THE THYSICAL PROPERTIES OF THE SOIL. 957 



a nature as to counterbalance one anotlier. The thermal conditions 

 can therefore be clearly determined only by taking into consideration 

 the course of the soil temperature. 



With a rising temperature (during exposure to sunlight by day or 

 during warm seasons) quartz warms up most strongly of any of the 

 soil constituents, then follow in order clay, chalk, and humus. With a 

 foiling temperature (during the night or during cold seasons) the heat 

 conditions are exactly the reverse, inasmuch as quartz cools most 

 quickly, humus most slowly, while clay and chalk are intermediate. 

 The reasons for this are to be found principally in the differences in 

 specific heat of the various constituents in the moist state and in their 

 conductivity for heat, as described above. 



In soil of the same constitution the warming depends on the nature 

 of the surface, and the size and arrangement of the particles, as well 

 as on the humidity. 



With regard to the influence of the nature of the surface we may 

 mention first that drying up of the top layer induces a rise in the soil 

 temperature owing to a reduction in the evaporation throughout the 

 mass but especially at the surface. The superposition of thin layers of 

 a soil which easily dries up (quartz sand) has the same result. If the 

 upper layers of the soil are stirred the temperature is lowered, because 

 the conductivity of the dry layer, owing to the large quantity of 

 inclosed air, is diminished and the fragments lying on top are only 

 partially in contact with the soil below for purposes of conduction. 

 The constitution of the soil being the same, the color has a decided 

 influence down to comparatively great depths. During the warmer 

 seasons, when the radiation is unhindred, the dark-colored soils are 

 warmer. The differences in temperature in dark and light colored soils 

 disappear more or less during colder seasons when the exposure to the 

 sun is imperfect. This influence of color diminishes as the humidity 

 increases, and as the other factors controlling the warming of the soil 

 obtain preponderance. Under greater differences in the physical prop- 

 erties, especially those due to a higher proportion of humus and to a 

 larger capacity for water, the influence of color may be entirely 

 eliminated. 



Kegarding the warming of the soil as affected by differences in 

 structure, it has been observed that the soil warms up during the sum- 

 mer and cools down during winter more strongly when the structure is 

 coarse. The principal reason of this is that with an increase in the 

 diameter of the particles are connected a decrease in the quantity of 

 water and an increase in conductivity. Crumbly soils are, as a rule, 

 warmer than powdery, but the differences are comparatively slight. 

 On the other hand, an increase in density of the mass of the soil has a 

 very important influence on its temperatnre, in that an increase of den- 

 sity causes a higher temperature during warmer seasons or with arising 

 temperature and a lower temperature during colder seasons or with a 



