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 Table of Contents

Group Title: Bulletin Florida Cooperative Extension Service
Title: Growth media for container grown ornamental plants
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00008512/00001
 Material Information
Title: Growth media for container grown ornamental plants
Series Title: Bulletin Florida Cooperative Extension Service
Physical Description: 13 p. : ill. ; 28 cm.
Language: English
Creator: Ingram, Dewayne L ( Dewayne Lebron ), 1952-
Henley, Richard W
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Place of Publication: Gainesville Fla
Publication Date: 1991
Subject: Plants, Ornamental   ( lcsh )
Potting soils   ( lcsh )
Plant growing media   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
non-fiction   ( marcgt )
Statement of Responsibility: Dewayne L. Ingram and Richard W. Henley.
General Note: Cover title.
General Note: "August 1991."
 Record Information
Bibliographic ID: UF00008512
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: ltqf - AAA6774
ltuf - AHY6664
oclc - 24683020
alephbibnum - 001674786

Table of Contents
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Full Text

fisi, f --
August 1991

Bulletin 241

Growth Media for Container Grown

Ornamental Plants

Dewayne L. Ingram and Richard W. Henley

Florida Cooperative Extension Service
Institute of Food and Agricultural Sciences
University of Florida
John T. Woeste, Dean

Table of Contents

Introduction ............................................. 1
Physical properties of container media ..................... 1
Chemical properties of container media ......................4
Soluble salts
Cation exchange capacity
Carbon to nitrogen ratio (C:N)
Organic components
Inorganic components
Formulating a growth medium for each production system ......11
Media mixing, handling and storage ........................13
Component storage
Mixing procedures
Media storage
Industry trends toward preblended media ....................14

Dewayne L Ingram is a former Environmental Horticulturist, and Richard W. Henley is Professor and Extension Environmental Horticulturist,
Central Florida Research and Education Center, Apopka, FL 32703.

Container production of nursery
stock has increased dramatically
within the last 30 years. Currently,
over 85 percent of the value of
Florida landscape and foliage crops
is from container produced mate-
rial, and the trend is similar
throughout the southern states.
Advantages of container production
compared to field production
include: convenient marketing
package, extension of the market-
ing/planting season, easier trans-
port and rapid crop turnover.
However, container plant produc-
tion requires more intense manage-
ment than field production. Roots of
container plants are exposed to
more rapid fluctuations and ex-
tremes in temperature than plants
grown in the ground. The large
surface area to volume ratio of
these containers provides little
buffering to environmental condi-
The purpose of a container
medium is to physically support the
plant and supply adequate oxygen,
water and nutrients for proper root
functions. The plant must be held
upright in the medium and the
medium must be heavy enough to
stabilize the container and keep it
in an upright position. The opti-
mum weight of container media
depends on the size and form of the
plant being produced and the
degree of air turbulence in the plant
production area. Excess weight
should be avoided since this ham-
pers handling and increases ship-
ping costs.
A balance between available
water and aeration in the growth
medium is essential for production
of quality plants in containers.
There must be adequate small pore
space to hold water for plant uptake
and enough large pores to allow
exchange of air in the medium to
maintain critical oxygen concentra-
tions. Anaerobic conditions (without

oxygen) do not allow the roots to
obtain energy from the respiratory
process and encourage disease
development. Energy is required for
root growth, proper hormone
balance and nutrient uptake as well
as maintenance of cell and or-
ganelle membranes and apparati
for basic physiological processes.
Plants can be grown in many
different media if proper manage-
ment is provided. The optimal
container medium will minimize
required management for quality
plant production. Plants can be
grown in nutrient solutions (hydro-
ponics) but the solution must be
aerated or circulated and changed
or replenished routinely. The
optimal container medium will thus
depend upon the specific plant
species or cultivar to be grown, the
size of container, environmental
conditions in the production area
(such as irrigation control, rainfall
distribution, irrigation water
salinity level, light intensity and
temperature), characteristics and
location of the markets and the
availability and cost of growth
medium components.
Since the growth medium relates
to every cultural practice in the
production of nursery crops in
containers, selection or formulation
of a container medium properly
suited for a given production system
is extremely important. A quality,
well-chosen medium is an invest-
ment that will pay great dividends
in terms of plant growth and
quality. Adjustment of cultural
practices such as disease or insect
control measures due to a poorly
chosen container medium may add
up to $200 in hidden or indirect cost
of a cubic yard of container medium
over the production period. Even
larger costs can be encountered
through losses in plants or reduced
plant quality due to a poor con-
tainer medium. It pays to purchase
or formulate a container medium
suited for each production system.

This publication presents the
principles of selecting and formulat-
ing growth media for container
plant production. Measurable
physical and chemical properties of
growth media are discussed. The
characteristics of selected growth
medium components and tech-
niques for customizing container
media for specific conditions are
presented along with recommenda-
tions and pitfalls relative to media
mixing, handling and storage.

Physical properties of
container media
Commonly measured physical
properties of container media
include total pore space, water-
holding capacity, air space, bulk
density and particle size distribu-
tion. Total pore space is the
volume of air in an oven-dry
growth medium expressed as a
percent of the volume. It is the
non-solid portion of the volume.
The water-holding capacity of a
medium is the volume of water
that is retained by a medium after
irrigation and drainage. The
amount of water held in a particu-
lar medium is dependent upon the
particle size distribution and the
container height. When a con-
tainer medium has been saturated
with water and allowed to drain
freely, the medium is said to be at
"container capacity." The volume
of the medium occupied by air at
this moisture level is termed the
air space. Some researchers also
refer to this air space at container
capacity as the aeration porosity
or drainable pore space.
Organic matter and minerals
comprise the solid portion of con-
tainer media. Approximately 20 to
40 percent of a growth medium will
be solids. This means the total pore
space averages 60 to 80 percent of
the container medium volume.
Particle size distribution and the
porosity of individual particles
determine the total pore space and


Figure 1. Container A Is filled with small
particles which fit together to form small
pore spaces while the large particles used
in container B form many larger pores.

the volume of water and air in a
container medium at container
capacity. Large particles fit to-
gether to create large pore spaces
(Figure 1). These large pores are
generally filled with air at container
capacity. When smaller particles
are mixed with the larger particles,
the volume of large pore space is
reduced and the volume of the
medium comprised of solids and
water after irrigation increases.
Thus, the particle size distribution,
or the relative volume of each
particle size range, determines the
water-holding and aeration proper-
ties of a container medium.
The internal pore space of a
particle obviously differs with the
type of particle. A perlite particle
essentially has no internal pore
space while 40 to 45 percent of a
pine bark particle volume is pore
space. Much of the intraparticle
pore space will be filled with water
at container capacity. A portion of
the water may be available for plant
uptake but a substantial percentage
will be unavailable. It has been
determined that water in container
media held at tensions greater than
100 centimeters of pressure is not
readily available to plants.
The amount of water present in a
growth medium decreases as
tension or suction is placed on the
water. By placing varying tensions
on the water in a medium, a water
release pattern can be developed

S2 PB: 1 P 1 S



25 50 75 100


Figure 2. Moisture retained by growth media at various tensions.

for that medium. This pattern is
usually referred to as the mois-
ture retention curve. Moisture
retention curves for various
growth media and media compo-
nents are presented in Figure 2.
Note that most of the water in pine
bark, peat and sand is held at less
than 25 cm of tension. Twenty to
35 percent of the total pore space
in a soilless growth medium may
be filled with tightly-held water
(greater than 100 cm of tension)
and is considered to be of little
value to container-grown plants.
When a containerized medium is
irrigated, there is a layer of nearly
saturated medium at the bottom of
the container (Figure 3). The
thickness of this excessively wet
layer depends upon the particle size
distribution, which determines the
water-holding capacity of the
medium. There are no capillary
pores to place tension on water at
the bottom of the container as one
would find at the same depth in a
field soil profile; therefore, water in
the bottom of a container is held

by very small tensions or in some
cases may be essentially free
water. If the container is tilted,
much of this free water would
drain from the lowest point of the
container. Water above the near
saturated medium at the bottom of
the container has tension placed on
it by the force of gravity. The
greater the distance above the
near saturated conditions, the
greater the tension exerted on the
water in that region of the con-
tainer medium. For example, water
between particles in the surface of a
15-centimeter tall container would
be held at tensions greater than 15
centimeters. Water between par-
ticles held at tensions less than 15

<-- content

*< content

Figure 3. Moisture gradient in container
media at container capacity.

centimeters would have drained
from this depth of the container
medium after irrigation. Water
held in pores inside a particle
may be held at greater tensions
or not be in contact with water in
or between adjacent particles and
not directly affected by the
Gravitational pull exerted by the
continuum of water from the base
of the container or from the top
of the saturated medium.
Therefore, the height of a
container will affect the air space
in the growth medium at container
capacity. The concentration of
water at a given height of the
medium from the bottom of the
container is not influenced by the
container height, only the medium
particle size. Therefore, increasing
the height of the container in-
creases the volume of medium with
the larger pore spaces filled with
air. You can demonstrate this
concept to yourself or others using
a sponge.
Obtain a large rectangular
sponge (about 1.5 inches thick, 5
inches wide and 12 inches tall).
Cut the sponge into 2 pieces,
approximately one-third and two-
thirds size portions. This sponge
represents a container medium
with a combination of large and
small pore spaces. Just as in a
container medium, a portion of
pore space in the sponge will be
filled with water after irrigation
and drainage. Remember that the
proportion of large and small pore
spaces in these two sponges is the
same; they were once one sponge.
Saturate the sponges in water then
stand them upright on a raised
stand of some sort over a pan.
Water will drain from the sponges.
This is the force of gravity pulling
water from the large pore spaces.
The concentration of water at a
given height in two sponges is the
same, even though one sponge is
taller than the other. After a few
minutes, squeeze the water from

the 2 sponges and measure the
amount of water obtained from
each. Try to squeeze the sponges
with the same pressure. The
volume of water squeezed from the
two sponges will be surprisingly
close. This illustrates the impor-
tance of consideration of container
height when formulating a con-
tainer medium. If approximately
the same amount of water was
obtained from each sponge, that
means a greater volume of the
pores in the taller sponge was
filled with air after drainage.
Some have suggested the
placement of gravel in the bottom
of containers improves drainage. In
fact, the gravel decreases the total
volume of medium with favorable
aeration. The pores at the interface
of the container medium and
gravel must be saturated before
water will move down into the
gravel. This means that a layer of
medium with near maximum
water content is positioned above
the gravel rather than on the
container bottom. Therefore, the
effective height of this container is
reduced by the depth of the gravel
in the bottom.
Root distribution in container
media can be influenced by the
particle size distribution. A me-
dium with high water-holding
capacity and low aeration may
result in a concentration of roots in
the top portion of the container,
especially if the medium in the
bottom portion of the container
remains saturated for extended
periods. Roots growing in poorly
aerated media are weaker, less
succulent and more susceptible to
micronutrient deficiencies and root
rot pathogens such as Pythium and
Phytophthora than roots growing
in well-aerated media.
Rapid temperature fluctuations
and extreme temperatures are
common in container media. The
high container surface area to
volume ratio provides little

buffering of environmental
fluctuations. Root-zone tempera-
tures on a bright, sunny day often
exceed the air temperature by
150C (270F) because of direct
solar radiation on container
sidewalls. Winter night tempera-
tures may be lower than air
temperatures because of rapid
heat loss from this large surface
area. These facts are particularly
evident in smaller containers.
The amount of water present in a
container medium will influence
how rapidly the temperature of
the medium changes. Water
buffers or reduces the rate of
medium temperature change,
although the extent of this buff-
ering is not clearly understood in
nursery containers.
The particle size distribution
and thus the water-holding capac-
ity and air space can change over
time in the container. As the
particle size is decreased through
biological degradation, the medium
volume decreases, the air space
decreases and the weight of solids
per unit of volume of the remain-
ing medium increases. This degra-
dation may or may not be accompa-
nied with a proportionate increase
in water-holding capacity. Smaller
particles may wash to the bottom
regions of containers over time.
This has been reported especially
with the use of large amounts of
sand in otherwise porous media.
Fine sand tends to accumulate in
the bottom thus clogging the larger
pore spaces, decreasing aeration in
the bottom portion of the container
and water-holding capacity in the
upper zones in the container
Container medium volume
generally decreases and general
physical properties change over
time due to compaction, shrinkage,
erosion and root penetration.
Decreases in the volume of
medium in a container result in
decreased drainable pore space

and readily available water.
Compaction refers to the reduction
in container medium volume
caused by settling or compression.
Compaction can occur as the result
of poor potting procedures,
breakage of particles, or compac-
tion from the impact of overhead
irrigation and/or other cultural
practices. Shrinkage occurs as a
result of particle degradation. As
certain medium particles decom-
pose, they become smaller and fit
closer together, thus decreasing the
total volume and the volume of air-
filled pores after irrigation and
drainage. Compaction and shrink-
age during the production period
should be less than 10 percent, but
slightly more may have to be
tolerated for plants requiring
multiple seasons for production.
Intense rainfall and/or irrigation
can splash or wash particles from
the container, and particles can be
lost during removal of weeds, etc.
Root volume increases often com-
pensate for losses in growth me-
dium volume.

Chemical properties
of container media
Chemical properties commonly
measured for container media and
media components include pH,
soluble salts, cation exchange
capacity and the carbon to nitrogen
ratio. These properties should be
thoroughly examined during the
growth medium selection and/or
formulation process.

Optimum pH of a container
medium differs with plant species
but generally a pH between 5.0 and
6.5 is desirable. The pH has a major
role in the availability of nutrient
ions. In production systems where
nutrients are added frequently in
forms that are normally absorbed
by the plant, the suitable pH range
may be much wider than research
with field soils would indicate. A

pH above 7.5 usually results in
chemical binding of micronutri-
ents and a pH below 4.0 could
result in toxic concentrations of
ions such as aluminum, zinc, or
copper. Field soils are limed to
maintain the pH above 5.0 or 5.5
to reduce the possibility of such
toxicities. However, the level of
aluminum in soilless container
media is generally too low to cause
problems and a pH below 5.0 can
be tolerated by many ornamental
Generally, growers should mix
the components together in the
ratio that yields the desired physi-
cal properties, then determine the
pH. Amendments to adjust pH such
as dolomitic limestone should be
added at the suggested rate to small
quantities of the medium and
allowed to incubate in moist,
aerated conditions for a few days
before the effect of the amendment
is determined. Procedures for
determining the pH of a growth
medium are presented in Florida
Extension Circular 556, Nursery
Laboratory Development and
The pH of some components will
change over time. Typically the pH
of a pine bark based medium will
decrease during the production
cycle. However, irrigation with
alkaline water can more than offset
this tendency. The growth medium
pH should be monitored regularly to
allow for adjustments. Elemental
sulfur, acid-producing fertilizers
and dilute acids can be used to
decrease pH. Liming material can
be applied to increase pH but a
change in pH from application of
liming materials to the surface of a
container medium is generally slow
because the effect tends to be
concentrated in the upper strata of
the medium. For best results,
amendments which are slowly
soluble and slowest to adjust pH are
best incorporated at the time of
media preparation.

Soluble salts
Care should be taken to avoid
using growth medium components
with high soluble salts levels.
Components such as sand, small
gravel and peat harvested from
areas high in soluble salts may not
be acceptable for use in container
media or may have to be leached
before used. Salts in sands and
small gravel can be leached by large
amounts of water while the mate-
rial is in a pile or storage bin. Salt
levels in peat may be more difficult
to reduce by leaching because of the
ability of these organic materials to
hold many ions. Soluble salts in the
range of 2500 to 4000 ppm are
considered high for most woody
crops and the moderate or sug-
gested range after fertilization is
1000 to 1500 ppm. However, some
plants are more sensitive to salt
levels than others. For example, the
optimum salt level for azaleas is
500 to 700 ppm.
There are three common methods
of measuring soluble salts of growth
media, including the saturated
paste extract, 2 to 1 dilution by
volume and a pour-through or
Virginia Tech extraction method. A
more thorough examination of the
measurement and interpretation of
soluble salts is provided in Florida
Extension Circular 556, Nursery
Laboratory Development and

Cation exchange capacity
The ability of a soil or growth
medium to retain nutrients against
leaching by irrigation water or
rainfall is estimated by measuring
the cation exchange capacity (CEC).
Most adsorption sites on growth
medium particles are negatively
charged and attract positively-
charged ions. Many nutrients
required by plants are positively
charged and thus are attracted by
these negatively-charged sites.
Sands and other low-surface area

materials have low cation exchange
capacities while organic components
have a greater ability to retain
cations. Pine bark has a cation
exchange capacity in the range of 10
to 13 milliequivalents per 100 cubic
centimeters while a CEC of approxi-
mately 1 is common for builders'
Although a high nutrient-holding
capacity is desirable, some thought
must be given to soluble salt
buildup which may injure plants.
Media with desirable water-holding
and aeration characteristics will
usually allow for periodic leaching
necessary to prevent or reduce salt
accumulation. Salt accumulation is
generally not a problem unless the
irrigation water is saline or the
fertilizer source, rate and/or sched-
uling result in excess salt concen-
trations. Container media with 50
to 60 percent peat or pine bark of
moderate particle size (1/8 to 3/8
inch; 0.3 to 0.9 cm) have proven to
have adequate CEC for efficient
production of woody plants in

Carbon to nitrogen ratio (C:N)
Rapid decay of organic matter in
container media can result in
decreased volume and a subsequent
decreased aeration of the medium.
Materials with a high cellulose
(carbon) to nitrogen content will be
decomposed rapidly by microorgan-
isms in the soil. Not only will
particles become smaller, but
nitrogen that would normally be
available for plant uptake will be
utilized by microorganisms. Saw-
dust and shavings have a higher
C:N than other organic matter such
as peat and bark. Sawdust has a
C:N ratio of about 1,000 :1 while
bark has of ratio of approximately
300 :1. Decomposition of these
organic particles is initially rapid
and the rate of decomposition
decreases with time. Therefore,
older or composted materials will
decompose more slowly than

freshly-produced sawdust. Man-
agement of fertilization to main-
tain the proper nutrient concen-
tration in the growth medium is
extremely important if optimum
plant growth is to be obtained
when using fresh organic material
with a high C:N. It is important to
eliminate fresh wood contamina-
tion of the bark. Remember the
C:N ratio of wood is three times
greater than of bark.

Organic components
Peat. The most common growth
medium component for container
production is peat moss. However,
there can be tremendous diversity
among the characteristics of peat
from different sources or different
locations within an individual peat
bog. Peat must have a high fiber
content to provide internal water-
holding capacity (small pores) yet
allow drainage of pores between
particles (large pores). If a peat
appears oily when wet or is slick
rather than fibrous when rubbed
between your fingers, it may not be
suitable for use in producing
container-grown plants.
Peat is a term which applies to a
type of soil formed from partially
decomposed mosses or sedges which
accumulate in bogs over a period of
hundreds or thousands of years.
Although the term "peat moss" is
widely used, it is not correct. The
correct designation should be "moss
peat," which indicates those peats
formed from moss plants. Sphag-
num peat is the preferred peat of
most greenhouse operators because
of its high water holding capacity,
adequate air space, high cation
exchange capacity and resistance to
decay. Sphagnum peat is formed
from sphagnum mosses in very acid
bog conditions which preserve
most of the plant fiber structure.
The acidity of many sphagnum
peats ranges from pH 3.0 to 4.0.
Hypnum peats are derived from
hypnum mosses and have a higher

and much broader pH range (4.0
to 7.5), and less persistent fibers
than sphagnum peat.
Other peats consist of fibers of
sedges, reeds and grasses. These
peats are especially susceptible to
decomposition, especially in the
preserice of fertilizer solutions.
Peats which break down rapidly
cause media shrinkage and compac-
tion, a condition which hampers
plant growth and makes the con-
tainerized medium difficult to
manage. Many Florida peats are
derived from sedges, reeds and
Peats with a fibrous quality are
better than those reduced to a
powdery consistency due to either
decomposition, plant origin or
harvesting and processing proce-
dures. Very fine grades of peat are
the least desirable unless mixed at
the proper ratio with larger, porous
particles because they have more of
a predominance of small pore space
than coarser grades. The peat
selected should have some fiber
structure and be brown in color
when dry. Material which has
decayed further, such as that found
in muck soils, is black and has a
powdery consistency when dry.
Muck is a very poor component for
any potting medium.
Peats derived from sedges, reeds
and grasses have the ability to bind
certain soil-applied, plant growth
regulators, such as Cycocel, more
than other types of peats. For this
reason soil-applied growth regulator
test reports should specify the type
of peat and other media component
Pine bark. In the southeast,
several pine species are important
forest crops. As the wood is utilized
the bark is removed mechanically.
For many years this bark was
regarded as a waste product that
required a disposal site where the
material could be stockpiled.

During the late 1960s about 20
percent of Florida's sawmills and
most of the pulp mills utilized pine
bark for fuel. The number increased
through the 1970s as oil and
related fuel prices soared. It is now
difficult to find operations which
generate large quantities of pine
bark that do not utilize it for fuel.
Pine bark has been recognized as
a suitable component for container
growth media since the 1960s and
in some cases it is a good single-
component growth medium. Pine
bark is preferred to hardwood
bark because it resists decompo-
sition and contains less leachable
organic acids than some hard-
woods. Research at the University
of Georgia has shown that milled
pine bark with 70 to 80 percent
of the particles by volume within
a range of 1/42 to 3/8-inch (0.6
to 9.5 mm) in diameter, with the
remaining particles less than 11
42 inch (0.6 mm), is a good
potting medium component.
Although many early users of pine
bark felt that aged pine was better
than fresh bark, most potting
media formulators today utilize
fresh material and supply a small
nitrogen charge, approximately
1/4 to 1 pound (113 to 454 g.) of
nitrogen per cubic yard of bark,
to offset the small surge in mi-
croorganism growth in fresh
Since the pH of pine bark ranges
between 4.0 and 5.0 and has a
tendency to decrease over time in
production systems with acidic or
neutral irrigation water, incorpora-
tion of a liming material such as
dolomitic limestone may be advis-
able. Approximately 5 to 9 pounds
(2.3 to 4.0 kg) of dolomitic limestone
will normally adjust a cubic yard of
bark to pH 6.0 to 7.0 over a 60-day
period. Hydrated lime may be
substituted for a portion of the
dolomite to raise the pH over a one-
week period, while coarse limestone
will extend the pH adjustment

A container medium of pine bark
has noncapillary pore spaces
between the large particles. Bark
particles have a relative high cation
exchange capacity, while most
particles have internal water-
holding capacity.
The large moisture content of
fresh bark makes it heavy, a
characteristic which limits its
shipment over long distances. Once
bark dries below 35 percent of its
total water-holding capacity, it
becomes difficult to rewet. Use of
a horticultural wetting agent
would be helpful for rewetting
bark. A moisture adjustment
period of several days is required.
Sphagnum moss. Sphagnum
peat should be not be confused with
sphagnum moss which is the whole
moss plant collected alive along
with connected dead, but non-
decomposed moss parts. Dried
sphagnum moss is not generally
used in potting mixes but may be
used as a top dressing of shredded
moss parts over seeds in germina-
tion trays. The moss is reported to
have some fungicidal activity.
Historically sphagnum moss has
been used extensively for packing
around roots of bare root plants and
for shipping plants. Another popu-
lar application of sphagnum moss
has been the lining of hanging
basket frames. This procedure is
still used to a limited extent, but
the solid plastic sidewall containers
have largely replaced hanging
basket frames. When frames are
used, a moistened layer of sphag-
num moss about 1 to 2 inches thick
is placed around the inside of the
wire mesh or plastic mesh frame to
contain the potting medium added
inside the lining.
Sphagnum moss is a source of
the fungus Sporothrix schenckii,
which causes sporotrichosis.
Sporotrichosis in humans usually
starts as a local skin disease of the
hands, arms and legs, but may

become generalized. Workers
handling sphagnum moss are
encouraged to wear gloves to
prevent injury to the skin surface
and prevent entry of the organism
through existing skin lesions.
Hardwood bark. Deciduous
hardwood bark is used extensively
in many areas of the country as a
container media amendment. In
Florida, hardwood tree species are
grown primarily in the northern
third of the state. Hardwood bark
differs greatly from pine bark in its
chemical and physical characteris-
tics. The pH range of fresh hard-
wood bark is 5.0 to 5.5. As the bark
ages in the presence of water, the
pH increases to 8.0 or 9.0, a condi-
tion much too alkaline for plant
production. Fresh hardwood bark
should never be used immediately
for potting plants.
Researchers at the University of
Illinois developed an effective
composting procedure for hardwood
bark in the 1960s which effec-
tively adjusts the pH and pasteur-
izes the bark, eliminating most
soil-borne pathogens. Prior to
composting, hardwood bark has
two other features which render it
unfit for plant production. Be-
cause hardwood bark decomposes
more rapidly than pine, there is
initially a high demand for nitro-
gen by microorganisms which will
induce a nitrogen deficiency in
plants growing in the fresh bark.
The second potential problem
relates to certain hardwood
species which have been reported
to have a phytotoxic effect on
plants grown in fresh bark or
plants drenched with extract from
fresh bark.
Hardwood bark should be me-
chanically processed to small
particles which will pass through a
1/2-inch (1.27 cm) mesh screen,
with 10 percent of the particles
larger than 1/8-inch diameter and
35 percent less than 1/32-inch (0.8
mm) diameter.

Composting procedures as
prescribed by the University of
Illinois researchers specify that for
each cubic yard of a 2 parts fresh
hardwood bark: 1 part sand (v:v)
mix the following should be added:
6 pounds (2.7 kg) ammonium
nitrate, 5 pounds (2.3 kg) super-
phosphate, 1 pound elemental
sulfur and 1 pound iron sulfate.
These materials should be blended
in the medium thoroughly, prefer-
ably in a tumbling type mixer, and
arranged in large deep piles, kept at
approximately 60 percent moisture.
Covering the pile with a plastic
sheet will help stabilize the mois-
ture content during the composting
period. The high level of microbial
growth in the presence of the fresh
bark and fertilizer causes the
temperature to approach 1500 F,
(660C) a temperature which elimi-
nates most pathogens. Turning the
pile of composting bark 3 to 5 times
during the 60-day process is recom-
mended to get a uniform product.
After composting, bark-induced
nitrogen deficiency problems and
phytotoxicity caused by bark from
certain tree species are eliminated.
Melaleuca bark. The bark or
bark and wood of Melaleuca
quinquenervia, Melaleuca or punk
tree, has been used successfully by
several University of Florida
researchers as a soilless growth
medium component. Melaleuca was
introduced to southern Florida
early this century from Australia
and has become a major weed in the
southern third of Florida. Because
it propagates so freely from seed
and grows rapidly in moist exposed
soil, it has become a serious threat
to the ecology of many areas in
southern Florida, including parts of
the Everglades.
The bark of melaleuca consti-
tutes nearly one half the bulk of
its small branches. When prop-
erly processed by special hammer
mills, the bark and wood together

are an excellent component for
soilless mixes. That which has
been milled to pass through a 1/2
or 3/4-inch (1.3 to 1.9 cm)
screen without excessive fines
seems to be an excellent product.
Because of the many thin layers
that constitute the structure of
melaleuca bark, it has an open
structure which provides excel-
lent aeration. Another desirable
characteristic of this bark and
wood is its resistance to decay
which provides particle size stabil-
Processed melaleuca bark and
wood is a suitable substitute for
pine bark in mixes containing up to
one-third pine bark, such as a blend
of equal volumes of pine bark, peat
and sand, for production of several
woody ornamentals. Increasing the
percentage of melaleuca bark
volume to 50 percent results in less
growth of juniper and Illicium
parviflorum compared to growth in
a 2 pine bark:1 peat:1 sand (v:v:v)
At the present time, most
melaleuca is being harvested and
processed for a bark and wood
landscape mulch that is too coarse
for most potting media applications.
As pine bark and peat become
scarce, use of melaleuca will become
more prevalent.
Animal manure. Animal
manure has been used by some
growers in potting mixes in the
past. Although manures do contain
most essential nutrients for plant
growth, the concentration of ele-
ments varies considerably with the
animal, mulching material used
(straw, etc), the technique of ma-
nure collection and storage, and
manure age. Moist manures are
heavy which makes them expensive
to transport long distances; there-
fore, they are usually limited to
rather local applications. Consis-
tency of supply has been a problem
for many horticultural operations
attempting to use animal manure.

Some potential dangers of
manure include: soluble salt
damage from high nutrient con-
tent, ammonia damage to roots and
foliage from steam pasteurized
manures and weed seeds, insects,
pathogens and nematodes contained
in non-pasteurized or non-fumi-
gated manures. For this reason,
popularity of animal manures in
potting mixes declined sharply
during the middle of this century.
If animal manure is to be in-
cluded in a potting medium, only
well-rotted material should be used.
Other materials such as straw or
shavings are combined with the
manure from many sources and the
degree and/or potential for degrada-
tion of these materials should be
considered. Cattle manure is
preferred over other animal ma-
nures because it has fewer nutri-
ents. If animal manure is used in a
potting mix, only a small amount,
about 10 to 15 percent by volume,
should be used and the soluble salts
level in the manure and the blended
potting media should be monitored
closely. Use of animal manures for
potting media is not recommended
by the authors due to the risk
factors mentioned.
Sawdust, wood shavings and
wood chips. Sawdust, wood
shavings and wood chips constitute
a rather broad category of wood
particles generated by sawmills and
other wood processing industries,
often involving a wide range of
particle sizes and several tree
species. Wood particles are gener-
ally less desirable for potting media
than bark because wood has a much
great C:N ratio; about 1:1,000 for
fresh wood compared to 1:300 for
bark. Addition of approximately 25
to 30 pounds of nitrogen per ton of
fresh sawdust or other relatively
fine wood particles will supply
sufficient nitrogen for microor-
ganisms to prevent nitrogen
deficiency during plant produc-

tion. Sawdust of hardwood species
ties up nitrogen and breaks down
about three to four times faster
than sawdust of softwood species.
In Florida, cypress sawdust is
preferred because it is slower to
decay than most other wood par-
ticles: Cypress wood products are
becoming scarce due to heavy
cutting of cypress stands and
government protection of wetlands.
The reverse trend is occurring
with melaleuca wood and bark.
Plant material currently harvested
in southern Florida yields up to 50
percent (by volume) bark from its
medium to large size branches. The
wood component of melaleuca has
been shown to be long-lived in a
growth medium. Since melaleuca is
spreading throughout southern
Florida, it can be viewed as a
developing resource which is
essentially renewable on a local
Composted municipal refuse.
Municipal refuse or garbage com-
post presents a major disposal
problem for most communities.
Garbage consists primarily of cloth,
glass, metal, paper, leaves, plastic,
rubber and wood. These materials
are usually sent to landfills for
disposal. A few communities are
studying proposals for composting
operations which involve, in some
cases, removing metals, paper and
rags, then grinding the garbage into
fine particles which can be mixed
with wood chips or some other
bulking agent and composted.
Composting is usually done with
the ground and blended material in
piles which are turned several
times over a period of a month or
The adoption of garbage
composting technology by munici-
palities is lagging behind that of
composting sewage sludge. Most of
the horticultural research with
composted garbage indicates that
major differences exist in the

quality of compost generated from
different locations, many of them
experimental, depending upon the
type and proportion of garbage
If composted garbage becomes
available in a given area, it should
be carefully evaluated on a small
scale before proceeding with large
batch utilization. The material
should be monitored to ensure
consistency from batch to batch.
Composted sewage sludge.
Several large communities through-
out the United States have adopted
sewage sludge composting opera-
tions. Due to the variety of systems
used in different communities to
process sewage, it is difficult to
describe a single pathway that fits
all situations. Sewage used for
composting in most cases is primary
sewage which has had most of the
water removed. Reduction of the
sewage water content from the 98 to
99 percent water range to between
30 and 80 percent water is accom-
plished by several different proce-
dures, depending upon the sewage
plant design. Names for partially
processed sludge include: drying
bed sludge, heat treated sludge or
dewatered sludge, depending upon
the process used to remove the
The composting process usually
involves mixing 2 to 3 parts wood
chips by volume with 1 part par-
tially dehydrated sewage sludge
and piling the material to a height
of 6 feet (1.8 m) or more. Height of
the piles depends upon the porosity
of the material, moisture content,
system of aeration and other
factors. Most operations in Florida
employ windows for composting
sludge which must be turned every
5 to 10 days, depending upon
moisture content, for a period of 1 to
3 months. Forced aeration of
stationary composting sludge piles
is used in some other parts of the

It is anticipated that composted
sewage sludge will become widely
available to Florida nurseries in the
future. Sludge composted with wood
chips has been used successfully to
replace or partially replace peat and
bark in media used to grow a large
number of different ornamental
plants. Some screening of the
product may be necessary to remove
large wood particles not suitable for
inclusion in growth media. The
amount of screening required will
depend upon the uniformity of
woody chip size used in the com-
post. Preliminary research has
indicated that some composted
sludges may react with manganese,
rendering it unavailable for plant
uptake. Therefore, manganese
deficiencies may appear in sensitive
Peanut hulls. At the present
time there are several sources of
peanut hulls in northern Florida
and other southeastern states
where hulling operations are
located. Peanut hulls have been
used by some flowering pot plant
growers in the past as an amend-
ment, primarily for mineral soil
based potting mixes. The hulls have
considerable fiber structure which
will initially provide additional
large pore spaces. The fibrous
structure of peanut hulls is rather
short lived in potting mixes due to
rapid decomposition of the hulls in
the presence of fertilizer and water.
While they may be suitable for a
crop production period of 6 to 12
weeks, peanut hulls are not recom-
mended for long-term crops. Rice
hulls have similar properties and
thus are not recommended for long-
term crops.
If peanut hulls are used, they
should be steam pasteurized or
chemically fumigated to eliminate
lesion nematodes which are known
to reside on the hulls for long
periods. Lesion nematodes attack
many ornamental plants.

Gasifier residue. Residues
from burning organic materials
such as wood and bark have rela-
tively stable particles. Research has
been conducted with such materials
from a few sources and it should be
noted that the physical and chemi-
cal properties of these residues will
differ with source. It has been
determined that a residue from the
gasification of wood chips and bark
can be an acceptable component for
production of woody ornamentals. It
should not exceed 1/3 the volume in
combination with peat and pine
bark. One definite disadvantage of
this residue is its high pH which
often exceeds 8.0 and is not appre-
ciably lowered by leaching. Sulfur
or other acidifiers must be used in
conjunction with this residue. It
would be essential to conduct tests
with various amendments, compan-
ion components and crop species
before using gasifier residues in
container media. Since there are
many other suitable components
with desirable characteristics
currently available in the nursery,
industry it is doubtful ifgasifier
residues will be used extensively in
container media for some time.
Bagasse. Bagasse is a fibrous
by-product of the sugarcane indus-
try. Although bagasse does initially
provide additional open pore space
in a mix, it tends to break down
rapidly with the addition of fertil-
izer and water. During the decom-
position process the medium will
shrink and much of the large pore
space will be lost. If bagasse is to be
considered as an amendment, it
should be restricted to small con-
tainers and short term crops which
would mature before its fibrous
quality is lost.
Bagasse is available from a few
of Florida's sugar mills, although
most of the product is burned by the
large mills to generate power.
Bagasse has also been utilized
successfully in Florida as a land-

scape mulch and a bulking agent
for composting sewage sludge.

Inorganic components
Polyphenolic foam. One
manufacturer of florist foam makes
a coarse-particle, open-pore poly-
phenolic foam which is very light
when dry and can hold a large
amount of water after irrigation.
Since the particles are approxi-
mately 3/8-inch in cross section and
of variable length, aeration of the
medium is quite good. At the
present time the product is de-
signed to be used as a single-
component medium for the produc-
tion ofcymbridium orchids.
Preliminary experimentation
suggests other ornamental plants
may also be grown in the material
and it may be blended with other
products such as polystyrene foam.
Low bulk density, particle persis-
tence and good water holding and
aeration properties make this
material potentially useful for long
distance shipping of plants where
weight is a major factor. There does
seem to be potential application of
the foam particles for production of
high value foliage plants for export
to distant markets where there are
rigid restrictions on importation of
plants grown in natural organic
base media. Price of polyphenolic
foam and the need to drench the
foam with a solution of potassium
bicarbonate to neutralize acidic
materials used in the manufacture
of the foam, may limit its use.
Hydrophilic gels. During the
past 20 years, several products have
been introduced to the horticultural
industry which are designed to
increase the water holding capacity
of growth media. These products,
which are called hydrophilic gels or
water absorbing polymers, are
capable of holding over 150 times
their dry weight when fully charged
with water. The products are
generally starch or acrylic polymers

which are formulated as granules
or flakes which can be easily
incorporated in potting mixes.
After the granules absorb water,
they swell and assume a gel-like
consistency. The swelling action of
a gel tends to maintain open pore
space in a mix because a mix
containing a small amount of gel
will increase in volume as the gel
swells. A gel also increases the
water-holding capacity of a mix,
although a portion of the water
held by the gel is held so tightly
that it is not available for plant
Due to cost of these gels, they
have been considered for use
primarily on high-value greenhouse
crops. Research in some cases has
not supported the cost effectiveness
of utilizing hydrophilic gels on
certain floricultural crops. One
factor which tends to override the
benefits of gels is that high quality
peat-lite mixes already have
excellent water-holding capacity.
Gel products should be evaluated
by growers on an individual crop
basis. Impact of gels on crop
production, crop shelf life and
nursery profits should all be
Perlite. Perlite is a light weight,
white, expanded, closed-pore
alumino-silicate mineral of volcanic
origin which has become widely
used in the horticultural industry
as a component to peat-lite mixes.
The ore is crushed and heated to
approximately 18000F (982C)
which causes the ore to expand.
Perlite has been well received by
the horticulture industry since
the 1950s when it became a
popular amendment for potting
media comprised of mineral soils
and peat. With adoption of peat-
lite mixes in the 1960s, usage of
perlite increased in commercial
Perlite is now utilized exten-
sively for its light weight, physi-

cal stability and ability to provide
non-capillary pore space in a
mix. Perlite has little water
holding capacity since the internal
pore structure is closed. It has
extremely low cation exchange
capacity, no nutritive value of its
own, and no notable influence on
pH of mixes in which it is em-
The bulk density of perlite is
approximately 6 to 8 pounds per
cubic foot (0.1 g/m3). The fine dust
associated with handling dry perlite
is irritating when airborne and
inhaled. The percentage of such
small particles in perlite should be
minimized by only obtaining a
horticulture grade of perlite. An
effort should be made to minimize
the physical movement of loose dry
perlite until it can be moistened or
incorporated with moist peat or
other amendments. Individuals
involved with considerable perlite
handling should wear a breathing
mask or respirator and goggles
while performing that task. A fine
spray of water on perlite as it is
being poured from the bag and the
use of properly placed exhaust fans
in an enclosed media blending area
will greatly reduce the perlite dust
Vermiculite. Vermiculite, an
aluminum-iron-magnesium silicate,
is a mica-like mineral which, when
heated above 1400F, expands to
an open-flake structure that
provides spaces for air and water.
Vermiculite has been used in-
creasingly as a potting mix
amendment since peat-lite mixes
were introduced in the 1960s.
Vermiculite particle size is
determined by the particle size of
the ore, prior to heating. Due to the
range of pore spaces of processed
vermiculite, it retains considerable
moisture upon wetting. The pH of
most of the vermiculite used in
horticulture falls within a range of
6.0 to 8.9. Although vermiculite
contains measurable amounts of

potassium, calcium and magnesium
available to plants, it should not be
regarded as a fertilizer. Vermiculite
also has good buffering and cation
exchange capacity.
One of the major shortcomings of
vermiculite is its poor physical
stability after wetting. Particles
which have been mixed, wetted and
compressed do not recover physi-
cally. Compression of moist ver-
miculite causes the expanded
particle to collapse and frequently
slip apart. This is particularly a
problem when the mix is handled
wet, when vermiculite containing
mixes are used in large containers
where the pressure is great toward
the bottom of the container, and in
situations where mixes are used on
a second crop such as in a propaga-
tion bed or recycled mix.
There are several grades or
particle sizes of vermiculite used
by horticulturists. Each manufac-
turer of vermiculite has its own
system of grades. The finer grades
are generally used in mixes formu-
lated for small pots and plug tray
applications, while coarser grades
are usually found in mixes designed
for larger containers.
Polystyrene foam. Polystyrene
foam is a plastic product manufac-
tured from resin beads which are
subjected to heat and pressure. The
polystyrene foam used in peat-like
mixes is usually derived from scrap
generated during the manufactur-
ing of polystyrene bead-foam such
as sheet insulation. The scrap
pieces are shredded by mechanical
rieans into small particles suitable
for blending with peat, bark,
vermiculite and other components.
StyrofoamR is one trademarked
brand of polystyrene foam. Ex-
truded polystyrene foam is much
denser than the bead-foam and is
generally not used in potting
Polystyrene foam is utilized in
potting mixes to improve drainage,

reduce water holding capacity,
reduce bulk density and serve as a
cost effective alternative to perlite.
The closed pore structure of the
foam makes it one of the least water
retentive components in use. The
foam has no appreciable cation
exchange capacity, and contains no
plant nutrients.
A desirable particle size range of
polystyrene beads for potting mixes
is 1/8 to 3/16-inch diameter and 1/8
to 1/2-inch (0.3 to 1.3 cm) for flakes:
Due to the extremely low bulk
density of the foam beads or chips
(0.75-1.0 lbs/ft3; 12 to 16 g/l) it
presents some handling problems.
It should be handled in areas where
there is little air turbulence to
prevent particle drift. The drift
problem is compounded by the
static charge of the foam particles
which causes them to stick to
objects and surfaces in the media
handling area. A small amount of
water plus a wetting agent applied
to the foam will reduce both han-
dling problems.
The light weight and durable
nature of polystyrene foam make it
an attractive alternative medium
component for crops in hanging
baskets and a variety of interior
plants which must be packaged and
shipped long distance.
Rockwool. Rockwool is manu-
factured from a mineral called
basalt through a heating and fiber
extrusion process. Although
rockwool is utilized primarily for
insulation, it can be utilized as a
rooting medium by itself or in
combination with other ingredients,
such as peat, bark, and perlite to
make a soilless growth medium.
Rockwool formulated into blocks,
cubes and slabs has been used
extensively in the production of
hydroponically-grown vegetables
and flowers in Europe. Only a
limited amount of rockwool blocks
are used in the United States for
ornamental production.

Within the past few years, loose
rockwool and more recently granu-
lated rockwool have been promoted
as components for soilless mixes.
Loose rockwool comes in rather
coarse wads which resemble the
formulation used for insulation
while the granulated formulation is
much finer with a crumb-like
appearance. The granulated prod-
uct was made to facilitate easier
blending with other components.
Those rockwool products formu-
lated for horticultural use vary
considerably in physical and chemi-
cal properties among manufactur-
ers. Some product lines have been
treated to make the wool more
hydrophilic (attract water),
while other lines are essentially
hydrophobic (repel water).
Blends of the two lines can be used
to achieve a specific water holding
Rockwool is utilized because it
can be manufactured to uniform
standards and does not break down
from bacterial or chemical action.
When protected from excessive
compaction, rockwool provides
aeration but lacks notable cation
exchange capacity and nutrient
supply of its own.
Adoption of rockwool as a compo-
nent for soilless mixes will depend
upon its cost effectiveness when
compared with other products
which are used to provide
noncapillary pore space in potting
mixes. The greatest potential for
this amendment is in the high
quality greenhouse pot plant
market. Rockwool is currently being
sold in the United States under the
names Grodana and HortwoolR.
Calcined clays. There are now
a number of companies in the
United States which quarry clay
and heat it in specialized kilns
which cause the clay to expand
under high temperature into a
highly porous fused structure which
is physically and chemically stable.

The next steps involve crushing
large chunks of calcined clay into
smaller particles which are
subsequently graded into specific
particle size ranges. Light weight
concrete products and road sur-
facing additives are two popular
Significant applications for
calcined clays in horticulture and
agronomy were developed during
the 1950s as it was demonstrated
that the substrate for heavy traffic
turf areas such as golf greens could
be improved through clay products
such as Turface". When a substan-
tial amount of calcined clay is added
to mineral soils which receive heavy
foot traffic, the calcined clay main-
tains good aeration and drainage
properties needed for turf growth.
During the 1950s and early
1960s, Turface was also marketed
to the Nursery industry as a
potting medium component with
limited success. Rather high bulk
density and cost of the product were
major factors limiting its acceptance
during a period when modified field
soils were used extensively for
potting media.
After a long period of rejection by
nurserymen, calcined clay is receiv-
ing some attention again by a few
commercial soil formulators as an
amendment in some of the highest
quality peat-like mixes. Although
the cost of calcined clays is still
high, many growers of long-lived
pot plants recognize that the
quality of the potting mix is fre-
quently the factor most limiting the
successful management of their
product once in the hands of the
Many calcined clays have proper-
ties which make them desirable as
potting media components. Those
clays which are receiving the most
attention are more porous and
therefore considerably lighter in
weight than Turface. Calcined clays
are essentially indestructible

particles, which provide non-
capillary pore space to a mix due
to the large spaces created be-
tween particles, and hold water
internally within their open-pore
particle structure. Most calcined
clays have good cation exchange
capacity which helps in the
retention of nutrients but have no
nutrient value of their own.
It is suspected that justification
for more extensive use of calcined
clay will come as the long term
management of tropical plants is
better understood by
interiorscapers. Potting mixes
which decompose and shrink once
installed in commercial
interiorscapes are difficult to
manage and often contribute to
premature plant replacements. The
cost of plant replacements and the
additional labor required to manage
interior plants growing in low-
quality mixes is far more costly in
the long term than paying a little
more for plants produced in high-
quality, physically-stable potting
mixes. Some large interior plants
can be kept in place for a period of 5
to 10 years with proper care and
use of a good potting medium.
In Florida, a calcined clay prod-
uct for potting mixes is available
from Florida Solite Company in
Green Cove Springs. The firm has
the ability to obtain a specified
particle size range through crushing
and screening, and sells a product
called SoliteR which is suitable as
a container medium component.
Calcined clays are also available
from other states, Europe and
South America.

Formulating a growth
medium for each
production system
It is possible to formulate a
growth medium for a specific
container size, growth environment,
management intensity and the
plant's requirements. It has been

noted that container depth directly
affects the percent of the growth
medium that is filled with air at
container capacity. A growth
medium for plants grown in a
greenhouse, where control of the
moisture level is possible, can have
a greater water-holding capacity
than a medium for plants exposed
to natural rainfall distribution.
During Florida's rainy season,
plants may receive an average of
one-half inch of rainfall per day for
30 days, which dictates using
container media with exceptional
drainage. Unfortunately, a medium
with exceptional drainage also has
relatively low water-holding capaci-
ties which requires frequent irriga-
tion during drier conditions. This
means that a container medium
must be designed to reduce stress
during the most severe conditions
expected for a given environment.
This directly influences the
required management intensity.
The first consideration in the
formulation of a growth medium is
the appropriate balance between
water-holding capacity and aera-
tion. A more porous medium is
required for a shallow container,
such as for propagation, than for
deeper containers typical of those
used in the production phase. For
outdoor production of woody crops,
a drainable pore space equal to 20
to 30 percent of the volume provides
the drainage buffer required for an
extended rainy period. The corre-
sponding water-holding capacity
ranges from 30 to 50 percent.
Greenhouse crops can be grown
effectively in media with 10 to 15
percent drainable pore space and a
much higher water-holding capac-
ity. More intense management of
the moisture relations is possible
when rainfall effects are eliminated.
Once the desired characteristics
have been determined and the
available components selected, a
medium can be formulated to meet
those characteristics. At the
present time the only method for

determining the correct formula-
tion is by trial and error, al-
though researchers are in the
process of developing computer
assisted models to predict the
medium characteristics based on
measured characteristics of
individual components.
A grower experienced with
particular components knows the
approximate component ratio
required. For example, a woody
plant grower in north Florida has
available pine bark, peat and sand.
Generally for outdoor production, a
medium consisting of 15 to 25
percent by volume of a coarse sand
or fine gravel is required to have
the weight necessary to keep the
containers upright when placed on
open production beds. The percent-
age of pine bark and peat required
to formulate a container medium
with 25 percent air space depends
most upon the particle size distribu-
tion of the bark. If the bark is
composed primarily of particles in
the range of 1/4 to 1/8-inch (0.63 to
0.32 cm) diameter with few fine
particles, a medium of 60 percent
pine bark, 25 percent peat and 15
percent sand would be an appropri-
ate medium to start testing. If
significant small particles are
present in the bark source, less peat
may be required. Once a sample of
the test medium has been prepared,
water-holding capacity and air
space after irrigation and drainage
must be determined in the con-
tainer size for which the medium is
being formulated. Step-by-step
procedures for these determinations
have been presented in Florida
Extension Circular 556, Nursery
Laboratory Development and
If water-holding and air space
characteristics of a given medium
are within the desired ranges,
record this formulation in an
appropriate record book and make
plans to prepare the volume
required for current needs. In
most cases, the characteristics of

the first trial medium will be
outside the desired range. If the
air space is too low, then more
larger particles must be added to
the formulation. In the above
example, more pine bark and
possibly less peat would be mixed
in the next trial medium. Keeping
good, permanent records of these
procedures will reduce the need
for future trials and ensure the
medium will be formulated con-
sistently with the desired charac-
teristics. If trial medium air
space is too high, then more small
particles should be added to the
next mixture. Peat or sand can be
added to reduce the size and possi-
bly the number of the larger pore
Such trial and error procedures
should be repeated until the desired
characteristics of media for various
container sizes are achieved. The
number of different media prepared
for a particular nursery should be
minimized. Only one growth me-
dium formulation may be required
for nurseries without tremendous
diversity in container sizes, envi-
ronmental conditions or plants. If a
variety of container sizes, ranging
from small to very large, and/or
different environments exist within
a single operation, media for the
different production systems must
be formulated.
Make sure the medium prepared
in one batch has the same water-
holding and pore space characteris-
tics as the next batch mixed from a
different load of components. The
particle size distribution of each
load of components should be
tested. If the particle size distribu-
tion is the same as the load from
which the medium was formulated,
then the grower can confidently
prepare the next batch using the
same formula. However, if the
particle size distribution is different
on subsequent loads, the formula
should be tested and adjustments
made as required.

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3 1262 04969 9837

Media mixing,
handling and storage
The ideal formula for a container
medium may be known, but proper
mixing and handling procedures
must be followed if optimum results
are to be obtained. Assuming
components arrive at the nursery
free of weeds, weed seed, pathogenic
fungi and insects and with a uni-
form and acceptable particle size
distribution, the nursery operator
must take steps to ensure the
quality is maintained.

Component storage
Components must be stored off
the ground and protected from
surface water. A concrete slab or
bin is ideal for components received
in bulk. The surface water patterns
around the concrete slab must be
adjusted to eliminate the possibility
that surface water, carrying patho-
gens, weed seeds and/or insects,
that could come into contact with
the medium component. Bulk
components should be covered with
black plastic film or other suitable
covering to prevent contamination
with wind-borne seeds, pathogens
and other pests when access is not
The length of the storage period
determines whether bagged compo-
nents are stored outdoors or at least
under cover. Most bags will remain
intact outdoors for 6 to 8 weeks, but
if an annual supply is purchased,
indoor storage is needed. Covering
bags stored outdoors with opaque
plastic film will extend the life of
the bags. Even if outdoor storage is
acceptable, consider the surface
water drainage pattern and the
ground surface because most bags
are not watertight.

Mixing procedures
When various components are
mixed together, a homogenous
mixture must be obtained. This
includes fertilizer amendments as

well as growth medium compo-
nents. Variability in a growth
medium batch or between batches
can result in differences in plant
growth and quality, because the
water-holding and aeration charac-
teristics and fertilizer concentra-
tions would differ from container to
container. Obtaining uniform
mixtures without altering the
particle size distribution of the
medium is not easy, but its impor-
tance can not be overemphasized.
Consideration must be given to
the reasons a nursery operator
would choose to mix media on the
site rather than purchasing media
prepared to certain specifications.
Media must be available upon
demand. Advanced planning is
usually more critical if pre-blended
media are purchased, but there
must be sufficient advanced plan-
ning even if components are pur-
chased individually. Cost is another
consideration. It might be more
economical for a small to medium
size nursery to purchase media
ready for use because of the high
cost of effective mixing equipment.
However, larger nurseries generally
mix adequate volumes of media to
justify the purchase and mainte-
nance of appropriate equipment.
A good system for mixing me-
dium components in a nursery
utilizes a rotary-type mixer, such as
a cement mixer commonly used on
ready-mix trucks, or a drum and
paddle type mixer. There appears to
be less breakage of the component
particles when rotary-type mixers
are used, but difficulties include
loading the mixer and retrieving
the mixture. Adjust rotating drum
speed so materials are carried well
up the drum wall before tumbling.
Drum and paddle-type mixers can
be used effectively if the mixing
duration is carefully monitored.
Stationary horizontal drum mixers
should not be filled above two-
thirds the auger diameter or the
top-added components can float and

not mix into the lower materials.
With prolonged mixing, the particle
size of some components can be
reduced significantly resulting in a
medium with unknown and possibly
undesirable water-holding and
aeration characteristics.
The proper mixing system can
also vary with the medium compo-
nents. Perlite can be easily crushed
during mixing, reducing the particle
size. Vermiculite is an expanded
material and if crushed, it will not
expand again. When the particle
size of such materials is reduced,
they do not serve the purpose for
which they were chosen. Resin-
coated fertilizers and other pellet-
ized fertilizers may be crushed by
prolonged mixing in some mixing
Systems are now available that
allow the components be placed in
large bins from which they drop
onto conveyer belts in layers or
directly into the mixer at the proper
ratio. Fertilizers and other chemical
amendments can also be applied in
this manner. Other systems require
loading the components into the
mixer with a front-end loader at the
proper ratio.
Some nursery operators utilize
front-end loaders to mix media by
turning the various components
piled on a concrete slab. This
system is inexpensive but simply
does not provide uniform mixing,
especially of fertilizer amendments.
It is impossible to uniformly distrib-
ute 1 to 3 pounds (0.45 to 1.4 kg)
amendment of all per cubic yard of
medium by sprinkling it on the
surface of a pile of growth medium
components to be turned by a front-
end loader. The problem with
adequate distribution of amend-
ments during mixing with a front-
end loader can be solved by pur-
chasing one of the components, for
example pine bark, with the
amendments already uniformly
distributed in the component at a

rate that will result in the proper
rate for the final medium. If 10
pounds of a fertilizer is desired
per cubic yard of medium and the
bark comprises 50% of medium
volume, then the fertilizer should
be added to the bL thfe Wi t of
20 pounds per cubic yard. Sanita-
tion during this type of mixing
procedure can also be a problem.
Shredder-mixers are also used
to prepare media. Such a system
can greatly reduce particle size
and is unsatisfactory for blending
fertilizers, especially controlled-
release fertilizers, into the
container medium.
Growth medium components
should have a relatively low to
moderate moisture content for
mixing. This is especially true if dry
fertilizers are to be added during
mixing. It is difficult to achieve an
uniform distribution of the dry
fertilizer particles in a moist me-
dium. If the fertilizer amendments
have already been added to one of
the components, a moderate
moisture level during mixing might
be satisfactory. Another consider-
ation is to add moisture after the
medium has been mixed. It is often
difficult to rewet pine bark, peat
and other components when they
have a moisture content below 30
percent. Chemical wetting agents
can be used effectively to reduce
this problem.

Media storage
The raised covered slab or
covered bin facilities suggested for
component storage can be used for
prepared media. Media prepared
with the proper fertilizer amend-
ments should generally be stored in
such a way to minimize leaching.
Since there can be release of fertil-
izers in the medium during storage
and salt levels could reach critical

levels, the salinity level of media
stored for several weeks should be
determined before it is used. Avoid
this problem by preparing or
purchasing only the amount of
media needed to sa sf- the short-
term demand. .

Common amendments to growth
media during mixing include
micronutrients, dolomitic limestone
for pH adjustment and pesticides.
An approved insecticide for the
control of fire ants must be incorpo-
rated in the growth medium of
container-grown plants to be
shipped out of Florida. Superphos-
phate has been routinely added to
media during mixing, but research
has shown that the phosphorus in
superphosphate is readily leached
from pine bark based media. Ad-
equate phosphorus for a growing
season can not be added during
media preparation by adding
superphosphate. Therefore, phos-
phorus should be applied periodi-
cally as a part of the overall fertili-
zation program.

Industry trends
toward preblended
Fifteen years ago most nurseries
obtained container media compo-
nents and blended them according
to their specifications. During the
past ten years, there has been a
strong trend among nurserymen to
purchase preblended potting mixes
from specialty firms. This trend
continues today and the specialty
firms can be divided into two rough
categories-those blenders which
use primarily native peats, barks,
sand, and those which employ
primarily imported peats, perlite,
vermiculite, calcined clay and other
relatively expensive components.

The trend toward utilization of
preblended media is most developed
in the expensive preblends which
are utilized extensively by green-
house container plant growers
producing plants in small to me-
dium size pots. These blends are
sold in bags or in bulk.
The cheaper mixes are used
primarily for landscape ornamental
production beyond the liner stage
and for large potted foliage plants.
Use of local materials including
peats, wood particles, bark and
sand constitutes a considerable
savings in the cost of components
and ultimate cost of the mix. These
mixes are generally less uniform
and consist of less persistent peat
and other particles than used in
mixes consisting of high quality
The important decision nursery
operators must make is to evaluate
the benefits of using nursery-made
mixes versus commercially
preblended products. Consideration
should be given to costs of media
components, labor (ordering prod-
ucts, mixing components and
quality control), and equipment for
blending (equipment purchase and
maintenance). Loss in crop value
from restricted growth, dead plants
or increased production time should
also be considered in determining
actual costs. The final decision
should be made on an economic
basis rather than holding with
company tradition or doing what
many of the other local nurseries
are doing.
Some companies have gone one
step beyond preblending potting
media and are prefilling pots with
specified preblended materials and
delivering them directly to the
nursery. This is another service
provided to some nurseries which
should be evaluated systematically.

in cooperation with the United States Department of Agriculture, publishes this information to further the purpose of the May 8 and June 30, 1914 Acts of
Congress; and is authorized to provide research, educational information and other services only to individuals and institutions that function without regard to
race, color, sex, age, handicap or national origin. Single copies of extension publications (excluding 4-H and youth publications) are available free to Florida
residents from county extension offices. Information on bulk rates or copies for out-of-state purchasers is available from C.M. Hinton, Publications Distribution
Center, IFAS Building 664, University of Florida, Gainesville, Florida 32611. Before publicizing this publication, editors should contact this address to determine
availability. Printed 8/91.

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