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.]