinkler. To wet the
soil thoroughly requires the equivalent of 2 inches of rainfall.
During prolonged dry periods in the summer, watering
should be repeated at intervals of 2—3 weeks. Coffee cans
placed near the sprinkler make handy gauges for measuring
the amount of water that has been applied. If water begins

to run off the surface before the intended amount has been
supplied, half of the volume should be applied one day and
the remainder the following day.

Other means of supplying supplemental water are soaker
hoses and root-watering needles. Soaker hoses are suited for
such limited areas as border, hedge, and foundation plant-
ings. A root-watering needle is conveniently used around
small trees or shrubs. The needle has the advantage of
injecting water into the immediate area of the roots;
however, since only a limited amount of soil is watered at
each injection, the needle must be moved at frequent
intervals.

Summary of recommendations

Recommendations for the fertilization of shade trees and
shrubs should be based on controlled experiments using
known plant species and known soil types; however, only a
limited number of such studies have been made. These
recommendations, therefore, are based primarily on
experiments by the authors and on information gleaned
from research in arboriculture, pomology, forestry, and
agronomy.

Illinois Natural History Survey Circular 56

Fertilizing

1. Measure accurately the area to be fertilized and determine
its size in square feet. For ease in calculating areas and
applying fertilizer, plot a square or rectangular area.

2. Weigh accurately the amount of fertilizer to be used.
A bucket and kitchen scales are useful.

3. Apply nitrogen fertilizers annually to the soil surface at
the rate of 6 pounds of nitrogen per 1,000 square feet.
Uniform applications can easily be made with spreaders
commonly used to apply fertilizer to lawns. Nitrogen
fertilizers are most effective when applied in April or early
May before trees break dormancy. To prevent grass burn,
wash fertilizer from grass blades immediately after
application.

4. Apply phosphorus and potassium fertilizers every 3—5
years. Phosphorus should be applied at the rate of 3.6
pounds of phosphoric acid (PO) and potassium at the rate
of 6 pounds of potash (KyO) per 1,000 square feet.

One method is to place dry fertilizer in a series of holes
12-15 inches deep at 2-foot intervals in parallel lines 2 feet
apart throughout the area to be fertilized. A second method
is to use water-soluble materials and inject them into the soil
using a hydraulic pump and a soil needle. Injections are 18
inches deep at 2 2-foot intervals in parallel lines 2% feet
apart throughout the area to be fertilized.

Phosphorus and potassium may be applied in spring or fall
but are often applied in spring when hole preparation and
needle injection are easier.

Fertilizing can often be continued indefinitely. Some woody
species, however, may become succulent or develop a
weeping appearance after prolonged fertilization. All
fertilized plants should be carefully observed each year, and
fertilization should be discontinued when it fails to
accomplish a purpose.

Fertilizing and Watering Trees

Watering

1. Prepare a dike 3—4 inches high around the planting hole
of a recently planted tree or shrub. During the growing
season and until the root system has become established, fill
this dike with water at 7- to 10-day intervals.

2. During droughts or during extended dry periods in the
summer, water established trees with a lawn sprinkler at
intervals of 2—3 weeks. Each watering should be the
equivalent of 2 inches of rainfall.

Selected references

Fertilizer-plant relationships

Brady, N.C. 1984. The nature and properties of soils, 9th ed. Collier

Macmillan Publishers, London. 750 p.

Donahue, R.L., R.W. Miller, and J.C. Shickluna. 1983. Soils: an
introduction to soils and plant growth, 5th ed. Prentice-Hall
Inc., Englewood Cliffs, New Jersey. 667 p.

Foth, H.D. 1984. Fundamentals of soil science, 7th ed. John Wiley
and Sons, New York. 435 p.

Jones, U.S. 1979. Fertilizers and fertility. Reston Publishing
Company, Reston, Virginia. 368 p.

McVickar, M.H., and W.M. Walker. 1978. Using commercial
fertilizers, 4th ed. Interstate Printers and Publishers, Danville,
Illinois. 363 p.

Thompson, L.M.,and F.R. Troeh. 1978. Soils and soil fertility, 4th
ed. McGraw Hill Book Company, New York. 516 p.

Tisdale, S.L., W.L. Nelson, and J.D. Beaton. 1985.Soil fertility and

fertilizers, 4th ed. MacMillan Publishing Company, Inc., New

York. 754 p.

Tree fertilization experiments

Chadwick, L.C. 1935. The fertilization of shade trees in the nursery.
Proceedings of the American Society for Horticultural Science
32:357-360.

Chadwick, L.C. 1937. Fertilizer trials with shade trees in the
nursery. Proceedings of the American Society for Horticultural
Science 34:664—668.

24 Illinois Natural History Survey Circular 56

Himelick, E.B., D. Neely, and W.R. Crowley, Jr. 1965. Experimental
field studies on shade tree fertilization. Illinois Natural History
Survey Biological Notes 53. 12 p.

Neely, D., E.B. Himelick, and W.R. Crowley, Jr. 1970. Fertilization
of established trees: a report on field studies. Illinois Natural
History Survey Bulletin 30:235—266.

Smith, E.M.,and C.H. Gilliam. 1980. Soil fertility practices are vital
for growing healthy landscape plants. American Nurseryman
151:15, 78-82.

Smith, E.M. 1981. Fertilizing Malus ‘Snowdrift’ in the landscape.
Arboricultural Journal 5:137—142.

van de Werken, H. 1981. Fertilization and other factors enhancing
the growth rate of young shade trees. Journal of Arboriculture
71:33-37.

van de Werken, H. 1984. Fertilization practices as they influence
the growth rate of young shade trees. Journal of Environmental
Horticulture 2:64—69.

Tree planting and care

Carter, J.C. 1970. Illinois trees: selection, planting and care. Illinois
Natural History Survey Circular 51. 123 p.

Dirr, M. A. 1977. Manual of woody landscape plants, revised ed.
Stipes Publishing Company, Champaign, Illinois. 536 p.

Harris, R.W. 1983. Arboriculture. Prentice-Hall Inc., Englewood
Cliffs, New Jersey. 688 p.

Himelick, E.B. 1981. Tree and shrub transplanting manual.
International Society of Arboriculture, Urbana, Illinois. 76 p.

Partyka, R.E., J.W. Rimelspach, B.G. Joyner, and S.A. Carver. 1980.
Woody ornamentals: plants and problems. Chemlawn
Corporation, Columbus, Ohio. 427 p.

Perry, T.O. 1982. The ecology of tree roots and the practical
significance thereof. Journal of Arboriculture 8:197—210.

Pirone, P.P. 1978. Tree maintenance, 5th ed. Oxford University
Press, New York. 608 p.
Schoeneweiss, D.F. 1966. Prevention and treatment of construction

damage to shade trees. Small Homes Council, University of
Illinois at Urbana-Champaign. 8 p.

Illinois Natural History Survey
Natural Resources Building
607 East Peabody Drive
Champaign, Illinois 61820

A Division of the Illinois Department of Energy and Natural Resources

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