RELATIONS OF THE SOIL TO HEAT. 187 
The mean annual temperature of the soil is the same as, 
or in higher latitudes a degree above, that of the air. The 
nature and position of the soil must considerably influence 
its temperature 
Sources of the Heat of the Soil.—The sources of that 
heat which is found in the soil are three, viz.: First, the 
original heat of the earth; second, the chemical process 
of oxidation or decay going on within it; and third, an 
external one, the rays of the sun 
- The earth has within itself a source of heat, which 
maintains its interior at a high temperature; but which 
escapes so rapidly from the surface that the soil would be 
constantly frozen but for the external supply of heat from 
the sun. 
The heat evolved by the decay of organic matters is 
not inconsiderable in porous soils containing much vegeta- 
ble remains; but decay cannot proceed rapidly until the 
external temperature has reached a point favorable to 
vegetation, and therefore this source of heat probably has 
no appre*iable effect, one way or the other, on the welfare 
of the p'ant. The warmth of the soil, so far as it favors 
vegetable growth, appears then to depend exclusively on 
the heat of the sun. 
The direct rays of the sun are the immediate cause of 
the warmth of the earth’s surface. The temperature of 
the soil near the surface changes progressively with the 
seasons; but at a certain depth the loss from the interior 
and the gain from the sun compensate each other, and, as © 
has been previously mentioned, the temperature remains 
unchanged throughout the year. 
Daily Changes of Temperature.—During the day the 
sun’s heat reaches the earth directly, and is absorbed by 
the soil and the solid objects on its surface, and also by 
the air and water. But these different bodies, and also 
the different kinds of soil, have very different ability to 
absorb or become warmed by the sun’s heat. Air and 
