Fertilizing Landscape Plants
Fertilization is a very important component of plant health care in the
landscape. Fertilization is necessary to supplement naturally occurring
essential mineral elements in the soil to maintain an optimum supply for
plant growth. Soil analysis (testing), combined with observations of plant
growth, are the keys for the home gardener to develop the most effective
nutrition program for the landscape. The mineral elements critical for
optimum growth and development of landscape plants must be present in the
soil and plant at proper levels.
The objective of this fact sheet is to help the gardening public make
informed decisions regarding the nutrition of their landscape plants.
Included is a brief review of soil analysis, soils, pH, essential elements,
fertilizers and fertilizer rates, timing, and methods of application.
Prior to planting, one of the first priorities is to have the soil
tested, simply because it is much easier to correct nutrition imbalances at
this stage. Additional soil tests every 2 to 3 years are highly recommended
to monitor the fertilizer program and prevent mineral element deficiencies
that could result in abnormalities or a decrease of optimum plant growth.
Samples should be taken from a minimum of 6 to 8 sites per area (tree and
shrub beds, vegetable garden, annual beds, etc.). The samples should be
combined and thoroughly mixed to provide uniformity. Dry the soil sample at
room temperature and place it in a self-mailer available from your local Extension offices in most counties or private soil testing
laboratories listed in the classified section of the telephone directory.
Results from the testing laboratory will include corrective
recommendations for soil pH, phosphorus (P), potassium (K), calcium (Ca),
and magnesium (Mg). Nitrate nitrogen (NO3N) and soluble salts
(EC, electrical conductivity) are not tested regularly by most laboratories;
however, these tests can be requested.
The physical and chemical properties of soils significantly influence the
growth of landscape plants. Fertilizer applications are dependent on organic
matter, soil texture (size of soil particles), and drainage.
Organic matter in soil may be a slow-release source of nutrients, may
contribute to desirable soil structure (arrangement of soil particles), and
increases total water available to crops. Organic matter increases the
water-holding capacity of sandy loam soils while increasing aeration of silt
and clay loam soils. As organic matter decomposes into humus, it becomes
colloidal in nature and cation exchange occurs (positively charged ions,
such as calcium and magnesium, are adsorbed on to negatively charged
particles). Incorporation of sphagnum peat moss, composted municipal sludge,
composted yard waste, pine bark chips, among other sources, is recommended
at planting if tests indicate less than five percent organic matter in the
Soil texture is determined by the relative amount of sand, silt, and clay
in the soil. Common soil textural classes are sandy loam, silt loam, and
clay loam. The surface area of soil particles is important and varies with
the size of these soil particles. Clay particles have 100 times the surface
area as the same volume of sand particles; therefore, clay that is
negatively charged has a greater capacity to attract positively charged soil
nutrients. Sandy loam soils must be fertilized more often than clay loam
soils because of their lower capacity to attract and hold (adsorb)
positively charged mineral elements.
As stated above, clay has a negative charge that can be measured to
indicate the exchange capacity for cations such as Ca++, Mg++, K+, and
others. This is called cation exchange capacity (CEC) and its determination
is included in many soil test results. The CEC is an indication of the
soil's capacity to provide nutrients for plant use, and is a measure of
nutrient leaching potential.
Soil drainage is critical to survival and growth of most landscape
plants, especially evergreen trees and shrubs. When the rate of water
movement through soil is restricted by fine-textured clay soils, sub-soil,
hard pan, or other material difficult to penetrate, a saturated zone may
develop in the root zone of plants. Spaces in the soil normally containing
air are filled with water, resulting in saturated soil. Wet soils cause more
problems to landscape crops than any other single cause. When drainage is
poor, roots are injured from the lack of oxygen, fertilizer uptake is
limited, and plant growth is reduced. Soil moisture problems can be solved
by installing surface and/or internal drainage.
Adjusting Soil pH
Mineral soil pH values between 6.0 and 7.0 result in the greatest number
of mineral elements to be available for uptake by plants. Several plants
such as certain conifers, most broadleaf evergreens, maples, oaks, sourgum,
and sweetgum should be grown in acidic soils with a pH from 5.5 to 6.0.
Other plants such as viburnum, hydrangea, and lilac grow best at neutral
(7.0) to slightly alkaline soil pH values. In most situations, mineral
element deficiencies can be avoided by proper soil pH management.
When the pH of a mineral soil drops below 4.5, aluminum (Al), iron (Fe),
and manganese (Mn) are very soluble. When this occurs, these elements are
absorbed in large quantities and may become toxic to certain plants, while
nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and/or magnesium
(Mg) may become limiting for plant growth.
As the soil pH increases, ions of Al, Fe, and Mn precipitate (settle out
of the soil solution) and the availability of these elements decreases to a
point where nutrients may become deficient for normal plant growth.
It becomes evident that a soil pH of 6.0 to 7.0 is generally desirable,
although slight adjustments are needed for specific plants. A soil test will
indicate the amount of lime needed to increase the pH of acidic soils or the
amount of sulfur needed to lower pH of alkaline soils.
Nine essential elements required in relatively large amounts for plant
growth are called macronutrients or major elements. Included are nitrogen,
phosphorus, potassium, calcium, magnesium, sulfur, carbon, hydrogen, and
oxygen. The last three are readily available in air and water. Seven other
essential elements required in small amounts by plants are called
micronutrients or minor elements and include iron, manganese, zinc, boron,
molybdenum, copper, and chlorine.
If an insufficient amount of any of these 16 essential elements is
lacking or in excess, plants will not grow properly. More or less distinct
symptoms occur for individual nutrient element deficiencies or excesses
because each element has its own role in the growth and development of the
plant. Once a deficiency or toxicity symptom is visible, plant growth has
been and will continue to be reduced until corrected.
The analysis or grade of a fertilizer refers to the minimum amounts of
nitrogen (N), phosphorus (as P2O5), and potassium (as K2O) in the
fertilizer, and is always printed on the bag, can, or bottle. A 10-10-10
fertilizer would represent 10 percent nitrogen, 10 percent P2O5,
and 10 percent K2O. Therefore, in 50 pounds of 10-10-10, there
are 5 pounds of N, 5 pounds of P2O5, and 5 pounds of K2O.
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 to become available for
plant use because the organic nitrogen (NH2) must be reduced (converted) by
micro-organisms to ammonium (NH4) or nitrate (NO3). The NH4 and NO3 forms
are useable by plant roots.
Inorganic or chemical fertilizers are either mixed or manufactured and
have the advantage of lower cost. High analysis, rapid solubility, and
availability necessitate some caution when applying these fertilizers.
Slow-release fertilizers may be either inorganic or organic. They are
characterized by a slow rate of release, longer residual, low burn
potential, low water solubility, and higher cost.
There are several fertilizer categories of slow-release nitrogen
fertilizers commercially available in garden centers including
urea-formaldehyde (UF) and related urea based formulations, isobutylidene
diurea (IBDU), sulfur coated urea (SCU), plastic coated (various
formulations such as MulticoteTM and NutricoteTM, salts
(MagAmpTM), and natural organics such as composted sewage sludge.
Water soluble or liquid fertilizer is applied either to the soil or on
the foliage. Many water soluble formulations are available for almost any
specific need from plant starter, high nitrogen fertilizers, to minor
element formulations. Chelated iron is used extensively for prevention and
control of iron deficiency of azalea, rhododendron, oak, and sweetgum, among
Rates of Application
Studies have shown that approximately three pounds of actual nitrogen per
1,000 square feet per year is needed to maintain the health of woody plants
in most landscape situations. If foliage color, annual growth, or general
vigor is not normal, the application rate should be increased to five pounds
of nitrogen per 1,000 square feet per year. Certain plants such as broadleaf
evergreens, dwarf conifers, and alpine plants should be fertilized with
one-half the above rates. If soil and foliar test results are available,
follow the recommendations provided, otherwise the suggested rates given
above could be used as a guide. Woody plants respond well to fertilizers
with a 4-1-2, 3-1-2, 4-1-1, or 3-1-1 ratio such as 24-6-12, 18-6-12, 20-5-5,
12-4-4, respectively. Landscape plants respond to 3 to 4 times as much
nitrogen as phosphorus, and twice as much potassium as phosphorus. An
application of three pounds of actual nitrogen per 1,000 square feet using a
3-1-2 ratio would include one pound of P2O5 and two pounds of K2O.
To convert from actual nitrogen to fertilizer, divide the amount of
actual nitrogen desired per 1,000 square feet by the percentage of nitrogen
in the fertilizer analysis or grade. Example: How much 18-6-12 is needed to
apply three pounds of nitrogen per 1,000 square feet? Answer: 16.6 pounds (3
/ 0.18 = 16.6 pounds).
Timing Fertilizer Treatments
In the landscape, fertilizing once a year is preferable to less frequent
applications, especially with newly planted materials. Applications twice a
year in light sandy soils or in seasons of excess rainfall are suggested.
The best time to fertilize in the northern United States is autumn,
generally after the first hard freeze in October and before the soil freezes
The next best time to fertilize landscape plants would be prior to growth
in early spring, between February and early April again in the northern
United States. If fertilizer was not applied during the autumn or spring
season, applications may be made up to July 1. Fertilizer applied after this
midsummer date is not recommended, as it could delay acclimation to winter
Methods of Application
Fertilizer can be applied in the landscape via 1) liquid soil injection,
2) drill or punch bar holes in the soil, 3) surface application, 4)
fertilizer stakes or spikes, 5) foliar sprays, and 6) tree trunk injection
or implantation. Each serves a specific role depending on the site and plant
health. Regardless of the method selected, the soil should be moist at the
time of fertilizing to prevent fertilizer injury.
Liquid Injection into Soil
Liquid injection of soluble fertilizer into the soil is rapidly absorbed
by the roots, and is an excellent method of correcting deficiencies quickly.
Injection sites should be 2 to 3 feet apart, depending on pressure, and 6 to
9 inches deep. Fertilizing deeper than nine inches may place the fertilizer
below the feeder roots. The addition of water to dry soil is desirable in
summer or during periods of drought.
Drill Hole or Punch Bar
A major advantage of the drill hole system is the opening of heavy,
compacted soils which allow air and fertilizer to penetrate the soil. This
technique and liquid injection prevent excess growth of grass in turf areas.
The drill holes should be placed in the soil in concentric circles or in a
grid system around the main plant stem beginning 2 to 3 feet from the main
stem and extending 3 to 6 feet beyond the dripline. Space the holes two feet
apart and drill them 6 to 9 inches deep. The recommended rate of fertilizer
for the area should be uniformly distributed among the holes.
After the fertilizer is applied, the holes can be filled with either
organic materials such as peat moss or compost or inorganic materials such
as gravel or calcined clay. The selection of organic or inorganic material
will depend on the greater need for either water or air after the fertilizer
Fetilizer Placement- Drill Hole Method
Fertilizing via the surface of the ground is as effective as most other
methods. However, this method should not be used in good quality turf, as
injury could occur, particularly if more than two pounds of actual nitrogen
per 1,000 square feet is applied at any one time. In turf areas, apply
fertilizer with either liquid injection or drill hole techniques.
Fertilizer Stakes or Spikes
Fertilizer stakes or spikes that are driven into the soil contain
satisfactory fertilizer materials. Unfortunately, the spacing of spikes is
such that very little fertilizer comes in contact with the root system. One
or two stakes per inch of trunk diameter does not represent adequate
fertilizer distribution because lateral fertilizer movement is limited in
Spraying liquid or water soluble fertilizer on the foliage should be a
consideration to correcting minor element deficiencies, especially of iron
or manganese. This method should not be considered adequate as a means of
providing all the macronutrients required by plants. To correct chlorosis,
several applications may be necessary during any given growing season.
Tree Trunk Injection or Implants
The infusion of liquid or implants of fertilizer is often the most
satisfactory method of correcting iron or manganese problems. In areas of
adverse soil pH, high moisture relationships, or locations where other means
of application are not practical, this method is often the most satisfactory
in obtaining desired results. Holes must be placed in the trunk root flare
which causes a wound that will close within a growing season.
1This factsheet was originally produced by Elton M. Smith,
Professor Emeritus, The Ohio State University for the Ohio Florists'
Association who has granted permission for its use and distribution. Dr.
Rose is the person currently responsible for the contents of this factsheet.