SOIL HEAT 315 



results in England. Wollny ^ finds a wet soil to be the 

 cooler in the daytime, the difference being roughly pro- 

 portional to the amount of water present. The effect 

 of the amount of water on the rate of evaporation is of 

 course influenced to a certain extent by texture, struc- 

 ture, and humus, since these factors exert such a marked 

 influence on water capacity and capillary movement. 



Tlie practical importance of a study of the effect of 

 evaporation on soil temperature lies in the fact that evap- 

 oration can be controlled to a certain extent under field 

 conditions. This is not so true, unfortunately, of radia- 

 tion and conduction. Thorough underdrainage is the 

 dominant operation in the prevention of cooling by 

 evaporation. By this removal of excess water the specific 

 heat is lowered, radiation is slightly retarded, and con- 

 vection is facilitated. This means a faster warming of 

 the soil, tending toward an optimum temperature rela- 

 tion as far as the plant is concerned. Optimum moisture 

 encourages optimum heat conditions, as well as other 

 favorable relations whether chemical, physical, or biologi- 

 cal. Drainage, lime, humus, and tillage figure in heat 

 control as well as in other phases of soil improvement. 



225. Effect of organic decay on soU temperature. — 

 Besides the effect of organic matter on color and its conse- 

 quent influence on the absorption of heat,_it may function 

 in another direction, namely, in producing heat of fer- 

 mentation. How far this liberation of heat under field 



^Wollny, E. Untersuehungen iiber den Einfluss des 

 Wassers auf die Boden. Forsch. a. d. Geb. d. Agri.-Plxysik, 

 Band IV, Seite 147-190. 1881. Also, XJntersueliiingen liber 

 den Einfluss der Oberfliaebliehen Abtroebnung des Boden auf 

 dessen Temperatur- und Feuobligkeitsverhaltnisse. Forseh. a. d. 

 Geb. d. Agri.-Physik, Band III, Seite 325-348. 1880. 



