SOURCES OF HEAT OF THE SOIL 451 



nitrates, which takes place most actively at a tempera- 

 ture of 80 to 100 Fahr., and ceases at about 40 Fahr. 



319. Chemical changes. In the soil chemical changes 

 are greatly accelerated by a high temperature, and are 

 correspondingly retarded by low temperature. But, 

 unlike biological activity, they never wholly cease as a 

 result of temperature changes, though the type of 

 change in the different compounds may be altered. 

 Warm temperatures increase particularly the solubility 

 of the soil constituents, by which they are made available 

 to plants. 



320. Physical changes. As a result of temperature, 

 physical changes are less marked than the chemical and 

 biological, except when the freezing point is reached, 

 when the soil moisture is solidified and renders nutrition 

 of higher plants impossible. The movement of moisture 

 and gases through the soil is greatly facilitated by the 

 higher temperatures within the range of plant growth. 



II. SOURCES OF THE HEAT OF THE SOIL 



There are three direct sources of heat which reach 

 the soil. These are: (1) Solar radiation. (2) Conduction 

 from the interior of the earth. (3) Organic decompo- 

 sition. 



Under field conditions, the first of these sources is 

 far the most important. 



321. Solar radiation. Solar radiation of heat reaches 

 the soil in three ways. 



(1) By direct radiation from the sun in the form 

 of sunshine. 



