CHAPTER XIV 

 SOIL HEAT 1 



Normal plant growth is practically suspended below a 

 temperature of about 40° F., while proper germination of 

 seeds does not proceed much below that temperature. 

 As a rule it is not desirable to place either seeds or plants 

 in a soil in which active growth does not take place almost 

 immediately, since certain molds and fungi, active at 

 low temperature, may sap their vitality and ultimately 

 cause their destruction. The desirable chemical reactions 

 in the soil are checked to a certain extent by lack of heat, 

 while the important biological activities are greatly im- 

 peded, if not brought entirely to a standstill, when the 

 soil temperature approaches 32° F. Such functions as 

 the decay and putrefaction of organic matter, the forma- 

 tion of ammonia from simple humic bodies, the building-up 

 of this ammonia into the nitrate form, and the fixation 

 of the free nitrogen either by free-fixing or symbiotic 

 bacteria, depend on an optimum soil temperature. 



A knowledge of the functions of heat, therefore, es- 

 pecially as to its relationship to plant growth and bac- 

 terial activities, becomes important; for the farmer can 

 to a certain extent control soil temperature. He is able 



1 For bibliography of the literature of soil heat, see Bouyoucos, 

 G. J. An Investigation of Soil Temperature and Some of the 

 Most Important Factors Influencing It. Michigan Agr. Exp. 

 Sta., Technical Bui. 17, pp. 194-196. 1913. 

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