AGRICULTURAL CHEMISTRY. 159 



The relations of the soil to heat are of the utmost importance in 

 affecting its fertility. The distribution of plants in general, is deter- 

 mined by differences of mean temperature. In the same climate and 

 locality, however, we find the farmer distinguishing between cold 

 and warm soils. 



The temperature of the soil varies to a certain depth with that of 

 the air; yet its changes occur more slowly, are confined to a narrower 

 range of temperature, and diminish downward in rapidity and amount, 

 until at a certain depth a point is reached where the temperature is 

 invariable. 



In summer the temperature of the soil is higher in day time than 

 that of the air; at night the temperature of the surface rapidly falls, 

 especially when the sky is clear. 



In temperate climates, at a depth of three feet, the temperature 

 remains unchanged from day to night; at a depth of 20 feet the an- 

 nual temperature varies but a degree or two; at 75 feet below the 

 surface, the thermometer remains perfectly stationary. In the vaults 

 of the Paris Observatory, 80 feet deep, the temperature is 50° Fah- 

 renheit. In tropical regions the point of nearly unvarying tempera- 

 ture is reached at a depth of one foot. 



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. 



The sources of that heat which is found in the soil are two, viz: 

 first, an internal one, the chemical process of oxydation or decay; 

 second, an external one, the rays of the sun. 



The heat evolved by the decay of organic matters is not inconsid- 

 erable in porous soils containing much vegetable remains; but this 

 decay cannot proceed rapidly until the external temperature has 

 reached a point favorable to vegetation, and therefore this source of 

 heat probably has no appreciable effect one way or the other on the 

 welfare of the plant. The warmth of the soil, so far as it favors ve- 

 getable growth, appears then to depend exclusively on the heat 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 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 season; 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. 



During a summer day the heat of the sun reaches the earth 

 directly, and it 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 ab- 

 sorb or become warmed by the sun's heat. Air and water are almost 

 incapable of being warmed by heat applied above them. Through 



