Growing tobacco seedlings in multipot trays

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BULLETIN 775

NOV 5 1979

JULY 1979

Growing Tobacco Seedlings in Multipot Trays

By H.C. De Roo and G.S. Taylor

THE CONNECTICUT AGRICULTURAL EXPERIMENT STATION

NEW HAVEN

Growing Tobacco Seedlings in Multipot Trays

By H.C. De Roo and G.S. Taylor

Growers of shade tobacco have recently changed
their method of growing seedlings. Instead of using
broadcast-seeded ground beds covered by sash to
produce their seedlings, they have begun using
mechanically seeded plastic trays sheltered in plastic
houses. Because this is a new method, we have conducted
experiments to learn the optimal growing conditions.

The most popular system employs 96-hole Multipots
(Multipot Company of America, 3325 Lawndale,
Chicago, 111. 60623). The seeds are coated with clay to
increase their diameter to 1.5 - 2mm to allow automatic
seeding equipment to be used. Once seeds are in the tray,
germination may be affected by environmental factors
such as light, water supply and temperature; and by the
nature of the potting mixture and fertilization.

To study the effects of the clay shell on germination,
we placed 50 unpelleted and 50 pelleted tobacco seeds
(strain WS-117, Brown) in each of five petri dishes at
23°C(75°F). Our daily counts show that germination of
pelleted seed is delayed about three days. However, the
effect of pelleting on overall germination (98%) was
negligible. (Fig. 1.)

Water. Salts in water can affect the growth of
seedlings. Well water may contain nitrate or other salts
which, together with fertilizer, may increase soluble salts
in the soil solution to injurious levels. Water with a
Solubridge reading below 0.75 millimhos/cm and a pH
near 6.0 is satisfactory. We also found that it is important
for the trays to be level. If they are not, some cups will

flood and others will dry out. Thus, some seedlings may
die from too much water and others in the same tray may
die from lack of water.

Light. To test for reaction to light we placed 100
seeds on wet filter paper in each of four petri dishes. Two
dishes were wrapped in transparent plastic and two in
black plastic sheeting. Germination at 20° C (70° F) is
shown in Fig. 2. Darkness retarded germination, but the

% GERMINATION
100

,__»--. — •

_j i — 1_

9
DAYS

12

Fig. I. The germination of 250 unpelleted ( unbroken line ) and 250
pelleted ( broken line ) shade tobacco seeds moistened and
incubated at 23°C(75°F).

Connecticut Experiment Station — Bulletin 775

final germination rate was the same for seeds in the dark
as for seeds in the light.

Temperature. Although tobacco seed is tolerant of
temperatures as high as 35° C (95° F), satisfactory
germination occurs at 20°-26°C (70°-80°F) (5). We
placed two petri dishes containing 100 pelleted seeds each
in a germination chamber at 24° C (76° F). After 7 days
they were removed to a greenhouse with a minimum
night temperature of about 1 5° C (60° F). Two other petri
dishes containing 100 pelleted seeds each were placed in
the greenhouse immediately for comparison. Fig. 3
shows that germination is earlier at the higher
temperature. However, after 24 days, the overall
germination rate was the same for both treatments.

COMPOSITION OF MIXES

Tobacco germinates and grows best in a relatively
fine textured, uniform medium. A good medium is
openstructured and well-aerated, but still capable of
holding water and moving it by capillarity. A good
medium also should be free of insects, pathogenic
organisms and weed seeds, and be light in weight to
facilitate handling.

Chemical Properties. Standard media are made up
of sphagnum peat moss, combined with sand or perlite,
and some vermiculite. As these materials are generally
free of pathogens, weed seeds, and insects, they do not
have to be sterilized. The media are amended with lime,
gypsum, and superphosphate, and supplemented with
standard fertilizer materials.

The Cornell peat-lite mixes, (2) containing peat and
vermiculite or perlite are satisfactory for tobacco from
germination to transplanting. These mixes are amended

% GERMINATION
100

% GERMINATION
100

Fig. 2. The germination of 200 pelleted tobacco seeds moistened and
incubated at 20° C (70° F) in the light ( unbroken line ) or in
the dark ( broken line ).

Fig. 3. The germination of 200 pelleted tobacco seeds that were
moistened and incubated at two different starting tempertures:
7 days at 24°C (76° F), thereafter in a greenhouse at a minimum
night temp of 15°C (60° F) ( unbroken line ); started and kept
in a greenhouse at a minimum night temp of I5°C (60°F)
( broken line )

with 5 lbs of ground limestone, 1-2 lbs of 20%
superphosphate, and fertilized with 1-1.5 lbs of calcium,
potassium, or ammonium nitrate per cu yd. Two to three
oz of a wetting agent, combined with a few gallons of
water, are added to reduce dust and facilitate wetting.
Jiffy Mix, Premier's Pro Mix, and Heco Mix are
commercially available alternatives. The principal
ingredient of these mixes is peat that has been limed and
fertilized. We used Heco mix in our trials because it is
readily available and is used by several tobacco growers.

Nutrient analysis using Morgan's methods (6)
indicated that Heco mix generally contains too much
fertilizer. High fertilization is shown by a high
concentration of soluble salts. We found soluble salt
levels of about 2.2 millimhos/cm using a Solubridge
according to the saturated-soil-extract method, which
measures electrical conductance at 25° C (1). We have
found that tobacco seed germinates best at a salt index of
0.8; seedling development is satisfactory when
fertilization does not raise the salt index beyond 1.8, a
level which allows ample nutrients. The pH of the Heco
mix varied between 5.3 and 6.2, which is satisfactory.

An experiment clearly demonstrated the inhibitive
effect of a high salt concentration. We diluted Heco mix
with 1 part and with 3 parts of peat and compared growth
in these mixes to growth of seedlings in undiluted Heco
mix and in unfertilized peat moss. We added lime to all
media to bring their pH to the 5.3 of the Heco mix. No
fertilizer was added. In half of two 96-hole Multipot
trays 12 cups were filled with each medium and seeded
with 2 pelleted seeds per cup. Thus, there were four
replicates. The trays were placed in the greenhouse on

Growing Tobacco Seedlings in Multipot Trays

June 3rd for 10 weeks. The soil was wetted with tapwater
as needed, and liquid fertilizer (Rapid-Gro) was applied
twice.

At five and one-half weeks, leaves of seedlings in
undiluted Heco mix were less than half as long as those in
the other three media. Clearly, they were stunted by the
high salt content (2.8 millimhos/cm). The number of
plants and the total fresh weight in each cup are shown in
Table 1. The best growth was obtained with a mix of 1
part Heco and 1 part peat.

Table 1. Growth of tobacco during 10 weeks in multipot trays
containing Heco mix and various dilutions with sphagnum peat
moss.

5'/2 weeks

10 weeks

Longest leaf

Mea

n fresh wt

Mean fresh wt

Variations

(mm)

per cup

(g)

per

plant (g)

1 Heco: 0 Peat

0.72

1.54

0.77

1 Heco: 1 Peat

1.64

2.31

1.24

1 Heco: 3 Peat

1.65

1.94

1.07

0 Heco: 1 Peat

1.43

1.93

1.15

L.S.D.

0.27

Physical Properties In other tests we attempted to
create an optimal physical environment for seed
germination and seedling growth. We experimented with
peat and rice hull amendments to keep the media loose
and open-structured after heavy watering. The mixes
used and their measurable physical properties are shown
in Table 2. As indicated by the bulk densities (BD), all
mixes are light in weight. The total pore space (TPS)
shows that they are also porous. We consider 75% to be
sufficient.

Retention of water by the media, following a
saturating watering, provides additional information
about their suitability for tobacco seed germination and
seedling growth (3, 4). Since oxygen is necessary for
germination and root growth, an important standard is
the amount of air that can enter the pore space after

drainage of free water by gravity. We measured this air
volume by measuring the water retained at a low tension
of 15 cm. The results in Table 2 (column AIR) indicate
that, after irrigation, the aeration of the pure peat mixes,
Heco and 1 : 1 Heco-peat is less than the 1 5% we consider
sufficient. A range of about 15-25% air-filled pore space
is desirable.

The amount of water retained by the mix that is
readily available to roots is also an important property.
The RAW column of Table 2 indicates that rice hulls did
not significantly reduce the volume of readily available
water. The data in column WBC indicate that the all
Heco mix released less water between tensions of 50 and
100 cm to the roots than did most of the other mixes,
although more water was still available.

From these data it appears that a rigid material that
does not decay readily, such as rice hulls or perlite, is
needed to keep the peat moss of Heco (medium 1) and
Heco-peat (medium 2) open-structured and well aerated.
Surface characteristics of the media such as texture
and structure also have a great effect on the germination
of pelleted tobacco seed. If, during the seeding of a tray, a
pelleted seed lands on a rice hull, germination will often
be delayed, abnormal, or incomplete because the
developing rootlet encounters the rigid rice hull and is
prevented from penetrating the medium. This action,
combined with the stretching of the hypocotyl, makes the
clay shell still clinging to the seed leaves (cotyledons)
appear to "pop up" from the medium surface. To
counteract this effect we covered the seeded rice hull
mixtures with a thin layer of screened, fine, peat moss. A
thick layer will become solid after irrigation and will
delay or restrict germination and stimulate the growth of
algae.

For our observations of this aspect of germination we
used the first four media shown in Table 2. Media 1, 2, 3,
and 4 were amended to a pH of about 5.6, with
respectively 0.5, 1.5, 2.0, and 2.5 lbs of hydrated lime per
cu yd of mix. Each cup was seeded with one pelleted seed.

Table 2. Physical characteristics of various media for tobacco seed germination and growth to transplant size, or volume weight (x 62.42 = dry weight
in lbs/cu ft); TPS = total pore space, % by volume; AIR = air space at 15 cm water tension; RAW = readily available water, released between 15
and 50 cm water tension; WBC = water buffer capacity, water released between 50 and 100 cm tensions.

Media

BD

g/cc

TPS

AIR

RAW

WBC

No.

Ingredients

Vol. ratios

Vol. %

1.

H

—

.11

92

9.8

22

2.4

2.

H:P

1:1

.09

94

5.2

20

6.4

3.

H:P:R

3:3:2

.12

91

22

20

4.3

4.

H:P:R

2:2:1

.11

93

22

17

6.9

5.

H:R

6:1

.11

93

25

18

5.2

6.

P:V:G

2:1:1

.14

88

28

15

4.5

Connecticut Experiment Station — Bulletin 775

After seeding, one flat of medium 3 and 4 was covered
with a thin layer of fine screened peat moss. The flats
were kept in a greenhouse with a minimum night
temperature of about 15°C (60° F). After 6 days we
added another set of flats. The observations and counts
made during both experiments were averaged and are
summarized in Table 3. About 2 weeks were required in
January- February 1975 for the first seeds to germinate.
The "popping up" count of media 3 and 4 vs. media 3C
and 4C clearly shows that the thin cover of fine, screened
peat moss nullified the undesirable effect of the rice hulls.
The "popping up" of medium I was probably caused by
the relatively high concentration of the Heco mix which
inhibited the penetration of this medium by the rootlets.
A dilution with unfertilized peat moss reduced the
"popping up" from 25% to 9%.

The germination percentage after 5 weeks tends to
reflect the abnormal, early germination in the various
media. Although all germination percentages are
acceptable, it is clear that those in the rice hull mixes were
improved by the surface dusting and in the Heco mix by
dilution with peat moss. The uniformity of the stand of
the tobacco seedlings was evaluated after 6-7 weeks. The
ratings on a scale of I to 4 reflect the germination pattern.
The higher the "popping up" percentage, the lower the
germination tends to be; and the more irregular is the size
of the tobacco transplants.

USE OF FERTILIZERS

Finally, we observed a fertilizer problem which we
believe is related to the aeration problem presented in
Table 2. During the tobacco seedling season of 1977,
extensive periods of cloudy, relatively cold weather were
experienced. As a result, seed germination and seedling
growth was slowed and many Heco mix samples were
submitted by growers for soil testing. The nitrogen levels

generally were high, particularly in ammonium nitrogen,
which are known to be phytotoxic at high fertility levels.
Under conditions of low light intensity, cool
temperatures, and the rather low air and high water
contents of the Heco mix, oxidation of the ammonium
nitrogen to nitrate nitrogen by soil bacteria was
apparently suppressed. Under such weather conditions,
fertilizers high in nitrate nitrogen should be more
satisfactory. An example might be a 1 5-0- 1 5 made up of 1
part potassium nitrate and 2 parts of calcium nitrate (by
wt). Commercial grades such as 16-32-16 or 20-20-20
contain about 65 to 72 percent ammonium nitrogen and
might be less satisfactory. For continuous fertilization
every I to 2 weeks, about 2 lbs. of 1 5-0- 1 5 to 1 00 gallons
of water should be satisfactory.

SUMMARY

An open-structured medium that did not collapse
and stayed well-aerated, and that was amended and
fertilized in a controlled manner, produced satisfactory
tobacco seedlings.

Literature Cited

1. Baker, K.F., ed. 1958. The U.C. system of producing healthy

container-grown plants. Calif. Agr. Exp. Sta. Manual 23.

2. Boodley, J.W. and R. Sheldrake, Jr. 1972. Cornell peat-lite mixes

for commercial growing. New York State Coll. of Agr. Inf. Bull.
43. 8 p.

3. De Roo, H.C. 1967. Artificial root media and fertilizations for

container-grown woody ornamental plants. Conn. Agr. Exp. Sta.
Bull. Soils 26. 12 p.

4. De Roo, H.C. 1975. Agricultural and horticultural utilization of

fermentation residues. Conn. Agr Exp. Sta. Bull. 750. 47 p.

5. Garner, W.W. 1951. The Production of Tobacco. 520 p. New York;

The Blakiston Company.

6. Lunt, H.A., H.G.M. Jacobson. and C.L.W. Swanson. 1950.

The Morgan Soil Testing System. Conn. Agr. Exp. Sta. Bull.
541. 60 p

Table 3. Germination of pelleted tobacco seed and seedling growth as affected by media and treatment.
Media mixtures are described in Table I; Pc = seed covered with screened fine peat moss, popping up =
pelleted seed raised above media surface, adversely affecting germination. Index I = very uneven; 2 = uneven;
3 = moderately uniform; 4 = uniform.

Media

Ingredients

Popping up %

1

All H

25

2

1 H: 1 P

9

3

1 H: 1 P: R

15

4

2 H: 2 P: 1 R

17

3c

1 H: 1 P: 1 R-

Pc

0

4c

2 H: 2 P; 1 R<

Pc

0

Germination %

78
82
74
81

82
88

Seedling development

2
3
2
2

3
3

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