=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.