CHAPTER XI 

 SOIL IIEA.T 



It is universally recognized that biological activity is an 

 energy expression and that such activity will not continue 

 unless certain temperature relations are maintained. With 

 higher plants this heat relation has two phases, the tempera- 

 ture of the air and that of the soil. The former is clearly 

 a climatic factor and, except on a small scale, is beyond the 

 control of man. The temperature of the soil, in a similar way, 

 is subject to no radical regulation, yet soil management meth- 

 ods provide means whereby certain small but biologically vital 

 modifications can be made, climatically unimportant but prac- 

 tically worthy of careful consideration. 



115. Importance of soil heat. — ^Normal plant growth is 

 practically suspended at a temperature of 40^ F., while the 

 germination of most seeds does not take place even at this 

 point. In general, it is poor practice to place certain seeds 

 and plants in soil where growth activities will not occur at 

 once, since bacteria and fungi, active at low temperatures, 

 may sap their vitality and ultimately cause their destruction. 

 Three groupings of higher plants may be made as far as their 

 temperature relationships are concerned. Wheat represents 

 the crops that germinate and grow at relatively low tempera- 

 tures. Maize requires a medium temperature for proper 

 growth, while pumpkins and melons typify crops, the heat 

 requirements of which are very high. The following data 



*For a bibliography of the literature of soil heat see Bouyoucos, 

 G. J., An ImesUgation of Soil Temperatiure and Some of the Mosrt; 

 Important Factors Influmcmg It; Mich. Agr. Exp. Sta., Tech. Bui. 17, 

 pp. 194-196; 1913. 



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