Group Title: Bulletin University of Florida. Agricultural Experiment Station
Title: Soil temperature studies with cotton
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 Material Information
Title: Soil temperature studies with cotton
Series Title: Bulletin University of Florida. Agricultural Experiment Station
Physical Description: 32 p. : ill., charts ; 23 cm.
Language: English
Creator: Camp, A. F ( Arthur Forrest ), 1896-
Walker, M. N ( Marion Newman ), 1900-
Publisher: University of Florida Agricultural Experiment Station
Place of Publication: Gainesville Fla
Publication Date: 1927
Copyright Date: 1927
Subject: Cotton -- Soils   ( lcsh )
Soil temperature   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
non-fiction   ( marcgt )
Bibliography: Includes bibliographical references (p. 32).
Statement of Responsibility: by A.F. Camp and M.N. Walker.
General Note: Cover title.
 Record Information
Bibliographic ID: UF00026436
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: ltuf - AEN4047
oclc - 18172892
alephbibnum - 000923496

Full Text


The publications in this collection do
not reflect current scientific knowledge
or recommendations. These texts
represent the historic publishing
record of the Institute for Food and
Agricultural Sciences and should be
used only to trace the historic work of
the Institute and its staff. Current IFAS
research may be found on the
Electronic Data Information Source

site maintained by the Florida
Cooperative Extension Service.

Copyright 2005, Board of Trustees, University
of Florida

Bulletin 189 September, 1927

Agricultural Experiment Station




I. Apparatus for the Control of Soil Temperature for Experi-
mental Purposes.

II. The Relation of Soil Temperature to the Germination and
Growth of Cotton.


Bulletins will be sent free upon application to the
Agricultural Experiment Station

P. K. YONGE, Chairman, Pensacola E. L. WARTMANN, Citra
E. W. LANE, Jacksonville J. T. DIAMOND, Secretary, Talla-
A. H. BLANDING, Leesburg hassee.
W. B. DAVIS, Perry J. G. KELLUM, Auditor, Tallahassee

WILMON NEWELL, D. Sc.. Director ERNEST G. MOORE, M. S., Asst. Ed.
JOHN M. SCOTT, B. S., Vice-Director IDA KEELING CRESAP, Librarian
S. T. FLEMING, A. B., Asst. to Di- RUBY NEWHALL, Secretary
rector K. H. GRAHAM, Business Manager

W. E. STOKES, M. S. Agronomist OUIDA DAVIS ABBOTT, Ph. D., Chief
W. A. LEUKEL, Ph. D., Asso. L. W. GADDUM, Ph. D., Asst.
C. R. ENLOW, M. S. A., Asst. C. F. AHMANN, Ph. D., Asst.
FRED H. HULL, M. S. A., Asst.
JOHN M. SCOTT, B. S., Animal A. N. TISSOT, M. S., Asst.
Industrialist H. E. BRATLEY, M. S. A., Asst.
R. M. BARNETTE, Ph. D., Asst. A. F. CAMP, Ph. D., Asso. Hort.
C. E. BELL, M. S., Asst. M. R. ENSIGN, M. S., Asst.
H. L. MARSHALL, M. S., Asst. G. H. BLACKMON, M. S. A., Asst.
J. B. HESTER, B. S., Asst.
W. A. CARVER, Ph. D., Asst. O. F. BURGER, D.Sc.. Plant Pathologist
"M. N. WALKER, Ph. D., Asst. G. F. WEBER, Ph. D., Asso.
E. F. GROSSMAN, M. A., A sst K. W. LOUCKS, B. S., Asst.
RAYMOND CROWN, B.S.A., Field Asst. ERDMAN WEST, B. S., Mycologist
C. V. NOBLE, Ph. D., Ag. Economist A. L. SHEALY, D.V.M., Veterinarian
BRUCE MCKINLEY, A. B., B. S. A,, D. A. SANDERS, D. V. M., Asst.
Asst. E. F. THOMAS, D. V. M., Lab. Asst.
M. A. BROKER, M. S. A., Asst.

W. B. TISDALE, Ph. D., Plant Pathologist, in charge, Tobacco Experiment
Station (Quincy)
Ross F. WADKINS, M. S., Lab. Asst. in Plant Pathology (Quincy)
JESSE REEVES, Foreman, Tobacco Experiment Station (Quincy)
J. H. JEFFERIES, Superintendent, Citrus Experiment Station (Lake Alfred)
W. A. KUNTZ, A. M., Assistant Plant Pathologist (Lake Alfred)
R. L. MILLER, Assistant Entomologist (Lake Alfred)
W. L. THOMPSON, Assistant Entomologist (Lake Alfred)
GEO. E. TEDDER, Foreman, Everglades Experiment Station (Belle Glade)
R. V. ALLISON, Ph. D., Soils Specialist (Belle Glade)
J. H. HUNTER, M. S., Assistant Agronomist (Belle Glade)
J. L. SEAL, M. S., Assistant Plant Pathologist (Belle Glade)
H. E. HAMMAR, M. S., Field Assistant (Belle Glade)
L. O. GRATZ, Ph. D., Assistant Plant .Pathologist (Hastings)
A. N. BROOKS, Ph. D., Associate Plant Pathologist (Plant City)
A. S. RHOADS, Ph. D., Assistant Plant Pathologist (Cocoa)
STACY O. HAWKINS, Field Assistant in Plant Pathology (Homestead)
D. G. A. KELBERT, Field Assistant in Plant Pathology (Bradenton)
R. E. NOLEN, M. S. A., Field Assistant in Plant Pathology (Monticello)
FRED W. WALKER, Assistant Entomologist (Monticello)



A number of the important diseases of cotton are carried in
the soil, and in studying these it was found necessary to con-
trol the soil temperature during certain experiments. This was
necessary not only to determine the relation of temperature to
the development of the disease, which is a very important point,
but also to be able to maintain the soil temperature at a point
favorable to the disease when studying the influence of factors
other than temperature. This latter point has proven to be
very important, since under ordinary greenhouse conditions the
soil temperature has fluctuated so greatly as to frequently make
it impossible to get consistent infection with some of the dis-
eases even when conditions other than temperature were fav-
orable; obviously, therefore, it was impossible to study possible
means of control unless the effect of extremes of temperature
was eliminated. In pursuing these investigations the soil tem-
perature control apparatus has been developed which is de-
scribed here and which has been unusually economical in opera-
tion under Florida conditions. Considerable information also
has been collected on the behavior of cotton at varying soil tem-
peratures, and, since this information is vitally connected with
the study of the various diseases, it is reported in this paper.

4 Florida Agricultural Experiment Station


The construction and use of soil temperature control appara-
tus, under the climatic conditions that prevail in Florida, is com-
"plicated by two factors, the temperature of the air and the tem-
perature of the water supply during warm weather. The tem-
peratures reached by both the air and the water are such as to
preclude the obtaining of constant temperatures below 30-32
degrees C. without the use of artificial refrigeration. This
means that with a series of soil tanks, of the type commonly
designated as the Wisconsin soil tank*, covering a range from
10 to 400 C., all of the tanks below 30-32 C. will have to be
supplied with automatically controlled refrigeration, which im-
plies a considerable expense per tank for controls, brine pump,
motor, etc.
To avoid the duplication of refrigeration apparatus, recourse
was had to the internally balanced system employed by Living-
ston and Fawcett ('20) for a series of incubation chambers.
This system makes use of a long metal "trough" divided into
compartments by metal crosswalls, the outside of the "trough"
being thoroughly insulated. One end of the apparatus is main-
tained at a constant high temperature and the other at a con-
stant low temperature and the intermediate compartments take
up a range of temperatures between these extremes, the inter-
change of heat through the metal crosswalls maintaining this
balanced condition. With this method of construction it is pos-
sible to have a series of temperature tanks covering practically
any range with a minimum of electrical controls, apparatus for
heating, refrigeration, etc. The apparatus as finally constructed
in the experimental greenhouses here, consisted of 8 soil tanks,
each containing 8 soil pots, one thermostatically controlled brine
pump and one thermostatically controlled heater. This equip-
ment can be easily adjusted to give a series of 8 soil tempera-
tures covering practically any range from 0 to 50 degrees C. In
the construction of the Livingston and Fawcett apparatus great
care was employed to eliminate air leaks and other breaks in the
insulation between the internal system and the outside atmos-

*This type of tank requires a separate unit for each temperature and
is described by Leukel ('24), and Jones, Johnson and Dickson ('26).

Bulletin 189, Soil Temperature Studies with Cotton 5

here and as a result of this care a very high degree of accu-
racy was obtained in their apparatus. One of the great diffi-
culties in the use of this type of apparatus for soil temperature
control lies in the loss or absorption of heat through the unin-
sulated soil surface combined with the almost unpreventable air
leaks around the soil pots. The success attained in the use
of the apparatus, in spite of these difficulties, is such however,
as to make it desirable to give a description of the apparatus
together with some notes on its operation.


A water-tight galvanized iron tank 25 feet long, 22 inches
deep and 30 inches wide was constructed. Riveted and soldered
crosswalls were placed so as to make a water-tight compartment
40 inches long at one end and one, 20 inches long, at the other;
the space between these end compartments was divided into
eight 30-inch water-tight compartments. This tank was insu-

Fig. 1.-Cross section of soil tank, showing general construction.

6 Florida Agricultural Experiment Station

lated on the outer surfaces of the sides and bottom with 3 inch
cork boards, sealed with asphalt, and with flooring on the out-
side of the cork, likewise sealed with asphalt. Covers for the
compartments were made by using 2-inch cork board between
two layers of flooring. The cylindrical, galvanized iron pots
for the soil were 5 inches in diameter and 16 inches deep with a
half inch flange around the top. The openings for the pots were
cut so that the pots hung by the flanged edge from the bottom
layer of flooring and the insulation and upper layer of floor-
ing were cut out to slope away from the top of the pots so that
the surface of the soil was never shaded by the edge of the in-
sulation. The covers were sealed in place with soft putty so as
to prevent air leaks and the leaks around the pots were reduced
by using a circle of small rubber tubing, though this was later
Subsequently a much better cover was designed which per-
mitted the practical elimination of the air space between the
water and the top, and which allowed the water to come up
higher on the pots. It also allowed the flanges of the pots to
rest flush on the top of the tank, thus obviating the necessity of
cutting away the insulation around the tops of pots to a funnel-
shaped opening as previously described. The general construc-
tion of this cover is shown in Fig. 1. It consisted of a covering
of light flooring resting on the edge of the tank and acting as
a support for the soil pots. During construction the flooring
boards may be held together by cleats across their ends and
these discarded when the cover is sealed to the tank, or if de-
sired, they may be retained to make the cover more readily re-
movable. Holes were cut in this for the pots and 1 inch cork
board was sealed to the under side with hot asphalt, the cork
being thoroughly covered so that water could not reach the cork
itself. After the holes for the pots had been cut through the
cork, and made slightly larger than the holes through the wood-
en cover to allow for the thickness of the asphalt layer, the en-
tire cover was placed cork downward in a bath of melted as-
phalt to be sure of a complete sealing of the surface of the cork.
By placing the cover in the asphalt bath in this manner the
upper surface of the boards was kept clean and free from as-
phalt while the cork was thoroughly sealed to the lower surface
of the boards. A small hole was countersunk in the cork to al-
low for the agitator. If desired, the top may have a cork patch

Bulletin 189, Soil Temperature Studies with Cotton 7

on the center of the upper surface to replace the insulation lost
in the countersinking for the agitator. With this type of con-
struction the water can be actually kept at a higher level than
the bottom of the cork without injury to the apparatus (see
Fig. 1).
An agitator was provided for each compartment and all of these
were operated from a drive shaft running in bearings attached
to the bottom of the tank. The drive was by gears rather than
by belts, the gears giving less trouble when once adjusted. The
general design of the agitators was that employed by Livingston
and Fawcett and eliminated the use of packing boxes; this type
of agitator is illustrated in Fig. 1. A pipe flange was riveted
and soldered on the inside of the bottom of each compartment,
approximately in the center, and all flanges in line from one end
of the tank to the other. A piece of 1-inch pipe was turned down
in a lathe for about 6 inches and threaded back of the turning.
The turned portion below the thread was small enough in diam-
eter to pass through the pipe flange and long enough to clear
the cork insulation and the framework below the tank. The
upper end of the pipe was cut just long enough to stand above
the water level and both ends of the pipe were bushed to fit the
agitator shaft. The agitator proper was made of 3/-inch strap
iron, bent as shown in Fig. 1. The agitator was threaded and
screwed on to the top of the vertical agitator shaft and locked
with a thin nut. The drive shaft was operated by a quarter
horsepower motor geared so that the agitators made about 15
revolutions per minute.
For heating the 20-inch end compartment a 1,000 watt bay-
onet heater with thermostatic control was used; a brine coil
and a thermostatically controlled brine pump, of the positive
gear type, were employed for cooling the other end compart-
ment. If the brine coil is properly proportioned, no counter
heater will be needed. Brine was supplied for the entire equip-
ment from a large brine tank which was cooled by the conven-
tional ammonia system. '
The entire tank was placed over a concrete trench about 36
inches deep and about 5 feet longer than the tank and with
edges flush with the floor. This allowed easy access to the
shaft and gears underneath the tank for greasing and repair
and at the same time allowed the tank to be low enough for easy

8 Florida Agricultural Experiment Station

handling of the pots. Where possible the tank should. be acces-
sible from both sides.
In the construction of such apparatus there are a number of
apparently minor points that should be carefully considered.
Compartments should be made just large enough and deep enough
to accommodate the pots and agitator; the compartments neces-
sarily contain large amounts of water and an effort should be
made to keep this quantity as small as possible. Construction
should be of such type as to aid in readily sealing the compart-
ments against air leaks. The construction throughout should be
rugged and should permit of hard usage. An equipment which
will give a moderately accurate control of temperatures over a
long period without a breakdown is much to be preferred to
one which gives a high degree of accuracy but which, due to its
delicacy, is liable to frequent trouble.
The operation of such a series of tanks depends upon a bal-
ance maintained between the end compartments, one hot and
the other cold. The heat and refrigeration are distributed through
the metallic crosswalls, each compartment thus having one warm
and one cool wall. There is likewise a certain amount of heat
interchange between the tank and the surrounding atmosphere,
through the insulation and unprotected soil surface, resulting
in absorption of heat in the cool compartments and loss of heat
in the hot compartments. The ratio between the rate of ex-
change through the insulated walls and the rate of exchange
through the crosswalls is the factor determining the efficiency
of the control. If the amount of exchange through the cross-
walls greatly exceeds the insulation losses the control will be
found to be excellent, but if the rate of exchange through the
crosswalls falls off or the amount of insulation loss increases, a
corresponding drop in the efficiency is to be expected. Since the
thickness and character of the crosswalls is constant for the ap-
paratus, variations in rate of exchange through the crosswalls
will have to be effected by varying the rate of stirring, though
increasing the rate of stirring increases correspondingly the
utilization of power both for heat and for refrigeration. With
the amount of loss to the outer atmosphere reduced as much as
possible, the rate of stirring should be sufficient to give the de-
sired accuracy of control, but there is no reason for raising it
above that point. If such a machine could be made with perfect
insulation the balance might be maintained very accurately, blt

Bulletin 189, Soil Temperature Studies with Cotton 9

under the conditions of operation for soil temperature work,
the balance between the hot and cold ends is upset by the in-
creased amount of imperfectly insulated surface, principally the
surface of the soil in the pots. In practice it was found, how-
ever, that the transfer of heat through the metal crosswalls
was much faster than the rate of loss through the soil and in-
sulation so that the accuracy of control was quite satisfactory.
In case it is known that proper refrigeration will be available,
the end compartment used for cooling can be much reduced in
size-the one in the equipment used being built with the pos-
sible use of ice in mind. It might be possible to eliminate the
end compartments entirely, but it is probably advisable to re-
tain them for heating and cooling alone, thus avoiding the com-
plication of pots and control apparatus in the same compart-
For the information of any one contemplating the building
of such a soil tank, some figures are given below as to the cost
of the various items. These costs will vary considerably with
the locality and the make of equipment used, but may be used
as a guide for consideration.
Tank of 24 gauge galvanized iron --........ --.......--.... $230.00
128 cylindrical Pots of same metal .-----............----......... 140 00
Cork for insulation ............-......- - -..... ..------ ...-..-- 60.00
Lumber ....--------......--.. ........- ---...... ----------- 000
Agitators, including machine work ------..... -.................... 150.00
1.000-watt bayonet heater ............-- .....--- --............- 20.00
One-quarter H.P. motor ........... .............------------.-- 20.00
One-half H.P. motor ...-....--...----....-.... .......-. .....------ 30.00
Brine pump ..............- -.....-- --------------.--... --------- 20.00
Two thermoregulators .---------............ ---... ......----- 30.00
Two A. C. relays ...----................-------- .......----... 10.00
Sundries-paint, nails, etc. ............-....... .- .... ...... --- 50.00
Labor, including carpenter work -----..................... 150.00-200.00
(Amount depends on grade of labor and amount of
"Where a supply of cold brine is not available it is possible to
utilize one of the small commercial units now on the market.
For apparatus as large as that described the ordinary refrig-
erator unit does not have sufficient capacity but there are sev-
eral machines made for use in large commercial refrigerators
that are sufficiently large for this purpose. If one of these is
used the cooling coils should be placed directly in the end com-
partment in place of the brine tank (some units have coils sur-
rounded with a brine tank which should be removed) and the

10 Florida Agricultural Experiment Station

motor should be attached to the conventional thermostat and re-
lay instead of the commercial control commonly supplied.


After the apparatus had been in operation for some time, a
study of the thermal conditions within the system was made.
Copper-constantan thermocouples were used to measure the
temperature at various points in the pots and in the water in the
compartments. The measurements were made by the null-point
method, using a potentiometer and keeping the comparison junc-
tions of the thermocouples in melting ice.

Time of

March 30 35.2 35.2 35.5 35.2 35.6 26.0.
3:15 P.M.
5:11 P.M. 35.3 35.3 35.5 35.4 35.6 25.2
7:40 P.M. 35.0 35.0 35.2 35.2 35.7 ........
10:15 P.M. 35.0 34.8 35.1 35.1 35,8 21.2
March 31 35.0 35.0 35.1 35.1 35.3 23.4
8:30 A.M.
10:30 A.M. 35.7 35.7 35.7 35.7 35.2 29.6
12:00 35.6 35.6 35.7 35.6 35.0 28.4
1:00 P.M. 36.0 36.1 36.1 36.1 35.0 28.6
3:30 P.M. 36.7 36.9 36.8 36.9 36.2 26.0
5:00 P.M. 36.3 36.3 36.3 36.5 36.5 23.4
7:20 P.M. 35.8 35.8 35.9 35.9 36.5 21.4
9:25 P.M. 35.5 35.5 35.6 35.7 36.4 19.4
April 1 34.9 34.8 35.0 34.8 35.7 18.4
9:00 A.M.
10:00 A.M. 35.2 35.1 35.4 35.1 35.6 23.0

Bulletin 189, Soil Temperature Studies with Cotton 11

The accuracy of the thermostatic control was tested by plac-
ing a thermocouple in the end compartment and about 10 inches
from the thermostat, which was of the bimetallic type. By mak-
ing frequent readings and observing the opening and closing of
the thermostat, it was found that the range permitted was
about 1 degree C. This range might have been narrowed con-
siderably by use of a mercury thermostat and more rapid stir-.
ring but the advantages to be gained would have been offset by
the greater delicacy of the apparatus. Greater precision in con-
trol would have been offset by the variation in the pots due to
heating of the soil surface by the direct action of sunlight.


Time of Depth in mm. Air
Day 8 65 85 190 235 Temp.

9:20 A.M ............. 22.3 21.8 21.8 21.7 21.6 24.8
9:27 A.M. ............... 22.3 21.9 21.8 21.6 21.7 24.9
9:53 A.M. ............... 22.6 22 21.8 21.6 21.6 26.4
10:34 A.M ............... 23.2 22.4 21.8 21.6 21.6 27.5
11:00 A.M. ............... 23.4 22.6 22 21.7 21.6 29
11:30 A.M ............... 23.7 22.6 21.9 21.6 21.5 30.3
1:40 P.M. ............... 23.7 22.9 22.2 22 22 29.6
3:30 P.M .............. 23.2 23 22.8 22.2 22.2 23
4:35 P.M. ........... 22.2 22.3 22.2 22.1 22.1 21.6
8:26 P.M. ...........- 21.7 21.8 22 22 22 21
9:00 P.M ............ 21.7 21.8 21.9 22 22 20.8
8:20 A.M. ................ 21.3 21.4 21.6 21.6 21.7 20.2
10:30 A.M. .............. 22.3 21.6 21.6 21.6 21.6 23.4
To determine the variation in temperature at different points
in the same compartment, thermocouples were suspended in the
four corners of the warmest compartment containing soil pots.
The maximum difference in the four corners at any one reading
was found not to exceed 0.3 degrees C. The procedure was re-
peated several times with the thermocouples placed in different

12 Florida Agricultural Experiment Station

positions in relation to the pots; i. e. inside the circle of pots,
outside the circle and between the pots, with the same results.
This procedure was repeated in other compartments with like
results. This indicated that the agitation was entirely adequate
to prevent a compartment from being warmer on one side than
on the other due to the effect of the adjoining compartments.
The effect of the agitators was to keep the entire body of water
in a compartment moving with a slow circular motion and it was
realized that if the motion was too slow inequalities might de-
velop due to the cooling and warming respectively at the oppo-
site ends of the compartment. The measurements were made
fairly close to the pots. If temperatures had been measured in
the extreme corners of the compartments, greater variation
might have been found.

Time of Depth in mm. Air
Day 1 65 1 85 1 190 235 Temp.

1:37 P.M. ............... 34.6 34.8 35.4 35.6 35.4 28.2
1:56 P.M ...............- 34.7 35.0 35.4 35.7 35.7 26.2
3:38 P.M ............. 34.4 35.0 35.5 35.8 35.8 25.8
4:54 P.M ....... 33.9 34.7 35.5 35.8 35.8 25.0
7:50 P.M. ...... .. 32.4 34.2 35.3 35.8 36.0 18.8
9:20 P.M. ......... 32.1 34.0 35.2 36.0 36.0 16.8
8:10 A.M........... 32.1 33.4 34.9 35.4 35.4 19.3
10:40 A.M ............ 33.4 33.8 34.4 35.3 35.0 24.8
11:05 A.M ................ 33.5 34.0 34.6 35.4 35.0 25.5
11:27 A.M. ................ 33.8 34.0 34.5 35.4 35.0 25.4
11:46 A.M. ....-......... 33.9 34.1 34.6 35.4 35.0 25.8
2:20 P.M. ................ 34.4 34.4 35.2 35.5 35.8 26.4
4:25 P.M .......... 33.8 34.4 35.1 35.7 35.5 26.0
4:40 P.M. ................ 33.7 34.4 35.1 35.7 35.6 25.2
8:00 P.M .. ..... 32.5 34.1 35.2 35.9 35.8 19.0
8:12 A.M............ 32.1 34.2 35.0 35.6 35.6 18.0

Bulletin 189, Soil Temperature Studies with Cotton 13

The variation in temperature between pots in the same com-
partment was determined by burying thermocouples three and
one-half inches deep in the center of the four corner pots and
placing a fifth thermocouple in the water of the same compart-
ment. In Table I are given the results of this experiment, to-
gether with the air temperatures during the same period. At
any one reading there was little variation of temperature be-
tween the four pots, further indicating the efficiency of the
agitation. The fluctuation in water temperature over a 24-hour
period, as shown in Table I, is undoubtedly due to heat loss or
absorption through the insulation and the soil surface. If it
were possible to control the temperature of the surrounding
air within reasonable limits it would be much easier to obtain
accurate control than is the case where the air temperature
fluctuations are exceedingly large. With constant air tempera-
ture the deviation from the theoretical temperature would be a
constant instead of a variable quantity.
It was realized, of course, that there would be considerable
variation in the temperatures at different places in the same pot
and at different times of day. Thermocouples were buried at
various depths in the same pot as follows, 8, 65, 85, 190, and
235 mm., and near the center of the pot. Readings were taken
with the pot in various compartments and over varying periods
of time. Observations were made also of the rate of warming

.. ......---- -------

....... ............. .

.. ..........'

Fig. 2.-Temperature at vari... ,-l .il.pi'- in the -;,lilt soil p.:.tb. Upper
chart for pot in warm (*ii tii. i t. !,:.' er. It. .r' i ,:n I:..:'l ci.rnIIrt-
ment. 8 mm. deep,.... 65 mm,, -- -- -- m., -
-- -- 190 mm., ------ 235 mm. deep. .

14 Florida Agricultural Experiment Station

when a pot was moved from a lower to a higher temperature.
The results of these measurements in two compartments are
given in Tables II and III and shown graphically in Fig. 2. It
will be noted that the thermocouple 8 mm. below the soil sur-
face, which was covered with a layer of cork about 1/2 inch deep,
showed a considerable Variation, following rather closely the
variation in the air temperature, though the magnitude of the
fluctuations was much less than that of the air. There is un-
doubtedly some insulating value in the ground cork used on the

- i
.1 14

i -


Fig. 3.-Graphical representation of temperatures at various depths in a
soil pot that is changed from a cool compartment to one at a higher
temperature, change made at 10:30 A,M. 8 mm. deep, .
65 mm., ----- 85 mm., --- 190 mm., - - - -
235 mm.
235 mm.

Bulletin 189, Soil Temperature Studies with Cotton 15

surface of the soil, but it is impossible to add sufficient cork to
eliminate this variation without interfering with the plants.
As the depth increases the variations are materially reduced and
follow more and more closely the water temperature. The soil
surface may lose or gain heat by both conduction and radiation,
though the former is probably the cause of most of the varia-
tions if the apparatus is located in a greenhouse in which the
direct rays of sunlight are guarded against by coating the glass,
as is commonly done here. The variations from the water tem-
perature are inherent in any type 'of soil temperature control
apparatus that is used in a greenhouse without air temperature
control. If the air temperature above the soil could be main-
tained at the same temperature as the water in the tank, and
direct sunlight were not allowed to strike the pots, this sur-
face variation would almost entirely disappear, or if a constant
temperature could be maintained over the entire tank the devia-
tion of the surface temperatures of the soil from the water tem-
perature, in those compartments where the water is at a differ-
ent temperature from the air, would be constant and conse-
quently greatly minimized in importance. Such air control
would be very difficult in Florida, since it would require large
amounts of refrigeration applied with great care as to distribu-
tion, control, etc. Generally speaking, however, the degree of
soil temperature control obtained without air temperature con-
trol has been very satisfactory and, it is believed, quite as satis-
factory as that obtained with the ordinary single tank unit.
In Fig. 3 are shown graphically the readings obtained when
a pot was removed from a low temperature to a higher tem-
perature. It will be noted that most of the change in tempera-
ture of the soil took place in the first hour and that the rate of
heating slowed rapidly toward the last and it was only after
about two and one-half hours that a complete balance was at-
tained. The rate of warming, under such conditions, is prob-
ably governed by the difference in temperature between the pot
and the water surrounding it. This general relation should be
borne in mind when examining data on experiments in which
pots are alternated in tanks of different temperature.
In Fig. 4 are shown the mean daily readings for soil in the
various compartments during one of the experiments. These
readings were taken with mercurial thermometers thrust about

16 Florida Agricultural Experiment Station

3 inches deep in the soil in the center of the pot. Considering
the probable variations in the depth of the various thermometer
bulbs and the fact that the readings were taken by several peo-
ple, the figures agree remarkably well.

Fig. 4.-Daily soil temperature readings made by means of mercurial ther-
mometers thrust in the soil. Along the base line the hours of the day
and the date are shown and on the ordinate, the temperature in de-
grees C.
On the whole, it is believed that the control of soil tempera-
ture attained is quite as satisfactory as that obtained by use of
the single unit tank, while both the original cost and the upkeep
J9 ^f^^II ____- .--'--______ .

cost are probably consider aly lower for the type of equipment
described here. If this type of equipment were utilized in a
ig.chamber of constant air temperature readings made by means o meruria ther-
mometers thrust in the soil. Along the base line the hours of the day
and the date are shown and on the ordinate, the temperature in de-
grees C.

degrOn the f whole, it i as believed temperature control of soil tempera-should
ture attained is quite as satisfactory as that obtained by use oferiod.
the single unit tank, while both the original cost and the upkeep
cust are probably considerably lower for the type of equipment
described here. If this type of equipment were utilized in a
chamber of constant air temperature it should give a very high
degree of effk'ieil.-y as regards temperature control and should
need a very small amount of adjusting over a long period.

Bulletin 189, Soil Temperature Studies with Cotton 17


During the course of work on soil-borne diseases of cotton,
considerable data have been collected on the rate of germination
and on the rate of growth of cotton seedlings at different soil
temperatures. Such information concerning the normal rela-
tions of the plant is fundamental to the interpretation of the
pathological condition and for this reason has seemed to justify
the detailed report given in this bulletin. In this connection all
of the checks in the various experiments are available and in
certain series of experiments where cultures proved non-infec-
tious the data on all the plants are also available. Particularly
desirable data are available for three series of experiments in
which seed of Lightning Express No. 3 was used and three se-
ries in which Express No. 432 was used. In addition to these,
one experiment was run especially to determine rate of germi-
nation, using seed of Express 432. Data on experiments in
which seed of other varieties were used are available, but are
not sufficient to use as a basis of comparison of several varieties.


In all the experiments reported here, steam sterilized soil was
used. After sterilization sufficient water was added to put it
in a good friable condition and the soil was thoroughly mixed
to insure it being uniformly moist throughout. The soil pots
from the tank were then filled, all the pots being filled to the
same weight. About one-half to three-quarters inch of ground
cork was placed on the surface, the same amount by weight be-
ing added to each pot. This was ground cork of the quality
commonly termed "doll cork" and was clean and bright and free
from cork dust.
The pots were then placed in the tank and allowed to remain
long enough for them to come to a temperature equilibrium be-
fore planting. The pots were weighed daily and the amount of
moisture lost by evaporation made up so that the soil contained
approximately the same percentage of moisture throughout
each experiment. The seed weie planted about one and one-
half inches deep, measuring from the surface of the soil rather

18 Florida Agricultural Experiment Station

than from the surface of the cork. In recording germination
the plants were considered as "up" when the cotyledons were
clear of the cork. It was found that the cork, being very light,
did not bother the plants and that they grew quite normally. In
the early experiments records were made only once each day so
that it is possible to have a considerable error in the recorded
time of germination at those temperatures where germination
takes place in less than 4 or 5 days. It might happen in two ad-
joining tanks that, in one, the plants would come up just after
the reading was taken, whereas in the other they came up just
before the next reading, thus causing an error in the neighbor-
hood of 20 hours in the record. In a final experiment this error
was eliminated by making readings every 2 hours, until ger-
mination was complete at the higher temperatures. In this ex-
periment the very first appearance of any part of the plant was
also recorded, the cork being moved about gently so as not to
injure the plants.
Measurements to determine the rate of growth were made on
the height of the plants, these measurements being made from
the level of the top of the pot to the terminal bud and a correc-
tion made for the distance from the edge of the pot to the soil
The measurements were made from the top of the pot instead
of from the soil level to avoid variations in the soil level due
to watering. The same correction was made for all plants in
the same pot but the correction varied slightly for different pots.
The figures given in the tables are the corrected figures and
indicate the distance from the soil surface to the terminal bud.
In the course of the experiments certain plants developed ab-
normally and all such plants were eliminated from the measure-
ments, such abnormalities as twisted or missing cotyledons,
plants obviously diseased, as indicated by abnormally slow
growth or other symptoms, were consequently discarded and
only those that showed a normal growth were used. In the lat-
ter series of experiments only those plants in a tank coming up
at the same time were measured, thus eliminating the factor of
unequal starting. The measurements were continued over a
comparatively short time, about two weeks, in order to avoid
the probable effects of pot binding after this time.
The temperatures in the various compartments varied a little
with the different experiments so that the temperatures of one

Bulletin 189, Soil Temperature Studies with Cotton 19

experiment may be different from those of another. The figures
given for temperatures -were the averages of the daily readings
obtained from a thermometer thrust about three inches deep in
the soil, in the center of the pot. Deviations from this tempera-
ture due to surface effects, as noted in the description of the
tank, are to be expected. At the higher temperatures it is prob-
able that during the night the temperature at the level of the seed
fell below the figure given and at the lower temperatures it was
slightly higher during the day than the figure given. The read-
ings themselves, from which the average was obtained, seldom
varied more than a degree from the mean.


In Figs. 5 and 6 the number of days it took cotton seed to
germinate is plotted against the temperatures of the soil. In
Fig. 5 we have the data on three different experiments in which
seed of Lightning Express No. 3 were used and in Fig. 6 the
data from the same num-
ber of experiments in
which Express 432 were
used. Since the observa-
tions were made only once
Sa day the num ber of days it
2 o took to reach 90% germi-
1 + nation at each temperature
1 x was used instead of the
9 average number of days.
8 xo Generally speaking the ex-
7 o perimental figures fall fair-
S+ 0 ly close together and when
5 .x+ x ox+ o x+
5 + + + 0 + allowance is made for the
24-hour readings as pre-
viously mentioned the fig-
ures are in very good
S2 2 22 28 30 agreement. Slight differ-
18 20 22 24 26 28 30 32 34 36 38 40'E.
ences in planting depth
Fig. 5.-Number of days required for might have occurred in dif-
Lightning Express No. 3 to reach
90 percent germination at various ferent experiments and it
soil temperatures. Each type of is possible also that there
symbol (X, O, +) gives the re-
sults of a single experiment, were some differences in

20 Florida Agricultural Experiment Station

soil texture and moisture. A further variable is to be found
in the weather which might have had more influence on the sur-
face temperatures of the soil in one experiment than it had in
In Fig. 7 we have plotted the data from an experiment in
which seven pots with six seed each were used at each tempera-
ture and the readings taken at frequent intervals (2-hour inter-
vals until germination was complete at the higher tempera-
I tures). Each plant was re-
17 corded separately as to the
16 + time of first appearance and
5 the time when the cotyledons
x were clear of the cork. Each
3 plant was noted separately
and the number of hours for
a germination was recorded,
9 + omitting seed which failed to
a germinate. The mean num-
7 x ber of hours at each tempera-
Sture with the probable error
4 o o of the mean is given in 'Fable
X +X +oX o o
x x I
z In all experiments where
soil temperatures of 400 C.
14 16 is 2D 22 24 Z6 28 3o 3 36 40-o or above were employed, no
Fig. 6.-Number of days required for germination took p lace
Express 432 to reach 90 percent though there was germination
germination at various soil tem-
peratures. Each type of symbol at temperatures as high as
(+, X, 0) shows the results of a 390 C. This establishes quite
single experiment.
satisfactorily an upper limit
for germination. At temperatures above 34-35 C. there is a
falling off in the rate and usually in the percentage of germina-
tion; if the seed tend to be "weak germinators" there are fre-
quently produced numerous abnormalities such as failure to
lose the seed coat, twisted cotyledons, etc. At the opposite ex-
treme it is much more difficult to set a limit. Germination has
been recorded at temperatures of 141/2 and 150 C. but at tem-
peratures below 140 C. no germination has been obtained. It is
believed, however, that if both seed and soil could be kept sterile,
germination might be obtained at lower temperatures. At tem-
peratures below 200 C. there is commonly a falling off in the

Bulletin 189, Soil Temperature Studies with Cotton 21

percentage of germination as shown in Table IV, as well as the
production of malformed plants. The reduction in germination
and the production of such abnormal plants at both high and low
temperatures is variable, depending to some extent on the var-
iety but probably more on the physiological condition of the par-
ticular lot of seed. Generally speaking we may say that the ex-
tremes of temperature tend to accentuate such weakness, result-
ing in lower germination and more abnormal forms. Some lots
of seed will maintain germination and uniformity remarkably
well throughout the range while another lot from the same var-
iety will show great variation in this respect. Under field con-
ditions it is doubtful if very much germination can be expected
when the mean soil temperature is below 150 C.


Temp. No. of Plants First Appearance of Cotyledons Clear of
Centigrade From 42 Seed Any Partof Plant ork

40 0 (no germination)
38.7" 37 58.6 0.932 71.2 0.823
34.8 39 54.9 0.683 65.4 0.574
31.80 40 61.4 0.504 72.2 0.803
28.3 41 78.8 0.708 89.1 0.911
25.20 41 95.9 0.911 109.6 1.351
22.4 33 131.6 2.23 145.7 2.195
19.0* 25 226.3 3.38 234.3 3.477

The establishing of the "optimum" temperature for germina-
tion is difficult especially in so far as the results charted in Figs.
5 and 6 are concerned. These results would seem to indicate
the effect of a limiting factor as evidenced by the flatness of
the curve at temperatures above 32 C., though there was evi-
dence that it was due to the length of the period between ob-
servations. On this account the subsequent experiment was
carried out as graphed in Fig. 7. In this experiment there was
distinctly slower germination at 38.7 than at 34.80 C. and as
in previous experiments no germination occurred at 400 C. Of


22 Florida Agricultural Experiment Station

the temperatures used the quickest germination occurred at
34.50 C. In Fig. 7 a smoothed curve has been drawn to coin-
cide as nearly as possible with the. experimental data (except
the results at 38.70) and observation would indicate that this
temperature (34.50) is at least very close to the "optimum" tem-
perature for germination. This is in good agreement with the
work of Balls ('19) who gave an optimum for the growth of
cotton roots of 330 C., based on observations under controlled



"o 0

Temp. 19 2021 22252425262728293031 323334353637383940
Fig. 7.-Mean number of hours required for germination of Express 432.
Concentric circles represent means for first appearance of any part of
the plant, single circles mean number of hours until cotyledons are
clear of the cork. Curve smoothed to show typical Van't Hoff curve
as based on data.

The long period between observations in the first six experi-
ments reported in this paper (Figs. 5 and 6) makes a compari-
son at the higher temperatures difficult, nevertheless they seem
to agree very well with the experiment in which the observa-
tions were closer together, the latter experiment merely clear-
ing up the points overlooked in the earlier ones.

Bulletin 189, Soil Temperature Studies with Cotton 23

Between temperatures of 150 and 340 C. all curves of germ-
ination agree very well with the so-called Van't Hoff rule.* The
factor (Q 10) increases at the lower temperatures and becomes
smaller at the higher portions of the range but this is quite in
agreement with the observations on many chemical reactions.
Considering the fact that it is a biological process of a very
complicated sort and dependent upon many environmental fac-
tors, the agreement with the Van't Hoff generalization for chem-
ical reactions is remarkably good.
At the higher temperatures (38-39 C.) there is a very de-
cided lengthening of the time required for germination. The
curve for rate of germination (if we define the rate as the aver-
age length of time necessary for germination), therefore, fol-
lows the curve commonly obtained when the relation of bio-
logical processes to temperature is studied, in that, at tem-
peratures below the optimum, the curve agrees readily with the
Van't Hoff generalization concerning the effect of increasing
temperature on the rate of chemical reactions; while at tem-
peratures above the optimum there is a rapid falling off in rate
as the lethal temperature is approached.
It is well to point out the comparatively narrow range be-
tween the optimum temperature for germination and the lethal
temperature, and the comparatively rapid germination even
at 38.80 C., only 1.20 below the lethal temperature.


Daily measurements of the heights of the plants were made in
several experiments but the results in all cases were so similar
that only one typical example will be discussed here. In Fig.
8 the daily average height, in millimeters, at each temperature
*The so-called Van't Hoff rule (see Van't Hoff ('96) ) for the relation
of chemical reactions to temperatures as commonly used by biologists
states that the rate of a chemical reaction doubles for a rise in tempera-
ture of 100 C. Actually it is a much broader generality and the state-
ment that the rate of a chemical reaction about doubles or trebles with
a rise in temperature of 10 degrees more nearly states the situation. The
temperature coefficient (Q 10) representing the relationship of rate to
.10 degrees rise in temperature actually varies quite widely and is com-
monly found to be as low as 1.5 for some reactions and as high as 4 or 5
for others though never falling as low as similar values for reactions
that are purely physical. In this respect a chemical reaction may be eas-
ily distinguished from a physical process, especially in biological pro-
cesses where other indications are obscured. (See Fawcett ('21.))


38.5 C 37" C 34 C 31 C 28.50 C 24.5C 22.50 C 19' C

3/8 42.41 0.71 44.990.90 43.660.75 33.582.11 17.8 0.89
3/9 45.08 -1.53 62.99-0.81 61.08-0.73 55.330.93 33.412.77 16.920.06
3/10 55.66 1.28 57.220.69 75.491.13 74.160.98 67.251.16 45.490.76 29.411.20
3/11 58.58 1.51 64.390.99 84.910.921 82.830.81 77.081.26 54.05-1.11 36.991.05
3/12 60.25 1.49 72.390.96 94.411.35 91.49-0.99 84.421.30 61.93+1.28 45.83-1.47 13.660.45
3/13 62.75 1.48 82.161.02 106.001.63 101.9 1.08 95.25+1.50 71.45-1.43 55.91-1.14 119.8 0.96
3/14 64.58- 1.6 91.911.35 115.6 1.88 112.05-1.15 104.4 11.68 80.16:1.50 63.66-1.22 28.001.29 t
3/15 66.91 1.17 101.241.54 125.6 2.21 119.8 1.35 112.6 1.93 87.86-1.69 70.911.27 36.661.69 C
3/16 69.99 1.19 107.77--1.58 134.052.51 128.2 -1.54 121.0011.87 95.081.66| 79.33-1.52 44.331.45
3/17 72.83 1.14 112.3 1.52 141.9 -2.67 137.25-1.67 129.6 1.90 101.9 1.68 86.49+1.66 50.331.23
3/18 73.99 1.01 116.8 -1.75 148.6 2.87 142.551.73 137.7 2.09 108.551.9 92.66-2.18 55.16-1.38
3/19 76.08 1.03 121.6 1.79 156.1 -2.97 149.8 1.77 145.88-2.16 117.1 1.96 99.8 2.62 60.661.31
3/20 78.91 0.97 128.661.82 167.4 -2.92 15 5.i.- 1.91 155.45-2.21 125.6 -2.04 107.4122.83 66.161.88
3/21 81.99 1.31 133.6 2.04 175.7 :.89 167.9 2.29 162.5 -2.29 133.9 -2.181 113.453.02 71.5 1.5
___________ _____________________________________

Bulletin 189, Soil Temperature Studies with Cotton 25

is plotted against the time in days; in Fig. 9 each day's measure-
ments of heights are plotted as a separate curve against the
temperature in degrees Centigrade; in Fig. 11 the mean daily
increments for each temperature are plotted against time;' in
Table V are found the mean heights at the various tempera-
tures from which these curves were plotted. From Fig. 8 then
we may follow the increasing heights of the plants at any one
200 340

2 .5
150 4

50 -

MAB.8 9 10 11 12 13 14 15 16 17 16 19ZO 21 2223 24
Fig. 8.-Growth in height of cotton plants at various soil temperatures.

temperature as a single line, whereas in Fig. 9 we may get a
bird's-eye view of the plants in all the temperatures on any par-
ticular day, the latter curve being particularly interesting. Fig.
10 shows a series of plants near the close of an experiment and
gives photographically the same information as is graphically
given in Fig. 9. There is a distinct peak in these experiments
which is made more apparent by the interpolation of an extra

26 Florida Agricultural Experiment Station

tank at 37 C. which was carried on at the same time as the rest
of the experiment but under separate control. If the -daily
curves shown in Figure 9 were smoothed the peak would seem
to fall between 330 and 340 C. and probably closer to 33. This
optimum would correspond well with that found for germina-
tion of cotton seed in previous experiments, and probably rep-
resents a very nearly true optimum for soil temperature as re-



"15 DEyC. 19 -225 22 3 34 3
Fig. 9.-Heights of cotton plants in mm. from day to day to show rela-
tive heights of plants at different temperatures throughout an experi-

lated to both germination and growth and agrees quite well with
Balls' data. This "optimum" as shown in Fig. 9 is extremely
marked and might easily be misinterpreted if the daily incre-
ments were not examined as plotted in Fig. 11. It will be seen
from Fig. 11 that except at the very low temperatures and the
very high temperatures there is comparatively little difference
in the "rate" of growth, if we may apply this term to the daily
increase in height. Thus from 25 to 350 C. there is but little
difference in the "rate" of growth, whereas within the same
range the "rate" of germination follows the Van't Hoff rule very

Bulletin 189, Soil Temperature Studies with Cotton 27

closely. This is probably to be interpreted as the effect of a
limiting factor in the atmospheric conditions surrounding the
aerial portion of the plant (see Blackman '05). In other words
we may say that when the leaves and stems of the plant reach
the air and take up their various functions there is some factor
which will not permit of growth as rapid as the soil temperature
would allow. This limiting factor might be any one of a num-
ber of factors such as illumination, air temperature, CO. (car-
bon dioxide) supply, or soil moisture, though it is believed that
there was sufficient moisture in the soil to take care of growth


Fig. 10.-Relative heights of cotton plants after approximately two weeks'
growth at different temperatures. Temperature in degrees C. shown
on each pot.

for the period of these experiments. Considering the fact that
these plants were grown in a greenhouse the illumination might
easily be the limiting factor, and CO0 supply has frequently
been reported as a limiting factor under such conditions.
The general relationship of the various factors in the above
experiment may be illustrated as follows: Suppose that there is
only sufficient CO in the air to permit of a growth rate which
we may designate as y, then if other factors than CO2 concentra-
tion are adjusted to permit a greater rate of growth than this,
the supply of CO0 becomes the limiting factor and the growth
does not exceed the value y. Now if we designate the rate of
growth allowed by any particular soil temperature (t), as t. x,
in which x is a variable denoting the relationship of growth to
temperature; we may say that as t varies below the optimum
temperature, t. x should vary according to the Van't Hoff gen-

28 Florida Agricultural E.,pI in;,,.,t Station

eralization. However, this relationship holds only as long as
t. x ( y, since, if the theoretical value t. x exceeds y the growth
rate will remain at the value y, for raising the soil temperature
does not increase the CO- supply. Now if the value of y is in-
creased by adding CO to the atmosphere until y) t. x, when the
value of t represents the optimum soil temperature, and if no
other limiting factors come into play, then we have a condition
in which the soil temperature is the effective factor throughout
the range from the lower limit of growth to the death point. In
studies on the effect of soil temperature on growth it would be

I 24\

10 570 / --

P. 9 10 II I 13 14 15i 16 17 i18 0 20 l
Fig. 11.-Mean daily growth increments, in mm., of cotton plants grown
at various soil temperatures showing probable effect of a limiting fac-
tor in the environment.

desirable to raise the value of "y" so that at the optimum soil
temperature t. x limiting factor throughout the range. However, with the com-
plication of factors affecting the plant it is by no means certain
that this could be accomplished.
In this connection, if the differences between the curve for
March 12 and the curve for March 21 in Fig. 9 were plotted, we
would obtain a fairly flat-topped curve representing the typical
effect of a limiting factor. When this is done it becomes appar-
ent that the peak in the succession of curves in Fig. 9 is based

Bulletin 189, Soil Temperature Studies with Cotton 29

very largely upon the initial stimulus obtained during germina-
tion. Thus, near the central portion of the temperature range,
plants grow at an almost constant rate when factors other than
the soil temperature come into play, but maintain their relative
heights as established by different periods of germination.
At the extremes of the temperature range we find a pro-
nounced effect of soil temperature, as would be expected. In
Fig. 11 it will be noted that there is a great difference in rate
of growth between 37 and 38.50 C., the rate at 37.00 being two
to four times faster than the rate at 38.50, whereas there was
comparatively little difference in the time required for germina-
tion between 34 and 38.50 C. This may possibly indicate a
cumulative effect of high temperature as mentioned by Black-
man ('05) and Balls ('19), or it may indicate that the effect of
temperature on root production is so pronounced that the plant
soon has insufficient root system to care for the top. Even in
the latter case it could easily be true that thermo-toxy as de-
fined by Balls ('19) could explain the reduction in rate of root
It is, perhaps, significant to note in this connection the ab-
normal characters of the underground parts of the young cot-
ton plants grown at the high temperatures, 380-390 C. When
plants grown at these temperatures were pulled up at the end of
an experiment, the tap roots were found to be browned and
shrunken with but one or two short brown and shrunken lat-
erals. Comparable plants grown at 34.50 C. showed none of
these characters, being clean and white and well filled out. The
laterals of these plants were much longer than those described
for the higher temperature and of a clear white color. None of
the repeated cultures made from the browned roots of plants
grown at 380-390 C. temperature showed the presence of a fun-
gus or bacterium, to which might be attributed the symptoms
The extent to which these temperature relations hold in the
field is at present unknown. We have available soil tempera-
ture data for sandy soil at Gainesville for two crop years, and
in Table VI are given the weekly mean soil temperatures for
March, April, and May of 1926 and 1927 taken 6 inches below
the surface. Cotton is usually planted from March 25 to April


30 Florida Agricultural Experiment Station


Week Ending Mean Week Ending Mean

(1926) (1927)
March 1st .............. 15 March 2nd .... 16
March 8th ................ 14 March 7th ........ 10
March 15th ................ 13 March 14th ...... 18
March 22nd ............. 16 March 21st ..... 23
March 28th ................ 18 March 28th .... 18
April 5th ................. 16 April 4th ....... 23
April 12th ................. 22 April 11th ...... 25
April 19th .................. 22 April 18th .... 24
April 26th ............... 24 April 25th ... ... 25
May 3rd ................. 24 May 2nd ............ 25
May 10th ............... 24 May 9th ............ 28
May 17th .... ...... 26 May 16th ...... 28
May 24th .............. 26 May 23rd ... 27

5, at which time the soil, at a 6-inch depth, is about 20 C. with
levels closer to the surface slightly warmer and also fluctuating
through a wider daily range as illustrated in Fig. 12 in which
specimen curves are given for the one and six inch depths. It
would appear from the temperature data that the above-men-
tioned planting period is about as early as is advisable, since
earlier plantings would be very distinctly affected by the soil
temperature, making both germination and growth very slow.
By the middle of April factors other than soil temperature have
probably become the limiting factors in growth-moisture supply,
food supply, air temperature, CO0 supply, etc.,-in fact there is
reason to believe that the food supply rapidly becomes the limiting
factor in growth, when moisture and other conditions are fav-
orable. Generally speaking, it is doubtful if soil temperature
is a limiting factor for germination or growth of cotton beyond

Bulletin 189, Soil Temperature Studies with Cotton 31

April 15 in normal years, while in some years other factors,
particularly soil moisture, may supersede it entirely by the end
of March.

1.' u"P" -

y' $1 .t.,. -p,.<
^ *^ "*-* rV is - 4 - s i.- .t?4 ^ J

r- , i, ' r : : r 1

: V.-4 '4- A "-- *- ,.1 :<^ 4" ,* .i 7* '*
I.(-.^) .,;: .....F /-- i. 11,, ,

o-- :n s a..nds.... ndy. .i.s t im t .
.'*" '-" '- -.-- ,- ....

Fig. 12.-Soil temperature (Centigrade) at 1 inch and 6 inch soil depths
on clean, sandy soil-specimen chart.

A soil temperature tank, based on an internally balanced sys-
tem and which is believed to be more economical than the sin-
gle unit type, is described and its efficiency discussed.
Experiments on the germination of cotton at various soil tem-
peratures showed an optimum of 33-34 C., and no germination
at 40 C. No germination was obtained at 140 C. or below, but
under more nearly sterile conditions it is believed that germina-
tion might be obtained. Between 150 and 350 C. the rate of
germination agrees very well with the Van't Hoff rule.

82 Florida Agricultural Experiment Station

Measurements of the growth of cotton seedlings indicated
that under the conditions of the experiments some environ-
mental factor other than temperature largely limited their
growth between approximately 200-350 C., temperature being
a limiting factor only above or below that range.
Outdoor soil temperature records for the last two years indi-
cate that the usual planting period (March 25 to April 5) is
about as early as strong germination of cotton seed can be ex-


Balls, W. Lawrence ('19) The cotton plant in Egypt. 202 pages. London,
Blackman, F. F. ('05) Optima and limiting factors. Ann. Bot. 19: 281-
295. 1905.
Fawcett, Howard S. ('21)~ The temperature relations of growth in cer-
tain parasitic fungi. U. of C. Pubs. in Ag. Sci. 4: 183-232. 1921.
Jones, L. R., James Johnson and James G. Dickson ('26) Wisconsin stud-
ies upon the relation of soil temperature to plant disease. U. of Wis.
Ag. Exp. Sta. Res. Bull. 71. 144 pages, 59 figs. 1926.
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