304 SOILS: PROPERTIES AND MANAGEMENT 



Besides the quite obvious effect of the dark color on 

 the rate of heat absorption, two other points are worthy 

 of notice. The first is the tendency of the soil tempera- 

 ture to lag behind the temperature of the air, and the 

 second is the almost equal minimum reached by the two 

 soils. The latter point would seem to indicate also that 

 color had little differential effect on the heat lost from 

 the soil by radiation into the air. 



217. Effects of texture and structure on heat absorp- 

 tion. Ordinarily the texture and the structure of a 

 soil, other conditions being equal, have little direct 

 influence on rate of absorption. Wollny 1 found with 

 dry and moist soil that the coarser the particles, the higher 

 is the temperature during warm weather. A loose, open 

 structure was always more favorable for high tempera- 

 tures than one more finely pulverized. Wollny's tem- 

 perature differences, however, were very small, and it is 

 probable that the experimental error, particularly due 

 to lack of moisture control, was greater than the observed 

 differences. Under normal conditions the practical effects 

 arising from the influence of texture and structure on 

 rate of absorption are probably entirely eliminated by 

 other factors. The importance of texture and structure, 

 as will be shown later, is in the direction of the control 

 of soil heat through their influence on soil moisture. 

 Moisture in turn is a potent factor in the ultimate soil 

 temperature, as it influences specific heat, radiation, and 

 evaporation to such an extent. 



218. Radiation of heat by soil. The principal loss 



1 Wollny, E. Untersuchungen tiber den Einfluss der Struk- 

 tur des Bodens auf dessen Feuchtigkeits- und Temperatur- 

 verhaltnisse. Forscli. a. d. Geb. d. Agri.-Physik, Band V, 

 Seite 145-209. 1882. 



