228 NATURE AND PROPERTIES OF SOILS 



Certain inherent qualities of the soil as well as its position 

 tend to influence its capacity to absorb radiant energy. The 

 effect may be measured in the resultant rise in temperature, 

 providing all variables are under control. The factors in- 

 volved are texture, structure, color, and position. Only the 

 last two are of practical importance. 



118. Influence of color on absorption. — It is well known 

 that a black surface absorbs more energy than a white 

 one under similar conditions and will register a more rapid 

 and a higher temperature rise. This is because of a difference 

 in reflection, the white surface being more effective in this 

 respect. The same principle has been shown by a numbei 

 of investigators to hold with soil.^ The addition of organic 

 matter, provided its decomposition has been of the proper 

 sort, will, other factors being equal, favor a higher soil tem- 

 perature. Wollny^ in experimenting with soil covered with 

 thin layers of different colored material obtained some inter- 

 esting field data. The black soil not only exhibited the high- 

 est temperature but also showed the greatest fluctuation. Min- 

 imum temperatures were the same regardless of color, while 

 temperature differences decreased with depth. The curves 

 in Fig. 42 are typical of Wollny's results on clear days. 



Besides the quite obvious effect of color on rate of energy 

 absorption, the curves exhibit two other points worthy of 

 notice. The first is the tendency of the soil temperature to 

 lag behind that of the air and the second is the equal minima 

 reached by the two soils. The latter tendency would seem 

 to indicate that color has little effect on the radiation of heat 



by soil. 



^ Bouyoucos, G. J,, An Investigation of SoiZ Temperature; Mich. Agr. 

 Exp. Sta., Tech. Bui. 17, p. 30, 1913. 



Lang, C., Pder WarTne-ab sorption und Bmission des JBoden; Forscli, 

 a. d. Gebiete d, Agr.-Physik., Band I, Seite 379-407, 1878. 



^Wollny, E., XJnterauchung uber den Einfluss der Farle des Bodens 

 auf dessen Erwanming ; Forseh. a. d. Gebiete d. Agri.-Physik., Band I. 

 Seite 43-69, 1878. Also, Untersuohungen uber den Einfluss der Farhe 

 des Bordens auf dessen Envarmung ; Forscli. a. a. G-ebiete d. Agr.-Pliys., 

 Band IV, Seite 327-365, 1881. 



