d 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 pe