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 CONTENTS
Page A
Introduction ....................... 3
Soil Factors Influencing Temperatures ............. 3
Plant Response to Soil Temperatures .............. 4
Corn ...................... 4
F Soybeans .................... 5
. Sorghum .................... 6
Cotton ..................... 6
Tobacco ..................... 7
Vegetable Crops .................. 7 .
Source of Soil Temperature Data ............... 8
Comparison of Soil Temperatures at Various Planting Depths .... 1 1
Annual Soil Temperature Cycle at Selected Locations ...... 12-21
Henderson .................... 12
Mayfield .................... 13 I
Greenville .................... 14
Glasgow ..................... 15
Bardstown .................... 16
Somerset .................... 17
Williamstown ................... 18
Flemingsburg ................... 19
Lexington .................... 21
Freezing of Soils During the Winter Season ............ 20 i
Literature Cited ...................... 23
ACKNOWLEDGMENTS
The authors acknowledge assistance of personnel of the National Cli-
matic Center, NOAA, for their evaluation of the soil temperature observa-
tions and to the University of Kentucky Computing Center for help in the
preparation of the annual temperature graphs.
2
 {5-.- —

 $0Il TEMPERATURE CUMATOEOGY of KENTUCKY
by JERRY D. HILL and ALLEN B. ELAM, ]R.*
Soils have several physical characteristics which make them suited or, in some cases, un-
suited for man’s use. In addition to a soil’s structure, texture, and profile, there are certain
thermal characteristics which determine the temperatures to be experienced at various depths
under given conditions of moisture and exposure. These conditions vary over a reasonably wide
range, but there are fairly narrow limits in which they will frequently fall. The purpose of this
publication is to discuss the effect of soil temperatures on several plant species, to present the .
temperatures normally measured in the soil at a variety of locations, and to give a reasonable
estimate of what might be expected at any particular time of year.
- SOIL FACTORS INFLUENCING TEMPERATURE
Except for a very small amount produced by biological processes, the source of all heat
reaching the soil is solar radiation. The nature of the soil surface determines the amount of this
radiation which will be absorbed and the amount reflected back into the atmosphere. The term
"albedo" is normally used to characterize the ratio of radiation reflected to the total initially
falling on the surface. Color plays an important role in determining the albedo, with dark soils E
absorbing about 80% of the radiation falling on them and light-colored soils absorbing only about
50%. In mid-latitude locations, such as Kentucky, the direct rays of the sun always strike level
soil at an angle and never from directly overhead, even in midsummer. Those soils on a south-
facing slope face the sun and, thus, receive somewhat more concentrated radiation and have more
heat available per unit area of surface than those on a level or north-facing slope.
The sun’s energy absorbed by the soil is used initially to raise the temperature of the surface
layer, then it penetrates downward to heat progressively deeper layers. The amount of heat
required to change the temperature of a given volume of soil by 1 degree is called its heat
capacity, while the rate at which it transfers heat downward is called the thermal conductivity.
Both properties vary somewhat in a given soil, owing to its moisture content and the amount of
air in the pore spaces.
As for heat capacity, a typical soil requires about 0.3 calorie of heat energy to raise the
temperature of a cubic centimeter of soil by I degree centigrade. For the water and air com- I
ponents of the soil, these values are:
water—1 calorie per cubic centimeter per degree Centigrade and
air—0.00026 calorie per cubic centimeter per degree Centigrade, thus indicating that water
requires a much greater amount of heat to raise its temperature by l degree than would an equal
volume of air.
As the soil surface warms, there is a transfer of heat downward, and the rate at which it
proceeds varies with the thermal conductivity as noted before. Again, the values vary consider-
ably, but some approximate magnitudes for thermal conductivity are:
Silt loam soil - 0.002 calorie per square centimeter per second per degree C.,
Water - 0.001,
Air - 0.00006.
*Advisory Agricultural Meteorologist and State Climatologist (retired), NOAA, National Weather Service, respectively.
3

 The low thermal conductivity of the air makes it a much less efficient transporter of heat than is
soil, while water is of the same order of magnitude as the soil particles themselves.
By comparing the values given for both the heat capacity and thermal conductivity, we see
that soil with a high percentage of water in it requires more heat to change its temperature than
does a dry soil; however, a wet soil conducts heat downward to the lower depths much better. A
very dry soil with a large proportion of air in the upper layers would tend to have very high
daytime temperatures near the surface but much less of a temperature change below 6 to 8
inches.
__ The soil surface in contact with the atmosphere is the point of greatest heat exchange.
ii During the day it receives heat from the sun, and at night the heat is radiated away. Since the
heat capacity of the soil requires that it lose much more heat than the air before its temperature
changes by 1 degree, the soil temperatures tend to remain much more stable than air tempera-
tures. Under a sod cover, the soil temperature will have less than l degree variation during the
day below a depth of about 20 inches. This means that at night and, usually, during the winter
the deeper depths are warmer than the surface and heat flows upward, while on sunny days the
surface is warmest and heat flows downward.
PLANT RESPONSE TO SOIL TEIVIPERATURES
Most biological responses take place at a rate determined directly by the temperature sensed
by the organism. For the plant, photosynthesis proceeds at a rate determined by the leaf
temperature. This is a complex interaction of air temperature, moisture availability, and the
amount of solar radiation falling on the leaf. Many of the other plant processes such as germi-
nation, nutrient uptake, and translocation, which take place below the soil surface, depend on
the soil temperature at that depth. Since the soil temperature undergoes a daily cycle at most
planting depths, we normally speak of a single figure, the daily average, which is the average of
the daily high and low temperatures at that point. n
Many researchers have studied plant response, particularly germination, in relation to soil
temperatures, and it would seem appropriate to list some of their findings here for the principal
crops grown in Kentucky. These studies have taken two main approaches, the first being a
determination of emergence time required at various temperatures. The basic reasoning here is ·
that a long period under the soil surface exposes seed to avariety of pathogens and insects, thus
reducing the chance of emergence and the probability of a satisfactory plant stand. The second
approach is to determine the threshold temperatures below which and above which germination
does not occur. Then an optimum range can be established where germination will be most rapid
and likely. Since seed is an expensive production item and sometimes in short supply, soil
temperatures are a key to obtaining maximum production from available resources.
Corn
The minimum temperature for germination of corn has been determined to be about 500 F,
although the rate here is slow and the optimum would be somewhere above 60°. It would seem
that corn planted in a seedbed at temperatures below 50° has very little chance for germination.
The upper lethal temperature for the seed is about lO7° which is not an unusual temperature for
a bare soil surface in mid-summer. Farmers who might be considering mid-summer planting of
short-season hybrids for silage should consider that temperatures at shallow depths in dry soil
could very likely exceed the lethal level on a sunny day and result in very poor stands.
Results of emergence studies of corn made at various depths and temperatures (l) are shown
in Table l.
4
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 TABLE 1.—CORN: DAYS TO 80% EMERGENCE.
Soil Temperature Planting Depth (inches)
(°F) 1 3 5
44 No emergence in 24 days
56 16 days 21 days 24 days*
68 8 7 8
80 4 4 4
*60% emergence at the end of 24-day test period.
There was very little variation in the emergence time at various depths, but temperatures had
a much greater effect in determining the time the seed remained below the soil.
Soybeans
l Emergence studies made on two varieties of soybeans grown in Kentucky (2) indicated 4
about the same response to temperature in both. The seedlings exhibited essentially no growth at
50° and 104° F, with a maximum growth rate at about 86° F. Figure 1 indicates the time
required to achieve 50% emergence using a planting depth of 1 inch.
20
15
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5
50 60 70 80 90 100 110
Mean Temperature (Degrees F) at Seed Level
Fig. l.—Days to 50 percent emergence at various soil temperatures.
5

 Sorghum
The sor hums have tro ical ori ins and as a result, seem to ow best at hi h tem eratures.
S P 8 , 8* S P A
Studies of emergence (6) show that they are less sensitive to depth than to temperature. There
appears to be little effect of planting depth on days to emergence except at the cooler tempera-
tutes where the deeper planting has a pronounced retarding effect (Table 2). The germination
percentage also shows a similar response (Table 3).
TABLE 2.—SORGHUM: DAYS TO EMERGE.
 
‘ -
Et-
Temperature Planting Depth (inches)
(OF) 0.5 1.5 2.5
 
59 6 9 11
68 7 8 8
77 5 4 5
86 4 4 4
95 4 4 5
  .
TABLE 3.-—SORGHUM: PERCENT GERMINATION.
 
 
Temperature Planting Depth (inches)
(°F) 0.5 1.5 2.5
 
59 71 58 56.5
68 80.5 75 72.5
77 79 80 78.5 ‘
86 77 83.5 82
95 82 82.5 74
 
Cotton ‘
Because of its long growing season, it would seem advantageous to plant cotton as early as
possible. However, cotton does not emerge as rapidly as some other plant species, so early
planting at cool soil temperatures is conducive to seed rot and poor stands. To reduce emergence
time to about 7 days, average soil temperatures should be 68° F or higher. Each 2 degrees added
to the soil temperature can reduce emergence time by about 1 day, as indicated by Fig. 2(5).
I2
I I
LD
E IO
P-
z B 9 . . .
4 Fig. 2.—Emergence time required by cotton (5).
-¤ 5] B 4
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PERIOD I899-ISM E7}!
INFORMATION COLLECTEDA kx
FROM UNOFFICIAL SOURCES . I
Fig. 15.—Average depth of frost penetration (inches), U.S.
• 0
WILLIAMSTOWN
• 14
FLEMINGSBURG
5 O
• 0 LEXINGTON
T O
• 0 IRVINGTON BARDSTOWN 3
HENDERSON O
BEREA
I
O
O · · O • 0 SOMERSET
'°'°""'CETON GREENVILLE GLASGOW cuM6I2m.AN¤ 0
GAP O
I
` Fig. 16.—Number of days in average year in Kentucky with minimum temperatures less than 32°l·` at 4 inches below sod surface.
Datapcriodz l967—71.
22

 TABLE 11.-LOW SOIL TEMPERATURES, LEXINGTON, KY., 1967-1971.
Number of days in Average Year the Soil Temperature Fell Below
320, 300, 280 F. and Lowest Temperature Recorded (°F)
Inches Days Below 320 Days Below 300 Days Below 280 Lowest Temperature
Below Under Under Under Under
Surface Bare Soil (Sod) Bare Soil (Sod) Bare Soil (Sod) Bare Soil (Sod)
1 33 12 19 1 12 * 150 140
2 26 8 13 O 8 O 170 300
4 13 5 7 O 4 0 180 300
8 8 * 0 O O 0 300 310
*Less than 0.5 day. Other data rounded.
_ Figure 16 and Table 11 do not indicate the actual depth of penetration of freezing tempera-
tures since observations were made only at certain points in the profile. However, they should be _
of value in supplying some indications of frequency and extent of penetration of freezing
temperatures. Penetration of freezing temperatures at a given site is dependent on a number of
factors—including moisture content of the soil, soil type, soil cover, and topography. Therefore,
it is suggested that for specific locations the data in the attached figures and table be supple-
mented by actual reports. For example, utility companies and cemetery custodians often can '
supply such information.
LITERATURE CITED
1. Alessi, _]. and Power, _]. F. 1971. Com Emergence in Relation to Soil Temperature and
Seeding Depth. Agron. _]. 63:717-719.
‘ 2. Egli, D. B., Hatfield, _]. L., Hill,]. D., and Tekrony, D. M. 1973. The Influence of Soil
Temperature on Soybean Seed Emergence. Agronomy Notes, University of Kentucky
Dept. of Agronomy, Lexington, Ky. 6(2). 3pp.
3. Hambidge, G. (editor). 1947. The Yearbook of Agriculture: Climate and Man. U.S. Govern- ·
ment Printing Office, Washington, D.C. 1,248 pp.
4. jaworski, C. A. and Valli, V. J. 1964. Tomato Seed Germination and Plant Growth in
Relation to Soil Temperatures and Phosphorus Levels. Proceedings of the Florida State
Horticultural Society 77: 177-183.
5. Riley, _]. A., Newton, D. H., Measells, _]. W., Downey, D. A., and Hand, L. 1964. Soil
Temperatures and Cotton Planting in the Mid-South. Miss. Agr. Exp. Sta. Bul. 678.
· 6. Shaw, B. T. (editor). 1952. Soil Physical Conditions and Plant Growth. Agronomy Mono-
graph No. 2. American Society of Agronomy. Published by Academic Press, New York,
N. Y.
7. Toole, V. K., Webster, R. E., and Toole, E. H. 1951. Relative Germination Response of Some
Lima Bean Varieties to Low Temperatures in Sterilized and Unsterilized Soil. Proceedings
of the Am. Soc. for Hort. Sci. 58:153-159.
23

 8. Wang, _]. Y. 1967. Agricultural Meteorology. Agricultural Weather Information Service, San
jose, Calif. 693 pp.
2M-11-73
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