41
Contents

2.2.1 Introduction 43

2.2.1.1 Terminology 43

2.2.1.2 The need for "artificial soil" 43

Restricted volume 43

Perched water table 43

Imbalance of soil microorganisms 43

Lack of soil structure 43

2.2.1 .3 History of artificial growing media 43

2.2.2 Functions of a Growing Medium 44

2.2.2.1 Water 44

2.2.2.2 Air 44

2.2.2.3 Mineral nutrients 44

2.2.2.4 Physical support 44

2.2.3 Characteristics of an Ideal Growing

Medium 45

2.2.3.1 Characteristics relating to seedling growth 45

Slightly acid pH 45

High cation exchange capacity 46

Low inherent fertility 47

Proper balance of pore sizes 49

Freedom from pests 53

2.2.3.2 Characteristics that affect nursery

operations 53

Reasonable cost and availability 53

High degree of uniformity and

reproducibility 54

Low bulk density 54

Dimensional stability 55

Durability and ease of storage 55

Ease of mixing and filling into containers 55

Ease of rewetting 55

Promotion of firm root plug formation 56

2.2.4 Components Used in Formulating

Growing Media for Tree

Seedlings 58

2.2.4.1 Organic components 58

Function of the organic component 58

Peat moss 58

Composted sawdust, bark, and other organic

materials 61

2.2.4.2 Inorganic components 63

Function of the inorganic component 63

Vermiculite 63

Perlite 64

Other inorganic materials 66

42

2.2.5 Selecting a Growing Medium 67

2.2.5.1 Interactions between growing media and

cultural practices 67

Irrigation and fertilization practices 67

Type of container 67

Growing season 68

2.2.5.2 Practical considerations 68

Cost and availability 68

Seedling performance 70

2.2.5.3 Commercial growing media 70

2.2.5.4 Custom-mixed growing media 70

2.2.5.5 Comparison of commercial and custom-mixed

growing media 71

Control over quality 71

Ability to "fine-tune" media 71

Time and labor 71

Incorporation of fertilizers and other

amendments 72

Uniform mixing 72

2.2.6 Procedures and Considerations for

Custom Mixing Growing Media 73

2.2.6.1 Equipment and procedures 73

Small-batch mixing 73

Mechanized mixing 73

2.2.6.2 Incorporation of fertilizers and other

materials 74

Limestone 74

Fertilizers 75

Surfactants 75

Super-absorbents 75

Mycorrhizal inoculum 76

2.2.6.3 Pasteurization or sterilization 76

2.2.6.4 Over-mixing and compaction problems 77

2.2.7 The Importance of Proper Growing 

Medium Compaction 79

2.2.8 Conclusions and Recommendations 81

2.2.9 Literature Cited 82
2.2.1 Introduction

2.2.1.1 Terminology

Many different terms have been used for the artificial soil used in 

container nursery culture, including potting soil, potting mix, soil 

mix, compost, and growth medium. It is inappropriate to use the 

term soil when referring to these materials, however, because soil 

is rarely a component of growing media in container tree nurseries 

in the United States and Canada. The term mix is sometimes used 

because many of these materials are composed of several 

different components, although some forest nurseries use only 

one material-for example, peat moss-as a growing medium. To 

avoid confusion and remain consistent, therefore, the terms 

growing medium (singular) and growing media (plural) will be 

used throughout this manual because they are the most 

appropriate terms and least likely to cause confusion. The 

aqueous solution absorbed by and surrounding the growing 

medium particles will be called the growing medium solution or

medium solution.

2.2.1 .2 The need for ":artificial soil"

When people first began to grow plants in containers, they used 

ordinary field soil but soon found that this practice created cultural 

problems. The very act of placing soil in a container produces 

horticultural conditions that are different from those of unrestricted 

field soil.

Restricted volume. Plants growing in containers have access to 

a very limited amount of growing medium compared to 

field-grown plants (Swanson 1989). Forest tree seedlings, in 

particular, are grown in small-volume containers, ranging from as 

small as 40 to over 700 cm' (2.5 to 45 cubic inches). This limited 

rooting volume means that seedlings have only small reserves of 

available water and nutrients and that these essential resources 

can change quickly (Van Eerden 1974).

Perched water table. By their very nature, containers create a 

perched water table because water cannot drain freely from the 

bottom of the container (Swanson 1989). The layer of saturated 

growing medium created at the bottom of the container has 

profound effects on the physical and horticultural properties of 

growing media. (See chapter 2, volume four of this manual.)

Imbalance of soil microorganisms. Native soils contain myriad 

microorganisms, some beneficial and some pathogenic. These 

organisms exist in a natural state of balance in field soils, but 

when these soils are placed in containers in the favorable growing 

environment of a greenhouse, many problems can develop. The 

high-fertility, high-moisture regimes used to promote rapid 

seedling growth favor the development of pathogenic organisms, 

such as damping-off fungi, but do not favor growth of most 

beneficial mycorrhizal fungi.

Lack of soil structure. Texture (particle size) and structure 

(particle aggregations) are basic physical properties of soil that 

create porosity. Although a given growing medium has tex ture, 

based on the size of the various component particles, the 

concept of soil structure does not apply to artificial growing media 

because the individual particles of the various components do not 

bind together. The regular cultivation program and organic 

amendments that farmers use to maintain favorable structure 

(tilth) in field soil are obviously impossible in container nursery 

culture. Therefore, the textural properties of growing media 

components must be carefully chosen and blended to produce 

the right mixture of porosity that will persist over the growing 

cycle.

2. 2.1.3 History of artificial growing media

Because of the problems with field soil in containers, growers 

began supplementing soil with other materials to develop a mixture 

that would be suitable for container culture. The first systematic 

search for a uniform, standardized growing medium began in 

England in the 1930's when the John Innes Horticultural Institute 

developed a loam-based compost that was amended with peat 

moss, sand, and fertilizers (Bunt 1988). By the early 1950's, the 

first truly artificial growing media were created at the University of 

California; they were composed of various proportions of fine sand 

and peat moss with supplemental fertilizers (Matkin and Chandler 

1957). The Cornell Peat-Lite Mixes, the predecessors of modern 

growing media, were developed at Cornell University in the 1960's 

using various combinations of peat moss, vermiculite, and perlite 

(Mastalerz 1977).

43
2.2.2 Functions of a Growing Medium

Plants growing in containers have certain functional 

requirements that must be supplied by the growing medium 

(Mastalerz 1977).

2.2.2.1 Water

Plants require a large, continuous supply of water for growth and 

other physiological processes, such as transpirational cooling, 

and this water must be supplied by the growing medium. Liquid 

water is held both externally and internally by the growing 

medium until it is needed by the seedling: externally in the 

relatively small pores between the particles and internally in the 

interior space of porous materials like peat moss. Because of the 

limited volume of small containers, the growing medium must 

have a high water-holding capacity to supply water to seedlings 

from one irrigation to the next.

2.2.2.2 Air

Plant roots consist of living tissue and must expend energy for 

growth and physiological processes, such as absorption of 

mineral nutrients from the medium solution. The energy for these 

physiological activities is generated by aerobic respiration, which 

requires a steady supply of oxygen. The byproduct of this 

respiration is carbon dioxide, which can accumulate to toxic levels 

if it does not disperse to the atmosphere. Therefore, growing 

media must be porous enough to provide efficient exchange of 

oxygen and carbon dioxide. Because oxygen diffuses through 

water only 1/10,000 as fast as through air, this gas exchange 

must take place in the larger, air-filled pores in the growing 

medium. These large pores are a direct function of media particle 

size, arrangement, and amount of compaction.

44

2.2.2.3 Mineral nutrients

With the exception of carbon, hydrogen, and oxygen, plants must 

obtain all 13 essential mineral nutrients from the growing medium 

solution. Many mineral nutrients, including the ammonium form of 

nitrogen (NH 4) potassium (K+), magnesium (Mg2+), and calcium 

(Ca2+), exist in the growing medium solution as electrically 

charged cations. These nutrient ions remain in the medium 

solution until plant roots take them up and utilize them for growth 

and tissue maintenance or, because of their positive electrical 

charge, they become adsorbed onto negatively charged sites on 

certain types of growing medium particles. This supply of 

adsorbed nutrients, which is measured by the cation exchange 

capacity (CEC), provides a reservoir of mineral nutrients to 

support plant growth between fertilizer applications.

2..2.2.4 Physical support

The final function of the growing medium is to anchor the seedling 

in the container and support it in an upright position. This support 

is a function of the bulk density (relative weight) and the rigidity of 

the growing medium. Weight is important for large, individual 

containers that stand alone (Maronek and others 1986) but is 

inconsequential for the small-volume, aggregated container types 

that are typically used in forest tree nurseries. The rigidity of a 

growing medium is a function of the compressibility and 

compaction of the growing medium particles, and the size of the 

container.
2.2.3 Characteristics of an Ideal Growing

Medium

There is no single growing medium that can be used for all 

purposes, but lists of general horticultural properties are provided 

by Hartman and Kester (1983), James (1987), and Swanson 

(1989). For container tree nurseries, a well-formulated growing 

medium will have most of the following properties, which can be 

separated into cultural characteristics (those that influence 

seedling growth) and operational characteristics (those that affect 

nursery operations).

2.2.3.1 Characteristics relating to seedling growth

The cultural characteristics of a growing medium are the 

properties that affect its ability to consistently produce crops of 

healthy tree seedlings under the cultural practices in a container 

tree nursery: slightly acid pH, high cation exchange capacity, low 

inherent fertility, adequate porosity, and freedom from pests.

Slightly acid pH. pH is a measure of relative acidity or alkalinity of 

a substance, on a logarithmic scale from 0 to 14; values below 7.0 

are acidic and values above 7.0 are alkaline. Materials used to 

formulate growing media can differ considerably in pH. Peat 

mosses are usually acidic in nature, depending on the constituent 

plant species and local water quality, whereas vermiculite can 

range from neutral (pH 7.0) to mildly alkaline (Bunt 1988). PH 

ranges for the four principal growing media components used in 

container tree nurseries are listed in table 2.2.1. The final pH of a 

growing medium will depend on the proportion of the ingredients, 

their original pH, and subsequent cultural practices, especially 

fertilization and irrigation. Irrigation water is usually circumneutral 

or slightly alkaline, and so a normally acidic growing medium will 

typically rise 0.5 to 1.0 unit of pH (that is, become more alkaline) 

through the growing season (Gladon 1988).

The principal cultural effect of pH in mineral soils is its influence on 

availability of mineral nutrients, especially micronutrients; several 

mineral nutrients can become unavailable or even toxic at extreme 

pH values. Comparison of the effect of pH on the mineral nutrient 

availability of mineral and organic soils (such as most growing 

media) shows that maximum availability for organic soils is one full 

point lower (pH 5.5) than for mineral soils (pH 6.5) (fig. 2.2.1). 

Control of pH is less critical in container nurseries, where all 

essential mineral nutrients can be supplied with fertilization. Most 

plants can grow within a relatively wide range of pH values if 

micronutrients are supplied in the proper form and in the proper 

ratio (Bunt 1988).

The pH can also affect the numbers and types of microorganisms 

in growing media, including fungal pathogens. Fusarium spp. are 

more virulent in neutral to alkaline conditions, and losses to 

damping-off fungi increase at pH values above 5.9 (Handreck and 

Black 1984). Most of this information applies to native soils, 

however, and less is known about the effects of pH on pathogen 

activity in artificial growing media. One recent study on a root 

disease of Douglas-fir container seedlings revealed that disease 

losses were more severe at pH 4.0 (94%) than at pH 5.0 (10%) or 

pH 6.0 (4%) (Husted 1988).

On an operational basis, therefore, container nursery managers 

should strive to maintain the pH of their growing media in the

slightly acidic range of pH 5.5 to 6.5. (See chapter 1 in volume 

four of this manual for a more detailed discussion.)

45
High cation exchange capacity. The ability of a material to 

adsorb positively charged ions, the cation exchange capacity 

(CEC), is one of the most important factors affecting the fertility of 

growing media. The CEC can be defined as the sum of the 

exchangeable cations, measured in units called milliequivalents 

(meq), that a material can adsorb per unit weight or volume—the 

larger the number, the greater the nutrient-holding ability. The 

primary cations involved in plant nutrition are calcium (Ca2+), 

magnesium (Mg2+), potassium (K+), and ammonium (NH4

+), listed 

in order of decreasing retention on the CEC sites (Bunt 1988). 

Many micronutrient ions are also adsorbed, including iron (Fe2+

and Fe3+), manganese (Mn2+), zinc (Zn2+), and copper (Cu2+). 

These nutrients are stored on CEC sites on growing medium 

particles until they are taken up by the root system (fig. 2.2.2).

46

CEC has traditionally been measured on a weight basis for field 

soils, but CEC per volume is more meaningful for soil-less growing 

media, because of the relatively low bulk density of most media 

and the small volumes of the containers. Realistically, plants grow 

in a volume rather than a weight of growing medium, and volume 

is now the generally accepted basis for measuring CEC for 

horticultural purposes (Bunt 1988). CEC values for some typical 

growing medium components are compared in table 2.2.1. 

Vermiculite and peat moss have the highest CEC values, whereas 

inorganic materials such as perlite and sand have very low CEC 

values. In a list of CEC values for some of the standard growing 

media (table 2.2.2) (Bunt 1988), the peat—vermiculite medium 

has the highest CEC by a considerable margin.
the peat-vermiculite medium retained more nutrient cations than the 

peat sand medium (fig. 2.2.3). Interestingly, the nitrate form of 

nitrogen (NO; ) was heavily leached from both types of growing 

medium because it is a negatively charged anion, which are repelled 

by the negatively charged cation exchange sites. Some materials 

have the ability to retain a small amount of anions—an anion 

exchange capacity. Whether this applies to nutrient ions like 

phosphate (PO4

-

) is unclear however. Black (1988) reports that 

considerably more soluble P was leached from a 1 :1 

peat-vermiculite medium than from a mineral soil. Bunt (1988), 

although stating that PO4

-

 is leached from most types of growing 

media, showed that vermiculite apparently has a limited ability to 

absorb P04

-

ions (fig. 2.2.3).

A growing medium with a high CEC has another desirable property 

that is of interest to the container nursery manager. Because it is 

able to selectively adsorb and release cations from the growing 

medium solution, such a growing medium can buffer the seedling 

root system against sudden changes in pH or salinity (Whitcomb 

1988).

Low inherent fertility. It may seem incongruous at first, but an 

initially low fertility level is actually a desirable attribute for growing 

media used in container nurseries (Mastalerz 1977, James 1987). 

Maintaining high levels of mineral nutrients, especially nitrogen, 

during seedling germination and emergence is unwise because of 

the possibility of encouraging damping-off fungi. In addition, many 

species of tree seedlings do not require any fertilization for the first 

few weeks of growth (except perhaps for phosphorus, which is best 

supplied through a nutrient injection system). Carlson (1983) 

reported that newly germinated seedlings took up few mineral 

nutrients until 2 weeks after germination, and Barnett and Brissette 

(1986) stated that the seed megagametophyte (endosperm) 

provides ample supplies of phosphorus and other essential mineral 

nutrients for early germinant growth.

The most important benefit of low initial fertility is that nursery 

managers can completely control mineral nutrient concentrations 

in the growing medium solution through fertilization. Inherently 

fertile growing media or media amended with incorporated 

fertilizers make it impossible to completely control seedling 

nutrition during the growing season. Growing media that contain 

slow-release fertilizers should not be stored for more than a few 

days because fertilizer salts can build-up and

47

High CEC values are desirable for growing media because they 

maintain a fertility reserve that supports plant growth between 

fertilizer applications. The CEC can also hold cations in the growing 

medium against leaching, which can be very significant with the 

high irrigation rates used in most container tree nurseries. Certain 

growing medium components are better at resisting leaching than 

others, and generally speaking, the higher the CEC value for a 

medium, the greater its resistance to leaching. Comparing the 

amount of mineral nutrients recovered from two different types of 

growing media in the same volume of leaching water shows that
possibly harm germinating seedlings (Handreck and Black 1984). 

Most container tree nurseries in North America are equipped with 

liquid fertilizer injection systems that allow custom fertilization at 

any time. Even though many brands of commercial growing media 

contain a starter dose of fertilizer, this practice cannot be 

recommended. A low initial fertility allows the container nursery 

manager to custom-fertilize at any time during the rotation and 

control seedling growth and phenology. The ability to completely 

leach all nutrients out of the growing medium and change nutrient 

ratios before the hardening period is often used to initiate bud set 

and cold hardiness (see chapter 1, volume four of this manual).

Most of the components of growing media used in container tree 

nurseries are inherently infertile (table 2.2.1), but there are some 

exceptions. Mastalerz (1977) reported that vermiculite contains 

"large amounts" of potassium and magnesium that are available 

for plant growth, although tests of peat-vermiculite media 

revealed only small amounts of these nutrients (table 2.2.3). 

Peat moss contains between 1 to 2.5%, nitrogen,

48
which is in an organic form and therefore not immediately 

available for plant uptake (Bunt 1988). The grade of peat moss is 

important, however, because the more decomposed forms, such 

as peat humus, may contain enough nitrogen to be a problem to 

seedling culture (see section 2.2.4.2).

Scarratt (1986) analyzed a standard peat-vermiculite growing 

medium for a variety of mineral nutrients and other chemical 

properties and found very low levels of all nutrients. Most 

micronutrients were present in very low concentrations, and 

copper (Cu) was completely absent (table 2.2.3). Commercial 

brands of growing media vary in their initial fertility levels, 

because many media contain supplemental amounts of fertilizer. 

Sanderson (1983) reviewed the fertility of 23 different brands and 

found that nitrate levels ranged from 3 to 154 ppm, phosphorus 

from 1 to 112 ppm, potassium from 8 to 244 ppm, and calcium 

from 100 to 3,160 ppm. Handreck and Black (1984) noted that 

growing medium components can contain varying levels of 

fertility, making it difficult to achieve uniformity between batches. 

These data indicate that growers should be aware of the fertility 

levels of their growing media and should routinely analyze 

growing media nutrient levels (see chapter 1, volume four of this 

manual).

Certain components of growing media can have a negative effect 

on fertility, however, and compete with the seedling for nutrients. 

Uncomposted organic materials, such as sawdust or bark, can 

tie-up a significant amount of nitrogen because the 

microorganisms that decompose these organics require high 

levels of this nutrient. Some types of pine bark can remove iron 

from the growing medium solution, and vermiculite can tie-up 

both iron and phosphorus (Handreck and Black 1984).

Proper balance of pore sizes. There is probably no other 

physical property of growing media that has been so thoroughly 

discussed and investigated as porosity. This attention is well 

deserved, however, because the relative pore space of a 

growing medium affects every aspect of seedling growth in 

containers. A properly balanced pore structure means good gas 

exchange for the root system, which directly affects all root 

functions such as water and mineral nutrient uptake. Milks and 

others (1989) state that plants growing in small containers often 

have growth problems due to poor aeration or low water-holding 

capacity of the growing medium. Aeration porosity is considered 

to be the most important

49

physical property of any growing medium (Johnson 1968, 

Bragg and Chambers 1988).

A growing medium is composed of solid particles and the pore 

spaces that exist between them; these pore spaces are as 

important horticulturally as the particles themselves. The amount 

of pore space is expressed in terms of percent porosity and is a 

function of the size, shape, and spatial arrangement of the 

individual growing medium particles in the container (fig. 2.2.4).

Porosity can be functionally divided into three parts—total 

porosity, aeration porosity, and water-holding porosity 

(Bethke 1986, Handreck and Black 1984):

Total porosity. Total porosity is the measure of the total pore 

space of a growing medium, expressed as the percentage of its 

volume that is not filled with solid particles. For example: 1,000 

ml of growing medium with a total porosity of 40% has 400 ml of 

pores and 600 ml of solid particles (fig. 2.2.4).

Aeration porosity. Aeration porosity is the measure of that part 

of the total pore space that is filled with air after the growing 

medium is saturated with water and then allowed to drain freely. 

The pores that contain air are relatively lame and are termed 

macropores (fig. 2.2.5).
Water-holding porosity. Water-holding porosity is the measure 

of that part of the total pore space that remains filled with water 

after the growing medium is saturated with water and allowed to 

drain freely. The pores that contain water are relatively small and 

are termed micropores (fig. 2.2.5).

The porosity characteristics of a growing medium (the relative 

proportion between aeration porosity and water-holding porosity) 

depend on the types and sizes of the growing medium 

components. Bugbee and Frink (1986) varied the size of peat 

moss and vermiculite particles to produce growing media with 

aeration porosities from

1 .0 to 33.6% by volume. Total porosity remained constant 

with the increase in aeration porosity, but water-holding 

porosity decreased linearly.

Porosity can be measured in several different ways—aeration 

porosity can be measured by either volumetric or gravimetric 

means (Bragg and Chambers 1988). A relatively simple 

procedure for estimating total porosity, aeration porosity, and 

water-holding porosity is described in table 2.2.4.

Recommendations for the total amount and type of pore space for 

container growing media vary considerably. Handreck and Black 

(1984) reported that well-formu lated growing media contain about 

60 to 80% total porosity. Havis and Hamilton (1976) stated that the 

total porosity of a growing medium should exceed 50%, and that 

the aeration porosity should be 20 to 25%; Whitcomb (1988) 

recommended a higher aeration porosity of approximately 25 to 

35% for container tree seedlings. Puustjarvi and Robertson (1975) 

recommended an even higher aeration porosity value of 45 to 50% 

because of the high oxygen demand of roots in greenhouse 

environments. This wide variation in porosity recommendations 

also reflects the physical characteristics of the different types of 

growing media and different ways in which porosity can be 

measured. A well-formulated growing medium will contain a 

mixture of macropores, for aeration and drainage, and micropores, 

for water-holding ability (fig. 2.2.5). The ratio of macropores to 

micropores varies considerably between different growing medium 

mixtures, however, and the porosity characteristics of a given 

medium cannot be predicted from the porosity of its individual 

components. Beardsell and others (1979) found that, although total 

porosity can be predicted from bulk density for certain growing 

media, the aeration and water-holding porosity cannot.

Factors affecting porosity. The porosity of a growing medium will 

vary with the characteristics of its components, the degree of 

medium compaction within the container, and the container height. 

In fact, container height is actually the principal factor controlling 

aeration porosity of growing media in a container (Milks and others 

1989). There are four factors that affect porosity characteristics in 

containers: individual particle size, particle characteristics, particle 

size mixture, and changes in porosity over time.

Individual particle size. In bareroot seedling nurseries, soil 

structure is one of the most important properties

50
affecting soil porosity but, in container nurseries, porosity is 

determined mainly by the range of particle sizes present in the 

growing medium (Handreck and Black 1984). Large particles do 

not pack together as closely as small particles and therefore 

produce greater total porosity. Aeration porosity and water-holding 

porosity have a complementary relationship: as the size of the 

particle increases, the water-holding porosity decreases and the 

aeration porosity increases (fig. 2.2.6).

The particle size that generates the proper mix of porosity 

apparently varies with the type of material. For

peat moss, a particle size between 0.8 mm (0.03 inch) and 6.0 mm 

(0.24 inch) is recommended. If the peat particles are smaller than 

about 0.8 mm (A in fig. 2.2.6), micropores predominate, and the 

growing medium can easily become waterlogged. As the particle 

size increases, the ratio of macropores to micropores increases 

until, above about 6.0 mm (B in fig. 2.2.6), macropores 

predominate and the growing medium will not retain enough water 

for plant growth (Puustjarvi and Robertson 1975). The ideal particle 

size range for pine bark, on the other hand, is somewhat smaller. 

Handreck and Black (1984) reported that particles smaller than 0.5 

mm (0.02 inch) had the most significant effect on the aeration and

51
water-holding characteristics of the growing medium. They found 

that aeration porosity was decreased most by bark particles smaller 

than 0.25 mm (0.01 inch), whereas available water was increased 

more by particles in the range 0.10 to 0.25 mm (0.004 to 0.010 

inch). The ideal size for sand is: 60% of the particles between 0.25 

and 1.00 mm (0.01 and 0.04 inches), with no more than 3% smaller 

than 0.1 mm (0.004 inch) or larger than 2 mm (0.08 inch) (Swanson 

1989).

Particle characteristics. The physical and chemical properties of 

the particles themselves also affect the porosity of a growing 

medium. Some components, such as peat moss and vermiculite, 

can be compressed, whereas others, such as perlite and bark, 

retain their original size under compression. Peat moss and 

especially vermiculite are fragile and can be easily broken into 

smaller particles during mixing and handling. Adding moisture to 

peat-vermiculite media during mixing can actually maintain the 

integrity of the particles (Milks and other 1989). The irregular 

surface texture of perlite particles generates macropore space 

(Moore 1988). Some organic components, especially

52

uncomposted sawdust and bark, shrink during decomposition.

The particles of many growing medium components are not solid 

but contain internal spaces; this internal porosity is important to

the horticultural properties of a growing medium. Some dense 

materials, such as sand particles, have essentially no internal pore 

space, whereas other materials, such as peat moss, vermiculite, 

and pine bark, have a substantial amount of internal porosity . The 

internal pore space of pine bark particles was determined to be 

about 43% of its total volume (Pokorny 1987). In addition to 

affecting the bulk density, this internal porosity may influence the 

moisture-holding and nutrient-holding properties of a growing 

medium. Although internally held water is considered by some 

researchers to be unavailable to plants (Spomer 1975), Pokorny 

(1987) demonstrated that seedling water uptake is increased by 

root penetration of pine bark particles. The spongy structure of 

sphagnum peat moss and vermiculite particles reflect high internal 

porosity, which is one reason for their popularity as growing 

medium components in container tree nurseries. Other porous 

materials, such as perlite, have sealed pores and therefore cannot 

absorb moisture. For example, perlite particles hold consistently 

less moisture and therefore provide more aeration than pumice 

particles of the same size (Johnson 1968). Because of these 

desirable properties, perlite is often added to a growing medium to 

increase the aeration porosity.

A comparison of the porosity characteristics for some common 

growing medium components (table 2.2.5) further illustrates the 

variation between different materials. Peat moss has the highest 

total and water-holding porosity because it has high internal pore 

space; fir bark has the lowest water-holding porosity because it is 

suberized and therefore water repellent.

Particle size mixture. Because growing media are usually a 

mixture of two or more components with a variety of particle sizes, 

the arrangement of the individual particles and their relationship to 

each other affects porosity. Whenever particles of different sizes 

are mixed together, the resultant volume is always less than the 

sum of the original volumes because the smaller particles fill in the 

pores between the larger ones. This is especially significant with 

angular particles such as some types of sand in which the 

pyramid-shaped particles are able to pack tightly together.
cause most of the commonly used components are considered to 

be pest-free (James 1987). Vermiculite and perlite are rendered 

completely sterile during manufacturing, when they are exposed 

to temperatures as high as 1 ,000 °C (1,832 °F). The pest-free 

status of peat moss is subject to debate (table 2.2.1). Bluhm 

(1978) reports that, although some peat moss products are 

advertised as "sterile" or "pest-free," peats have been shown to 

contain pathogenic fungi, weed seeds, and nematodes. Baker 

(1985) stated that water molds and other root-rotting fungi have 

been found in commercial peat moss from several geographic 

areas, including Canada. Bunt (1988) concluded that peat is not 

technically sterile, but because the microorganisms that it contains 

are not usually pathogenic, it is not usually sterilized before use 

(see chapter 1, volume five of this manual).

Some recent studies have found that some types of sphagnum 

peat moss actually inhibit certain pathogenic fungi, such as 

Pythium spp. Wolffhechel (1988) inoculated Pythium into samples 

of sphagnum peat moss from five different locations and found 

considerable variability in their receptivity to the pathogen. 

Because it could be destroyed by heat pasteurization or fungicide 

treatment, this ability to suppress Pythium was attributed to the 

presence of beneficial microorganisms. Growing media 

formulated with this disease-suppressive peat moss could inhibit 

the development of Pythium-related damping-off and root rot. One 

commercial supplier, Ball Seed Company, has already released a 

peat-based growing medium that is "naturally suppressive" to 

Pythium diseases.

2.2.3.2 Characteristics that affect nursery operations

In addition to container characteristics that influence seedling 

growth, nursery managers must consider growing media 

properties that relate to the operational aspects of container tree 

nursery management. Factors such as cost and availability, 

uniformity and reproducibility, bulk density, dimensional stability, 

durability and ease of storage, ease of mixing and loading, 

rewettability, and the ability to produce a firm root plug are also 

important in the selection of a growing medium or individual 

components.

Reasonable cost and availability. Cost is one of the most 

significant, yet often overrated, factors in selecting growing media 

or their components. Because growing media have such an 

overriding effect on container seedling growth, nursery managers 

should not make cost

 53

Changes in porosity over time. The porosity of a given growing 

medium also changes over time because of decomposition of 

medium particles, siltation from irrigation and gravity, and root 

ingrowth. Langerud (1986) concluded that a physically stable 

medium is needed that can maintain the critically important 

balance between aeration and water-holding ability. Beardsell and 

others (1979) found that fine particles of pumice settled to the 

bottom of the container where they filled in between the larger 

particles and caused drainage problems. Many growers fail to 

appreciate the amount of pore space that becomes filled with 

roots; in fact, tree seedlings are expected to produce enough roots 

to form a relatively firm root plug at the end of the growing season. 

As new roots are produced, they penetrate and then expand to fill 

up the larger air-filled pores of the growing medium, gradually 

reducing the aeration porosity. This problem is more serious for 

larger tree seedlings that are grown for more than one growing 

season. To counteract this effect, growers should use a coarse, 

well-aerated growing medium that, although requiring more 

irrigation when the seedlings are small, will provide adequate air 

exchange when the seedlings mature.

Freedom from pests. One of the most serious problems with 

soil-based growing media is that native soil can contain a variety 

of pests, such as pathogenic fungi, insects, and nematodes, and 

weed seeds. Because of these pests, soils need to be pasteurized 

with heat or sterilized with chemicals before they are used in 

growing media. With the advent of artificial growing media, the use 

of pasteurization has decreased substantially be
the determining factor when selecting a medium. Swanson 

(1989) stated the case well-price should not be the first priority in 

choosing a commercial growing medium or medium 

components, unless it is essentially prohibitive.

Availability has an effect on other operational aspects of a 

growing medium, especially uniformity and cost (Swanson 1989). 

Most of the materials used to formulate growing media are 

inexpensive in themselves but are bulky and sometimes heavy 

and therefore expensive to ship. Primarily because of shipping 

costs, a material that is inexpensive in one location may be 

prohibitively expensive in another. Many components are 

produced in restricted geographic areas and are either difficult to 

obtain or prohibitively expensive in other areas. Sphagnum peat 

moss is easily accessible and relatively cheap in Canada and the 

northern United States but is considered uneconomical in many 

parts of the world. Sand, on the other hand, is a growing medium 

component that is found worldwide at a relatively low cost. Prices 

of media components also may change: materials that are initially 

inexpensive, such as pine bark, may become costly due to high 

demand and competition with other uses (Whitcomb 1988).

High degree of uniformity and reproducibility. Both the 

components and the resultant growing media must be uniform in 

quality and reproducible from batch to batch. Some materials, 

such as peat moss and sand, can vary considerably between 

locations. Although peat moss is used as a generic term, the 

quality of this popular growing medium component varies with 

respect to the type of plant that composes the peat and the climate 

under which the peat was deposited. Mastalerz (1977) 

recommends standardization of growing media and medium 

components to ensure that each batch has the same physical, 

chemical, and biological properties. A growing medium that varies 

in physical or chemical characteristics can cause serious problems 

with irrigation, fertilization, and other cultural practices (see section 

2.2.5.1). A good growing medium must also be reproducible in 

order to guarantee crop uniformity and maintain crop production 

schedules (Whitcomb 1988). The ability to consistently reproduce 

a uniform growing medium from crop to crop is one of the most 

important cultural factors in successful container tree nursery 

management.

Low bulk density. The bulk density of a growing medium is 

defined as weight per unit volume and is

54

usually expressed in grams per cubic centimeter (g/cm3) or 

kilograms per cubic meter (kg/m3); the English units are pounds 

per cubic foot. The bulk density of a specific growing medium is a 

function of three factors:

• The bulk densities of the particles composing the 

medium.

• The compressibility of the particles.

• The arrangement of these particles with respect to one 

another.

Bulk densities of different growing medium particles vary 

considerably depending on their chemical composition and 

physical structure. Although Handreck and Black (1984) 

estimated the bulk density of average mineral particles at 

approximately 2.6 g/cm3 (162.3 pounds per cubic foot) and 

average organic matter particles at 1 .55 g/cm3 (96.8 pounds per 

cubic foot), such averages are of limited usefulness because the 

compressibility and arrangement of the particles also affect bulk 

density values. Mineral particles can vary from solid sand to 

highly porous vermiculite, and organic particles from rigid bark to 

spongy peat moss.

The bulk density of a growing medium in a filled container is also a 

function of the internal arrangement of the individual particles. 

Although Beardsell and others (1979) found that total porosity 

could be predicted from bulk density for some types of growing 

media, aeration and water-holding porosity are often related to 

how densely the medium is compacted during the filling process 

(compaction is discussed in detail in section 2.2.7).

Bulk density is traditionally measured on an ovendry weight basis, 

but wet bulk density is also important for operational reasons. 

Some materials, such as peat moss and vermiculite, are able to 

absorb many times their own weight in water. Nelson (1978) 

reported that a growing medium consisting of vermiculite and 

perlite had a bulk density of around 0.51 g/cm3 (32 pounds per 

cubic foot) when saturated, compared to a dry weight of only 0.10 

g/cm3 (6.5 pounds per cubic foot). Growers interested in keeping 

their media light in weight may want to consider perlite as a 

component because this material is relatively hydrophobic. A 

growing medium consisting of a mixture of peat moss and perlite 

would, therefore, have a saturated weight much lower than that of 

a peat moss-vermiculite mixture (Whitcomb 1988).
Both dry and wet bulk densities for common growing medium 

components are listed in table 2.2.5. Some materials, such as 

peat moss and vermiculite, have much higher wet bulk densities 

than dry bulk densities. Sand has obviously the highest values 

because of its higher particle density, and the small difference 

between the wet and dry values reflects a low water-holding 

capacity. In ornamental nurseries, the high bulk density of sand 

adds stability to single, free standing containers so that top-heavy 

plants do not blow over; this property is inconsequential for the 

small, aggregated containers used in most forest tree nurseries. 

Dry bulk density is operationally important for shipping and 

handling of dry medium components, but wet bulk density affects 

the handling and shipping of the container seedlings when the 

growing medium is saturated. Based on ease of handling, 

therefore, components with low bulk densities would have an 

advantage in container tree nurseries.

Dimensional stability. A growing medium should not shrink or 

swell excessively during use. Bilderback (1982) lists three different 

types of volume changes that can occur.

1. Shrinkage due to expansion and contraction.

Materials such as peat moss can shrink during periods of 

alternating wetting and drying. Harlass (1984) reported that if 

some peat-based media are allowed to dry out excessively, they 

can pull away from the container wall, making uniform rewetting 

difficult.

2. Decomposition of organic materials. Organic 

components that were not properly composted will lose volume 

as they decompose (see following section on durability and ease 

of storage).

3. Space-volume shrinkage. Small particles can fill in the 

pore spaces between larger particles; the greater the difference in 

particle sizes, the greater the change in volume. Whitcomb (1988) 

discussed the poor aeration that resulted when fine vermiculite 

particles were carried to the bottom of the container with 

percolating irrigation water, filling the pore spaces and raising the 

level of the perched water table. Sand or pumice with a range of 

particle sizes is also not recommended because the smaller 

particles gravitate to the bottom of the container over time and 

reduce porosity.

Durability and ease of storage. Most of the popular 

components of growing media are durable and will not 

decompose or otherwise change over time. Uncomposted organic materials can decompose considerably during the 

cropping cycle and are therefore not recommended for growing 

media (Nelson 1978). Composted organic materials, including 

sawdust, wood chips, and bark, can be used, however. Pine bark 

does not change volume appreciably because of its slow 

decomposition rate, but even so most nursery managers prefer 

aged or composted bark (Bilderback 1982). Peat moss can vary 

considerably in its degree of decomposition, which can be rated 

on a 1 to 10 scale using the von Post system IPuustjarvi and 

Robertson 1975).

Initially sterile materials that are ordered in bulk quantities can be 

contaminated with weed seeds or other pest propagules during 

storage. For this reason, batches of premixed growing medium or 

medium components should be purchased in sealed plastic 

packages and stored out of direct sunlight, which will rapidly break 

down the plastic covering.

Ease of mixing and filling into containers. This factor is 

particularly important for nursery managers who custom mix their 

own batches of growing media. Uniform mixing can be difficult to 

achieve because of differences in the bulk density, particle size, 

and moisture content of ]he various components. Growing medium 

components should not aggregate or clump during storage and 

should flow easily during the mixing and filling operations. Peat 

moss and pine bark are shipped relatively dry but must be 

thoroughly wetted before mixing; this can be operationally difficult, 

however, because these organic materials are often hydrophobic at 

low moisture contents (see section 2.2.5.4 for more information on 

wetting agents). Flowability is especially important when filling the 

small-volume containers typically used in forest tree nurseries. 

These containers have correspondingly small top diameters that 

"bridge" easily and prevent the medium particles from flowing and 

uniformly filling the container.

Ease of rewetting. Some components, such as peat moss and 

bark, can become hydrophobic if allowed to dry out excessively, 

which can reduce irrigation infiltration rates. This problem is 

particularly serious during the dormancy induction period, when 

container managers withhold irrigation to induce seedling moisture 

stress. Bunt (1988) stated that the difficulty in rewetting peat is due 

to a film of air trapped on the surface of the peat particles and to 

the presence of iron humates, which repel water. Bark particles 

can also be difficult to rewet, and Pokorny (1979) stated that this 

hydrophobicity is

55
caused by both physical and chemical factors: most bark particles 

are covered with organic chemicals that resist water absorption 

and their rough surfaces create a surface tension that also 

physically repels water. Bilderback (1982) recommended using 

surfactants if the hark is below 35% moisture content. Surfactants, 

also called wetting agents, are routinely added during the mixing 

procedure to increase wettability of growing medium components 

(see section 2.2.5.4 for more information on surfactants).

Promotion of firm root plug formation. This operational 

characteristic is unique to forest tree nurseries because the 

shippable seedlings are removed from the container before 

planting. Container seedlings are expected to maintain a firm root 

plug during handling, shipping, and planting. A cohesive, resilient 

plug is especially important when the seedlings are planted with a 

dibble or other planting tool that makes a hole the same size and 

shape as the container. The growing medium and other cultural

practices must be designed to promote adequate root growth 

throughout the container so that a firm root plug develops by the 

end of the growing season (fig. 2.2.7A). Growing media that do 

not pack evenly in the container or inhibit aeration can result in a 

poorly formed root system that does not extract easily from the 

container or does not maintain the integrity of the root plug during 

handling and shipping (fig. 2.2.713). Lackey and Alm (1982) rated 

"styroplug quality" of conifer seedlings grown in 6 different types 

of growing media and found that two brands of commercial 

growing media produced plugs with the highest quality rating. 

Tinus (1974) evaluated the survival and growth of ponderosa pine 

seedlings with different "root plug integrity" ratings; both survival 

and growth decreased significantly as the root plug deteriorated 

(table 2.2.6).

56
57
2.2.4 Components Used in Formulating

Growing Media for Tree Seedlings

Although pure peat is used in some northern container tree 

nurseries, most modern growing media consist of two or more 

different components that are selected to provide certain 

physical, chemical, or biological properties. Other amendments, 

such as fertilizer or wetting agents, are sometimes added during 

the mixing process. For purposes of clarity, a growing medium 

component usually constitutes a large percentage (>10%) of the 

mixture, whereas an amendment is defined as a supplemental 

material that contributes less than 10%, to a medium (Gladon 

1988) (amendments will be covered in section 2.2.6).

The Container Nursery Survey revealed that only 5 materials 

were used for growing media in forest tree nurseries in the 

United States and Canada: peat moss, sawdust, sand, 

vermiculite, and perlite.

A typical growing medium used in horticulture today is a 

composite of two or three components. Mixtures of organic and 

inorganic components are popular because these materials have 

opposite, yet complementary, physical and chemical properties. 

Beardsell and others (1979) found that some of the physical 

properties of growing medium mixtures, such as water-holding 

ability, can be predicted from the characteristics of the individual 

components.

2.2.4.1 Organic components

Function of the organic component. Organic materials are 

desirable components of growing media because they generate a 

large proportion of micropores, and thus produce a high 

water-holding capacity, yet are resilient enough to resist 

compaction. Organic matter also has a high CEC and can 

therefore retain nutr ient ions against leaching and provide a buffer 

against rapid changes in salinity.

The amount of organic material used in growing media varies 

considerably, generally ranging from 25 to 50% (by volume), but 

sometimes up to 100% (Mastalerz 1977). Joiner and Conover 

(1965) found that the best proportion of organic material was from 

40 to 50%, and Harlass (1984) reported that mixes containing 

more than 50% organic matter may actually have less pore space.

Although many different types of organic matter have been 

used for growing media in ornamental container crops 

(Mastalerz 1977, Bunt 1988), peat moss is the most commonly 

used organic material in forest tree

58

nurseries. The only other organic material reported in the 

Container Nursery Survey was sawdust, which was mentioned 

by only two nurseries.

Peat moss. Although samples of peat moss may appear similar, 

they can have very different physical and chemical properties. 

Peats are formed when partially decomposed plants accumulate 

under water in areas with low temperatures and low oxygen and 

nutrient levels (Peck 1984). Peats can be composed of several 

species of plant including mosses, sedges, and grasses. The 

species of plant, its degree of decomposition, variation in local 

climate, and water quality all contribute to differences in peat 

moss quality and determine its value as a growing media 

component (Mastalerz 1977).

There are several different systems for classifying peat; for 

horticultural purposes, the principal plant species and the degree 

of decomposition are most important. The American Society for 

Testing Material (ASTM) uses a 5-class rating system based on 

the type of plant composing the peat and the organic fiber content 

(Bunt 1988, Mastalerz 1977, Hellum 1975). Analyzing the physical 

and chemical properties of peat moss is time consuming and 

technically demanding, and therefore nursery managers generally 

rely on the information provided by the suppliers. One newly 

developed analytical procedure, infrared spectroscopy, can 

determine the botanical composition, degree of humification, 

cation exchange capacity, nitrogen content, and other physical 

and chemical properties of a peat sample (Lehtovaara and others 

1988). Some physical and chemical characteristics of the principal 

horticultural grades of peat moss are provided in table 2.2.7, and 

the four grades can be described as follows.

1. Sphagnum peat moss. This classification requires a 

minimum 90% organic matter on a dry weight basis, with over 75% 

of this material being mosses of the genus Sphagnum. There are 

approximately 335 species of sphagnum moss in the world 

(Puustjarvi 1975); Hellum (1975) reports that there are 25 species 

in Alberta alone and that S. fuscum is one of the most desirable. 

The stems and leaves of sphagnum peat moss are characteristic, 

and the fibrous, unicellular structure of the foliage can be 

diagnostic (fig. 2.2.8). The leaves of sphagnum moss contain a 

large number of pores that form an internal capillary system 

capable of storing large volumes of available water; in fact, Peck 

(1984) estimates that 93% of the water in this internal pore space 

is available to plants.
States contain a low proportion of sphagnum moss, with the 

exception of those from northern states such as Minnesota, 

Michigan, Maine, and Washington (Lucas and others 1965).

There is considerable variation in peat moss quality, even in the 

type harvested from sphagnum bogs; Carlson (1983) stated that 

peat moss quality varied not only between different bogs, but even 

vertically within the same bog. Scagel and Davis (1988) evaluated 

the physical and chemical properties of peats used in British 

Columbia container tree nurseries and found highly variable 

results. A laboratory comparison of the physical properties of four 

commercial brands of sphagnum peat moss revealed significant 

differences in many of the properties (table 2.2.8). Carlson (1983) 

provided standards for physical property tests and recommended 

that such tests be performed on all potential peat sources.

Peck (1984) distinguished between two different types of 

sphagnum peat moss: light peats and dark peats. Light 

sphagnum peats are so-named because they are light in both 

color and weight; they have a high volume of internal pores, a 

large proportion (15 to 40%) of which is classified as aeration 

porosity. Dark sphagnum peats are twice as heavy as light peats 

and contain less total pore space, with correspondingly less 

aeration porosity. The CEC of dark peats is over twice that of light 

sphagnum peats. Peck (1984) considers dark peats less suitable 

for long-term container seedling culture because they are less 

durable and less resilient than the lighter colored peats.

59

In addition to their physical characteristics, types of peat moss can 

be identified by their geographic origin. Sphagnum moss only 

grows in cool, northern climates. Hellum (1975) stated that 

desirable sphagnum bogs are found only in the boreal forest and 

that peat bogs in the southern part of Alberta contain a low 

proportion of sphagnum moss because of alkaline water and soil 

conditions. Most peat mosses produced in the United
2. Hypnum peat moss. The organic matter content of this 

type of peat moss must exceed 90% of the ovendry weight, more 

than 50% of which must be composed of mosses of the Hypnum 

genus (Bunt 1988). Hypnum peat moss is generally less 

expensive than sphagnum moss but may contain weed seeds or 

plant pathogens because of the conditions under which it was 

formed (Whitcomb 1988). Mastalerz (1977) reported that much of 

the peat moss produced in the northern United States contained a 

high proportion of hypnum mosses. Hypnum mosses decompose 

more rapidly than sphagnum mosses but are used in the growing 

media for some horticultural crops, especially for acid-intolerant 

plants (Peck 1984). Growing media containing a high percentage 

of hypnum peat moss are not recommended for container tree 

seedlings.

3. Reed and sedge peat moss. These peats are formed 

from rushes, grasses, sedges, and similar marsh plants; an 

ovendry sample must contain a minimum 33% of these materials 

on a dry weight basis (Bunt 1988). Reed-sedge peat mosses are 

generally finer textured, more decomposed, and less acid than 

sphagnum peat moss. Mastalerz (1977) considers this peat moss 

class unsatisfactory for use in growing media because of its rapid 

rate of decomposition, fine particle size, and low fiber content.

60

4. Peat humus. Peat humus includes all types of peat 

moss that are in such an advanced stage of decomposition that 

the constituent plants are not recognizable. Peat humus is 

usually derived from reed-sedge or hypnum peat moss (Lucas 

and others 1965) and is composed of less than 33% total peat 

fiber (Bunt 1988). Because this grade of peat moss often 

contains a high proportion of foreign material such as silt and 

clay, it is considered undesirable for use in growing media 

(Mastalerz 1977).

Although other grades of peat moss have some horticultural 

applications, sphagnum peat moss is the only type that can be 

recommended for production of container tree seedlings (fig. 

2.2.9A). Operational trials comparing growing media containing 

sphagnum peat moss and media containing other peat moss 

types have produced startling growth differences (fig. 2.2.913).

In addition to the type of plants, peat texture is determined by the 

manner in which the peat is harvested and processed. Peat can 

be harvested from bogs by several processes, including block 

cutting and hydraulic mining. Block cutting involves cutting slabs 

of peat from the bog and then shredding the peat to the proper 

texture, whereas hydraulic mining mechanically shreds the peat 

and then dredges the liquid slurry from the bog (Helium 1975). 

The harvesting method can affect the physical characteristics of 

peat moss, especially the porosity (Wilson 1985). The hydraulic 

mining process breaks down the structure of the peat particles, 

causing it to pack more tightly with a resultant loss in total and 

aeration porosity (table 2.2.9). Block-cut peat is usually preferred 

for container growing media because of its coarser texture; the 

ASTM defines coarse peat as having particles larger than 2.38 

mm (about 0.1 inch) (Bunt 1988).
Composted sawdust, bark, and other organic materials.

Although peat moss is the most common organic component used 

in growing media in container tree nurseries in the United States 

and Canada, other organic materials have potential, especially in 

warmer climates where the cost of sphagnum peat moss can be 

prohibitive (fig. 2.2.10). Wood residues, including sawdust, bark, 

and wood chips, are alternative organic materials that can be used 

for growing media, depending on local availability and cost. 

Sewage sludge and mushroom compost have also been tested in 

ornamental nursery

61
Bark. Tree bark is probably the most promising of alternative 

organic materials, and when prepared properly, both pine and 

hardwood bark have found wide acceptance as components of 

growing media in ornamental container nurseries (Bunt 1988, 

Mastalerz 1977, Bilderback 1982). Pokorny (1979) reviewed the 

horticultural properties of pine bark, and Stewart (1986) described 

the production of a commercially available pine bark product, 

marketed internationally as Cambark®. A similar bark-based 

product (Peatgro®) is being used to produce container seedlings 

in South Africa (Nelson 1989).

Pine bark is naturally acidic, has a low initial fertility, and also 

possesses many other beneficial properties (table 2.2.1). Bark is 

usually added to a growing medium to increase its air porosity; as 

the proportion of bark in bark-vermiculite media was increased, the 

percentage of air space was significantly increased (Lennox and 

Lumis 1987). Pokorny (1987) found that bark particles have 43% 

internal porosity, which contributes water for plant growth. 

Composted bark has a much higher CEC

62

applications (for example, Chong and others 1988). Lippitt (1989) 

tested rice hulls as an organic component in a forest nursery and 

reported that they are cheap, readily available, consistent in 

quality, resistant to decomposition, and mix well. Bunt (1988) and 

Mastalerz (1977) discussed alternative organic components that 

have been used in growing media for ornamental nurseries.

Because of their high carbon to nitrogen ratio (C/N), wood 

residues must be composted with supplemental nitrogen 

amendments before use (table 2.2.10). Fresh bark can have a C/N 

ratio of 300:1 and so is frequently composted before use. Organic 

materials vary considerably in decomposition rates and in the 

amount of nitrogen required during composting, even within one 

genus of tree (table 2.2.1 1). An excellent overview of the 

biochemistry and methodology of composting is provided by 

Poincelot (1972), and composting is also discussed in detail in 

Mastalerz (1977), Bunt (1988), Whitcomb (1988), and Nelson 

(1978).

Sawdust. Sawdust is commonly used in horticultural growing 

media and is usually composted before use. Because of the 

inherent differences in chemical properties between different 

woods, however, the suitability of sawdust as an organic growing 

media component is extremely variable. Mastalerz (1977) stated 

that sawdust from incense-cedar, walnuts, or redwood is known to 

have direct phytotoxic effects, and Gates (1986) related that 

sawdust from western red cedar is toxic to many horticultural 

plants. Stewart (1986) reported that conifers grown on peaty soil 

types accumulate high levels of manganese, and that sawdust 

from such trees is phytotoxic if these materials are used for 

growing media. Worrall (1976) investigated the properties of 

sawdust

from several species of Eucalyptus and found that, even within this 

one genus, the presence of chemical toxins varied between tree 

species (table 2.2.11). Only sawdust from sawmills should be 

considered for growing media because other wood residues may 

contain preservatives or other harmful chemicals. Sawdust from 

coastal sawmills can contain high levels of salts, which can also be 

harmful to seedling growth (Gates 1986). Obviously, sawdust 

should be chemically tested before it is used in growing media. 

Another potential problem is uniformity of particle size; particle size 

analysis of sawdust revealed a considerable amount of variability 

(Scagel and Davis 1988).
than raw bark and also has been shown to suppress the activity of 

pathogenic fungi (Hoitink 1980). Compared to standard 

peat-vermiculite mixes, growing media containing pine bark had 

significantly less seedling mortality after inoculation with species of 

Pythium and Fusarium (Pawuk 1981). In other situations, bark can 

replace peat for reasons of economy or availability. Milbocker 

(1987) estimated the cost of pine bark at half the cost of sphagnum 

peat moss.

Bark is usually incorporated in a mixture with other components. 

Stewart (1986) recommends adding 25 to 50% pine bark to peat 

moss to form a well-structured growing medium. The size of the 

bark particles is important, and Whitcomb (1988) recommends 

running bark through a hammermill with a 2- to 2.5- cm (0.75- to 

1.0-inch) screen to produce a desirable range of particle sizes. 

Handreck and Black (1984) recommended a mixture of pine bark 

sizes, including approximately one-quarter to one-third in the < 0.5 

mm (0.02 inch) range.

One of the biggest drawbacks of bark is its variability. Nurseries 

have reported problems in obtaining regular supplies of bark of 

consistent quality (Lippitt 1989). Two other potential problems with 

the use of bark are nitrogen deficiency and the presence of organic 

or inorganic toxins (Bunt 1988). Composting bark with 

supplemental nitrogen fertilizer supplies the nitrogen that 

microorganisms require during decomposition. The possibility and 

degree of phytotoxicity depend on the age of the bark, the season 

of collection, the species of tree, and the geographical location. 

Phytotoxicity is apparently related to the monoterpene or 

manganese content of the bark. More information and treatments 

for phytotoxicity are provided by Bunt (1988).

Sewage sludge. Sewage sludge is another organic material that 

has been used as a growing medium component in horticultural 

nurseries. Sludge is an extremely variable material depending on 

the raw materials involved and the stage of processing. Chong and 

others (1988) grew two ornamental woody plants in both primary 

and secondary sludge from two different paper mills. They reported 

that the mineral nutrient content of these sludge products was 

variable, especially for nitrogen, which caused irregular plant 

growth. Simpson (1985) tested a compost of sewage sludge and 

wood-waste for production of conifer seedlings but found that this 

medium was inferior to the standard peat-vermiculite growing 

medium.

2.2.4.2 Inorganic components

Function of the inorganic component. Inorganic materials are 

added to growing media to produce and maintain a structural 

system of macropores that improves aeration and drainage and 

decreases water-holding ability (Mastalerz 1977). Many 

inorganic components have a very low CEC and provide a 

chemically inert base for the growing medium. Inorganic 

materials with high bulk densities, such as sand, are used to 

provide stability to large, free-standing containers in ornamental 

nurseries.

Three different materials are routinely being used as inorganic 

components in growing media in container tree nurseries in the 

United States and Canada: vermiculite, perlite, and sand. Based 

on the 1984 Container Nursery Survey, vermiculite was by far the 

most commonly used, perlite was second, and sand was 

mentioned by only one nursery.

Vermiculite. Vermiculite is an aluminum-iron-magnesium silicate 

mineral, mined in the United States and Africa, that consists of a 

series of thin, parallel plate s. After the raw vermiculite mineral is 

mined, it is subjected to intense heat (up to 1,000 °C, or 1,832 

°F),a which expands the vermiculite particles to 15 or 20 times 

their original volume and gives them an accordion-like structure 

(Bunt 1988) (fig. 2.2.1 1).

Vermiculite has many unique properties that make it useful for 

horticultural purposes (table 2.2.1): it is lightweight and its 

plate-like structure generates a very high surface to volume ratio, 

producing a high water-holding capacity. The plates contain 

numerous cation binding sites, both externally and internally, 

which produce a high CEC; this property is unique for inorganic 

growing medium components, which are typically chemically inert. 

Bunt (1988) reported that, although vermiculite does not 

apparently have any anion exchange capacity, it can adsorb 

phosphate in available forms. Vermiculite contains some 

potassium and magnesium that are slowly released for plant 

uptake. Because of the high processing temperatures, vermiculite 

is completely sterile. The pH of vermiculite is variable, generally 

ranging around neutral (pH 7.0), too high for most conifer 

seedlings. This is not a problem, however, because vermiculite is 

normally mixed with a more acidic organic material such as 

sphagnum peat moss (Biamonte 1982, Mastalerz 1977).

63
water repellents, however, and should not be used. Obviously, 

growers should test vermiculite or any other growing medium 

component before using it on a large scale.

Vermiculite is produced in four grades, based on particle size, 

which determines the relative proportion of aeration and 

water-holding porosity (table 2.2.12). Grades 2 and 3 are most 

commonly used in growing media; grade 2 is preferred when more 

aeration porosity is desired, whereas grade 3 produces more 

water-holding porosity. It should be emphasized that a vermiculite 

grade usually contains a range of particle sizes, depending on the 

size of sieves used by the manufacturer. Several nurseries in the 

northwestern United States have compared conifer seedling 

growth in growing media containing either grade 2 or 3 vermiculite 

and have found better growth with grade 2. Tinus and McDonald 

(1979) recommended grade 1 for containers of 164 cm; (10 cubic 

inches) or larger, and grade 2 for smaller containers. Vermiculite 

particles are structurally unstable in a moist medium and will 

compress over time (Ward and others 1987). For this reason, 

vermiculite should not be used alone or with sand but should be 

mixed with perlite or peat, which provide resistance to compaction 

(Bunt 1988).

Perlite. Perlite is an alumino-silicate mineral of volcanic origin 

that is mined in several countries, including the United States 

and New Zealand. After mining, the raw mineral is crushed and 

exposed to temperatures as high as 1,000 °C (1,832 °F), 

producing White, lightweight particles (fig. 2.2.12).

Perlite possesses many useful characteristics that make it 

desirable for growing media (table 2.2.1). One of these unique 

properties is its closed-cell structure: water adheres only to the 

surface of the perlite particles and therefore growing media 

containing perlite are well-drained and lightweight. Perlite is also 

rigid and does not compress easily, which promotes good porosity. 

Compared to other inorganic components, such as sand and grit, 

perlite increases the aeration porosity of peat-based growing 

media (Ward and others 1987). Because of the high temperatures 

at which it is processed, perlite is completely sterile. Perlite is 

essentially infertile-it contains almost no plant nutrients (table 

2.2.13)-and has a minimal CEC (Bunt 1988, Moore 1988). The pH 

of perlite ranges around neutral (table 2.2.1), but this is not 

significant because it is usually mixed with acidic sphagnum peat 

moss (Nelson 1978).

Mastalerz (1977) recommended that only horticultural grades of 

vermiculite be used for growing media because insulation grades 

of vermiculite are often treated with water-repellent chemicals. The 

author, however, has used insulation grade vermiculite without any 

problems, and Goodwin (1975) also recommended its use in 

container tree seedling growing media. Tinus and McDonald 

(1979) stated that vermiculite sold for poultry litter or attic 

insulation is not only acceptable but cheaper. "Block fill" 

vermiculite has been treated with

64
According to the Container Nursery Survey, perlite is a minor 

component of growing media in forest tree nurseries, usually 

constituting from 10 to 30% of the mix. Perlite is usually added to 

organic components, such as peat moss, to increase aeration 

porosity, which is particularly important in the smaller volume 

containers used in container tree nurseries. Perlite grades are not 

standardized, but grades 6, 8, or "propagation grade" are normally 

used in growing media (table 2.2.13). Perlite grades are also not 

uniform and contain a range of particle sizes, depending on the 

sieve sizes used during manufacturing.

Perlite has a couple of operational drawbacks. Horticultural grades 

of perlite can contain considerable amounts (4% by weight) of very 

fine particle sizes (Maronek and others 1986) that cause eye and 

lung irritation during mixing if the perlite is not pre-moistened. 

Because of its closed- cell structure, perlite has a tendency to float to

65
66

Sand was one of the most common materials used in many of the 

original horticultural growing media recipes. It is one of the most 

readily available materials that can be used for growing media and 

is relatively inexpensive. Size recommendations vary considerably: 

Whitcomb (1988) recommended a uniform particle size of 2 to 3 

mm (0.06 to 0.12 inch), whereas Matkin and Chandler (1957) 

specified a fine sand with a diameter of 0.05 to 0.5 mm (0.002 to 

0.02 inch). Swanson (1989) recommended that 60% of the sand 

particles be between 0.25 and 1.00 mm (0.01 to 0.04 inch), with 

less than 3% smaller than 0.1 mm (0.004 inch) or larger than 2 mm 

(0.08 inch). Although sands are often used to increase porosity, 

small sand particles can lodge in existing pore spaces and reduce 

aeration and drainage (Ward and others 1987). Sand does add 

stability to freestanding containers. Some sands are also 

contaminated with calcium carbonate, which can raise pH and 

cause nutrient availability problems (Bunt 1988). Although pH is not 

always a good indication of CaCO3 content (Ward and others 

1987), growers can test sands by adding a drop of dilute acid or 

even strong vinegar; a fizzing reaction indicates the presence of 

CaCO3

the top of the medium during irrigation (Mastalerz 1977); this is 

normally not a problem with the small proportions used in 

container tree seedling growing media. Gates (1986) reported 

that perlite particles tended to bind to the walls of Styrofoam block 

containers, which may damage root plugs when the seedlings are 

extracted.

Other inorganic materials. Other inorganic materials that have 

been used in growing media in horticultural nurseries include sand, 

pumice (fig. 2.2.13), scoria, cinders, calcined (heat-expanded) 

clays, rock wool, polystyrene flakes, and foam particles. The use of 

any of these materials will depend on cost and availability, but it is 

doubtful if any will supplant vermiculite and perlite as the primary 

inorganic components in North American container tree nurseries.
2.2.5 Selecting a Growing Medium

2.2.5.1 Interactions between growing media and

cultural practices

Because of the diverse characteristics of the various growing 

media components, a container nursery manager can formulate a 

growing medium with almost any desired property. The physical, 

chemical, and biological properties of each growing medium are 

different, however, and are also affected by cultural practices used 

in the nursery, particularly irrigation, fertilization, and type of 

container. The timing of the growing season should also be 

considered. Scagel and Davis (1988) concluded that growers must 

adjust their cultural practices because of physical and chemical 

variability between different media. Carlson (1983) evaluated five 

different brands of Canadian sphagnum peat moss and found that 

each brand required slight modifications of cultural practices to 

produce optimum growth of spruce and pine seedlings.

Irrigation and fertilization practices. The water-holding and 

nutrient- supplying properties of a growing medium are a function 

of the different growing medium components and their 

interrelationships . Hoyle (1982) studied the effect of irrigation 

technique, type of fertilizer, and type of growing medium on the 

growth of yellow birch seedlings. Using two different peat 

moss-based growing media, he grew yellow birch seedlings under 

two irrigation systems and three fertilizer treatments. Seedling 

growth varied significantly between the two growing media, 

depending on which irrigation and fertilization treatment was 

applied (fig. 2.2.14). The container tree nursery manager must

tailor irrigation and fertilization regimes to fit the characteristics of 

the growing medium and will have to alter these cultural practices 

if the medium is changed. Colombo and Smith (1988) grew two 

species of conifer seedlings in growing media containing peat 

moss from local or commercial sources over a range of different 

fertilization rates (fig. 2.2.15). Although the growth responses 

differed between the tree species, the largest seedlings were 

those grown in the commercial peat moss-vermiculite medium; the 

authors attributed this enhanced growth to better aeration and 

drainage. Irrigation practices had to be adjusted by the end of the 

growing season because of changes in the water-holding capacity 

of a peat-perlite growing medium (Langerud and Sandvik 1988). 

Scagel (1989) concluded that many growing media problems can 

be attributed to poor irrigation practices, rather than to the media.

Type of container. Because of the perched water table that is 

inherent in container culture, the type of container will affect the 

performance of the growing medium. Short containers have a 

higher proportion of their volume in the saturated condition and will 

therefore require a growing medium with a greater aeration 

porosity than a taller container. Containers with porous sidewalls, 

like paperpots, have different water and nutrient relationships than 

impermeable containers and will therefore require a growing 

medium with more aeration porosity and a different irrigation 

regime than containers with solid walls. (See chapter 2, volume 

four of this manual for further discussion on water management in 

containers.)

67
Growing season. Many container tree nurseries produce more 

than one crop of tree seedlings per ear and the type of growing 

medium may need to be formulated differently for different 

cropping cycles Harlass (1984) reported that growing media with 

high water-holding capacities should not be used during the low 

light periods of fall or winter because the low, evapotranspiration 

rates during these periods can lead to water-logged conditions. A 

more porous growing medium is best for fall and winter crops, 

whereas growing medium with a higher water-holding cap city is 

better for spring and summer crops.

2.2.5.2 Practical considerations

Container tree nursery managers are faced with many different 

considerations when selecting a growing medium that will match 

their cultural regimes. The cultural and operational 

characteristics that were discussed in section 2.2.3 (table 

2.2.14) must be evaluated and compared so that the resultant 

growing medium will have the desired properties. On a practical 

basis, however, two factors are the most relevant hen selecting 

growing media: cost and availability of tie medium, and seedling 

performance.

Cost and availability. Regardless of the properties of specific 

growing medium components, a nursery manager must be able to 

find and afford the materials before they can be used. Cost is a 

relative factor, however, and managers should consider all 

aspects of the situation. Most materials used for growing me is are 

not inherently expensive; the cost of a specific growing medium 

component is more likely related to transportation costs, which are 

a function of characteristics such as weight and volume. Bulk 

ordering of large quantities of growing medium or components can 

greatly redo a unit cost (fig. 2.2.16). Materials such as sand are 

readily available and cheap but are so heavy that shipping and 

handling costs are often prohibitive. Other growing medium 

components, such as vermiculite, have a large relative volume that 

also increases transportation costs. Some component materials 

are unique to one particular area of the country and so nursery 

managers should first consider local materials before importing 

growing medium components from soiree other location. rowers 

have developed many innovative uses for organic waste materials, 

which can be substituted for peat moss in growing media 

(Mastalerz 1977). Container tree nursery

68
managers in some Pacific Islands are investigating native 

materials for growing media, such as volcanic pumice and the 

fibrous bark of tree ferns, because any imported material is 

prohibitively expensive.

The decision to purchase a ready-made commercial growing 

medium or to mix one's own from individual components (custom 

mixing) is primarily a question of the costs of materials and 

availability of mixing equipment. Bags of peat moss and 

vermiculite can usually be purchased more cheaply than a 

corresponding volume of commercial medium. Sanderson (1983) 

reported that prepackaged growing media could cost four times 

as much as custom-mixed media, and Goodwin (1975) estimated 

that a custom peat-vermiculite medium could be prepared for 

one-third as much as a commercial growing medium. The real 

economic consideration is the cost of labor and equipment to do 

the mixing; because labor and equipment costs vary between 

nurseries, there can be no standard comparison of costs, and 

nursery managers must do their own calculations.

69
Whitcomb (1988) stated that custom mixing batches of growing 

media can involve significant labor costs. Kusey (1989) listed 

several different hidden costs that must be considered when 

comparing commercial and custom-mixed growing media.

Again, because the growing medium is just one in a series of 

interrelated cultural factors that affect plant growth in a nursery, 

the prudent grower must consider the overall picture before 

making a decision about the economics of one particular 

practice. The combination of cultural practices that produces the 

best-quality seedling in the shortest period of time and at an 

acceptable cost will be the most economical in the final 

evaluation. Kusey (1989) concluded that minor differences in 

growing medium quality can be reflected ultimately in significant 

losses in seedling quality.

Seedling performance. Because of the complex relationships 

between growing media and cultural practices, growers should 

conduct operational trials of different growing media under their 

own cultural regimes. Obviously, each nursery cannot afford the 

time or space to conduct a complicated series of experiments on 

all the different types of growing media so, initially, managers 

should rely on recommendations from the literature or from other 

nurseries.

Small operational trials using different growing media should be 

attempted on each species of seedling and growing cycle. 

Nurseries that have implemented such trials have often 

discovered considerable differences in seedling performance 

(fig. 2.2.913).

In the final analysis, the best growing medium for a particular 

nursery will be a compromise of all the various factors in table 

2.2.14, and will be custom fitted to the cultural regime of the 

specific nursery.

2.2.5.3 Commercial growing media

There are many commercial brands of growing media on the 

market. Harlass (1984) listed 54 brands of growing media, Judd 

(1983) reviewed the components of 32 brands, and Sanderson 

(1983) discussed the components and fertilizer properties of 23 

different commercially available products. Some of these

products are specially formulated for one specific horticultural 

crop whereas others are more generic in nature. A few 

companies offer a growing medium specifically designed for tree 

seedlings (fig. 2.2.17); whether these

70

specialized media are better than generic types can only be 

determined by operational testing at individual nurseries. The 

Container Nursery Survey revealed that only 35% of the container 

nurseries in the United States and Canada purchase commercial 

brands of growing media.

When evaluating different brands of commercial growing media, 

only sphagnum-based media should be considered for growing 

forest tree seedlings. Lackey and Alm (1982) evaluated five 

different types of growing media including two commercially 

prepared brands contained sphagnum peat moss. The red pine 

seedlings that were raised in each growing medium were 

measured for a variety of growth parameters, and also rated for 

plug formation and seedling quality. The commercial media were 

consistently better: the Forestry Mix® was superior for all growth 

factors, whereas the Jiffy Mix® was only significantly better than 

the custom media in stem height. The plug quality rating and 

seedling quality indices were uniformly better for the seedlings 

grown in sphagnum-based commercial growing media (table 

2.2.15).

2.2.5.4 Custom-mixed growing media

Based on the Container Nursery Survey, 65% of the forest tree 

nurseries in the United States and Canada mix their own growing 

media; five different materials are being used: sphagnum peat 

moss, sawdust, vermiculite, perlite, and sand. Peat-vermiculite 

blends were by far the most popular (78%), pure peat media were 

second
in popularity (11%), followed by peat-vermiculite-perlite (6%) and 

peat-perlite (2%). The ratio of sphagnum peat moss to vermiculite 

in the growing media ranged from 1 :1 to 3:1, with the 1 :1 ratio 

being the most popular.

For custom-mixed growing media, the author recommends a 

mixture of coarse-textured sphagnum peat moss and vermiculite. 

Barnett and Brissette (1986) reviewed the literature and concurred 

that sphagnum peat moss-vermiculite mixes consistently produce 

the best quality seedlings. Sphagnum peat moss is the only type of 

peat moss that is recommended, although other types of peat 

moss are less expensive. The vermiculite should be grade 2 or 3, 

the former used for a more porous, well-drained medium and the 

latter for more water-holding capacity. A small proportion of perlite 

(10 to 30%) can also be added to increase the aeration porosity of 

the mix.

2.2.5.5 Comparison of commercial and custom-mixed

growing media

Control over quality. Most commercial suppliers have 

established standards so that the quality of different batches of 

growing media will remain constant, but nursery managers must 

rely on the integrity and reputation of the commercial producer. 

Kusey (1989) advocated the use of commercial media because 

producers have rigorous quality control programs, and some even 

have their own testing and research facilities. In custom

mixing, however, container tree nursery managers have direct 

control over the properties of their growing media because the 

quality of each component can be specifically evaluated.

Ability to "fine-tune" media. Although there is a wide selection of 

types and brands of commercial growing media available, growers 

have reported problems in obtaining prepackaged media with 

specialized properties. Obviously, it is uneconomical for large 

commercial producers to alter their components and mixing 

operations to meet small requests. Some local firms have begun to 

produce custom-mixed growing media, however, and will work with 

growers to meet their individual needs (fig. 2.2.17). On the other 

hand, because the nursery manager has control over the 

proportions and properties of the various components, small 

batches of custom-mixed growing media can be developed with 

specific chemical and physical properties, such as pH or aeration 

porosity, to meet the biological requirements of a particular crop.

Time and labor. Commercial media can be ordered in advance in 

large quantities, saving time and labor during the container filling 

operation. Small container tree nurseries, which often cannot afford 

to invest in mixing equipment or hire the extra personnel to do the 

mixing, usually find commercial media the most economical and 

convenient. Although custom-mixing can be done in advance, most 

nurseries mix their media as part of the container filling process. 

The extra labor and cost of the

71
specialized mixing equipment must be considered in the total cost 

of the media.

Incorporation of fertilizers and other amendments.

Many brands of commercial growing media contain limestone to 

raise the pH of the mix to some optimum value, and others contain 

a starter charge of nutrients to promote early seedling growth. 

Surfactants, or wetting agents, are often added to commercial 

growing media to improve their wettability. The fertilizers, wetting 

agents, and limestone added to commercial growing media may 

not be desirable and, in fact, may even be detrimental to tree 

seedling growth; many brands are formulated for crops other than 

tree seedlings (see section 2.2.6.3 for more detail).

Uniform mixing. Producers of commercial growing media have 

the proper equipment and knowledge to produce a well-mixed 

growing medium. Nursery managers, however, may find it 

difficult to obtain uniform mixing and even distribution of 

fertilizers and other incorporated materials with some of the 

equipment used for custom-mixing. Inexperienced personnel 

tend to over mix batches of media; over mixing breaks down the 

physical structure of growing medium particles, leading to 

compaction problems later in the growing season (see sections 

2.2.6.4 and 2.2.7).

72
2.2.6 Procedures and Considerations for

Custom Mixing Growing Media

The mixing process is one of the most important steps in the 

formulation of custom growing media; the best quality components 

are of no use if the growing medium is improperly mixed. 

Whitcomb (1988) emphasized that improper mixing is one of the 

major causes of variation in container plant quality. The proper 

operating procedures are just as important as purchasing the right 

type of mixing equipment. Kusey (1989) stressed that mixing 

should be performed by diligent, experienced workers who will 

faithfully monitor the quality of the growing media. Incorporation of 

fertilizers and supplements must be considered, as well as ways 

of treating growing media for soil-borne pests. Growing media 

compaction deserves special consideration because it has been 

shown to be one of the most serious, yet difficult to diagnose, 

problems in container nursery culture.

2.2.6.1 Equipment and procedures

Small batch mixing. Smaller nurseries often cannot afford to 

invest in specialized mixing equipment and prefer to prepare 

small batches of growing medium by hand. Nelson (1978) states 

that batches of up to 0.25 m3 (5 or 6 cubic feet) can be mixed on 

any clean, hard surface by workers with hand shovels.

To mix components, pile them on top of one another and 

broadcast any amendments over the pile. Then work around the 

edge of the pile with a large scoop shovel, taking one shovel full of 

material at a time and turning it over onto the top of the pile. As 

this material is added to the top, it tumbles down all sides of the 

pile and is mixed. Make sure that the center of the pile is mixed by 

gradually moving the location of the pile to one side during the 

mixing procedure. Mist the pile with water at frequent intervals 

during the mixing process to make the medium less hydrophobic. 

Continue this procedure until samples from the pile appear well 

mixed.

Mechanized mixing. Nurseries that require larger quantities of 

custom growing media on a regular basis should purchase a 

mixer. Several brands of mixers and combination mixers/container 

fillers are commercially available. Whitcomb (1988) recommended 

paddle-type mixers in which the drum is stationary and internally 

mounted paddles blend the components together. Handreck and 

Black (1984) preferred belt mixers that automatically feed each 

component onto an adjustable flow conveyor; the mixing occurs 

when the components fall from the end of the conveyor into a 

slowly rotating drum or directly into the container mixer/filler. 

Auger

73

type mixers, grinders, and soil shredders are not recommended 

because they break down particle structure and destroy porosity 

(Judd 1984, Bartok 1985). Modified mixing equipment, such as 

portable cement mixers (fig. 2.2.18A) with a capacity of 0.1 to 0.2 

m3 (3 to 6 cubic feet) or concrete trucks (fig. 2.2.1813) with a 

capacity of 4 to 8 m3 (5 to 11 cubic yards), can be converted into 

growing medium mixers. Single batch mixers can produce from 0.2 

to 9.2 m3 (0.25 to 12 cubic yards) of growing medium per hour 

compared to continuous mixing systems, which deliver up to 38.2

m3 (50 cubic yards) per hour (Bartok 1985).
Any mixing equipment can be specially modified with spray 

nozzles so that the growing medium can be gradually moistened 

and with aerated steam injectors for pasteurization (Nelson 1978). 

Sterility should be maintained during the entire mixing process for 

both the individual components and the final product (Bartok 

1985).

2.2.6.2 Incorporation of fertilizers and other materials

A variety of materials are routinely added to growing media during 

the mixing process; these include fertilizers, lime, surfactants, and 

mycorrhizal inoculum. Bartok (1985) stated that 0.76 m3 (1 cubic 

yard) of well-mixed growing medium for an ornamental container 

nursery may contain from 0.45 to 0.91 kg (1 to 2 pounds) of 

macronutrient fertilizer, 2.27 to 9.08 kg (5 to 20 pounds) of 

limestone, and 56.7 g (2 ounces) of micronutrients. Uniform 

incorporation of these materials is important because tree seedling 

roots only have access to a limited volume of growing media in the 

relatively small containers used in forest tree nurseries. It is 

particularly difficult to incorporate small volumes of dry material, 

such as micronutrient fertilizer, into a moistened growing medium. 

The probability of obtaining equal distribution of each fertilizer 

amendment, particularly the small amount of micronutrient 

fertilizer, to each container is low. Gladon (1988) stated that many 

early season growing problems are due to improper incorporation 

of growing medium amendments, and Whitcomb (1988) concluded 

that uneven mixing of incorporated fertilizers is one of the major 

factors causing uneven growth in container nursery stock.

Incorporated chemical amendments may separate out during 

subsequent handling if dry fertilizers are mixed into dry growing 

medium components (Bartok 1985). Prewetting the growing 

medium components, such as peat moss and vermiculite, with 

warm water containing a surfactant will allow the fertilizer and the 

growing medium particles to aggregate during the mixing process 

and stop this problem.

Limestone. Limestone, traditionally called lime in horticulture, has 

been added to growing media in ornamental container nurseries to 

raise the pH and supply calcium for plant nutrition. Actually, 

agricultural limestone [calcium carbonate (CaCO3) or dolomite 

(CaCO3 • MgC03)] is used rather than burnt lime (CaO) or slaked 

lime (CaOH) in horticultural applications (Bunt 1988). Liming is a 

carryover from the days when soil74

based media were common; Williams and others (1988) stated 

that chemical reactions are different in artificial growing media, 

however, and therefore liming should be discontinued in modern 

container nurseries. Liming is still practiced in some forest tree 

nurseries, such as those in coastal British Columbia, where the 

irrigation water contains low calcium levels (Gates 1986).

Addition of lime to growing media is not recommended in this 

manual for several reasons:

• It is operationally difficult to distribute the limestone uniformly 

during the mixing process (see section 2.2.5.5).

• The naturally acidic pH of most peat-vermiculite media 

should not have to be greatly increased for most tree 

seedling crops. Sphagnum peat moss has a pH of 3.5 to 4.0 

and vermiculite from 6.0 to 7.6 (table 2.2.1); a mixture of 

these two components should produce a growing medium 

reasonably close to the ideal pH range of 5.0 to 6.0. In fact, 

the initial pH of three typical peat-vermiculite medium had a 

pH of 4.06 (Scarratt 1986) (Table 2.2.3). It is culturally much 

easier to raise an initially low pH than lower one that is too 

alkaline; an acidic growing medium can easily be raised into 

the ideal pH range through routine irrigation and fertilizer 

injection. In fact, the pH of container media becomes slightly 

more alkaline over time due to the effect of bicarbonates in 

the irrigation water and alkaline fertilizers (Bunt 1988). 

Gladon (1988) estimated that the pH of a growing medium 

will rise from one-half to one full unit over the growing 

season.

• Calcium nutrition can be supplied much more quickly and 

easily with water-soluble calcium fertilizers (for example, 

calcium nitrate) than with slowly available limestone or 

dolomite. Calcium deficiency can be a serious problem with 

young seedlings because of their restricted root systems, 

and liquid fertilizer injections would be more effective early in 

the growing season than incorporated granular fertilizers.

Liming has caused some real problems in operational container 

tree nurseries. Dangerfield (1978) found that addition of 

dolomitic limestone to a peat-vermiculite growing medium 

induced lime chlorosis in Douglas-fir
seedlings, and he concluded that the practice of incorporating lime 

should be discontinued. Even with broadleaf species, which prefer 

slightly less acidic conditions, liming is hard to justify. Saligna 

eucalyptus seedlings were stunted and deficient in several 

micronutrients, most notably copper, following the addition of 

dolomite to a peat-vermiculite medium (Miyasaka and others 

1983). Chrustic and Wright (1983) concluded that there was no 

advantage to liming a pine bark-based medium if all the mineral 

nutrients are provided in the proper concentration and balance. 

(The nutritional aspects of liming are discussed in more detail in 

chapter 1, volume four of this manual.)

Fertilizers. Incorporation of fertilizers is a common practice for 

ornamental stock and sometimes for container tree seedlings, 

particularly when liquid fertilizer injection equipment is not available 

or seedlings are grown outdoors in areas with heavy rainfall. 

Incorporation of micronutrients is more common because artificial 

growing media are seriously deficient in micronutrients. (A complete 

discussion of the merits of fertilizer incorporation is provided in 

chapter 1, volume four of this manual.)

Surfactants. These chemical additives, which are also called 

wetting agents (fig. 2.2.19), break down the surface tension of 

water and increase the wettability of hydrophobic organic 

materials such as peat moss and pine bark. Unfortunately, the 

chemical and physical effects of these substances on growing 

media is poorly understood, and some may even be detrimental 

(Ward and others 1987). Whitcomb (1988) warned that not all 

wetting agents are safe to use: some are phytotoxic to certain 

woody plants. Barnett and Brissette (1986) reviewed the literature 

and reported that in some cases the recommended application 

rates were too high for tree seedlings. One widely used product 

(Aqua-gro®) reduced germination of four species of southern pine 

seed when applied at the recommended 0.1 % application rate; 

reducing the rate to approximately 0.02 to 0.04% provided 

adequate wetting action without causing phytotoxic effects 

(Barnett 1977). Pokorny (1979) tested 24 commercial surfactants 

on pine bark media and found that only 9 were both safe and

effective.

Incorporated surfactants may become inactive over time Ward 

and others (1987) reported that, although initially effective, 

surfactants may have to be reapplied during the growing season.

Superabsorbents. Superabsorbents are cross-linked polymers 

than absorb many times their own weight in water. They have 

been proposed as additives to increase the water-holding capacity 

of growing media. Several different products are available, 

however, and not all are suitable for horticultural applications. One 

class of superabsorbents (propenoate propeamide copolymers) 

has been shown to increase water-holding capacity, improve 

aeration and drainage, and reduce the irrigation requirements of 

growing media (Erazo 1987).

Although they have been added to growing media in ornamental 

nurseries, superabsorbents are not widely used in forest tree 

nurseries at the present time. Lennox and Lumis (1987) 

investigated the use of hydrophilic gels in growing media and 

found that one additive (Terra-Sorb®) increased the water-holding 

capacity by only 5%, which is probably not significant for 

container tree seedling production. In another study, Terra-Sorb 

was found to increase the water-holding capacity of the growing 

medium and delayed wilting in tomato seedlings (Adams and 

Lockaby 1987). Superabsorbents may

75
not fill a need in most container nurseries at the present time 

because of regular irrigation scheduling, but their use may be 

justified in some applications, especially if water use is restricted.

Mycorrhizal inoculum. One method of inoculating container 

tree seedlings with beneficial mycorrhizal fungi is to incorporate 

a specially prepared fungus inoculum into the growing medium 

at the time of mixing. (The benefits of mycorrhizae and the 

detail, of the inoculation procedure are discussed in chapter 2, 

volume five of this manual.) In the future, other beneficial 

microorganisms, such as plant-growth-promoting; rhizobacteria, 

may be inoculated into growing media (Digat 1988).

2.2.6.3 Pasteurization or sterilization

It has been commonly assumed that artificial grow ink; media are 

free from soil-borne pests, but that assumption has been 

challenged in recent years. The common inorganic components of 

growing media, such a,, vermiculite and perlite, are inherently 

sterile, but peat moss and other organic components are suspect. 

A recent outbreak of root disease in conifer seedlings in the 

Pacific Northwest has been traced to a fungal pathogen in 

peat-vermiculite growing media (Husted 1988). The exact source 

of the contamination is not always clear. Peat contamination can 

sometimes be traced to certain bogs, or areas of bogs, or certain 

stages during the handling or processing of the peat. The actual 

extent of growing media contamination is not known, but growers 

should take certain precautions in the interim.

Some brands of commercial growing media do treat their products 

and advertise that their mixes are essentially sterile (fig. 2.2.20A). 

Sterilization refers to the complete elimination of all living 

organisms in the medium whereas pasteurization is not as drastic. 

Completely sterile growing media may not be particularly desirable 

because many beneficial microorganisms, including bacteria, 

actinomycetes, and fungi that are normally found in growing media 

can actually be antagonistic o pathogens (Wolffhechel 1988). 

Steam heat (fig. 2.2.20B1 can be used to either pasteurize or 

sterilize the media, depending on the temperature. Heat 

pasteurization is generally thought to be preferable if the proper 

equipment is available (Bunt 1988); the standard recommendation 

is to heat the growing medium to 60 to 82 °C (140 to 177 °F) for a 

minimum of 30 minutes (fig.

76
Both pasteurization and fumigation are costly and time-consuming 

and, like all pest control treatments, have their drawbacks (Bunt 

1988). The most prudent procedure may be to have samples of 

peat moss or premixed growing medium tested for pathogens so 

that seriously contaminated batches can be identified and treated, 

or rejected. Purchasing specifications can also be written to 

require testing for pathogens. These preventative measures, like 

all pest control treatments, should be part of an overall pest 

management strategy. (See chapter 1, volume five of this manual 

for more information on soil-borne pathogens and treatment of 

growing media.)

2.2.6.4 Over mixing and compaction problems

Over mixing can break down the structure of the growing medium 

particles, which promotes compaction and destroys the aeration 

porosity of the medium. Fragile materials, such as vermiculite and 

peat moss, are easily damaged during mixing. Milks and others 

(1989) found that moistening peat-vermiculite before mixing 

actually prevented over compaction, although overly wet compo77

nents are heavy and will compact easier. In a typical batch of 

peat-vermiculite medium, the individual peat fibers and vermiculite 

particles should still be visible (fig. 2.2.21), and the medium should 

feel spongy in texture; a medium that appears fine or powdery has 

probably been damaged. Some growers prefer fine-textured 

growing media because it is easy to load into small containers, but 

such media tend to pack too tightly in the container and reduce 

porosity. Overly compacted growing media have reduced drainage 

and aeration, often resulting in root growth problems (see section 

2.2.3.1).

Mechanized mixing can be easily overdone if the mixers are run 

too long or are overfilled, or if the components are too wet 

(Whitcomb 1988, Handreck and Black 1984). Certain types of 

mixers are more damaging than others (see section 2.2.6.1). Most 

mixers will do an adequate job in 3 or 4 minutes if they are filled 

only about three-quarters full (Whitcomb 1988). Operational trials 

conducted on the effect of different mixing times on the size of 

peat moss particles showed that particle size was severely 

reduced if mixing times exceeded 5 minutes (table 2.2.16).
78
2.2.7 The Importance of Proper Growing

Medium Compaction

Filling the containers with the growing medium is a critical 

process. It is important to achieve the proper degree of media 

compaction, because poorly compacted media can negate the 

beneficial cultural properties of even the best growing media. 

Under compaction is rarely a problem and can be easily 

diagnosed and corrected. Over compaction, however, is fairly 

common due to over mixed media or to excessive mechanical or 

manual compression during the container filling process (Bunt 

1988).

Over compaction can have several effects on the physical, 

chemical, and biological properties of a growing medium. 

Although total porosity is naturally less in compacted media, the 

more important effect is the reduction or even elimination of the 

large pores that control aeration and drainage (Bunt 1988). In 

studies of conifer seedling growth in peat-based growing media 

that were compressed to various densities, Mitchell and others 

(1972) found that root activity varied inversely with peat 

compaction.

The ideal degree of media compaction will vary with the type of 

medium, type of container, and irrigation practices at each 

nursery. Matthews (1983) recommended a growing medium 

density of 0.1 g/cm3 of usable container volume for Styrofoam 

block containers. Hocking and Mitchell (1975) studied the effects 

of growing medium density in extruded peat containers and found 

that 0.2 g/cm3 gave the best seedling growth. Container nursery 

managers should conduct operational trials to determine the 

proper growing medium density for their own conditions.

Media compaction is difficult to assess in the small containers 

used in tree nurseries, and there is presently no reliable technique 

for measuring it. Several direct observations can be helpful in 

evaluating growing media compaction, however. Because of the 

resultant increase in bulk density of compacted media, containers 

that are unusually heavy should be suspect. During the filling 

process, the medium should not settle excessively when

79

a container is shaken or tapped on the table. The medium in 

properly filled containers should still feel springy to the touch. 

Obviously, evaluation of media compaction in containers is an 

imprecise process that can be improved only through operational 

experience. Unfortunately, a container nursery manager only 

becomes aware of a media compaction problem after seedling 

growth problems become apparent or during extraction, when 

poorly formed root plugs are apparent.

The deleterious effects of over compaction on tree seedling growth 

are often subtle and difficult to diagnose because of the myriad 

effects of compaction on the complex rooting environment in a 

container. Another factor that complicates diagnosis is variation in 

the degree of compaction that can occur between different blocks of 

containers, or even between cells within a block, which produces a 

mosaic of normal and symptomatic seedlings (fig. 2.2.22A). The 

symptoms of root injury from over compacted growing media can 

include foliar chlorosis, leaf drop, root browning, and eventual 

death. Because it first affects root function, the initial symptoms of 

over compaction injury can mimic drought stress, over watering, or 

even mineral nutrient deficiency. Mineral nutrient uptake is impaired 

when roots are not functioning properly due to an over compacted 

growing medium; iron chlorosis is just one of the nutritional 

disorders that can develop (Faber 1982). Roots that become 

weakened in over compacted media are particularly susceptible to 

opportunistic root pathogens such as Pythium or Fusarium spp. (fig. 

2.2.22B). Langerud (1986) reported a root dieback disease of 

container seedlings that was attributed to poor porosity of the 

growing medium.
80
2.2.8 Conclusions and Recommendations

The selection of a growing medium is one of the most important 

decisions in the culture of container tree seedlings. The 

physical, chemical, and biological characteristics of a growing 

medium affect not only seedling growth, but also other aspects 

of nursery operations as well. Container nursery managers, 

therefore, should carefully consider both biological and 

operational aspects when evaluating different types of growing 

media.

Deciding whether to purchase a commercial brand of growing 

media or to custom-mix will depend on many factors, including 

availability of components and mixing equipment and the size of 

the nursery operation. Several good commercial brands of 

growing media are available but, for complete quality control, 

container nursery managers should consider custom-mixing their 

own media.

Whether purchasing a commercial media or custom mixing, the 

selection of growing medium components is critical. For container 

tree seedlings produced in North America, a growing medium 

consisting of sphagnum peat moss and vermiculite is 

recommended, if these materials are available and reasonable in 

price. The proportion of peat moss to vermiculite on a volume/ 

volume basis can range from 1 :1 to 3:1. Coarse grade peat moss 

should be used whenever possible, and the coarser grades of 

vermiculite are preferred. A small proportion of perlite (10 to 30%) 

can be substituted for part of the vermiculite if a more well-drained 

medium is desired. Tree bark, especially composted pine bark, 

has shown promise as a growing media component in horticultural 

applications, but more information is needed on this material in 

forest tree nurseries. Substitution of alternative organic materials 

for peat moss should be approached cautiously, and only 

composted organics should be considered.

81

Chemical amendments to growing media are not usually 

warranted. Incorporation of limestone or other fertilizer materials 

into the growing medium is not recommended unless conventional 

fertilization techniques are unavailable. If possible, pH and mineral 

nutrient levels should be controlled through injection of acid and 

liquid fertilizers into the irrigation system. Even though most 

components are considered sterile, nursery managers should 

routinely have their growing media tested for pathogenic fungi.

Because the growing medium is just one in a series of interrelated 

cultural factors that affect plant growth in a nursery, the prudent 

grower must consider the overall situation before making a 

decision about the economics of one particular practice. The 

combination of cultural practices that produces the best-quality 

seedling in the shortest period of time and at an acceptable cost 

will be the most economical in the final evaluation. Small-scale 

operational trials with new types of growing media are always 

recommended; if trial results appear promising, then the medium 

can be used on a larger scale.
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82

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85
87

Index of Common and

Scientific Names

Trees

ash

green ash Fraxinus pennsylvanica Marsh 32, 33

baldcypress Taxodium spp. 10

birch

yellow birch Betula alleghaniensis Britton 67

casuarina Casuarina spp. 27

"cedar"

Alaska-cedar Chamaecyparis nootkatensis

(D.Don) Spach 14

Incense-cedar Libocedrus decurrens Torr. 62

western redcedar Thuja plicata Donn ex

D.Don 14, 33, 62

Douglas-fir Pseudotsuga menziesii (Mirb.)

Franco 9, 10, 33, 45, 62, 74

eucalyptus

saligna eucalyptus Eucalyptus saligna Sm. 62, 75

tallowwood eucalyptus E. microcorys F. Muell. 62

blackbutt eucalyptus E. pilularis Sm. 62

E. andrewsii 62

E. radiata 62

Fir

white fir Abies concolor (Gord. & Glend.) Lindl. ex

Hildebr. 62

Pacific silver fir A. amabilis Dougl. ex Forbes 33

Hemlock

western hemlock Tsuga heterophylla (Raf.) Sarg. 33

larch

western larch Larix occidentalis Nutt. 14

oak

red oak Quercus spp. 32, 33

redwood Sequoia sempervirens (D.Don) Endl. 62

pine

Caribbean pine Pinus caribaea Mill. 33

jack pine P. banksiana Lamb. 12, 26

jeffrey pine P. jeffreyi G rev. & Balf. 26

loblolly pine P. taeda L. 8, 11, 12, 30

lodgepole pine P. contorta Dougl. ex. Loud. 5, 6,

8, 10, 32-34

longleaf pine P. palustris Mill. 8, 12

ponderosa pine P. ponderosa Dougl. ex Laws. 32,

33, 57, 62

red pine P. resinosa Ait. 12, 26, 70

slash pine P. elliottii Engelm. 12

spruce

black spruce Picea mariana (Mill.) B.S.P. 12

white spruce P. glauca (Moench.) Voss 5, 8-10

walnut Juglans spp. 62

Others

Pests

Fusarium spp. 45, 63, 79, 80

Pythium spp. 53, 63, 79

grey mold Botrytis cinerea Pers.:Fr. 10, 30

"Sphagnum mosses"

Hypnum spp. 59, 60

Polytrichum spp. 59

Sphagnum cuspidatum 59

S. fuscum 58