RELATIONS OF SOILS AND PLANT GROWTH TO HEAT.
_The Temperature of Soils._—The rapid germination of seeds, as well
as the development of plants to maturity, is essentially dependent
upon the maintenance of the appropriate temperature. The temperature
most favorable to germination or growth, as well as the degree of
tolerance of high and low temperatures, varies greatly with different
plants, governing mainly what is known as their climatic adaptation.
A knowledge of these points with reference to the several crops is
therefore of no mean importance to the farmer; for, to a certain
extent, he can control the temperature in the soil itself, and he can
mostly choose for sowing and planting, the time when the soil shall
have the proper temperature for rapid germination or maturity. As a
rule, it is not desirable to have either seeds or seedling plants in
the ground for any length of time when the temperature is too low for
active vegetation; for while they rest, other, lower organisms (fungi
and bacteria), adapted to low temperatures, may continue in full
activity at the expense of the vitality of the crop plant.
_Water exerts controlling Influence._—Since the capacity of water for
heat is approximately five times greater than that of the average
soil, equal weights being considered, it follows that the temperature
of soil-water must exert a controlling influence over that of the
soil. Taking the case of a cubic foot of loamy soil, fully saturated
with water, in which one-third of the volume may be assumed to be
water: the weight of the dry soil being about eighty pounds per cubic
foot, calculation shows that the amount of heat required to raise the
temperature of the water contained, one degree, will be fully twice
as great as for that required for the soil itself. It is thus obvious
that the control of soil-temperature is largely dependent upon the
control of the water-content of the same, which has been discussed in
a former chapter. Even in the condition of moisture known to be most
favorable to plants, viz., one-half of the maximum water capacity, the
influence of the water-content upon the temperature will still be as
great as that of the entire soil mass. This consideration emphasizes
the importance of such control.
_Cold and Warm Rains._—It is not surprising then that the occurrence
of cold or warm rains or the use of cold or warm irrigation water at
critical periods, may largely determine the success or failure of
the crop. It is well known that the occurrence of a cold rain after
vegetation has started actively in early spring, may not only destroy
the season’s fruit crop by preventing the setting, or thereafter
causing the dropping, of the fruit, but may even, if the suppression
of vegetative action be continued for some length of time, result
in serious injury to, or death of trees. Widely extended disastrous
experience of the kind was had in California in February and March,
1887, resulting in the death of tens of thousands of fruit trees and
vines during that and the following season. It is obvious that in
such a case as this the rapid draining-off of the cold water through
underdrains would have materially mitigated, if not wholly prevented,
such injury.
_Solar Radiation._—Aside, however, from such overwhelming influences
as the above, the soil temperatures are measurably controlled by the
extent to which they receive and absorb the sun’s heat rays, whether
directly or through the mediation of the air. The direct effect of the
sun’s rays upon the surface is, upon the whole, the most generally
potent, although warm winds may occasionally exert a very strong
influence. The varying influence of the sun’s rays depends primarily
upon the change of seasons, which themselves result from the varying
angles at which the sun’s rays strike the surface; as well as upon the
duration of the day. The greater or less cloudiness or fogginess of the
sky, of course, exerts a decided effect in this connection.
_The Penetration of the Sun’s Heat into the Soil._—In the temperate
regions of the earth the daily variations of temperature cease to be
felt at depths ranging from two to three feet, according to the nature
of the soil material and its more or less compacted condition. The
monthly variations, of course, reach to greater depths; while the
annual variations do not disappear in the temperate zone, _e. g._, at
Paris, Zürich and Brussels, at a less depth than seventy-five feet.
At these depths of constant temperature we find approximately the
same temperature as that which we can deduce from the thermometric
observations as the annual mean. From similar causes the mean annual
temperature of any place may be approximately deduced from the
observation of the water of wells and springs derived from moderate
depths. For below the level of constant annual temperature the latter
begins to ascend steadily as we progress downward, owing to the
interior heat of the earth.
_Change of Temperature with Depth._—The following table of observations
made at Brussels illustrates the decrease of annual range of
temperature with depth:
Average Annual
At feet: temperature. range.
(°C.) (°C.)
3.25 7.2 10.5
15.6 13.5 4.5
30.8 16.4 1.3
75.0 17.0 0.0
It is interesting to compare with this record that of a well sunk by
Ermann at Yakutzk, Siberia, where the mean annual temperature is—9.7 C.
(14.6 F.). This temperature was found a few feet below the surface. At
50 feet the temperature was—7.2 C. (19° F.); at 145 feet—5° Cent. (23°
F.) at 350 feet—.9° C. (30.8° F.) showing that the ground was below the
temperature of freezing water for some distance farther down; so that
the search for liquid water was abandoned.
We thus see that in the Arctic regions, owing to the presence of water
in the form of ice, the melting of which impedes the access of solar
heat, the level of no variation is found at the distance of a few feet
below the surface, despite the great variations in temperature between
the short but hot summer and the extremely cold winter. In the tropics,
also, the annual temperature-variation disappears at a less depth than
2 feet, in consequence of the very slight difference between the two
seasonal extremes of temperature.
_Surface—Conditions that influence Soil-Temperature._ Among these,
_color_ has already been mentioned, and to a certain extent discussed.
While it is true that, broadly speaking, dark-colored soils absorb more
of the sun’s heat than light-colored ones, other things being equal, it
must still be understood, that the nature of the color-giving substance
exerts a very material influence upon the amount of heat absorbed. Thus
charcoal is among all known substances the one absorbing and radiating
the sun’s heat rays most powerfully, and all kinds alike; so much so,
that its absorbent power is taken as 100. But other substances which
to the eye appear equally black, have by no means the same absorbing
power. The heat absorption by black humus is high, though not quite
equal to that of charcoal; and many gray soils, though appearing to
the eye of rather light tint, really absorb more heat than others
which, to our perception, have the darker tint, but are colored by
other substances. Gardeners and especially vine growers in the colder
portions of Europe often take advantage of the powerful absorbing power
of carbon by spreading charcoal or black slate powder over the surface
of the soil where early maturity is specially desired; and slate powder
is similarly used by the peasants at Chamouni to hasten the melting of
the snow.
_Heat of High and Low Intensity._—It must also be kept in view that
the surfaces, and especially the colors that favor absorption of the
intense rays of the sun, may comport themselves quite differently
toward heat rays of low intensity, such as those thrown back from the
soil at night when it cools. Were this otherwise, a soil that absorbs
much heat in the daytime would lose it with corresponding rapidity at
night. But this is true only of charcoal; in the case of most other
substances, there is a material difference in favor of the retention of
the heat, of low intensity, by slower radiation into a “heat-trapping”
atmosphere.
_Reflection vs. Dispersion of Heat._—Theoretically, a smooth surface
reflects more heat than a rough one, and warms much more slowly by
absorption; as is strikingly shown by the use of polished metal screens
placed on walls to prevent their being overheated by a flue near by.
In the case of soils, also, the condition of the surface affects
materially the absorption of heat, but not in accordance with the
above rule so far as the result is concerned. For it is found that,
other things being equal, a loose or cloddy surface disperses in many
directions the heat it receives, and does not permit it to penetrate
by conduction to so great extent as would a more compact soil, whose
smooth surface would waste less of the heat received by radiation.
King has called special attention to the difference of
temperature existing between soils smoothed and compacted by
a roller, and the unrolled soil having a loose surface. He
found that the former at a depth of one and a half inches
was as much as 5.5°C. (10°F.) warmer than the loose soil,
and that even at a depth of three inches a difference as
high as 3.5°C. (6.5°F.) existed between the two. He observed
at the same time that the temperature of the air over the
unrolled ground was considerably warmer than above the
rolled, thus corroborating the differences observed in the
soil itself. But at night the heat is given out more rapidly
from the rolled than from the unrolled surface, the latter
acting as a non-conductor and keeping the soil warmer than
that of the more compact rolled land. King gives as the
average difference observed between rolled and unrolled land
on eight Wisconsin farms, 1.6°C. (3°F.) in favor of the
rolled land between 1 and 4 p.m.
It will thus be seen that the loose tilled layer, while impeding the
penetration of the sun’s heat into the deeper portions of the soil
during the day, on the other hand serves to retain it at night better
than a more compact soil. This obviously places it within the power of
the farmer to exert considerable control over the soil-temperature at
critical times; restraining or favoring the access of the sun’s heat in
accordance with the requirements of the climate or season, as the case
may be.
_Influence of a Covering of Vegetation, and of Mulches._—A cover of
either living or dead vegetation depresses the temperature of the soil
as compared with the bare land, as elaborately shown by Wollny and
Ebermeyer. In the monthly averages these differences rarely exceed
.8° C. (1.5 degrees F.), and are mostly below .50° C. (1° F.), but
during different parts of the day they may rise to 2.2 to 2.5° C. (4
to 4.5° F.), at 4 inches depth. In summer they are greater than at
other seasons. Of course the density of the vegetation or the thickness
of the mulch influences them materially. Forests exert the greatest
cooling influence upon the soil, and next to these the dense herbaceous
crops, such as clover, and the legumes generally.
_Influence of the Nature of the Soil-Material._—Aside from the
surface condition, the nature of the material itself exerts a certain
influence, not only upon the rate of introduction of heat, but also
upon the amount taken up. Thus quartz sand having the highest density
(greatest weight per cubic foot) and also the highest capacity for
heat among the usual mineral soil-ingredients, will, mass for mass,
experience a smaller rise of temperature than would clay or loam soil,
of less density or volume-weight, and also of lower heat-capacity.
While this holds good theoretically, differences corresponding to
this consideration rarely occur in nature, for the reason that the
much greater influence of the mechanical condition of the soil mostly
overbalances these effects. Thus Wollny has shown that while quartz
is a better heat-conductor than clay, quartz cobbles or gravel will
materially increase the temperature of the soil in which they are
imbedded. Yet compact clay is a better conductor of heat than loose
sand; hence the latter, when exposed to the intense heat of the summer
sun in the desert, becomes intensely hot on the surface, yet allows of
the existence of abundant moisture at a depth of ten or twelve inches;
while clay in the same region, being usually in a compacted condition,
will show a lower surface-temperature and will be warmer and drier
at a depth at which sand will still retain abundant moisture, and be
comparatively cool (See chap. 13, p. 257.) So much indeed depends upon
the state of mechanical division and flocculation in which the several
soils may happen to be, that a hard-and-fast statement in regard to
their relations to heat cannot and should not be given, as it would
only lead to disappointments and practical mistakes; the more as in all
cases the moisture-condition exerts an influence predominating by far
over that of the dry material itself, and this moisture-condition is
subject to rapid changes, owing to intrinsic differences in the several
classes of soils. Wollny states as the result of his experiments, that
in summer sandy soils are warmest; then humous, lime and loam soils;
while in winter humous soils are warmest, then loams; and sand coldest.
_Influence of Evaporation._—In treating of the Conservation of
Soil Moisture (chapter 13), the effects, conditions and control of
evaporation from the soil have already been discussed from several
points of view; so that a summary review of the subject must suffice in
this place.
It has been stated above that in the case of an average loam soil
saturated with water, the heat required to raise the temperature of the
water one degree would be about twice that needed to so change the dry
soil material itself. But if it is required to _evaporate_ the same
amount of water from the soil, nearly ten (9.667) times that amount of
heat will be required; or in the case assumed, twenty times as much
as would suffice to raise the temperature of the dry soil through an
equal interval of temperature. While in a few cases the cooling of the
soil by evaporation is desirable, in the vast majority of cases it is
injurious to the progress of vegetation, and should be restricted as
much as possible by the means outlined in a former chapter.
_Formation of Dew._—There is, however, another aspect of evaporation
from the soil which has been long misunderstood, although the true
state of the case was partially recognized long ago. Dew is in common
parlance said to “fall,” it being supposed that, like rain, it is
derived from the atmosphere. While this is partially true, inasmuch
as from very moist, and notably from foggy air dew is frequently
deposited on grass and foliage generally, as well as on wood and
other strongly heat-radiating surfaces; yet as a matter of fact, in
by far the majority of cases, as shown by H. E. Stockbridge[105] and
confirmed by everyday observation, dew is formed from the vapor rising
from the warmer soil into a colder atmosphere, and condensed on the
most strongly heat-radiating surfaces near the ground, such as grass,
leaves both green and dry, wood, and other objects first encountering
the rising vapor. In manifest proof of this it will be noted that very
heavy dews may be seen on the ground, when the roofs of houses as well
as the higher shrubs and trees remain perfectly dry. In winter this
may be most strikingly seen in the deposition of hoar-frost in and
immediately around the cracks of plank sidewalks, whose surface remains
dry.
[105] “Rocks and Soils,” pp. 175-189.
_Dew rarely adds Moisture._—Candid observations will convince any one,
therefore, that in most cases the supposed addition to the moisture of
the soil from dews is an illusion. Whatever dewdrops fall on the ground
are in general simply the return to the soil of a part of what came
from it; while the dew that evaporates from the bedewed leaves or other
objects represents simply a delayed outgo of moisture from the soil,
which for a time retards evaporation direct from the soil, and thus
effects a slight saving of moisture.
But while this is measurably true of inland and especially
of continental areas like the great plains of North and
South America, it is also true that in deep moist valleys,
and on the rainy and foggy coast regions of continents,
dews are found to both fall and rise, not uncommonly to
such an extent as to be equivalent to a not inconsiderable
aggregate precipitation. Thus in the moist coast belt of
Oregon and Washington lying west of the Cascade range of
mountains, the morning dews of summer are frequently so
copious that the water falls in showers from the lower trees
and shrubs, so as to necessitate the use of water-proof
clothing when traversing the woods in the morning, quite as
much as though rain was actually falling. In hilly and more
especially in mountainous regions the cold air descending
from above and flowing down in the ravines will often cause
a heavy condensation of dew in these, while the bordering
ridges, which rise above the cold currents, remain free from
dew. These descending currents as a rule not only bring no
surplus moisture with them, but in their downward course
become warmer by contraction and therefore relatively drier.
In these cases also, therefore, the dew is purely moisture
derived from the ground, which in rising encounters the cold
air and is thus condensed.
The fact that dew is most commonly derived from the soil could have
been foreseen from the other fact, long ascertained and known, that
during the night the soil is as a rule warmer than the air above it; as
has been shown by the earlier observers, as well as more specifically
by Stockbridge.
_Dew within the Soil._—It is obvious that whenever dew is formed above
the surface of the soil, the air within the latter must be at or near
its point of saturation with vapor, as in fact is usually the case
a few inches below the surface. It follows that when a depression
of temperature occurs within the soil, _e. g._, at night, dew must
be deposited within the soil down to the depth to which the nightly
variation reaches, increasing at that depth as the vapor from the
warmer soil below rises, to be in its turn condensed. There is thus
formed at that level a zone of greater moisture, which may sometimes
be noted in digging pits, by a deeper tint, without any corresponding
variation in the nature of the soil. The daily repetition of this
process, at varying depths, and its greater or less recurrence at or
near the limit-levels of monthly and even annual variations, must
exert a not inconsiderable influence upon the vertical distribution
of moisture in the soil; which instead of being usually found in
horizontal bands or zones of varying moisture-contents, is usually
remarkably uniform for considerable depths, despite the fitfully
recurrent additions from rains. It is at least probable that this
process of dew-formation within the soil materially assists capillarity
in effecting a measurably uniform vertical distribution of moisture.
(See also page 207, chapter 11).
_Plant-development under different Temperature Conditions._—In the
arctic regions the ground, frozen in winter to unknown depths, may
thaw to only three to five feet during the summer, notwithstanding
the great length and continuous sunshine of the arctic day. The
shallow-rooted arctic flora develops very rapidly under the influence
of the continuous daylight and heat, in the course of from five to
eight weeks. The seeds of these plants must, of course, be capable of
germinating at very low temperatures; and as a matter of fact, we find
that both in the arctic regions and in the higher mountains, certain
plants are found growing and blooming on slopes flecked with snow; each
plant surrounded by a small circle of bare ground, where the snow has
been melted under the influence of the dark-tinted earth and leaves.
It is clear that here germination has occurred, the foliage has been
formed, and the roots have been exercising their vegetable functions,
in ground soaked with water practically ice-cold.
_Germination of Seeds._—While wild plants of special adaptation may
thrive in very low (or high) temperatures, it is also true that few of
our cultivated plants will germinate, and still less grow thriftily,
at such low temperatures. The limit below which most cultivated plants
may be considered as remaining practically inactive lies between
4.4 and 7.2° C. (40 and 45° F.). Few tropical plants will germinate
much below 23.8° (75° F.) and in some cases not below 35° Cent. (95°
F.). Even maize and pumpkins, according to Haberlandt, germinate
most rapidly between 35 and 38.3° C. (95 and 101° F.), while for
wheat, rye, oats and flax the best temperature for germination lies
between 21.1 to 26.1 (70 to 79°). Under the most favorable conditions
of temperature and moisture, some small seeds which readily absorb
moisture will germinate in from twenty-four to forty-eight hours, while
at a lower temperature they may require from three days to two weeks.
Thus Haberlandt found that while oats would germinate in two days at
a temperature of 17.2 to 17.5° C. (63° to 63.5°), it took a full week
for germination when the temperature was only 5° C. (41° F.). It is
obvious that seeds remaining inert in the soil for such lengths of time
will be subject to a variety of vicissitudes that may injure or destroy
their vitality. There are many bacteria and fungous parasites which at
low temperatures are perfectly capable of attacking and destroying the
water-soaked seed. There is thus for each plant, from the lowest to the
highest, a certain temperature most favorable to development; and both
above and below this, the vegetative activity is seriously interfered
with or wholly checked. A knowledge of these limits is manifestly of
the utmost practical importance.
The influence of too high a temperature in preventing the
germination of cinchona seed from India, was curiously
exemplified when it was subjected to a supposedly favorable
steady temperature of 23.8° C. (75° F.) under otherwise most
favorable conditions. Not a single one came up in the course
of six weeks, and the box in which it had been sown was put
away outside of the hothouse as a failure. Within two weeks
a full stand of seedlings was obtained, at temperatures
ranging between 12.7 and 15.5° C. (55° and 60° F.). The fact
that the cinchona is a tree of the lower slopes of the Andes
(three to five thousand feet) although at home strictly
within the tropics, explains the apparent anomaly.
PART THIRD.
CHEMISTRY OF SOILS.