SOIL TEMPERATURE
We learned that roots need heat for their growth and development. Now
what is the relation of the different kinds of soil toward heat or
what are their relative powers to absorb and hold heat?
=Experiment.=--Some days before this experiment, spread on a dry floor
about a half bushel each of sand, clay and decayed leaf mould or black
woods soil. Stir them occasionally till they are thoroughly dry. When
they are dry place them separately in three boxes or large flower pots
and keep dry. In three similar boxes or pots place wet sand, wet clay,
and wet humus. Place a thermometer in each of the soils, placing the
bulb between one and two inches below the surface (Fig. 31). Then
place the soils out of doors where the sun can shine on them and leave
them several days. If a rain should come up protect the dry soils.
Observe and make a record of the temperatures of each soil several
times a day. Chart the average of several days observations. Fig. 32
shows the averages of several days observations on a certain set of
soils.
It will be noticed that the temperature of the soils increased until
the early part of the afternoon and after that time they lost heat.
[Illustration: FIG. 31.--SOIL TEMPERATURE EXPERIMENT.
Thermometer in pot of soil.]
HOW SOILS ARE WARMED
=Experiment.=--Hold your hand in bright sunlight or near a warm stove
or radiator. Your hand is warmed by heat radiated from the sun or warm
stove through the air to your body. In the same manner the rays of the
sun heat the surface of the soil.
=Experiment.=--Take the stove poker or any small iron rod and hold one
end of it in the fire or hold one end of a piece of wire in a candle
or lamp flame. The end of the rod or wire will quickly become very hot
and heat will gradually be carried its entire length until it becomes
too hot to hold. This carrying of the heat from particle to particle
through the length of the rod is called heating by conduction. Now
when the warm rays of the sun reach the soil, or a warm wind blows
over it, the surface particles are warmed and then pass the heat on to
the next ones below, and these in turn pass it to others and so on
till the soil becomes heated to a considerable depth by conduction.
A clay soil will absorb heat by conduction faster than a sandy soil
because the particles of the clay lie so close together that the heat
passes more readily from one to another than in the case of the
coarser sand.
If the soil is open and porous, warm air and warm rains can enter
readily and carry heat to the lower soil.
You have noticed how a pile of stable manure steams in cold weather.
You doubtless know that manure from the horse stable is often used to
furnish heat for hotbeds and for sweet potato beds.
Now the heat which warms the manure and sends the steam out of it, and
warms the hotbed and sweet potato bed, is produced by the decaying or
rotting of the manure. More or less heat is produced by the decay of
all kinds of organic matter. So if the soil is well supplied with
organic matter, the decay of this material will add somewhat to the
warmth of the soil.
HOW SOILS LOSE HEAT
Wet one of your fingers and hold your hand up in the air. The wet
finger will feel colder than the others and will gradually become dry.
This is because some of the heat of your finger is being used to dry
up the water or change it into a vapor, or in other words to evaporate
it.
In the same manner a wet soil loses heat by the evaporation of water
from its surface.
=Experiment.=--Heat an iron rod, take it from the fire and hold it
near your face or hand. You will feel the heat without touching the
rod. The heat is radiated from the rod through the air to your body
and the rod gradually cools. In the same way the soil may lose its
heat by radiating it into the air. A clay soil will lose more heat by
radiation than a sandy soil because the clay is more compact.
CONDITIONS WHICH INFLUENCE SOIL TEMPERATURE
It will be noticed that the dry soils are warmer than the wet ones.
Why is this? Scientists tell us that it takes a great deal more heat
to warm water than it does to warm other substances. Therefore when
soil is wet it takes much more heat to warm it than if it were dry.
It will be seen that of the dry soils the humus is the warmest. Why?
=Experiment.=--Take two thermometers, wrap the bulb of one with a
piece of black or dark colored cloth and the bulb of the other with a
piece of white cloth, then place them where the sun will shine on the
cloth covered bulbs. The mercury in both thermometers will be seen to
rise, but in the thermometer with the dark cloth about the bulb it
will rise faster and higher than in the other. This shows that the
dark cloth absorbs heat faster than the white cloth. In the same
manner a dark soil will absorb heat faster than a light colored soil;
therefore it will be warmer if dry.
Why was the dry clay warmer than the dry sand?
Because its darker color helped it to absorb heat more rapidly than
the sand, and, as the particles were smaller and more compact, heat
was carried into it more rapidly by conduction.
Why were the wet humus and clay cooler than the wet sand?
As they were darker in color and the clay was more compact than the
sand, they must have absorbed more heat, but they also held more
water, and, therefore, lost more heat by evaporation.
[Illustration: FIG. 32.
Charts showing average temperature of a set of dry and wet soils
during a period of five days. _H_, humus; _C_, clay; _S_, sand.]
[Illustration: FIG. 33.
To show the value of organic matter. 1 contains clay subsoil; 2, clay
subsoil and fertilizer; 3, clay subsoil and organic matter. All
planted at the same time.]
Of the dry soils, then, the humus averaged warmest, because, on
account of its dark color, it absorbed heat more readily than the
others. The dry clay was warmer than the sand on account of its color
and compact texture. Of the wet soils the sand was the warmest,
because, on account of its holding less moisture, less heat was
required to raise its temperature and there was less cooling by
evaporation, while the other soils, although they absorbed more heat
than the sand, lost more on account of greater evaporation, due to
their holding more moisture. Why are sandy soils called warm soils and
clay soils said to be cold?
How may we check losses of heat from the soil?
If we make a mulch on the surface of the soil evaporation will be
checked and therefore loss of heat by evaporation will be checked
also. The mulch will also check the conduction of heat from the lower
soil to the surface and therefore check loss of heat by radiation from
the surface.
VALUE OF ORGANIC MATTER
Figure 33 illustrates a simple way to show the value of organic matter
in the soil. The boxes are about twelve inches square and ten inches
deep. They were filled with a clay subsoil taken from the second foot
below the surface of the field. To the second box was added sufficient
commercial fertilizer to supply the plants with all necessary plant
food. To the third box was added some peat or decayed leaves, in
amount about ten per cent. of the clay subsoil. The corn was then
planted and the boxes were all given the same care. The better growth
of the corn in the third box was due to the fact that the organic
matter not only furnished food for the corn but during its decay
prepared mineral plant food that was locked up in the clay, and also
brought about better conditions of air and moisture by improving the
texture of the soil. The plants in the second box had sufficient plant
food, but did not make better growth because poor texture prevented
proper conditions of air and moisture. "And that's another witness"
for organic matter. Decaying organic matter or humus is really the
life of the soil and it is greatly needed in most of the farm soils of
the eastern part of the country. It closes the pores of sandy soils
and opens the clay, thus helping the sand to soak up and hold more
moisture and lessening excessive ventilation, and at the same time
helping the roots to take a firmer hold. It helps the clay to absorb
rain, helps it to pump water faster, helps it to hold water longer in
dry weather, increases ventilation, favors root penetration and
increases heat absorption. We can increase the amount of organic
matter in the soil by plowing in stable manure, leaves and other
organic refuse of the farm, or we can plow under crops of clover,
grass, grain or other crops grown for that purpose.