394 NATURE AND PROPERTIES OF SOILS 



under a considerable range of temperature. Freezing, while 

 rendering bacteria dormant, does not kill them, and growth 

 begins slightly above that point. 1 It has been shown that 

 some nitrification occurs at temperatures as low as from 37° 

 to 39° F. It is not, however, until the temperature is con- 

 siderably higher that bacterial functions are pronounced. 

 From 70° to 110° F. their activity is greatest, and it dimin- 

 ishes perceptibly below or above those points. The thermal 

 death point of most forms of bacteria is between 110° and 



Fig. 56. — Some important decay organisms found in soils, (a), Acti- 

 nomyces threads; (b), a colony of Actinomyces; (e) and (d), Pro- 

 teus vulgaris; (e), B. fluorescens; (f ), B. subtilis. 



160° F., but the spore forms even resist boiling. Only in 

 some desert soils does the natural temperature reach a point 

 sufficiently high actually to destroy bacteria, and there only 

 near the surface. In fact, it is very seldom that soil tempera- 

 tures, other conditions being favorable, become sufficiently 

 high to curtail bacterial activity. 



The presence of a certain amount of organic matter is es- 

 sential to the growth of most, but not all, forms of soil bac- 



1 In the seasonal study of bacteria it has been repeatedly noticed that 

 the counts increased during the winter, especially after a freeze followed 

 by a thaw. It was considered for a time that a special winter flora was 

 present, and was able to multiply in the soil-water which failed to freeze. 

 It is now considered that this increase is only apparent, the freezing 

 having disrupted the bacterial clumps, thus increasing the number of 

 colonies appearing on the plates during incubation. 



