DELATIONS OP* THE SOIL TO HEAT. 187 



The mean annual temperature of tlie 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 v/liieh is found in tlie 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 Avithin itself a source of heat, which 

 maintains its interior at a high temperature ; hut 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 t<?mperature has reached a point favorable to 

 vegetation, and therefore this source of heat [trobably has 

 no appr?,"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, njipears 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 

 unchangi'd throughout the year. 



Daily (hanj?es 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 



