SOIL HEAT 317 



on soil temperature, so far as the total bulk is concerned. 

 There are without doubt certain localized influences, 

 both chemical and biological, but how important they 

 may be it is rather difficult to say. From what is known 

 at the present time it seems that organic matter exerts 

 its greatest temperature effects through the darkening of 

 the color and the increase in moisture capacity of the 

 soil. 



226. Relation of slope to soil temperature. — The rela- 

 tion of exposure to soil heat is the last phase to be con- 

 sidered, with the exception of meteorological factors, 

 which are external in their relationships rather than intrin- 

 sic as have been most of the phases already discussed. 

 The slope of a surface varies the amount of heat 

 absorbed from the sun, without affecting, of course, the 

 absorptive power of the surface involved. The greater 

 the inclination of a soil from a right-angle interception 

 of the heat rays, the less rapid will be its rise of tempera- 

 ture in a given unit of time, the source of heat remaining 

 constant. This is because the greater the inclination, 

 the greater is the amount of surface a given amount of 

 heat must serve. It is evident that a less amount of 

 heat will reach each unit of soil surface, and a conse- 

 quent slower rise in temperature of the soil so situated 

 will result. Under normal conditions, therefore, any 

 inclination that will cause a surface to approach a right- 

 angle interception of the sun's rays will not only increase 

 its rate of temperature rise but at the same time will 

 increase its average seasonal temperature. In the North 

 Temperate Zone this of course is a southerly inclination. 

 The following diagram, illustrating the conditions on the 

 42d parallel at noon on June 21, makes clear this rela- 

 tionship : — 



