[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