1
4.1.6 Methods of Fertilizing Container

Seedlings 26

4.1.6.1 Incorporating dry fertilizers into the growing 

medium 26

Macronutrient fertilizers used for 

incorporation 27

Micronutrient fertilizers used for 

incorporation 29

4.1.6.2 Injecting liquid fertilizer solutions into the irrigation 

system 30

Types of nutrient injection systems 31

Commercial fertilizers for liquid fertilizer 

injection 33

Custom fertilizers for liquid fertilizer 

injection 33

Foliar fertilization 34

4.1.7 Formulating Liquid Fertilizer

Solutions 35

4.1.7.1 Determining the nutrient status of the irrigation 

water 35

4.1.7.2 Controlling irrigation water pH 35

4.1.7.3 Liquid fertilizer calculations for commercial

fertilizers 36

Macronutrients 37

Micronutrients 38

4.1.7.4 Liquid fertilizer calculations for custom

fertilizer mixes 39

Selection of chemicals 39

Mixing the stock fertilizer solutions 39

Example of calculations 42

4.1.8 Applying and Scheduling Liquid

Fertilizer Solutions 50

4.1.8.1 Application techniques 50

4.1.8.2 Timing of the first fertilizer application 50

4.1.8.3 Scheduling liquid fertilizer applications during

the growing season 51

Contents

4.1.1 Essential Mineral Nutrients 4

4.1.2 Mineral Nutrients and Seedling

Growth 7

4.1.2.1 Nutrient uptake and utilization 7

4.1.2.2 Nutrient deficiency symptoms 8

Nitrogen deficiency symptoms 8

Phosphorus deficiency symptoms 9

Magnesium deficiency symptoms 12

Micronutrient deficiency symptoms 12

4.1.3 Physical and Chemical Factors

Affecting Nutrient Availability 14

4.1.3.1 Growing media 14

4.1.3.2 Container volume 15

4.1.3.3 Relationship between pH and mineral nutrition 15

A practical definition of pH 15

Effect of pH on nutrient ion availability 15

The optimum pH for tree seedling growth 15

4.1.3.4 Water content in the growing medium 16

4.1.3.5 Salinity of the growing medium solution 16

4.1.4 Characteristics of Fertilizers Used in

Container Tree Seedling

Nurseries 17

4.1.4.1 Macronutrient fertilizers 17

Form 17

Grade or analysis 17

Nitrogen source 17

Nutrient release rate 18

Effect on chemical properties of the growing

 medium 18

Plant-use efficiency 19

Cost 19

4.1.4.2 Secondary nutrient fertilizers 20

4.1.4.3 Micronutrient fertilizers 20

Inorganic salts 20

Synthetic chelates 20

4.1.5 Determining Fertilizer Needs 22

4.1.5.1 Nutrient concentration 22

Units 22

Nitrogen level 22

4.1.5.2 Nutrient balance 23

4.1.5.3 Adjusting for seedling growth stages 25

2
3

4.1.9 Monitoring Container Seedling

Nutrition 53

4.1.9.1 What to measure when monitoring

fertilization 53

Electrical conductivity 53

Mineral nutrient levels 53

pH 53

4.1.9.2 Stages to monitor during fertilization 53

Irrigation water 53

Fertilizer stock solution 54

Applied fertilizer solution 54

Growing medium solution 54

Leachate 57

4.1.9.3 Seedling nutrient analysis 58

Sample collection and handling 58

Interpretation of seedling nutrient analysis test

results and standards 58

4.1.10 Effects of Heavy Fertilizer Use in

Container Tree Seedling

Nurseries 60

4.1.10.1 Luxury consumption of nitrogen and seedling

quality 60

4.1.10.2 The ideal nitrogen fertilization level63

4.1.11 Conclusions and

Recommendations 63

4.1.12 References 64
4.1.1 Essential Mineral Nutrients

The importance of mineral nutrition on both the quality and 

quantity of growth of container tree seedlings cannot be 

overemphasized. Probably more than any other cultural practice, 

with the possible exception of irrigation, fertilization controls both 

the rate and type of growth in container tree seedling nurseries.

The terminology of plant nutrition contains several terms that are 

technically inaccurate. In the jargon of plant science, "mineral" 

nutrients are the essential elements that plants obtain from the 

soil. Under the strict chemical definition, however, the term mineral 

refers to a compound, rather than a group of single elements. The 

term "nutrient" is also commonly used to refer to an essential 

element although this is not the exact scientific definition of the 

term (Jones 1983). Although they may not be etymologically 

correct, we will use established conventional plant nutrition terms, 

such as mineral nutrient, in this text.

The beneficial effects of adding "mineral" substances, such as 

wood ash or lime, to the soil to improve plant growth have been 

known for more than 2,000 years. It was not until the 19th century 

that the observations and speculations of Justus von Liebig lead to 

the "mineral element theory;" which stated that elements such as 

nitrogen, sulfur, phosphorus, and others are "essential" for plant 

growth (Marschner 1986). The terms essential mineral element

and mineral nutrient were proposed by two University of California 

plant physiologists in 1939. In this landmark paper, Amon and 

Stout (1939) established three criteria for essentiality (Jones 1983, 

Marschner 1986):

1. Omission of the element must result in abnormal growth, 

failure to complete all phases of the life cycle, or premature 

death of the plant.

2. The function of this element must be specific and

not replaceable by another element.

3. The element must exert a direct effect on plant growth and 

metabolism; be a component of a plant constituent, such as 

an enzyme; or be required for a distinct metabolic process, 

such as an enzyme reaction. According to this definition, 

mineral elements that have indirect effects on plant growth are 

not considered essential.

4

Thirteen elements have been identified as being essential for the 

growth of higher plants, although chlorine has been proven to be 

essential for only a limited number of species (Marschner 1986). 

For convenience' sake, the thirteen elements are classified as six 

macronutrients, which are used by plants in relatively large 

amounts, and seven micronutrients, which are required in very 

small quantities (table 4.1.1). Macronutrients are either 

constituents of organic compounds, such as proteins or nucleic 

acids, or act in osmotic regulation and are therefore found in 

relatively large amounts in plant tissue. Micronutrients, on the 

other hand, are primarily constituents of enzymes and are found 

in relatively mi nor proportions in plant tissue (tables 4.1.1 and 

4.1.2).
5
6
4.1.2.1 Nutrient uptake and utilization

The actual process of mineral nutrient uptake is complicated and 

well beyond the scope of this manual. Nutrient uptake by plants 

can be divided into active and passive absorption. Passive 

absorption means that ions are carried into the plant root along 

with the transpirational water stream. The factors controlling 

passive absorption are the volume of water moving into the plant 

(transpirational demand) and the concentration of the ions in the

4.1.2 Mineral Nutrients and Seedling

Growth

An understanding of how fertilization affects the growth of 

container tree seedlings is essential to the design and 

implementation of a nursery fertility program. Fertilizers break 

down into nutrient ions in an aqueous solution: e.g., ammonium 

sulfate [(NH4)2S04] yields ammonium (NH4

+) ions and sulfate 

(S04

2-

) ions. These nutrient ions become adsorbed on 

ion-exchange sites on particles in the growing medium, such as 

peat or vermiculite, or remain in the growing medium solution until 

they are taken up by the seedling root system (fig. 4.1.1). A tree 

seedling, like all plants, obtains these mineral elements from the 

soil solution as ions, although some nutrients can also be taken up 

as molecules or organic complexes. The molecule urea, which is a 

soluble form of nitrogen, can be absorbed by plant roots as well as 

some chelated complexes of micronutrients, such as FeEDTA (see 

section 4.1.4.3) (Jones 1983).

growing medium solution that surrounds the roots. Active 

absorption occurs when nutrient ions are taken up against the 

osmotic gradient that normally exists between the root cells and 

the growing medium solution. The biochemical processes 

involved in active uptake are not completely understood, but 

there is general agreement that active uptake is the dominant 

mechanism. There are three things that we do know about nutrient uptake by plants (Jones 1983).

1. The plant is able to take up nutrient ions selectively, even 

though the ion concentration and ratio of the surrounding 

solution may be quite different than that in the root cells.

2. Ions accumulate in the root across a considerable

gradient.

3. Active ion uptake requires energy, which is generated by cellular metabolism.

For a complete treatment of the biochemical aspects of mineral 

nutrient uptake by plants, the reader is referred to Marschner 

(1986) or Black (1968).

In the soil or growing medium, mineral nutrient availability is 

affected by the passive movement of ions with the soil solution, by 

diffusion, and by the growth of plant roots (Barber 1962). The 

passive movement of nutrient ions towards the plant root with the 

soil water during transpirational uptake is called "mass flow" (A in 

figure 4.1.2), the rate of which is controlled by transpirational 

demand. Within the growing medium solution surrounding the 

roots, nutrient ions are taken up from the rhizosphere (B in fig. 

4.1.2), either by passive diffu sion (movement from a relatively high 

concentration to lower concentration) or by active absorption 

processes. Plants also reach mineral nutrients through root extension (C in fig. 4.1.2), where the plant root tip grows into new 

supplies of mineral nutrients (Jones 1983). These processes are 

much simplified with liquid fertilizer applications in container tree 

seedling nurseries because the seedling roots are periodically 

bathed with a fresh supply of a complete nutrient solution.

There is a characteristic relationship between the concentration 

of a nutrient ion in seedling tissue and its growth (fig. 4.1.3). 

When a nutrient is present in low concentrations in seedling 

tissue, it is said to be defi cient and limiting to growth. At the 

lower end of this

7
deficiency range, the plant often exhibits certain visible 

abnormalities and these "deficiency symptoms" are characteristic 

of the specific nutrient deficiency (see section 4.1.2.2). At slightly 

higher tissue concentrations, the nutrient is still deficient enough to 

limit seedling growth but not enough to produce deficiency 

symptoms (fig. 4.1.3). This condition is called "hidden hunger" 

because, although plant growth is reduced, the nutrient

8

deficiency is difficult to diagnose from deficiency symp toms 

alone. Seedling nutrient analyses are often helpful in identifying 

this condition.

When the mineral nutrient supply is no longer limiting, the growth 

of the seedling reaches a plateau (fig. 4.1.3), termed the optimum 

nutrient range. The actual width of this optimum range varies 

between different nutrients but it is in this range of nutrient 

concentration that max imum growth occurs. (Normal mineral 

nutrient concentrations that are considered adequate for container 

tree seedlings are discussed in section 4.1.9.3) When mineral 

nutrients are present in the growing medium in surplus quantities, 

seedlings may continue to take up additional nutrients even though 

there is no measurable increase in growth; this condition is termed 

"luxury consumption" (fig. 4.1.3). Luxury consumption is relatively 

common in container tree seedling nurseries because of the ideal 

growing environment and the lack of the environmental factors that 

are limiting to growth in the natu ral environment.

When nutrient concentrations in seedling tissue reach extremely 

high levels, nutrient toxicity can occur and seedling growth will 

decrease (fig. 4.1.3); in extreme cases, excessive nutrient 

concentrations in seedling tissue can even cause death. (Luxury 

consumption and the cultural implications of heavy fertilizer use in 

container tree seedling nurseries are discussed in detail in section 

4.1.10.) The challenge is to keep the growing medium solution 

charged with optimum levels of all 13 mineral nutrients to 

maximize seedling growth.

4.1.2.2 Nutrient deficiency symptoms Nutrient deficiencies are 

characterized by specific, observable symptoms. Although there is 

considerable variation between symptoms for different tree 

species, a list of typical nutrient deficiency symptoms for the 13 

mineral nutrients is provided in table 4.1.3. These deficiency 

symptoms are somewhat useful in diagnosing mineral nutrient 

deficiencies, but many of the symptoms (e.g., chlorosis) can be 

caused by deficiencies of several nutrients and, therefore, seedling 

nutrient analysis is often necessary for an accurate diagnosis (see 

section 4.1.9.3). Also remember that by the time nutrient 

deficiency symptoms appear, a significant amount of growth may 

have already been lost (see hidden hunger, section 4.1.2.1).

Nitrogen deficiency symptoms. Nitrogen (N) deficiency 

symptoms include chlorosis and stunting (fig.
4.1.4A); chlorosis usually appears first on the lower leaves or 

needles, because N is a mobile element within the plant and is 

transferred to the newer foliage (fig. 4.1.4B). (Compare this pattern 

to that of iron chlorosis.) N stunting is usually easy to diagnose, 

and subsequently correct, because deficient seedlings rapidly 

respond to applications of N fertilizer.

Phosphorus deficiency symptoms. Phosphorus (P) deficiency 

symptoms are extremely variable between species and, therefore, 

this problem is difficult to diagnose from symptoms alone. 

Because P is required relatively early in seedling development, 

one of the classic symptoms of P deficiency is "purple heart;" a purpling of the new 

primary needles (fig. 4.1.5A). Foliar P deficiency symptoms range 

from no color change to dark-green, pink, or purple (fig. 4.1.5B), 

general chlorosis, and marginal scorch (fig. 4.1.5C), mottled 

interveinal chlorosis (fig. 4.1.5D), and chlorosis of the lower 

leaves (fig. 4.1.5E). Swan (1971) found that P deficiency 

symptoms varied between two different species of spruce: white 

spruce showed the characteristic stunting and purple foliar 

symptoms whereas red spruce, although equally stunted, showed 

almost no purpling. Obviously, foliar deficiency symptoms should 

not be considered diagnostic for P deficiency.
10
11
Magnesium deficiency symptoms. Magnesium is another 

mineral nutrient whose lack produces a characteristic chlorosis, 

but the chlorosis, in this case, is almost always strongly patterned: 

needle tips in conifers (fig. 4.1.6A) and interveinal in hardwood 

seedlings (fig. 4.1.6B).

Micronutrient deficiency symptoms. Micronutrient deficiency 

symptoms are extremely variable between nutrients and species. 

Iron (Fe) chlorosis is a relatively common disorder in horticultural 

nurseries, and some species are particularly sensitive (Bunt 

1976). Chlorosis is the first symptom of minor iron deficiency and 

this condition is usually expressed first in the newer foliage

12

(fig. 4.1.7A). In severe cases, the entire seedling becomes 

chlorotic and stunted and the disorder is almost impossible to 

correct at this stage. Copper (Cu) deficiency is commonly seen in 

plants grown in peat-based growing media and one of the 

characteristic symptoms is the twisted, chlorotic needles at the 

termi nal of the seedling (fig. 4.1.7B). Micronutrient deficiencies 

are difficult to diagnose and correct because the symptoms are 

often a result of an imbalance between several different 

micronutrients. Seedling nutrient analysis can be helpful in 

confirming visual symptoms, although foliar levels of some 

micronutrients, notably iron, can be higher in symptomatic 

seedlings.
13
of lower leaves. The roots may have orange-brown spots 

with dead tips.

2. Phosphorus leaching-Because artificial media lack the iron 

and aluminum oxides and other chemicals that fix P in natural 

soils, this essential nutrient can rapidly leach out of soil-less 

growing media and may lead to P deficiency.

3. Micronutrient deficiency-Artificial growing media do not 

contain the normal complement of micronutrients that is found 

in most natural soils. Some micronutrients, such as iron, 

copper, and boron, can become unavailable for plant uptake 

because they become fixed to insoluble humic acids as the 

organic component of the growing medium decomposes. Iron 

deficiency is particularly common in artificial media, and 

Scarratt (1986) found mild chlorosis and low foliar Fe levels in 

jack pine seedlings grown with most general-purpose 

fertilizers.

4. Lower pH requirement-Soil-less growing media should be 

kept at a lower pH to keep micronutrients available (see 

section 4.1.3.3).

Both peat and vermiculite have very high cation-exchange 

capacities, which means that the growing me dium can maintain a 

nutrient reserve of cations, such as NH4

+, K+, Ca 2+ and Mg2+. 

Vermiculite has even been reported to have some degree of 

anion exchange capacity and may retain significant amounts of 

P04

3-

 (Bunt 1976). While these adsorbed nutrient ions supply a 

fertility buffer under normal conditions, they can be flushed from 

the media during heavy leaching, which permits changes in 

fertility programs or induction of a nutrient stress if desired.

The primary benefit of an infertile growing medium therefore, is to 

allow the grower to supply all thirteen mineral nutrients in the 

proper concentrations, in the proper balance, and at the proper 

time. This is a considerable advantage over the standard bareroot 

nursery, where the nursery manager has to constantly deal with 

the complicated and variable nutrient relationships in a mineral 

soil. The ability to design and apply a well-balanced fertility 

program is one of the primary reasons for the rapid seedling 

growth rates that can be achieved in container seedling nurseries. 

(Growing me dia are discussed in greater detail in volume two of 

this series.)

4.1.3 Physical and Chemical Factors

Affecting Nutrient Availability

There are several factors that make the nutrient relations of 

container seedlings different from a natural forest wildling or a 

seedling in a bareroot nursery bed. These factors must be 

considered when designing a fertilization program because they 

have a significant influence on the mineral nutrient availability.

4.1.3.1 Growing media

The majority of container tree seedlings produced in North 

American nurseries are grown in soil-less or "artificial" growing 

media (artificial media are composed of materials other than soil). 

Many early growing media formulations such as the John Innes 

composts contain a substantial portion of loam soil, which 

provides some supply of mineral nutrients; this slight advantage is 

far outweighed by other drawbacks including the excessive 

weight of soil and the need for pasteurization to eliminate disease 

organisms and weed seeds.

The exact proportions may vary, but most artificial growing media 

are composed of an organic component, usually peat moss, and 

an inorganic component such as vermiculite. The high percentage 

of peat moss makes these growing media fall into the class of 

organic soils. Although vermiculite contains some mineral 

elements (5 to 8% K and 9 to 12% Mg), and sphagnum peat moss 

does contain some N, the nutrients are released so slowly that 

peat-vermiculite growing media can be considered infertile for all 

practical purposes (Bunt 1976). Scarratt (1986) analyzed a 

peat-vermiculite growing me dium and found very low levels of all 

macronutrients (1.56 ppm NH4 -N, 1.3 ppm P, 5.2 ppm K, 1.8 ppm 

Ca, and 1.2 ppm Mg); the growing medium also contained small 

amounts of most micronutrients except Cu.

The shift from soil-based to artificial growing media has produced 

some problems in plant nutrition (Nelson, as quoted in Appleton 

1986):

1. Ammonium sensitivity--Artificial media typically have relatively 

low pH values, slowing the bacterial conversion of ammonium 

to nitrate. Some plants suffer ammonium toxicity because 

they take up and store excessive amounts of 

ammonium-nitrogen; plants can store up to 20,000 ppm 

N03-N, compared to only a few hundred parts per million of 

NH, -N. Ammonium toxicity often causes injury to the root 

system, and injured plants show typical root injury symptoms: 

a wilted appearance, leathery-textured or curled leaves, and 

chlorosis or necrosis

14
ject of considerable debate. Gingrich (1984) supports the 

widely held proposition that pH is the "most important 

aspect" of container seedling nutrition, whereas Whitcomb 

(1983) states that pH has "little effect" on the nutrition of 

container plants as long as proper fertilization practices are 

followed. Tinus (1980) states that, ex cept at the extreme 

values where root injury may occur, pH does not directly 

affect seedling growth.

One reason for this difference of opinion can be attributed 

to the type of growing medium that is used; growing media 

that contain native soil are more affected by pH than are 

those containing exclusively artificial media such as 

peat-vermiculite mixes. The negative effects of extreme pH 

levels on aluminum and manganese ion toxicity and the 

availability of micronutrients in mineral soils is well 

established. The situation is different, however, in organic

soils. Studies on nutrient availability have shown that 

maximum nutrient availability occurs at approximately pH 

6.5 in mineral soils, but is a full pH value lower (pH 5.0 to 

5.5) in organic soils (Lucas and Davis 1961). Peterson 

(1981) found that 5.2 to 5.5 was the optimum pH range for 

nutrient availability in an arti ficial medium.

On an operational basis, the effect of pH on nutrient 

availability is not as critical in a container nursery as it is in 

a bareroot nursery. Natural soils contain a variety of 

chemical ions that react with nutrient ions, especially 

micronutrients, and make them unavailable to plants. This 

is not the case with artificial growing media, however, 

because of their inherently low nutrient status (Whitcomb 

1983). As long as a well-balanced fertilizer is regularly 

applied, pH effects on nutrient availability should not be a 

concern in container nurseries. Whitcomb (1984) reports 

that even a pH-sensitive plant like the azalea can be 

successfully raised in artificial media ranging from pH 3.0 

to 8.2 as long as an adequate nutrient supply is provided.

The optimum pH for tree seedling growth. Even though 

tree seedlings are able to tolerate a relatively wide range of 

pH values, it has been well documented that conifers grow 

best at around pH 5.5 whereas hardwoods prefer a slightly 

higher pH of 6.5. Maintenance of the growing medium 

solution within one-half pH unit on either side of these 

targets is recommended. Control of pH in the irrigation 

water and the growing medium solution is discussed in 

section 4.1.7.2.

15

4.1.3.2 Container volume

One of the most important considerations in container 

seedling fertility is the relatively small volume of most forest 

seedling containers. Most containers used in container tree 

seedling nurseries vary from 40 to 492 cm3 (2 to 30 cubic 

inches), and this small capacity means that seedlings only 

have a limited amount of nutrient reserves. During the rapid 

growth phase, both the nutrient concentrations and the 

balance between nutrients in the small volume of growing 

medium can change rapidly. To achieve optimum growth 

rates, growers must insure that the growing medium 

contains a constant and balanced supply of all the essential 

mineral nutrients (Jones 1983). (Other container effects are 

presented in volume two of this series.)

4.1.3.3 Relationship between pH and mineral

nutrition

A practical definition of pH. The pH of the growing

medium is one of the most commonly discussed, yet

least understood, factors affecting mineral nutrition of

container tree seedlings. An excellent discussion of the

chemical background of pH is given in Tinus (1980),

and the definition is straightforward enough: a relative

measure of the hydrogen ion (H+) concentration

expressed on a logarithmic scale. pH values range from

0 (very acidic) to 14 (very alkaline), with 7 being neutral. 

Because pH units are logarithmic, a solution with a

pH 6 has 10 times more H+ ions, a solution of pH 5

has 100 times more, and a solution with a pH 4 has

1,000 times more H+ ions, than a solution with a neutral pH 

(7.0).

In actual practice, the pH of a solution involves more than 

just H+ or OH- ions; in growing media solutions, for 

example, pH is often a reflection of the activity of other ions, 

notably C03

2-,

 HCO3, NH4

+, NO3-, SO42-

, P04

3-

, HPO4

2-

, and 

H2PO4-. Obviously, many different ions, whether naturally 

occurring in the water source or added as fertilizer, have an 

effect on the pH reading. The important thing to remember 

about nutrition for container tree seedling nurseries is that 

the pH reading is a symptom and not a cause: a high pH 

reading usually indicates the presence of an accessory ion 

such as CO3

2-

 or HCO3

-

 rather than the OH- as such.

Effect of pH on nutrient ion availability. The importance of pH on container seedling nutrition is the sub-
16

The typical peat-vermiculite growing medium used in container 

tree seedling nurseries does not contribute appreciably to the 

soluble salt problem, although lesser grades of peat moss can 

release salts upon decomposition. The small container volume 

also affects the salinity of the solution surrounding the particles of 

the growing medium. The salinity will drop as the seedling takes 

up fertilizer salts, or salts are leached from the growing me dium 

during irrigation. Salinity can also reach damagingly high levels 

under high evapotranspiration conditions if the growing medium is 

allowed to dry out (Furuta 1978).

Although soluble salts can affect plants in several ways, high 

levels of specific ions can adversely affect the uptake of certain 

nutrients, and upset the nutrient balance that is needed for 

optimal growth. Nutrient ion interactions include competition, 

antagonism, and synergism and are discussed in greater detail in 

section 4.1.5.2. Salinity effects on seedling growth can be found in 

section 4.1.9.1 and Rohsler and Wright (1984). (For aspects of 

salinity of irrigation water, see section 4.2.4.2.)

4.1.3.4 Water content of the growing medium

Because nutrient ions are dissolved in the aqueous solution 

surrounding particles of growing medium, the moisture content of 

the growing medium affects both nutrient availability and uptake. 

Fertilization is therefore greatly dependent on nursery irrigation 

practices, and a low water content in the growing medium 

drastically reduces the effectiveness of fertilization. Squire and 

others (1987) found that N and P fertilization only affected 

Monterey pine seedling growth at relatively high growing medium 

moisture levels (0.00 to -0.10 MPa). Even within this range, 

seedling dry weight decreased radically with increasing moisture 

stress (fig. 4.1.8). The implications of these findings should be 

obvious: the water content of the growing medium should be maintained at optimum levels for maximum effectiveness.

4.1.3.5 Salinity of the growing medium solution

The chief sources of soluble salts in container tree seedling 

nurseries are fertilizer residues, irrigation water, and the growing 

medium (Rohsler and Wright 1984). Inorganic fertilizers are 

chemically considered salts, and soluble salts are also introduced 

with the irrigation water.
4.1.4 Characteristics of Fertilizers Used in

Container Tree Seedling Nurseries

Fertilizers can be categorized in several different ways, but for 

practical purposes there are three types of fertilizers used in 

container tree seedling nurseries: macronutrient fertilizers, 

which supply N, P, and K; secondary nutrient fertilizers, which 

supply Ca, Mg, and S; and micronutrient fertilizers, which 

supply any one or a combination of the seven essential 

micronutrients. Sanderson (1987) itemizes the following 

characteristics of macronutrient fertilizers; secondary nutrient 

fertilizers are discussed in section 4.1.4.2 and micronutrient 

fertilizers are discussed in section 4.1.4.3.

4.1.4.1 Macronutrient fertilizers

Form. Both liquid and dry forms of fertilizers are used in 

container tree seedling nurseries; dry fertilizers can be 

purchased in either granules or pellets. Liquid fertilizers are 

generally injected through the irrigation system, whereas dry 

fertilizers are incorporated into the growing medium or used as 

a top-dressing. (See section 4.1.6 for more information on 

fertilizer application techniques.)

Grade or analysis. Fertilizer manufacturers are required by law 

to state the guaranteed nutrient content of the three primary 

macronutrients (N-P-K) on the fertilizer container: N is specified 

as percent N, but P and K are specified as the oxide form of the 

element, P as percent P2O5, and K as percent K2O. For 

example, a 20-20-20 fertilizer would contain 20% N, 20% P205

(8.8% P), and 20% K2O (16.6% K). (Conversion factors are 

listed in table 4.1.4.) High-grade fertilizers refer to the total 

amount of mineral nutrients in the fertilizer; for the 20-20-20 

fertilizer example, the total analysis would be 45.4%. The 

remainder of the content is composed of accessory chemicals 

that are not nutrients, although some fertilizers often contain 

other unspecified secondary nutrients, including Ca and S. 

High-grade fertilizers are the only type generally used in 

container nurseries, especially for liquid injection, because 

lower grades contain an unacceptable amount of inert material, 

which may cause solubility problems in the nutrient so17

lutions. The relative proportions of N-P-K determine the 

suitability of a fertilizer for specific species or growth stages 

(see section 4.1.5.3).

Nitrogen source. Nitrogen is by far the single most important 

mineral nutrient in a fertilization program because it most 

frequently limits the growth of container plants. N is available in 

many different organic and inorganic forms. Organic N forms are 

rarely used in container tree seedling nurseries because of their 

variable release rate, low nutrient analysis, and relatively high 

cost (Sanderson 1987), but many different inorganic sources of N 

are available. There are two different inorganic N ions that are 

taken-up by plants: ammonium, a positively charged cation 

(NH4

+), and nitrate, a negatively charged anion (N03-

).

Nitrogen type has an effect on availability, possible nutrient 

toxicities, and the pH of the growing medium (fig. 4.1.9). Nursery 

managers should check the analysis on the fertilizer container to 

determine which N form is
most common. Some fertilizers contain only NO3

-

, such as calcium 

nitrate, whereas others are composed exclusively of NH,' 

(ammonium phosphate). Ammo nium nitrate contains equal 

amounts of both nitrogen ions. Urea [(NH2) 2CO] is a widely used 

form of N that is commonly used to formulate high-analysis 

fertilizers for container tree seedling nurseries. Urea is microbially 

broken-down to NH4

+ and then to NO3-

 in the growing medium. 

These reactions are temperature-sensitive, and the artificial 

peat-based media used in most nurseries may not contain the 

proper number or type of microorganisms for these conversions. 

Scarratt (1986) speculated that nitrification of NH4

+ may be 

inhibited under the lower light conditions and wetter growing media 

that often occur during winter cropping seasons. Etha- zole 

(Truban®), a commonly used fungicide, also inhibits the growth of 

these microorganisms. Under the high temperatures usually 

occurring in greenhouses in summer, the conversion to nitrate may 

occur too rapidly; the cool soil temperatures in winter may cause 

toxic levels of NH4

+ to accumulate. Certain plant species are damaged by high levels of either ammonium or nitrate (Sanderson

1987).

Nutrient release rate. The rate at which the nutrients are 

released by the fertilizer is also important. Most of the high-grade 

fertilizers used for liquid fertilizers are highly soluble in water and 

therefore the nutrient ions are quickly released. Other fertilizers, 

such as the types used for incorporation in growing media, are 

specially formulated to release their nutrients over time. Nutrient 

release rate is affected by fertilizer formulation, temperature, 

moisture, and microbiological activity. (See section 4.1.6.1 for 

more discussion on slow-release fertilizers and section 4.1.6.2 for 

fast-release fertilizers)

Effect on chemical properties of the growing medium.

Fertilizer type can affect the pH and soluble salt levels of the 

growing medium, which can seriously affect seedling growth. 

Certain species are pH-sensitive, and high soluble-salt levels can 

cause "fertilizer burn;" which is particularly serious with germinants 

and small seedlings.

Fertilizers can be rated according to a relative salt index (table 

4.1.5), which is a measure of the ability of the fertilizer to produce 

soluble salts in the growing medium solution and potentially cause 

injury (Rohsler and Wright 1984). Nursery managers should select 

fertilizers that can provide the desired nutrient at the lowest relative salt index: e.g., to supply N, calcium nitrate (salt index = 53) 

would be potentially less injurious than

18

sodium nitrate (salt index = 100). Calcium nitrate also provides 

two different mineral nutrients (Ca and N), whereas sodium 

nitrate, besides N, contributes potentially harmful sodium (Na) 

ions to the growing medium solution.

The effect of a fertilizer on pH is specified on the fertilizer label as 

"potential acidity;" which is the amount of calcium carbonate 

needed to neutralize a ton of fertilizer. Potential acidity is useful in 

selecting a fertilizer for a specific crop, or for adjusting the pH of 

the growing medium during the growing season (fig. 4.1.9). Even 

slow release fertilizers can affect medium pH. Sanderson (1987) 

reports that Osmocote® 14-14-14 produces a more acid reaction 

than Pro-Grow® 25-10-10, probably a result of the nitrogen 

source. Ammonium-based fertilizers, such as ammonium nitrate, 

ammonium sulfate, and ammonium phosphate, are generally 

acid-forming, whereas nitrate-based fertilizers, such as calcium 

nitrate or potassium nitrate, will raise the pH of the growing 

medium. Urea, on the other hand, does not generally affect pH in 

soils, but in a solution will slowly hydrolyze and raise the pH.

Most inorganic fertilizers, especially the high-grade fertilizers used 

for liquid fertilization in container tree seedling nurseries, are 

chemically considered salts (e.g., calcium nitrate releases two 

nutrient ions: Ca 2+ and NO3

-

.
ply all of their P fertilizer requirement by using this 

technique.

Actually, the cost of the fertilizer itself can be relatively minor 

compared to the labor cost involved in mixing and application 

(Sanderson 1987). One of the real advantages of using 

incorporated fertilizers is the labor savings, compared to the 

bi-weekly or even daily costs of applying liquid fertilizers.

After considering all these factors, the choice of the best fertilizer 

may seem a difficult, complicated decision. On the contrary, 

deciding which fertilizer to use in container tree seedling nurseries 

should be based on one criterion alone: seedling growth response 

under the existing cultural and environmental conditions.

19

Indiscriminate use of any salt-producing fertilizer can cause salt 

injury. (See section 4.1.9.1 for procedures for monitoring soluble 

salts in nurseries and section 4.2.4.2 for more discussion on 

salinity effects.)

Slow-release fertilizers produce lower salinity than standard 

fertilizers because, by definition, they gradually release nutrient 

ions into the growing medium solution. The type of N used to 

formulate the slow-release fertilizers can also effect the salinity of 

the growing medium. Sanderson (1987) reports that Pro-Grow 

25-10-10, with its urea-formaldehyde base, produces less salinity 

than Osmocote 14-14-14, which is composed of inorganic nitrogen 

sources.

Plant-use efficiency. It has been estimated that plants use only 

one-tenth of the nutrients applied in a typical liquid fertilizer 

application (Furuta, quoted in Sanderson 1987). Liquid fertilizer 

applications require that some level of excess fertilizer must 

always be maintained in the growing medium solution. 

Slow-release fertilizers, on the other hand, provide nutrients at a 

rate that is more compatible with plant uptake. Whitcomb (1984) 

states that as soon as the nutrients are released from the 

slow-release fertilizer they are used by the plant. This means that 

salinity (electrical conductivity) measurements of the growing 

medium solution may appear abnormally low.

Sanderson (1987) estimates that plants lose ten times more 

nutrients from water-soluble fertilizer than from a slow-release 

fertilizer. This is not only wasteful, but such heavy leaching of 

soluble fertilizers also contributes to wastewater pollution. (See 

section 4.2.8 for discussion of wastewater disposal.)

Cost. Fertilizer cost differences can be attributed to analysis, type 

and source of raw materials, manufacturing costs, transportation, 

quantity purchased, and other factors. (For a comparison of the 

costs of the various chemical grades of ammonium nitrate, see 

section 4.1.7.4.) The individual nutrients also vary in cost, with N 

being the least expensive, and water-soluble sources of 

phosphorus the most expensive. Because superphosphate 

(0-46-0) is one of the cheapest forms of P, many nursery 

managers try to save money by incorporating this fertilizer into the 

growing medium (Sanderson 1987). A more practical approach is 

to supply P in the form of phosphoric acid (H3P04), which is 

primarily used to lower the pH of the irrigation water (fig. 4.1.10). 

Many container tree seedling nurseries are able to sup-
Inorganic salts. The most common inorganic micronutrient 

fertilizers are sulfates of the metal micronutrients (Fe, Mn, Cu, and 

Zn); other oxide and chloride forms of the metal micronutrients are 

also available but generally the sulfate forms are preferred. The 

major water-soluble forms of B include sodium tetraborate (borax) 

and Solubor®, whereas sodium molybdate and ammonium 

molybdate are the principal fertilizer forms of Mo (California 

Fertilizer Association 1985). (For a discussion of commercial 

micronutrient fertilizers, see section 4.1.6.1; for a list of the major 

fertilizer chemicals used for formulating custom liquid fertilizers, 

see section 4.1.7.4; for the micronutrient content of a "complete" 

commercial fertilizer, see section 4.1.7.3)

Synthetic chelates. A chelating agent is a compound, usually 

organic, that can chemically combine with a metal ion, forming a 

ring-like structure (fig. 4.1.12). The resulting molecule is called a 

chelate. Chelates are frequently used to protect metal 

micronutrients from the chemical inactivation that so often occurs 

in alkaline soils. Micronutrient fertilizers can contain several different types of chelating agents, but the stability of these 

compounds is variable. Chelate stability varies with the metal ion 

and the chelating agent. The common micronutrient chelating 

agents used in nurseries, with their much-needed chemical 

names, are:

EDTA = ethylenediaminetetraacetic acid

EDDHA = ethylenediaminedihydroxyphenylacetic acid

HEDTA = hydroxyethylethylenediaminetriacetic acid

Obviously, the chemistry of synthetic chelate fertilizers is quite 

complicated and the reader is referred to Mortvedt and others 

(1972) for more information.

Chelated micronutrient fertilizers are available for single nutrients 

or in specially blended mixtures. (Micronutrient fertilizers are 

discussed further in section 4.1.6.1; for a list of the common 

micronutrient chelates used to formulate custom liquid fertilizers, 

see section 4.1.7.4.)

4.1.4.2 Secondary nutrient fertilizers The secondary 

macronutrients (Ca, Mg, and S) are generally supplied by the soil 

and water and are therefore not often added as fertilizers in

bareroot nurseries. Adequate amounts of Ca and Mg are often 

supplied by irrigation water, especially in areas with "hard" water. 

Ca is supplied by calcitic limestone and both Ca and Mg are 

supplied by dolomitic limestone; both these materials are 

commonly used to raise the pH of acidic soils or peat moss in the 

case of container nurseries. S is also supplied in relatively small 

quantities by the breakdown of organic matter, most river water, 

rainwater, and from many pesticides (California Fertilizer 

Association 1985).

The secondary nutrients are also present in many chemi cals used 

for fertilizers and commercial fertilizer formu lations. Ammonium 

sulfate, (NH4) 2SO4, contains S and calcium nitrate, Ca(NO3) 2, 

contains Ca. (For a discussion of the secondary nutrient 

composition of the principal chemicals used for formulating custom 

liquid fertilizers, see section 4.1.7.4, and for the secondary nutrient 

composition of a typical commercial fertilizer, see section 4.1.7.3.)

4.1.4.3 Micronutrient fertilizers

In bareroot nurseries, seedlings obtain micronutrients from the 

soil, and micronutrient fertilizers are not applied unless a specific 

problem, such as high pH or calcium levels, is identified. 

Micronutrient fertilizers are definitely needed in the artificial 

growing media that are typically used in container tree seedling 

nurseries (see section 4.1.3.1). The balance of different 

micronutrients in the growing medium is also considered critical 

because high levels of one micronutrient can interfere with the 

availability of another (fig. 4.1.11).

Micronutrients can be supplied from either inorganic or organic 

sources, and the properties of the various sources vary 

considerably. The California Fertilizer Association (1985) classifies 

micronutrient fertilizers as: 1) inorganic salts, 2) synthetic chelates, 

and 3) natural organic complexes. Naturally occurring organic 

complexes are byproducts of the wood pulp industry but are not 

recommended for container tree seedling culture because they are 

less stable than synthetic chelates and are more readily broken 

down by microorganisms.

20
21
22

28.4 g (1 ounce) of fertilizer dissolved in 378.5 liters (100 gallons) 

of water equals 75 ppm. For example, if you want 225 ppm N in 

the applied fertilizer solution, add 85.2 g (3 ounces) of actual N or 

426.5 g (15 ounces) of bulk 20-20-20 fertilizer to 378.5 liters (100 

gallons) (Bonaminio 1983). (For a handy table for determining the 

amount of actual bulk fertilizers to add per standard volume of 

fertilizer solution, see section 4.1.7.3.)

Nitrogen level. N is one of the most important nutrients affecting 

plant growth and is the most frequently applied fertilizer element. 

Most fertilizer programs are based around the N concentration, 

and the levels of all the other nutrients are generally established 

relative to N. The best N level for seedling growth has been the 

source of much discussion among nursery scientists, and the 

levels prescribed in the literature show considerable variation 

(table 4.1.7). Rather high N levels of around 200 ppm or more 

have been commonly used, probably because much of the early 

work was based on fertilizer regimes for horticultural species. 

Another reason for the high N levels reported in the literature is 

that many N recommendations are for periodic fertilizer 

applications and are therefore higher than for continuous 

fertilization (see section 4.1.8.3). The Container Nursery Survey 

revealed that North American nurseries use N levels ranging from 

55 to 260 ppm for the period of most rapid seedling growth (rapid 

growth phase, table 4.1.7).

In recent years, however, more research has been done on tree 

seedling nutrition and the latest recommendations are for lower N 

concentrations during the rapid growth phase. Ingestad (1979) 

recommended N concentrations of 20 to 50 ppm for Scotch pine 

and 60 to 100 ppm for Norway spruce when all other nutrients 

were in proper proportion. Phillion and Libby (1984) grew spruce 

seedlings at N concentrations ranging from 12 to 500 ppm and 

concluded that 100 ppm was best in terms of seedling color and 

growth. Working with container jack pine, Scarratt found that 

constant fertilization with 100 ppm N produced excellent growth 

rates, and that higher N rates reduced seedling quality. The 

general trend has been to reduce total N levels from 200 to around 

100 to 150 ppm during the rapid growth phase to control shoot 

growth and produce a more balanced seedling. The general 

nutrient targets for container tree seedlings (table 4.1.8) will need 

to be adjusted for individual species as more knowledge becomes 

available. (See section 4.1.10 for a discussion of effects of heavy 

fertilizer use in container tree seedling nurseries.)

4.1.5 Determining Fertilizer Needs

A fertilization program for a conta iner tree seedling nursery should 

be designed to maintain specific concentrations of different 

mineral nutrients in the growing me dium, keep them in balance, 

and should also be designed to allow necessary nutritional 

changes during the growing cycle.

4.1.5.1 Nutrient concentration

The concentration of each mineral nutrient in the growing 

medium solution is the most important aspect of nursery 

fertilization. Too low a concentration and seedling growth will be 

reduced, whereas succulent container seedlings can be easily 

damaged by the high salt levels caused by high nutrient 

concentrations. High lev els of certain nutrients, especially N, can 

also affect seedling quality (see section 4.1.10).

Units. Mineral nutrient concentrations can be described in several 

different ways (table 4.1.6). Proportional units [parts per million 

(ppm)] are most useful for describing and comparing nutrient 

regimes, whereas the weight per volume [milligrams per liter (mg/I) 

or ounces per gallon (oz/gal)] are necessary for th e actual fertilizer 

calculations. Parts per million refers to the concentration of a 

material without regard for any specific unit of weight or volume 

(Bonaminio 1983). To describe chemical activity, milliequivalents 

(meq) or millimoles (mmol) per liter are most accurate. The 

conversion between parts per million and milligrams per liter is 

quite easy for aqueous solutions: 1 ppm = 1 mg/I because 1 liter of 

water weighs 1 kg (1 mg/kg = 1 ppm).

Although parts per million are the most useful units for calculating 

liquid fertilizer application rates, many nursery managers prefer to 

use the weight of actual bulk fertilizer to the amount of water. 

Although not scientifically accurate, many growers use the rule of 

thumb that
4.1.5.2 Nutrient balance

The relative proportion of the various mineral nutrients to one 

another in the growing medium solution is the next factor to 

consider when designing a container tree seedling nursery 

fertilization program. Jones (1983) states that nutrient 

concentration ratios are more important than the absolute 

concentration of any one element. The balance between the 

various mineral nutrients is biologically important for two reasons:

1. Excesses of certain nutrient ions in the growing me dium 

solution may affect the uptake and utilization of other nutrients 

(figure 4.1.11).

2. The ionic balance affects the pH of the growing medium solution.

One feature of active nutrient uptake (see section 4.1.2.1) is that 

ion uptake from the growing medium solution involves ion 

competition, antagonism, and synergism. Monovalent ions, such 

as potassium (K+), are taken up more rapidly than divalent or 

trivalent ions. If the principal form of nitrogen in the growing 

medium solution is nitrate (N03-), then the certain cations (K+, 

Ca2+, and Mg2+) are taken up in greater amounts than if 

ammonium (NH4

+) is present. The presence of NH4+ has been 

shown to increase the uptake of NO3

-

, whereas CI- ions inhibit 

NO3

-

 uptake (Jones 1983).

Steiner (1980) states that most plants will grow well in one

universal nutrient solution if certain ratios of cations and anions are 

followed (table 4.1.9). He has designed a "universal nutrient 

solution" that is based on relative cation and anion ratios, total 

ionic concentration, and pH (Jones 1983). One of the most widely 

used nutrient balance theories for tree seedling culture is based on 

the work of Ingestad, who sets the ratios of all other nutrients in 

relation to N. Ingestad (1979) proposed "nutrient ratios" for several 

conifer and hardwood seedlings; the ratios for Douglas-fir are 

listed in table 4.1.10, along with the actual nutrient concentrations 

for all 13 mineral nutrients at 100 and 200 ppm N.

The ratio between the nitrate (NO3

-

) and ammonium (NH4

+ ) ions 

has been investigated, and researchers found that the total 

concentration of these two ions was not as important as their 

relative balance. Seedling growth was reduced if either NO3

-

 or 

NH4

+ was used exclusively for the N source. van den Driessche 

(1978) found that equal proportions of both N ions resulted in the 

greatest seedling growth at pH 5.5, although root growth may be 

greater with higher NO3

-

levels. The relative proportion of NH4

+

and NO3

-

in the growing medium solution is also important as 

these two ions have a significant influence of the pH level of the 

solution (see fig. 4.1.9). Seedling root systems may be damaged 

by high NH4

+ levels in artificial growing media

23
and Nelson (as quoted in Appleton 1986) recommends that 

nursery managers use fertilizers with less than 40 to 50% 

ammoniacal nitrogen.

The question of mineral nutrient balance in fertilization solutions is 

obviously one of the most confusing aspects of plant nutrition, for 

much of the published research appears contradictory. The 

situation is analogous to that surrounding vitamins and human 

health: a nutrition- conscious consumer will be completely baffled 

by published vitamin recommendations that range from those that 

state that no supplemental vitamins are necessary to those that 

advocate megadoses of certain vitamins. The practicing nursery 

manager should realize, therefore, that no "hard and fast" 

recommendations of the best nutrient balance can be made. The 

best course is to be aware that nutrient ratios are important and try 

to design

24
25

The proper form of N to supply during the hardening phase has 

been the subject of much discussion but operational experience 

with western conifer seedlings suggests that nitrate-based 

fertilizers should be preferred over ammonium-based fertilizers. 

NH4

+ is thought to stimulate succulent shoot growth and delay 

hardening, and therefore fertilizers such as calcium nitrate are 

often used during the hardening period. Gingrich (1984) states 

that NO3

-

fertilizers should be used during the low-light periods of 

late fall or winter. Matthews (1987), however, reports that conifer 

seedlings in British Columbia container nurseries have been 

grown with NH4

+ based fertilizers during the hardening period with 

no apparent problems.

The recommended ratios of the three principal macronutrients 

(N-P-K) also vary between the three growth stages; Carlson 

(1983) reports different ratios for different species of conifer 

seedling. Hahn (1978) recommended a N-P-K ratio of 1:5:1 during 

early seedling growth and a 3:1:1 ratio during the rapid growth 

phase. Recent fertilizer experiments, however, cast some doubt 

on the need for special fertilizers during the different growth 

stages. Scarratt (1986) reports that special "starter;" "grower;" and 

"finisher" fertilizers showed no significant improvement over 

standard "general purpose" fertilizers for growing jack pine 

seedlings in containers. Different seedling species react differently 

to different fertilizer regimes, however, although some specific 

recommendations have been published (Tinus and McDonald 

1979).

a well-balanced fertilization program that produces acceptable 

seedlings based on trial-and-error in an operational nursery 

situation. Obviously, more research on the best mineral nutrient 

ratios for growing tree seedlings is needed but, in the meantime, 

nursery managers should try to develop fertilization programs that 

work under their own cultural regimes.

4.1.5.3 Adjusting for seedling growth stages

Because of the pronounced effect of fertilization on plant growth, 

nutrient levels are traditionally adjusted for the various growth 

stages during seedling development. For development of seedling 

growing schedules, three separate growth stages can be 

recognized: the establishment phase, which covers emergence 

and seedling growth through the cotyledon stage; the rapid growth 

phase, when seedlings grow in height at an exponential rate; and 

the hardening phase, which begins when the seedlings have set 

their terminal bud and shoot growth ceases but caliper and root 

growth increase.

Control of the N level is the most important factor for manipulating 

seedling growth, and recommended N fertilization levels vary 

considerably during these different growth stages (table 4.1.7). 

The results from the Container Nursery Survey show that nursery 

managers are using a wide range of N levels during each growth 

stage. Although the optimum N level will vary between nursery 

and tree species, the trend is towards levels similar to those 

recommended by Mullin and Hallett (1983): moderate N during the 

establishment phase, higher levels during the rapid growth phase, 

and low N levels during the hardening phase (table 4.1.7).
4.1.6 Methods of Fertilizing Container

Seedlings

According to Handreck and Black (1984), there are three basic 

ways to apply fertilizers in a container nursery:

1. Incorporating a slow-release fertilizer into the growing medium.

2. Injecting a liquid fertilizer solution into the irrigation

water.

3. Top dressing solid fertilizers to the surface of the

growing medium.

In North American container tree seedling nurseries, liquid 

fertilizer injection is the most common fertilization technique 

followed by incorporation of slow-release fertilizers (Container 

Nursery Survey). Top dressing is usually impossible because of 

the small top opening of the containers used for growing tree 

seedlings and so will not be considered in this manual.

4.1.6.1 Incorporating dry fertilizers into the growing

medium

Incorporation of a dry fertilizer into the medium is most

commonly used with plants grown in large volume containers in ornamental nurseries but is also used in some

container tree seedling nurseries. The Container Nursery

Survey revealed that, although no nurseries used incorporated fertilizers exclusively, 26% used them in combination with liquid fertilizer injection. Matthews (1982)

reports that, when seedlings are being grown with liquid

fertilizer injection, incorporating a low rate of slow

release fertilizer into the growing medium improved

seedling growth.

Incorporating fertilizers into the growing medium has several 

advantages in container nurseries:

1. No specialized fertilizer injection equipment is

necessary.

2. Low labor costs involved in the frequent mixing and

application of liquid fertilizers.

3. Mineral nutrient levels can also be maintained during wet 

months when irrigation is not required and nutrient leaching 

can be a problem.

Incorporation of slow-release fertilizers is necessary in nurseries 

that do not have well-designed irrigation systems or nutrient 

injectors. The labor costs of incorporating fertilizers are less than 

with liquid fertilizer injection,

26

as labor is required only during the initial mixing process. 

Formulating, mixing, and applying liquid fertilizer solutions requires 

a certain amount of training and must be done at least weekly. In 

outdoor growing areas, many mineral nutrients (N in particular) are 

subject to leaching during periods of high rainfall because 

irrigation, and therefore liquid fertilizer injection is not necessary. 

In British Columbia container nurseries, slow-release fertilizers are 

commonly incorporated into the growing medium in containers that 

are grown in open, uncov ered compounds where heavy rainfall 

causes severe leaching losses (Matthews 1982).

There are three major drawbacks to incorporating fertilizers into 

the growing medium in container tree seedling nurseries:

1. It is impossible to control the concentration and balance of 

mineral nutrients in the growing medium solution.

2. It is difficult to obtain even distribution of the fertilizer particles 

throughout the growing medium with the mixing equipment 

commonly used in container nurseries.

3. Incorporation requires extra mixing of the growing medium, 

which may breakdown particle size and cause compaction 

problems.

One of the real advantages of container seedling culture is that 

growth can be precisely controlled through all phases of seedling 

development, especially during the critical hardening period. 

Complete control of all 13 mineral nutrients is one of the most 

effective "tools" available to the container nursery manager, but 

this control is sacrificed by fertilizer incorporation. Nutrient release 

rates of slow-release fertilizers are controlled by factors, such as 

temperature, moisture content, and microorganism activity, that 

are beyond the control of the nursery manager. Once the nutrient 

charge has been introduced into the medium, it is impossible to 

completely regulate nutrient availability. This is particularly 

important for N because shoot growth and dormancy are sensitive 

to N availability, and it may be difficult to set bud and induce 

dormancy if residual N from incorporated fertilizers continues to 

be released into the growing medium solution.

The problem of uneven distribution of incorporated fertilizers is a 

real concern, particularly with small capacity
Several commercial brands of especially formulated 

"slow-release" fertilizers are also available.

Limestone or dolomite. Because of their insolubility, Ca and Mg 

are difficult to supply in liquid fertilizers and are therefore added as 

calcitic limestone (Ca only) or dolomite (Ca and Mg) during the 

mixing of the growing medium. [Many nursery managers 

erroneously use the term "lime" when referring to limestone; lime 

is calcium oxide (CaO), whereas limestone is calcium carbonate 

(CaCO 3).] Recommended incorporation rates range from 3 to 5 

kg/m3 (5 to 8 pounds per cubic yard) (table 4.1.11). The particle 

size of the limestone is also impor27

containers. It is always difficult to achieve even mixing when 

working with materials of two different sizes or textures because 

such materials tend to separate during storage or handling. It may 

be hard to obtain even distribution of a small volume of small 

fertilizer particles in a much larger volume of larger, lighter-weight 

peat- vermiculite particles. This problem is made worse when dry 

fertilizer is added to a moist growing medium-the dry fertilizer may 

adhere to the wet particles of the me dium and not mix evenly. The 

even distribution of fertilizer particles is of special concern when 

dealing with the small-volume containers used in container tree 

seedling nurseries; the chances that each small 41- or 66-cm3 

(2.5- or 4-cubic-inch) container will receive the proper amount of 

fertilizer when a charge of dry bulk fertilizer is mixed into a batch of 

moist growing medium are relatively low.

Unnecessary mixing of growing media is always discouraged 

because of the fragile nature of the peat and vermiculite particles; 

overmixing of growing media can lead to reduction of particle size, 

compaction, and subsequent root growth problems. Many types of 

mixers do not perform an adequate job of mixing without damaging 

the texture of the mix and for that reason, incorporation of dry 

fertilizers should be done only during the manufacture of the 

growing medium. Matthews (1982) states that mixing equipment 

must be able to incorporate slow-release fertilizers into the 

growing medium without breaking or fracturing the fertilizer prills 

(the effects of overmixing growing media is discussed in the 

chapter on growing media in volume two of this series).

Coated slow-release fertilizers, such as Osmocote® and 

Nutricote®, should not be mixed into the growing me dium in 

advance of the actual sowing process, because leakage from the 

fertilizer pellets can raise the electrical conductivity (EC) of the 

growing medium solution to dangerous levels (fig. 4.1.13). 

Handreck and Black (1984) state that, whenever possible, media 

containing incorporated slow-release fertilizers should be used immediately after mixing. Other incorporated fertilizers, such as 

gypsum or dolomite, do not affect the salinity of the growing 

medium (fig. 4.1.13).

Macronutrient fertilizers used for incorporation. Several 

mineral nutrients can be mixed into the growing medium during the 

container filling process. The most common type of fertilizers used 

for incorporation into growing media contain nutrients that are 

relatively insoluble and do not leach readily, such as P, Ca, and 

Mg.
tant because the chemical effects on pH and nutrient release rate 

are faster with smaller particles. Agricultural grade dolomitic 

limestone generally has a particle size of 60 mesh or finer (Ruter 

and van de Werken 1986).

These fertilizers are sometimes added to peat-based growing 

media during the mixing process to raise the inherently low pH. 

Some manufacturers of commercial growing media routinely add 

limestone or dolomite to their mixes, so it would be wise to inquire 

about this practice when purchasing commercial media. Ruter 

and van de Werken (1986) and Whitcomb (1984) present a good 

discussion of all the aspects of this traditional practice and 

conclude that nursery managers should consider all aspects 

before adding dolomitic limestone to the growing medium. Factors 

to consider include:

1. Levels of Ca and Mg in the irrigation water source—Waters 

rated as "hard" contain significant amounts of calcium and 

magnesium. Ruter and van de Werken (1986) report that 

acceptable levels range

28

from 40 to 120 ppm Ca and 6 to 24 ppm Mg. Whitcomb 

(1986) states that irrigation water with only 40 ppm Ca can 

supply most, if not all, of the calcium needed for container 

plant production. Whitcomb (1984) presents a table to 

calculate the amount of dolomite to add to a growing medium 

for irrigation waters containing different amounts of Ca.

2. Ability to supply Ca and Mg in soluble forms-Calcium 

carbonate and dolomite are very insoluble in water (table 

4.1.12) and Whitcomb (1986) estimates that, under the 

irrigation regimes used in container nurseries, it would take 

over 7 years to release all the calcium in dolomite whereas 

the magnesium would be released after one-third of the 

growing season. This disparity in nutrient release rates could 

lead to an imbalanced calcium/magnesium ratio and resultant 

nutritional problems (Whitcomb 1984). Many nurseries inject 

all their Ca and Mg as the soluble fertilizers calcium nitrate 

and magnesium sulfate (table 4.1.12). Liquid fertilizer 

injection allows
the nursery manager to control both the total level and 

nutrient balance of these important macronutrients.

3. Individual species response to pH and Ca and Mg 

levels-some acid-loving species have relatively low 

requirements for these nutrients and can actually be harmed 

by high Ca levels.

Addition of dolomite to artificial growing media has resulted in 

growth problems with some conifer seedlings. Dangerfield (1978) 

found that the addition of dolomite to peat-vermiculite media 

induced lime chlorosis in Douglas-fir seedlings, and Hathaway 

and Whitcomb (1984) report that dolomite significantly reduced 

shoot height and weight of Japanese black pine.

Phosphorus. P is another nutrient that is sometimes 

incorporated into growing media. P can be supplied by either 

single or triple (concentrated) superphosphate (table 4.1.11). 

Single superphosphate (0-20-0) supplies calcium, phosphorus, 

and sulfur whereas triple superphosphate (0-45-0) contains no 

sulfur (Gingrich 1984).

Slow-release fertilizers. Other fertilizer types that are commonly 

mixed into growing media include slow-release or 

controlled-release formulations. Masta lerz (1977) and Sanderson 

(1987) provide an excellent discussion of the various types of 

slow-release fertilizers and their characteristics, and Sanderson 

(1987) groups them into five different categories, three of which 

are commonly used in container nurseries.

Coated water-soluble fertilizers. Such fertilizers consist of dry 

N-P-K fertilizer encapsulated in a plastic resin

sphere: the capsule allows water to pass in and dissolve the 

nutrients, which are then released osmotically to the seedlings 

(fig. 4.1.14). Osmocote, Nutricote, and sulfur-coated urea are 

examples of this group. Sulfur-coated urea is seldom used in 

forest tree nurseries because of concern about the nitrogen 

release rate, but Osmocote and Nutricote are used in some 

nurseries.

Both ammonium or nitrate forms of N are used in the various 

formulations. Irrigation frequency and growing medium 

temperature are the principal environmental factors controlling the 

rate of nutrient release (Sanderson 1987). The nutrient formulation 

also affects the release of nutrients; Crowley and others (1986) 

found significant differences in the cumulative release of fertilizer 

salts from three different Osmocote 8-9 month formulations (fig. 

4.1.15).

Osmocote is available in several different formulations of N-P-K, 

with release rates varying from 3 to 14 months. A new microprilled 

formulation of Osmocote and a formulation that contains all macroand micronutrients (Sierra®) are now available (fig. 4.1.14). 

Matthews (1983) recommended that Osmocote be used as the 

principal fertilizer for container tree seedlings grown outdoors and 

recommends incorporating the 18-6-12 formulation into the 

growing medium (table 4.1.11).

Inorganic fertilizers of low solubility. MagAmP® is a commercially 

available fertilizer of such low solubility that it can go through 

steam sterilization with little nutrient release. This fertilizer is 

available in two different particle sizes (coarse and medium), 

which is the factor controlling nutrient release. Nitrogen is supplied 

as magnesium ammonium phosphate (hence the trade name), but 

Sanderson (1987) reports that the N release rate is too slow for 

some ornamental plants.

Organic fertilizers of low solubility. This group is represented by 

the urea-formaldehyde fertilizers, such as Agriform® tablets and 

IBDU, which slowly decompose by hydrolysis or biological activity. 

The nutrient release of these fertilizers is controlled by the type of 

growing medium, medium pH, and temperature, and the microorganism population (Sanderson 1987). Consequently, nutrient 

release is poor with growing media at low temperatures and or 

with low microorganism populations, such as artificial growing 

media.

Micronutrient fertilizers used for incorporation. Several 

commercial micronutrient fertilizers can be incorpo-
rated into growing media (table 4.1.11). Micromax® is a balanced 

blend of micronutrients in a soluble sulfate form that is slowly 

released when it is mixed with the growing medium (Whitcomb 

1984). Fritted micronutrients (FTE No. 555®) are also frequently 

premixed into the medium; this fertilizer consists of glass shards 

that contain the full range of micronutrients. The micronutrients are 

then slowly released into the growing medium solution during the 

growing season, providing a constant, balanced micronutrient 

supply (Bunt 1976). Esmigran® is another micronutrient fertilizer 

that consists of a mixture of micronutrients adsorbed on clay particles, which slowly release the nutrients into the soil solution.

Several brands of chelated micronutrient fertilizers are available. 

Chelates are organically bound forms of the metal micronutrients 

(Fe, Mn, Zn, and Cu) that prevent these ions from being 

chemically tied up by other nutrients such as P, Ca, Na, and 

HC03. Chelated micronutrient fertilizers have proven to be 

effective but have sev eral drawbacks (Whitcomb 1983):

1. They are broken down by certain other micronutrients.

2. They are expensive.

3. The micronutrients in some chelates can be replaced by other 

elements, such as Ca, in the growing medium.

30

The advantages of chelated fertilizers over other soluble 

micronutrient fertilizers in container tree seedling nurseries have 

not been studied, although they should prove beneficial in 

nurseries with special problems such as saline irrigation water.

Broschat and Donselman (1985) investigated the extractability of 

Fe, Mn, Zn, and Cu from an artificial medium that had been 

amended with several micronutrient fertilizers and found that 

availability varied between the different micronutrients. Zn, Mn, 

and Cu were readily extractable with the sulfate, oxide or 

clay-adsorbed fertilizers, but Fe availability was restricted in the 

Esmigran® formulation. Chelated forms of the micronutrients 

(Sequestrene®) were effective, especially for Fe, although they 

leach out of the growing medium over time. Whitcomb (1983) 

studied the addition of different micronutrient fertilizers to artificial 

media and found that soluble fertilizers were more effective than 

fritted forms.

4.1.6.2 Injecting liquid fertilizer solutions into the

irrigation system

The most popular method of applying fertilizers in container tree seedling nurseries, and the one recommend

ed by the author, is direct injection of liquid fertilizers

into the irrigation system (fig. 4.1.16). The benefits of

this technique are considerable:
1. Precise control of both the concentration and balance of all 13 mineral nutrients in the applied irrigation water.

2. The ability to completely change the nutrient solution at any time.

3. A very low chance of overfertilization and resultant

salt injury.

Although liquid fertilizer injection is highly 

recommended, there are certain requirements for this 

procedure that may be drawbacks for small nursery 

operations:

1. The need for specialized injection equipment.

2. A higher labor cost associated with the frequent

mixing and applying of liquid fertilizers.

3. A well-designed, automated irrigation system is essential to insure even fertilizer application.

Types of nutrient injection systems. There is a good 

selection of nutrient injection equipment available to the 

container seedling grower that vary in terms of features and 

cost. The choice of a fertilizer injector is often influenced by 

cost: the price of a new injector can range from as little as 

$10 to over $10,000 (table 4.1.13). The injector must also fit 

into the overall irrigation system so the grower must choose 

a model that is compatible with the existing irrigation 

equipment (fig. 4.1.16).

Nelson (1978) discusses the components of a basic nutrient injection system (fig. 4.1.17). The injector should not 

be installed on the main irrigation line but on a by pass, so 

that ordinary irrigation water can be applied when desired. 

A backflow preventer is legally required in many areas to 

prohibit accidentally introducing fertilizer into domestic 

water systems. Automated nutrient injection systems can 

be designed to install a soluble salt me ter in the output line 

to monitor the strength of the applied fertilizer solution.

The major types of liquid fertilizer injection systems in

31
use in container tree seedling nurseries include the following 

(Mastalerz 1977, Furuta 1978):

Tank and pump system. This is an economical, yet me chanically 

simple, way to apply liquid fertilizers. The system consists of a 

large mixing tank, in which the liquid fertilizer is mixed at "applied 

strength;" and a pump to force the solution through the irrigation 

system. A major advantage of the tank and pump system is that it 

is impossible to over fertilize and burn seedlings because the 

solution is always at applied strength. A major drawback is that the 

fertilizer solution tank must necessarily be large to hold enough 

application strength fertilizer to cover the entire area to be 

fertilized.

Venturi suction injector. This is one of the oldest and simplest 

types of injection systems and operates on the principle of the 

venturi tube. When water is forced through a narrow section of 

pipe, it gains velocity and creates a reduction in water pressure. 

When a smaller diameter side tube is attached at this narrow 

section, the

32
33

reduced pressure will cause a liquid in the side tube to be sucked 

into the irrigation pipe. These venturi injectors can therefore be 

used as liquid fertilizer injectors; they are accurate as long as 

water flow and pressure are consistent, but the injection rate may 

change with variable conditions. The Hozon® injector, a 

commercially available example of this type, injects at a ratio of 

approximately 1:16 (table 4.1.13).

Displacement injector. This type of injector, exemplified by the 

Gewa®, operates on the basis of a portable tank with a collapsible 

plastic liner. The fertilizer solution is placed into the lined tank, 

which is then sealed and connected to the irrigation system. The 

pressure of the water entering the tank forces the fertilization solution out of the plastic liner and into the irrigation line at a specified 

injection ratio. The Gewa injector has a variable injection ratio that 

can be set from 1:20 to 1:300 (table 4.1.13).

Positive displacement pump injector. Most of these injectors 

use water-driven pumps to inject a fixed amount of fertilizer 

solution into the irrigation line and operate at a speed that is 

proportional to water flow. They meter accurately at any line 

pressure or flow rate and generally do not require a mixing tank. 

There are several brands of positive displacement pump-type 

injectors available and the Smith Measuremix® injector is a typical 

example; they are generally more expensive than other types and 

are available in a variety of injection ratios (table 4.1.13).

Commercial fertilizers for liquid fertilizer injection.

There are several types of completely water-soluble fertilizer that 

are commercially available for use in liquid injection (fig. 4.1.18). 

Some formulations contain only N-P-K, whereas others also 

contain S and the micronutrients. These fertilizers may not 

contain Ca or Mg because these nutrients form insoluble 

precipitates in concentrated fertilizer stock solutions. The nutrient 

ratios are fixed, which means that although the total nutrient concentration can be changed there is no way to adjust the balance 

between nutrients. Commercial mixes are relatively expensive but 

are easy to use; another advantage is that only one stock solution 

is necessary and therefore a less expensive injector with only one 

head can be used.

Custom fertilizers for liquid fertilizer injection. Many growers 

choose to formulate their own custom fertilizer mixes from 

technical or fertilizer grade chemicals to supply all 13 mineral 

nutrients (fig. 4.1.19). These separate chemicals are generally 

cheaper than commercial fertilizers but require more storage 

space because more bags of chemicals are necessary. The 

custom mixes have to be made up regularly so there is more labor 

required compared to using commercial fertilizers, and most custom mixes require a nutrient injector with two heads. The primary 

appeal of custom mixes is that a complete and balanced fertilizer 

program can be designed around the nutrients already in the 

irrigation water and that the concentration of each nutrient can be 

individually adjusted at any time during the growing season. The 

calcu-
lations to formulate a custom fertilizer mix are relatively 

complicated compared to the commercial fertilizer mixes, but once 

they are completed, the actual mixing of the fertilizer stock 

solutions is simple.

Foliar fertilization. Liquid fertilizer injection has also been used to 

apply foliar fertilizers and has found limited application in 

ornamental seedling nurseries. Foliar sprays can be used to treat 

minor deficiencies of some mineral nutrients but can never be 

used as the only fertilizer source because mineral nutrient uptake 

rate is very much slower through leaf tissue than through the root 

system. Foliar fertilization should logically be more effective with 

broadleaved species because the waxy cuticle found on many 

conifer needles would slow nutrient uptake. Alexander and 

Schroeder (1987) present a good review of the practical 

applications of foliar fertilization in horticulture.

Foliar fertilizers are applied as dilute solutions. In formulating foliar 

fertilizers, urea is the preferred source of N. Handreck and Black 

(1984) present foliar spray recipes for N, K, and some 

micronutrients (Fe, Zn, Mn, Cu, and Mo). Foliar fertilizer 

formulations are also available iron, some commercial suppliers. 

Application equipment must be able to produce a fine mist, and a 

surfactant is often used to make sure that the fertilizer solution is 

spread evenly over the foliage surface. Foliar sprays are usually 

applied under humid conditions, often in the evening so that the 

solution remains in contact with the foliage for an extended period 

of time. Some nursery

34

managers allow the regular applied fertilizer solution to remain on 

the seedling foliage to promote foliar uptake. This practice is 

discouraged because the more concentrated standard ferti lizer 

solutions may burn succulent foliage. In container tree seedling 

nurseries, foliar fertilization has primarily been used to treat 

micronutrient deficiencies such as iron chlorosis but can also be 

used to provide a quick "green-up" before seedlings are shipped 

(fig. 4.1.20). As with all new cultural techniques, growers who 

want to test foliar fertilization should try the procedure on a small 

scale first.
monitoring fertilization is discussed in more detail in section 

4.1.9.

4.1.7.2 Controlling irrigation water pH

Once the base nutrient level of the water is known, the buffering 

capacity of the water should be determined by acid titration. 

Titration is a process in which small increments of an acid are 

added to a sample of irrigation water to determine the amount of 

acid that will be required to lower the pH of the water to the 

desired level (pH 5.5). Titrations can be done by any water testing 

lab or by nursery personnel using a pH meter and a burette or 

pipette. Any acid can be used for titrating as long as the normality 

is known so that conversions for relative acidity can be made.

35

4.1.7 Formulating Liquid Fertilizer Solutions

4.1.7.1 Determining the nutrient status of the

irrigation water

Regardless of the type of fertilizer used, the first step in

developing a liquid fertilizer injection program is to determine which nutrients are present in the irrigation water and in what concentration (Vetanovetz and Knauss

1988). Unfortunately, this important step is often neglected by many growers because they mistakenly assume that irrigation water is uniform in quality. Natural

waters usually contain appreciable concentrations of

several plant nutrients, especially Ca and Mg, and some

"hard" waters may contain enough of these nutrients to

partly or even completely supply plant requirements

(Jones 1983, Whitcomb 1986). The Container Nursery

Survey found that only a small percentage of container

nursery managers ever analyzed their irrigation water for

the complete range of mineral nutrients.

Mineral nutrient analyses of irrigation water can be performed by 

most analytical testing laboratories, but growers should be sure to 

specify that they want a nutrient analysis, instead of a standard 

water quality test. Actu ally, since many of the ions are the same in 

both tests, it would be wise to have water quality checked at the 

same time. A complete water analysis for both nutrients and 

quality should cost around $50 to $100. (Water quality testing is 

discussed in more detail in section 4.2.4.2.)

A nutrient analysis of the irrigation water at Mt. Sopris Nursery, 

Carbondale, CO, is listed in table 4.1.14. The concentrations of all 

the nutrient elements in the irrigation water are reported in parts per 

million. When requesting a water test, all nutrients should be 

reported in parts per million although the lab report may give the 

values in milligrams per liter (mg/I), an equivalent unit of measure 

(see table 4.1.6). In table 4.1.14, both the NO3

- and NH4+ forms of 

N were analyzed but, for most purposes, a test for total N is 

sufficient. Note that some macronutrients, such as P, and many 

micronutrients may exist at very low concentrations in natural 

waters, some of which may be too low to be detected by the 

analytical instruments. The pH and electrical conductivity (EC) of 

the water are routinely reported along with the mineral nutrients. 

Electrical conductivity is the relative concentration of dissolved salts 

in the irrigation water, and is reported in units of micro-Siemens per 

centimeter (μS/cm). Each nursery should have its own pH and EC 

meters and should perform its own tests regularly to monitor 

changes in water quality;
To keep the calculations simple, one procedure is to use a 1 % 

phosphoric acid (H 3PO 4) solution, because either 75 or 85% 

phosphoric are often used in custom fertilizer mixes to lower pH 

and add nutrient F A 1 H3P04 solution can be made from stock 

85% H 3PO4 by adding 11.75 ml of 85% acid to enough distilled 

water to make up 1 liter of solution. For safety reasons, always add 

acid to water: partially fill the flask with water, add the acid to it, 

and then add the rest of the water. Once the amount of 1 % H 3PO4

needed to lower the pH of the water sample is known, the 

conversion to either 75 or 85% stock acid solution is made by 

dividing by either 75 or 85.

Titration curves for the irrigation water at Mt. Sopris Nursery and 

the Colorado State Forest Service Nursery in Ft. Collins, CO, are 

given in figure 4.1.21. Note the difference between the two curves: 

the steeper the slope of the line, the lower the buffering capacity 

of the water. The water at the Colorado State Nursery has a very 

low buffering capacity and requires only 3 ml of H 3PO4 to lower 

the pH of 1 liter of irrigation water to the desired level, whereas the 

Mt. Sopris Nursery water requires almost 16 ml of 1 % H 3PO4. 

Because the amount of acid will probably need to be adjusted for 

seasonal changes, regular pH monitoring is necessary. The pH 

will also change after the fertilizer chemicals have been added to 

the fertilizer solution, so other minor adjustments may be 

necessary.

Several acids have been used for acid injection in container tree 

nurseries including nitric, sulfuric, phosphoric, and even acetic acid. Phosphoric acid should be used 

whenever possible because 1) it is the safest acid to handle, 2) it 

does not react violently with water or produce toxic fumes, and 3) 

it is a source of fertilizer P Jaramillo and Owston (1977) concluded 

that phosphoric acid is safest to use even though it is more 

expensive and recommended "food" or "reagent" grade because 

other grades may contain phytotoxic heavy metals. Considering 

how little acid is used in acidification, however, the possibility of a 

serious phytotoxicity problem is remote. (For a list of chemical 

grades, see section 4.1.7.4.) If the irrigation water is very alkaline, 

sulfuric acid may have to be used because the amount of 

phosphoric acid that would be required to lower the pH will add an 

excessive amount of P to the solution.

4.1.7.3 Liquid fertilizer calculations for commercial

fertilizers

Based on the results of the Container Nursery Survey,

88% of the container nurseries in North America use

commercial brand fertilizers, either alone or in combination with custom mixes for injection into the irrigation

system. Some fertilizer brands contain both macronutrients and micronutrients whereas others contain only

the major fertilizer elements. Check the label to deter

mine the exact nutrient composition; nutrients supplied

by a typical fertilizer (Peters Conifer Grower) at a

100 ppm N rate are listed in table 4.1.15.

36
37

6. Using the conversion factor from table 4.1.4, convert from 

the oxide form (P 2O 5) to the elemental form:

35 PPM P 2O 5x 0.4364 = 15 ppm P

7. Using a similar process as in steps #5 and 6, compute the 

amount of K that the 20-7-19 fertilizer supplies to the applied 

fertilizer solution (the fertilizer contains 19% K 2O):

500 mg/I x 0.19 = 95 ppm K 2O

95 ppm K 2O x 0.8301 = 79 ppm K

Record the concentrations of the other nutrients that are listed on 

the fertilizer label for that N concentration. For example, the actual 

nutrient concentrations for a typical fertilizer at 100 ppm N are 

listed in table 4.1.15.

A set of tables has been developed to quickly and easily 

determine the amount of bulk fertilizer to add for a range of 

desired N concentrations and for different nutrient injector ratios 

(table 4.1.17). Remember that these tables only provide N levels 

and that levels of P, K, and other nutrients are determined by the 

chemical make-up of the bulk fertilizer.

Macronutrients. An example of the nutrient content and mixing 

instructions for a typical fertilizer is provided in table 4.1.16. Note 

that the nutrient analysis is given in total nitrogen (N), available 

phosphoric acid (P 2O 5), and soluble potash (K 20). The two latter 

terms can be converted to %P and %K using the conversion 

factors in table 4.1.4. The weight of fertilizer to add to 1 liter of 

water to make 1 liter of liquid fertilizer concentrate are given at the 

bottom of table 4.1.16. To calculate the concentrations of all the 

nutrients, use the following procedure:

1. Set the target N level for the applied fertilizer solu

tion (100 ppm, for example).

2. Determine how much bulk fertilizer must be used to produce 

the target concentration (100 ppm)-the fertilizer in our 

example is 20-7-19, or 20% N:

100 ppm = 100 mg/I = 0.20

= 500 mg/I bulk fertilizer

3. Adjust for the nutrient injection ratio (1:200, for

example):

500 mg/I bulk fertilizer x 200

= 100,000 mg/I bulk fertilizer

4. Convert from milligrams per liter to grams per liter:

100,00 mg/I = 1,000 mg/g = 100 g/I bulk fertilizer

If using English units, convert grams per liter to ounces 

per gallon:

100 g/I x 0.1334 = 13.34 ounces of bulk fertilizer

per gallon water

(Note that this value agrees with the value in the mixing 

instructions in table 4.1.16.)

5. Now that we have established the amount of 20-7-19 bulk 

fertilizer (step #2) needed to supply our N target (step #1), 

we need to calculate how much P will be contained in the 

applied fertilizer solution (the fertilizer contains 7% P 2O 5):

500 mg/I x 0.07 = 35 ppm P 2O 5
Micronutrients. Some commercial fertilizers do not contain a 

complement of micronutrients. Supplemental fertilization with 

micronutrient fertilizers is usually necessary, therefore, because 

artificial growing media contain few, if any, micronutrients (see 

section 4.1.3.1). Iron deficiency is one of the most common 

micronutrient deficiencies in container tree seedling nurseries and 

is expressed as chlorosis and stunting (see fig. 4.1.7A). Scarratt 

(1986) reports mild chlorosis and low foliar Fe levels for container 

jack pine grown under commercial "general-purpose" fertilizers. 

Biweekly applications of ferrous sulfate are prescribed to 

supplement standard commercial fertilizers and prevent iron 

chlorosis in British Columbia nurseries (Matthews 1982). Chelated 

iron fertilizers are sometimes preferred over iron sulfate because, 

although more expensive, chelates are more avail38

able for plant uptake under a wider range of growing media 

conditions. Copper deficiency may also be a problem with 

peat-based growing media and deficiency symptoms have been 

identified in some conifer seedlings (see figure 4.1.7B). Scarratt 

(1986) reports that, in a chemical analysis of a standard 

peat-vermiculite growing medium, copper was the only 

micronutrient that was totally absent.

Several soluble micronutrient fertilizers are available in either 

single element or micronutrient mixtures; mixes are 

recommended over individual micronutrient fertilizers unless a 

specific deficiency has been identified. Overfertilization with a 

single-element fertilizer can result in direct toxicity or a nutrient 

imbalance that can inhibit the uptake of other micronutrients. 

Micromax® is
39

The specific chemicals for custom fertilizer mixes are chosen 

based upon four criteria:

1. Availability and ability to supply the desired nutrient-Commonly available soluble chemicals suitable for use as 

fertilizer are listed in table 4.1.19; note that some nutrients, 

such as N, are available in several soluble compounds 

whereas others, such as Ca, have only one soluble form.

2. Solubility in water and compatibility with other nutrient 

chemicals-The solubilities and compatibilities of some of the 

major fertilizer chemicals are compared in fig. 4.1.22.

3. Cost per supplied nutrient-Several chemicals contain more 

than one nutrient and are therefore more cost efficient and 

contain fewer unwanted ions. Multiple nutrient chemicals 

make fertilizer calculations slightly more complicated, 

however, because of the need to keep the other nutrients in 

balance.

4. Absence of potentially toxic ions-Some commonly used 

fertilizer chemicals contain undesirable secondary ions in 

addition to the nutrient ion. A good example of this is KCI, 

which contains 47% CI; because of the high concentration of 

K+ desired in fertilizer solutions, the accompanying CI- ions 

could become toxic.

Mixing the stock fertilizer solutions. The fertilizer chemicals are 

dissolved in water to form concentrated nutrient stock solutions. It 

is a good idea to use hot water for the stock solutions, because 

most chemicals disa sulfate-based mixture that is generally incorporated into the 

medium but can be used as a liquid topdressing if the foliage is 

rinsed immediately after application (Whitcomb 1984). Soluble 

trace element mix (S.T.E.M.®) is a mixture of sulfur and six 

micronutrients that is derived from sulfates and other inorganic 

chemi cals. Many nutrition specialists feel that chelated forms of 

micronutrients remain more available in the medium, and a mix of 

soluble chelated micronutrients (Compound 111®) is available for 

liquid fertilizer injection. Several commercial formulations of 

individual chelated micronutrients are also available, such as 

Sequestrene 330 Fe® (EDTA chelate) and Sequestrene 138 Fe® 

(EDDHA chelate).

Application rates are provided on the fertilizer label, usually 

specified as amount of micronutrient fertilizer to use per amount of 

macronutrient fertilizer (micronutrient fertilizers are also discussed 

in section 4.1.4.3).

4.1.7.4 Liquid fertilizer calculations for custom

fertilizer mixes

Selection of chemicals. Custom fertilizer mixes utilize

bulk chemicals to supply all the mineral nutrients necessary for tree seedling growth. There are several grades of

commercial chemicals that are classified according to

use (table 4.1.18); the technical or purified grades are

most practical for custom fertilizer mixes in terms of purity and cost. Fertilizer grade chemicals are not recommended for soluble fertilizer mixes because they contain

a relatively high percentage of impurities. The Container

Nursery Survey found that 12% of the container nurseries used custom mixes exclusively and another 6% used

them in combination with commercial fertilizers.
solve more quickly and completely in hot water. Masta lerz (1977) 

recommends preparing the stock solution in one container with 

hot water and then siphoning or pouring the solution into a second 

container so that undissolved particles are screened out. Stock 

solutions are also highly corrosive and so the stock solution tanks 

should be plastic rather than metal, with covers to keep debris out 

of the solution.

40

For safety's sake, always add the acid component to the solution 

first. Certain nutrients, especially Ca and Mg, cause problems in 

concentrated stock solutions because they form precipitates with 

other nutrients such as sulfates or phosphates: Ca + SO 4 = 

CaSO 4 (gypsum) (fig. 4.1.23). Three separate stock solutions are 

often used to supply all nutrients while maintaining the optimum 

solubility of chemicals in each solution. Note that Ca is
41
added to the micronutrient solution because it will form 

precipitate s with P or SO4 in the macronutrient solution:

Stock solution 1 (SS#1). Dilute acid—lowers the irrigation 

water pH and, if phosphoric acid is used, supplies P.

Stock solution 2 (SS#2). Macronutrients minus Ca---

supplies all macronutrients except Ca, which must be 

added to SS#3 for solubility reasons.

Stock solution 3 (SS#3). Micronutrients plus Ca—supplies all micronutrients and Ca.

In this example, a complete fertilizer solution would consist 

of stock solution #2 and stock solution #3, each

42

injected through its own injector head. Several nutrient injectors 

are available with two heads and they can be ordered with 

different injection ratios. The macronutrient solutions are usually 

injected at a 1:200 ratio whereas the micronutrient solutions can 

be more concentrated and injected at ratios as high as 1:500.

Examples of calculations. The following example was 

designed for Colorado State Nursery irrigation water, using a 

modification of the procedure from Tinus and McDonald (1979). 

The nursery uses a Smith® injector with two 1:200 heads and 

stock solution tanks of 200 liters (50 gallons) capacity:

SS#1 (dilute acid). Because the irrigation water at many nurseries 

is neutral to slightly alkaline, this stock solution can be injected 

with every irrigation to lower the water pH to 5.5, the ideal level for 

conifer seedling growth. Constant acid irrigation will keep the pH of 

the growing media low during seed germination, thus helping to 

reduce the occurrence of damping-off diseases. Phosphoric acid is 

commonly used to lower irrigation water pH because it also 

provides a constant supply of P, which is beneficial during early 

seedling growth before normal fertilization is initiated. If the 

irrigation water is very alkaline then a stronger acid, such as 

sulfuric acid, is sometimes used in addition to the phosphoric acid. 

Use of this acid solution with each irrigation also keeps calcareous 

deposits from forming on the irrigation nozzles in areas with "hard" 

water.

The proper amount of acid to use is determined by an acid 

titration of the greenhouse irrigation water using phosphoric or 

sulfuric acid (see section 4.1.7.2). Based on an average of three 

separate titrations with 1 H3PO4, 3.2 ml of 1 % acid lowered the 

pH of 1 liter of irrigation water to 5.5 (fi g. 4.1.21). Using 

commercial 85% H3PO4, it would take 1/85 as much of the 

concentrated acid to treat 1 liter of greenhouse water:
43

The recipe for SS#1, therefore, will be to add 1.5 liters of 85% 

H3PO4 to enough irrigation water to make 200 liters of stock 

solution.

The accuracy of these computations should be checked by 

collecting some of the applied irrigation water and testing its pH (a 

direct test of the pH of the stock solution is not indicative of the pH 

of the applied irrigation water). Because the quality of irrigation 

water will change during the year, the amount of acid added to the 

stock solution may have to be adjusted occasionally.

SS#2 (macronutrients minus Ca). The computations for this 

fertilizer solution are provided in table 4.1.20A. (A blank form is 

provided in table 4.1.20B.) The upper portion shows the target 

nutrient concentrations in parts per million, the amount of each 

nutrient in the irrigation water and the amount needed to add as 

injected fertilizer. The chemicals used to supply nutrients and their 

contribution in parts per million are shown in the left column. The 

final column on the right shows the total amount of the chemical 

that would be present in the applied fertilizer solution as it comes 

out of the irrigation nozzle.

The total parts per million of each nutrient must be converted to 

the weight of the chemical that needs to be added to each liter of 

water. This conversion is simple because 1 liter of water weighs 1 

kg by definition. Therefore, on a weight per volume basis, 1 mg/I 

equals 1 ppm.
44
45
46
The addition of MgSO4 completes the nutrient formula because the 

concentrations of all nutrients in the "totals" row reasonably meet 

the target concentrations (table 4.1.20A).

The recipe for all the ingredients is given in the "applied solution" 

column in table 4.1.20A; this is the actual concentration of fertilizer 

that is applied to the seedlings. These values are carried down to 

the "Applied solution" column at the bottom of the table, where the 

conversions are made for the nutrient injector and the stock 

solution. The adjustment for the nutrient injector (1:200) consists 

of multiplying the applied solution values by 200 and then converting milligrams to grams. The 

calculations for MgSO4 will serve as an example:

380 mg/I x 200 = 76,000 mg/I = 76 g/I

The next step again involves multiplying by 200 to compute how 

much chemical is needed for the 200-liter stock solution tanks and 

converting to kilograms:

76 g/I x 200 = 15,200 g = 15.2 kg

Remember that the CaNO3 will not be added to this stock 

solution (because of solubility problems) but to the 

micronutrient solution (SS#3).

SS#3 (micronutrients plus Ca). The calculations for this solution 

are basically the same as for SS#2, using the information in table 

4.1.24 to determine the amount of nutrient that each chemical 

provides. Chelated forms of the metallic micronutrients (Fe, Mn, 

Cu, and Zn) are more expensive than the sulfate forms but are 

thought to be more available under a range of soil conditions. 

When peat-vermiculite growing media are used, however, 

micronutrient availability should not be a problem.

47
48
49

The final recipes for all three stock solutions are given in table 

4.1.25, which shows the amount of each chemical that should be 

added to make 200 liters of solution.
4.1.8 Applying and Scheduling Liquid

Fertilizer Solutions

4.1.8.1 Application techniques

Once the stock fertilizer solutions have been prepared, the next 

step is to inject them into the irrigation system. It is always a good 

idea to stir the stock solution tanks each time before injection to 

insure that all the chemicals are well-mixed and in solution. If 

there is an excessive amount of sediment at the bottom of the 

container (more than a couple of inches), then the fertilizer solution should be reformulated because some of the fertilizer 

chemicals are reacting with the natural Ca" and Mg 2, ions in the 

irrigation water to form precipitates (fig. 4.1.23).

Liquid fertilizer applications should be scheduled as early as 

possible in the day to allow the foliage time to dry before nightfall, 

so that the possibility of foliage diseases such as botrytis blight is 

reduced. The key consideration in applying liquid fertilizers is to 

apply enough solution each time to completely saturate the 

growing medium profile and flush out excess fertilizer salts. 

Mastalerz (1977) recommends that an extra 10% of the solution be 

applied each time to insure that container capacity has been 

reached and that the solution drains from the container. He cites a 

figure of 20 liters of solution per m2 (0.5 gallons per square foot) of 

greenhouse bench space to properly fertilize a growing medium of 

approximately 15 cm (6 inches) depth. Carlson (1983) suggests 1 

liter (0.26 gallon) of solution per 100 cavities of 40 cm3 (2.5 cubic 

inches) capacity. Many growers have their own system for 

regulating the amount of fertilizer solution that is applied such as 

controlling the time that the sprinklers remain on, specifying the 

number of times that the irrigation boom passes over the benches, 

or limiting the quantity of solution that is applied during each 

fertilizer application.

The final step in liquid fertilizer injection is a "clear water rinse" to 

wash the fertilizer solution off the foliage to prevent possible 

"fertilizer burn." This is particularly important with certain fertilizers 

such as potassium chloride (KCI) that have a high- salt rating (see 

table 4.1.5). The duration of this rinse will vary with the size of the

seedlings but usually takes only 15 to 20 seconds (Carlson 1983).

4.1.8.2 Timing of the first fertilizer application

Traditionally, nursery managers delay the first application of 

fertilizer until the germinant has become established usually when 

the seed coat is shed from the cotyledons of conifer seedlings (4 

to 8 weeks after sowing). The reason given for this fertilization 

delay is that damping-off fungi

50

are stimulated by the fertilizer (Tinus and McDonald 1979) or that 

the concentrated fertilizer solution may "burn" the succulent 

germinants. If good growing media and proper sanitation 

procedures are used, however, damping-off should not be a 

serious problem in container tree seedling nurseries, and fertilizer 

burn should not be a problem if the fertilizers are promptly rinsed 

from the seedling.

Some nursery scientists feel that early fertilization is not 

necessary because the seed endosperm contains enough 

nutrition for initial growth and establishment. Carlson (1983) 

states that newly germinated seedlings take up few mineral 

elements until 10 to 14 days after germination. Barnett and 

Brissette (1986), however, report that a delay in initial fertilization 

can have a considerable effect on seedling development: a 

3-week delay can decrease loblolly pine dry weight by nearly 20% 

(fig. 4.1.24).
Many nursery managers incorporate a small amount of dry or 

slow-release "starter" fertilizer into the growing medium to 

support the germinant until liquid fertilizers can be applied. 

Brix and van den Driessche (1974) have reported that 

container seedling growth during the first few weeks could be 

helped with the incorporation of slow-release fertilizer into the 

growing medium. Edwards and Huber (1982) state that some 

nursery managers in the prairie provinces of Canada begin 

liquid fertilizer applications with high P formulations at 1 week 

after sowing to stimulate root development. Etter (1971) 

found that, although high N or P levels did not increase root 

growth of white spruce during the first 6 weeks of growth, a 

fertilization program using 50 to 60 ppm N produced 

significant increases in foliar and total seedling dry weight.

Observations made by Barnett and Brissette (1986) indicate 

that the timing of the initial fertilizer application should be 

reevaluated and that the starter fertilizer practice may have 

merit (fig. 4.1.24). Application of dilute liquid fertilizer during 

the germination and emergence period may stimulate growth 

without harmful side effects. Early fertilization, like any 

change in cultural practice, should be first implemented on a 

small scale to see if it is practical on an operational basis.

4.1.8.3 Scheduling liquid fertilizer applications during

the growing season

There are two basic ways to apply liquid fertilizers: constant 

and periodic fertilization. The application of a dilute fertilizer 

solution each time the crop is irrigated is known as constant 

fertilization (Mastalerz 1977); the concentration of this applied 

fertilizer solution is exactly the nutrient concentration desired 

in the growing me dium solution. Periodic fertilization consists 

of applying a more concentrated fertilizer solution according 

to some fixed schedule, such as once a week or every other 

irrigation. The applied fertilizer solution during periodic 

fertilization may therefore be several times more 

concentrated than the constant fertilization solution, which is 

the same as the nutrient levels desired in the growing 

medium solution. An example of a periodic fertilization 

schedule is given in table 4.1.26. The Container Nursery 

Survey found that 64% of the nurseries used periodic 

fertilization, 25% preferred constant fertilization, and the 

remaining 11 % scheduled fertilization based on their 

monitoring of crop development or growing medium nutrient 

levels.

A conceptual view of the effect of constant and periodic fertilization 

on the nutrient level in the growing medium solution or the 

seedling foliage is charted in figure 4.1.25. Because a more 

concentrated fertilizer solution is applied during periodic

fertilization, the nutrient level in the growing medium solution will 

fluctuate more than it would during constant fertilization. If this 

fluctuation reaches extreme levels, seedling growth could be reduced because of nutrient deficiencies or toxicities. Constant 

fertilization keeps the nutrient concentrations in the growing 

medium closer to the target concentrations that should accelerate 

seedling growth. Gingrich (1984) states that periodic fertilization is 

becoming less popular in container nurseries because of the large 

fluctuations in nutrient levels and EC in the growing medium 

solution.

Periodic application of liquid fertilizers is widely practiced in 

container tree seedling nurseries and obviously produces 

acceptable seedlings. This cultural practice should be carefully 

monitored, however, to avoid salt build-up in the growing medium 

solution (see section 4.1.9), and foliage rinses should always 

follow fertilization. Scarratt (1986) found that both constant and 

periodic fertilization produced good seedling growth but the 

moisture level of the growing medium was carefully monitored and 

leached to avoid salt build-up.

Mullin and Hallett (1983) discussed the two fertilizer application 

techniques and listed the following advantages to the constant

fertilization (replacement) technique:

1. Regular flushing of the growing medium prevents

the build-up of fertilizer salts.

51
2. Nutrients are evenly distributed throughout the me

dium profile.

3. Nutrient levels in the growing medium solution can

be changed quickly to correct imbalances.

4. Crops cannot be overfertilized because the applied solution is 

exactly the proper concentration for ideal growth.

5. Growing medium nutrient levels are returned to target 

specifications with each liquid fertilizer application.

Disadvantages of constant fertilization are slightly higher costs, in 

terms of fertilizer chemicals and labor, and the problem of 

disposing of the fertilizer solution runoff. Because the numerous 

advantages outweigh the few disadvantages, the constant 

fertilization procedure for liquid fertilizer application is usually 

preferable.

52

Exponential fertilization is a third way of applying liquid fertilizers 

that has recently been tested in container tree seedling nurseries. 

Exponential fertilization consists of starting with a low fertilization 

rate when the seedlings are small, and increasing the amount of 

fertilizer at an exponential rate that is commensurate with 

seedling growth. Timmer and Armstrong (1987b) showed that a N 

rate that was gradually increased from 5 to 125 ppm N over the 

fertilization period produced better seedling growth, particularly 

root growth, compared to conventional fertilization techniques. 

Exponential fertilization would also have other operational 

benefits, such as less chance of salinity buildup in the growing 

medium and better fertilizer-use efficiency, than traditional 

constant fertilization methods. On the negative side, exponential 

fertilization schedules are more complicated to compute, and the

applications would be more time-consuming than conventional 

liquid fertilizer applications (Timmer and Armstrong 1987b).
53

4.1.9 Monitoring Container Seedling

Nutrition

Because proper mineral nutrition is so important to seedling 

growth in conta iner tree seedling nurseries, growers should 

regularly monitor mineral nutrient levels at various phases during 

the fertilization operation, and nutrient levels in the seedlings 

themselves. Growers can monitor several different factors during 

the fertilization procedure (table 4.1.27).

4.1.9.1 What to measure when monitoring

fertilization

Mineral nutrient levels can best be monitored by measuring the 

electrical conductivity (EC) and pH of the various fertilizer 

solutions, and through chemical analyses of the solutions for 

specific nutrient levels (table 4.1.27).

Electrical conductivity. EC is a measure of the salinity (total salt 

level) of a solution and therefore gives an indication of the fertilizer 

salts that are present. An EC meter measures the electrical charge 

carried by the ions that are dissolved in a solution-the more 

concentrated the ions, the higher the reading (figure 4.1.26A). EC 

meters read salinity in units of conductance called mhos (pronounced "mows") or Siemens, which are the SI units. Most testing 

instruments measure millimhos (1/1,000 mho) or micromhos 

(1/1,000,000 mho) and must be adjusted for solution temperature. 

Following the SI convention, the EC units in the manual will be 

microSiemens per centimeter (μS/cm), which are equivalent to 

micromhos per centimeter.

Mineral nutrient levels . Chemical analysis of the nutrient 

solutions can be performed by testing laboratories or with portable 

testing kits. Laboratory tests are the most accurate but are 

time-consuming and costly. Several companies have developed 

testing kits for horticultural

use, but their use requires experience for meaningful 

interpretation. The authors recommend that growers use testing 

services whenever possible because the cost of the analysis is 

relatively small when compared to the value of the seedling crop.

pH. The relative acidity of the untreated irrigation water and 

applied irrigation solutions can be monitored with a pH meter (fig. 

4.1.26B). Monitoring the pH of fertilizer solutions can tell you 

whether the fertilizer stock solutions are properly formulated and 

whether the fertilizer injector is functioning, but is of little value in 

determining mineral nutrient levels. The pH of the growing medium 

solution changes with the uptake of certain ions such as K+, NH4

+, 

and NO3

-

, the production of carbon dioxide by root respiration, and 

the release of root exudates.

4.1.9.2 Stages to monitor during fertilization

Mineral nutrient levels can be monitored at five different steps 

during the fertilization process, starting with the native nutrient 

salts in the irrigation water and ending with the leachate solution 

that drains from the bottom of the container (fig. 4.1.27). The 

techniques for monitoring fertilization in nurseries differs, 

depending on whether solid fertilizer incorporation or liquid 

fertilizer injection is employed. Nurseries that inject liquid fertilizers 

can monitor the fertilization process at all five steps, whereas 

those incorporating solid fertilizers are restricted to only three of 

the five (table 4.1.27).

Irrigation water. The irrigation water should be checked for EC, 

mineral nutrient concentrations, and pH; this should be done 

during the initial fertilizer solution calculations (see section 

4.1.7.1). Mineral nutrient
levels in the irrigation water will normally not change appreciably 

over time, but EC and pH should be tested weekly. (Irrigation 

water quality standards are discussed in more detail in section 

4.2.4.2.)

Fertilizer stock solution. The efficiency of the nutrient injector can 

be checked by making an "applied strength" dilution of the fertilizer 

stock solution and measuring the EC level (table 4.1.27). For a 

1:200 injector, add one part of stock solution to 200 parts of 

irrigation water; the EC reading of the diluted fertilizer solution 

should be approximately the same (within 10%) as the EC of the 

applied fertilizer solution. If the actual injection ratio is consistently 

different than the stated ratio, then the stock solution formulas can 

be recalculated for the actual ratio. If the measured injection ratio 

varies excessively, then the injector is not functioning properly and 

may need to be serviced. Erratic changes in water pressure can 

sometimes cause nutrient injectors to malfunction; water pressure 

regulators can be installed in the incoming water line to moderate 

pressure changes.

Applied fertilizer solution. The applied fertilizer solution is the 

most important of the fertilization checks because this solution 

actually contacts the seedling foliage and enters the root zone 

(fig. 4.1.27). The applied solution is collected directly from the 

irrigation nozzle (fig. 4.1.28) and should be checked for EC, 

nutrient concen54

trations, and pH (table 4.1.27). The EC reading of the applied 

fertilizer solution should be approximately the sum of the base 

salinity of the irrigation water plus the salts added by the 

fertilizer stock solution. The pH of the applied solution should be 

near the target pH of 5.5, or adjustments should be in the 

amount of acid added to the fertilizer stock solution.

The nutrient concentration of the applied fertilizer solution should 

be determined by submitting a sample to an analytical lab, and 

these values should be compared to the target values. As an 

example, test results from Mt. Sopris Nursery (see table 4.1.14) 

show that, although most of the applied solution levels are 

reasonably close to the target levels, the N-P-K levels are 

consistently low. A check of the injector efficiency revealed that 

the actual injection ratio was closer to 1:220 instead of 1:200, 

which explains the discrepancy. The SO4 level is high in the 

applied solution because extra SO4 was added with the 

ammonium sulfate used to supply ammonium.

Growing medium solution. This is the fertilizer solution that 

surrounds the seedling root system in the container (fig. 4.1.27), 

and so it is one of the most critical solutions to monitor. The EC 

and pH of the solution in the growing medium are essential 

measurements (table 4.1.27), because the medium solution is in 

immediate
55
contact with the root system of the seedling. The growing medium 

solution will be chemically different from the applied fertilizer 

solution because fertilizer cations are adsorbed at different rates 

on the cation exchange sites of the growing medium particles and 

are also taken up at different rates by the seedling. The chemical 

composition of this solution will determine the overall health of the 

root system, and the kind and rate of mineral nutrient uptake.

The mineral nutrient status of the growing medium solution can be 

monitored by either of two different procedures: growing medium 

extracts or medium solution displacement.

Growing medium extracts. These extracts are prepared by 

adding a specified amount of water or chemical extracting solution 

to a sample of media and then extracting and chemically 

analyzing the medium solution. The saturated medium extract 

procedure consists of adding enough distilled water to a sample of 

growing medium (about 500 cm3) to reach the point of saturati on; 

after allowing the suspension to equilibrate for a period of time 

(about 1.5 hours), the solution is extracted with a vacuum filter 

and chemically analyzed (Warncke 1986).

56

To overcome the need to vacuum-extract the medium solution, 

other testing procedures make growing me dium suspensions by 

adding a larger amount of liquid to the medium sample. Nelson 

(1978) reports standards for 1:2 and 1:5 dilutions (medium/water) 

for monitoring the fertility of the growing medium. Other 

procedures such as the Spurway method (Markus 1986) or 

Morgan system (Mastalerz 1977) use chemical extractants to estimate the concentration of "exchangeable" nutrients contained in 

the growing medium. The problem with these dilution techniques 

is that the true chemical characteristics of the original medium 

solution are often modified because the dilutions are not 

straight-line relationships.

Growing medium solution displacement. This procedure 

consists of physically displacing and chemically analyzing the 

actual solution within the growing me dium (Nelson and Faber 

1986). Although solution displacement techniques are considered 

the most representative of mineral nutrient status at normal 

moisture conditions, they are too technically difficult to recommend for routine nursery monitoring. The "pour-through" 

technique is a modified procedure in which a specified amount of 

distilled water is added to the growing medium surface and the 

leachate is collected from the bottom of the container and then 

analyzed (Wright 1986).

Most authorities consider the saturated medium extract procedure 

the best to use because it is apparently well correlated to 

container plant growth (Bilderback 1986). EC standards for a 

peat-vermiculite growing medium using this procedure are given in 

table 4.1.28. The salinity of the growing medium solution reflects 

the applied salinity load (irrigation water plus fertilizer) plus the accumulated salts from unused fertilizer. This measurement is one 

of the most important monitoring tests that a grower can perform 

because many tree seedlings, especially conifers, are very 

sensitive to high salinity and damage can occur very quickly (fig. 

4.1.29). For instance, Phillion and Bunting (1983) recommend an 

EC rating between 1,200 and 2,500 /μS/cm for spruce seedlings 

growing in peat-vermiculite medium. Remember that salinity 

increases whenever the medium solution is allowed to dry below 

saturation, and so irrigation should be frequent, particularly during 

seed germination and seedling emergence.

A saturated medium extract sample can also be analyzed for 

individual nutrients. This nutrient analysis, which can be done 

with portable analysis kits or by
Unfortunately, research on the best nutrient levels for forest tree 

seedlings is limited, although Timmer and Parton provide some 

tentative guidelines for the major nutrients (table 4.1.28). Hallett 

(1982) has developed recommended nutrient levels for 

macronutrients based on growing medium extracts, and concluded 

that chemi cal analysis of the growing medium solution is an effective tool for greenhouse managers.

Leachate. The final test for fertility involves taking EC and pH 

readings on the "leachate" solution that drains from the bottom of 

the containers (fig. 4.1.27). This leachate can be obtained by 

taping a test tube or other container to the drain hole of the 

container during liquid fertilization. It may be necessary to collect 

leachate from several different containers before there is enough 

solution to operate the EC meter. A procedure for calculating the 

salt level in the leachate solution was developed by Tinus and 

McDonald (1979) (table 4.1.30). The EC reading of the applied 

fertilizer solution is subtracted from the EC reading of the leachate 

and this value (X) is rated according to the scale in table 4.1.30. If 

the EC of the leachate exceeds the EC of the applied fertilizer so7

sending the sample to a commercial laboratory, determines the 

actual levels of nutrient ions in the growing medium solution. 

Many different recommendations on the ideal mineral nutrient 

status in artificial growing me dia have been published for 

ornamental crops, and the recommendations vary between 

different analytical labs (table 4.1.29).
lution by 1,000 μS/cm, then excess salinity is building up in the 

growing medium and proper leaching is not occurring. In this 

situation, the containers should be irrigated immediately with 

water rinses until the EC reading returns to normal. If the leachate 

EC reading remains consistently high, then the amount of solution 

applied during irrigation or liquid fertilization should be increased 

to insure proper leaching.

4.1.9.3 Seedling nutrient analysis

One of the best ways to monitor the fertilization program in a 

container tree seedling nursery is to chemically analyze the foliage 

of seedlings and determine its nutrient status. The mineral nutrient 

concentration of the seedling foliage is a true measure of the 

effectiveness of a fertilization program because seedling nutrient 

analysis (SNA) reflects the actual uptake of mineral nutrients, 

compared to tests of the growing medium solution, which only 

measure the "available" nutrients in the root zone. Analytical 

laboratories are able to accurately and precisely measure the 

levels of all 13 mineral nutrients in a small sample of plant tissue. 

These testing laborato ries are either government-owned, located 

at agricultural universities or research facilities, or privately owned. 

Most laboratories work primarily with agricultural or horticultural 

crops, however, so it is important to work with one that has 

experience with tree seedlings. These laboratories are familiar with 

the best analytical procedures for tree seedlings and can provide 

useful interpretation of SNA results.

Sample collection and handling. The accuracy of SNA depends 

on the care with which the samples are collected and handled 

prior to the actual testing. For tree seedlings, the best tissue for 

SNA is the entire shoot of young seedlings or just the foliage of 

larger trees. The sample should be clean and collected randomly 

from the seedling population of interest (one bench in a 

greenhouse or one particular seedlot). Because

58

container seedlings are often small, tissue samples for nutrient 

analysis are usually submitted as a composite of individual 

seedlings (Landis 1985). About 60 g of fresh or 10 g of dried 

tissue is usually required for the actual test, so the composite 

sample should consist of a minimum of 20 to 50 seedlings 

(Aldhous 1975, Solan 1980). Seedling nutrient analyses usually 

cost from $10 to 50 per sample, depending on which tests are 

performed. The seedling sample is usually stored and shipped in 

a cooler, although some laboratories suggest drying the sample. It 

is wise to contact the testing laboratory prior to collecting the 

samples so that the proper sampling and handling procedures can 

be established.

Interpretation of seedling nutrient analysis test results and 

standards. It is relatively easy and inexpensive to have SNA 

performed, but interpretation of these test results can be difficult. 

Interpretation consists of comparing the test results with standard 

nutrient levels to determine if the seedling nutrient levels are 

adequate, deficient, or excessive. Many nutrient standards are 

published in the literature, but many of these are for mature trees 

or were not developed under container nursery conditions. 

Mineral nutrient standards and nutrient ratios for conifer seedlings 

are given in table 4.1.31, and should serve as general guidelines. 

Each nursery should begin to develop its own SNA standards, 

however, because of the variation that exists between species, 

seedling growth stage, and cultural practices. One option is to 

have seedlings from an ideal crop analyzed at several 

developmental stages and use these data for comparison. Hallett 

(1982) has developed guidelines for N concentration levels in 

white and black spruce and jack pine seedlings at various stages 

of development, and these data illustrate that N concentrations 

normally decrease during the growing season due to the dilution 

effect.

SNA test results can be confusing or even appear contradictory to 

the novice; nursery managers who are considering SNA for the 

first time should contact a fertilization consultant to help with test 

interpretation. Interpretation of SNA is actually more of an art than 

a science because of the complex interrelationships between 

mineral nutrients and the changes in relative nutrient 

concentrations that occur during seedling growth. A more 

complete discussion of seedling nutrient analysis and the relationship between nutrient levels and seedling quality is provided by 

Landis (1985).
59
4.1.10 Effects of Heavy Fertilizer Use in

Container Nurseries

Container nursery managers routinely promote rapid seedling 

growth with high fertilization rates because greenhouse space is 

expensive and some growers must produce more than one crop 

per year. Seedling shoot growth can be "forced" with heavy 

fertilizer applications, especially high N levels, but nursery 

managers should consider the potential side-effects of 

overfertilization: luxury consumption of mineral nutrients by seedlings and wastewater pollution. The first of these is discussed in 

the following sections, and wastewater pollution is discussed in 

section 4.2.8.2.

4.1.10.1 Luxury consumption of nitrogen and seedling

quality

High fertilization rates often lead to luxury consumption of mineral 

nutrients in the ideal growing environment of container tree 

seedling nurseries. Nutrient uptake by container seedlings is high 

because conditions in the growing medium, such as temperature, 

moisture level and pH, are conducive to rapid ion uptake. (See 

section 4.1.2.1 and fig. 4.1.3 for a discussion on nutrient uptake.)

Luxury consumption of any of the 13 mineral nutrient elements is 

possible, but seedlings accumulate high levels of certain nutrients 

(N, K) more than others. Some nursery scientists do not consider 

luxury consumption to be harmful but merely a waste of fertilizer. 

However, there is mounting evidence that high foliage nutrient 

levels, especially N, can be damaging. Timmer and Armstrong 

(1987a) actually refer to the negative growth effects of high N 

fertilization as "nitrogen toxicity."

N uptake is most likely to reach luxury consumption levels during 

the rapid growth phase, when high N levels are commonly used. 

Seedlings take up N readily and appear to store it in the foliage, 

because foliar N concentrations continue to increase without 

additional growth (Gilliam and others 1980). Ingestad (1979) grew 

two species of conifer seedlings at N levels from 20 to 1200 ppm 

and found that both species could accumu late high concentrations 

of N (2.7 to 3.4%) before seedling growth declined and mortality 

occurred (fig. 4.1.30). Seedling nutrient analysis at the end of a 

large-scale fertilizer trial, using a variety of liquid fertilizer and 

slow-release fertilization treatments, revealed that N levels ranged 

from 1.9 to 5.5%, with an average of 3.6% N (Matthews 1986). As 

Swan (1971) considers foliar lev els greater than 2.5 to 2.8% N to 

be luxury consump tion, these data indicate that many operational 

nurseries are applying too much N in their fertilization programs.

60

Certain hazards have been associated with luxury N 

consumption in both container and bareroot forest tree

seedlings:

1. High N fertilization rates may adversely affect seedling 

morphology because N produces excess shoot growth at the 

expense of root growth. Cornea (1982) studied the effects of 

N concentration on the growth of container ponderosa pine 

seedlings and found that the seedling root to shoot ratio 

decreased with increasing N fertilizer levels. Timmer and 

Armstrong (1987b) found that high N fertilization rates may 

reduce the weight and fibrosity of the root system. Torbert 

and others (1986) found that loblolly pine seedlings grown at 

lower fertilization rates had root systems that were longer, 

more fibrous, and had a higher degree of branching.

2. Prolonged tissue succulence and production of lammas 

shoots late in the growing season may increase the possibility 

of cold injury. Gilliam and oth ers (1980) found that high (150 

to 300 ppm) N levels caused second growth flushes 

("lammas growth") and delayed leaf defoliation in red maple 

seedlings for 3 weeks, compared to lower (50 ppm) N levels. 

Aronsson (1980) studied frost hardiness of Scotch pine and 

found a clear correlation between foliar N concentration and 

cold injury: seedlings with a foliar N level from 1.3 to 1.8% 

suffered less frost damage than those with N levels greater 

than 2.0%. Larsen (1978, as reported in Aronsson 1980) 

found
that Douglas-fir had the highest cold hardiness with foliar N 

concentrations from 1.3 to 1.4% (table 4.1.32). Hallett (1985) 

reported that balsam fir seedlings with a foliar N 

concentration greater than 2.2% were more subject to frost 

injury.

3. High levels of fertilizer salts in the growing medium have been 

shown to affect the formation of some mycorrhizae of 

container tree seedlings. Torbert and others (1986) found 

that, although higher fertilizer rates produced larger loblolly 

pine seedlings, my corrhizal colonization was 3 times higher at 

the lower fertilization rates. Cornea (1982) found that N 

fertilization level and mycorrhizal infection of ponderosa pine 

container seedlings was inversely correlated: mycorrhizal 

infection decreased linearly with increasing N concentration. 

Crowley and others (1986) studied the effect of slow-release 

fertilizers on mycorrhizal formation in shortleaf pine seedlings 

and found that the rate of fertilizer application, fertilizer 

release rate and N-P-K ratio all had an effect. Various 

mycorrhizal fungi respond differently to fertility, and some 

fungi such as Thelephora spp. actually appear to thrive in the 

high-fertility environment of container tree seedling nurseries. 

The effects of fertilization and other cultural practices on container seedling mycorrhizae are detailed in volume five of this 

manual.

4. High N fertilization has been shown to adversely affect 

seedling survival and growth after outplanting. Etter (1969) 

studied the effects of three N levels on drought survival of 

lodgepole pine seedlings and found that seedlings grown at 

higher N levels (500 ppm) had poorer outplanting survival 

than seedlings grown under standard N fertilization. 

Outplanting survival of Douglas-fir and white spruce 

seedlings was found to have a curvilinear relationship with 

foliar N concentration, with the greatest survival rate around 

2.1 % N (fig. 4.1.31). Cornett (1982) concluded that the poor 

root to shoot ratio caused by high N fertilization would be a 

disadvantage for ponderosa pine seedlings outplanted on 

droughty sites.

5. High foliar nutrient levels may result in increased animal 

browsing damage after outplanting. Many reforestation 

specialists have noticed that container seedlings are 

preferred by deer and elk compared to bareroot seedlings.

See the chapter on dormancy and hardiness in volume six of this 

series for more information on factors affecting seedling quality.

61
62

4.1.10.2 The ideal nitrogen fertilization level

The ideal N level for the production of high-quality container 

seedlings has been the subject of much discussion among nursery 

managers. The best N fertilization level will vary with many factors, 

but actual nursery production experience with several species of 

western conifer seedlings supports a moderate N level of around 

100 to 150 ppm during the rapid growth phase to encourage 

development of sturdy, hardy tissue. Extremely high N levels (> 

250 ppm) generally promote succulent shoot growth and an 

unbalanced shoot to root ratio, and should therefore be avoided. 

Scarratt (1986) found that N levels over 100 ppm commonly lead 

to excessive shoot elongation and poor shoot to root ratios of container jack pine. Phillion and Libby (1984) studied the effect of N 

fertilization on black spruce growth using liquid fertilizers; they 

grew seedlings at N levels from 0 to 500 ppm and monitored 

seedling height, diameter, and dry weight (table 4.1.33). Based on 

these growth trends and foliage color, they concluded that 100

ppm

N produced the best seedling. Scarratt (1986) found that, under 

constant fertilization, 100 ppm N was adequate for jack pine 

seedlings on a 10- to 12-week production cycle. For slow-release 

fertilizers, Crowley and others (1986) recommend that ferti lizers 

with extended release rates (8 to 9 months) should be used at a 

rate of 4.5 kg/m3 (7.5 pounds per cubic yard) to produce the ideal 

combination of good seedling growth and mycorrhizal 

development.

The best N fertilization level will vary with many cultural factors 

and between individual species, but maintaining a moderate N 

level of around 100 to 150 ppm during the rapid growth phase 

encourages sturdy, hardy tissue development. The best way to 

monitor N fertility is through regular SNA during the growing 

season in conjunction with seedling growth monitoring both at the 

nursery and after outplanting. Target foliar N levels should be 

maintained around 2.0% for best survival and growth after 

outplanting (fig. 4.1.31).
4.1.11 Conclusions and Recommendations

Fertilization is one of the most important cultural practices in a 

container tree seedling nursery, and growers should carefully plan 

and regularly evaluate their fertilization programs to insure that 

they are producing seedlings of the highest quality. A fertilization 

program for a container tree seedling nursery should be designed 

to maintain specific concentrations of the 13 different mineral 

nutrients in the growing medium and keep them in balance but 

should also be designed to allow necessary nutritional changes 

during the growing cycle.

The choice of a fertilizer depends on a multitude of factors: 

fertilizer form, grade, nitrogen source, nutrient release rate, 

interaction with the growing medium, plant-use efficiency, and cost. 

If possible, container nursery managers should conduct their own 

nursery fertilization trials because fertilization rates, like all cultural 

practices, are related to many nursery-specific factors. Irrigation 

type and frequency and characteristics of the growing medium 

both affect mineral nutrient uptake by container tree seedlings. 

Because of these complex interrelationships, fertilization practices 

may have to be periodically adjusted in response to seedling 

growth and outplanting performance.

Although incorporation of dry fertilizers into the growing medium 

can be justified in some instances, direct injection of liquid 

fertilizers into the irrigation system is recommended whenever 

possible. The benefits of this technique include precise control of 

both the concentration and balance of all 13 mineral nutrients, the 

ability to completely change the nutrient solution at any time, and 

a very low chance of overfertilization and resultant salt injury.

The constant fertilization (replacement) technique is recommended because it minimizes the chances of overfertilization, 

promotes regular flushing of the growing me dium to prevent the 

buildup of fertilizer salts, returns growing medium nutrient levels to 

target specifications with fertilization, and allows nutrient levels in 

the growing medium solution to be adjusted quickly.

Mineral nutrient levels can be monitored at different stages of the 

fertilization process, starting with the native nutrient salts in the 

irrigation water and ending with the leachate solution that drains 

from the bottom of the container. Growers should obtain their own 

pH and electrical conductivity meters to monitor the fertilization 

process. Seedling nutrient analyses should be performed regularly 

by analytical laboratories to track mineral nutri63

ent utilization. Each nursery should begin to develop its own 

seedling nutrient analysis standards because of the variation that 

exists between species, seedling growth stage, and cultural 

practices.

The high fertilization rates used in some container tree seedling 

nurseries to promote rapid shoot growth may actually be 

detrimental. The potential side effects of overfertilization include 

luxury consumption of some mineral nutrients (especially N), 

inhibited mycorrhizal development, and wastewater pollution. 

Hazards associated with luxury N consumption include excess 

shoot growth at the expense of root growth, prolonged tissue 

succulence and production of lammas shoots late in the growing 

season, and poor seedling survival and growth after outplanting. 

The best way to monitor N fertility is through regular chemical 

analysis of the seedling tissue during the growing season and 

after outplanting. Target foliar N levels should be maintained at, 

or slightly below, 2.0% for best survival and growth after 

outplanting
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