194 Report of the Department of Bacteriology of the 



of bacteria present in the soil. It is very difficult to obtain any 

 evidence bearing; on this point. The only evidence so far obtained 

 is the fact that bacteria have generally been found to reach their 

 highest numbers two or three weeks after freezing, while if the 

 increase were actually due to a breaking-up of the bacterial masses 

 in the soil, the process ought to stop as soon as the soil is completely 

 frozen. Moreover, if this be the true explanation, it is extremely 

 unlikely that the count immediately after the thaw would be so 

 nearly the same as it was before the freeze. 



If this increase in germ content is due to an actual multiplication, 

 the results seem to imply, on first thought, that soil organisms are 

 able to use congealed water in their physiological activities. The 

 soil-temperature, however, a few inches below the surface is seldom 

 much below freezing in this climate, and the denser portions of the 

 soil solution may never freeze. In these unfrozen portions, the 

 growth of certain kinds of bacteria may be favored. This possibility 

 was mentioned in the writer's first publication on the subject (1910). 10 

 Later it was further discussed by Brown and Smith. 11 



That certain bacteria can multiply at temperatures as low as this 

 has been recognized for some time, cold storage conditions having 

 been found insufficient to prevent all bacterial growth. It is strange, 

 however, that low temperatures should seem to be more favorable than 

 higher ones for the soil flora. The optimum temperature for nearly 

 all of the soil bacteria which the writer has isolated has proved to 

 be between 20° and 30° C. These bacteria show greater differences, 

 however, in respect to their minimum temperature of growth. This 

 fact is the basis of the theory already advanced to explain the increase 

 in numbers while the soil is frozen. If we assume that there are two 

 hostile classes of bacteria in the soil, one able to grow at tempera- 

 tures below freezing, the other with its minimum temperature con- 

 siderably higher, it is plain that sufficiently low temperatures would 

 suppress one class and allow the other to increase with more than 

 normal rapidity, even though these same organisms prefer higher 

 temperatures in pure culture. 



This theory ought not to be difficult to prove. If a distinct 

 difference could be shown between the kinds of bacteria in frozen 

 soil and those found when unfrozen, it would make this explanation 

 seem extremely probable. No such difference, however, has been 

 found. As the writer has elsewhere remarked, 12 there is a surprising 

 similarity between the predominating bacteria found in different 

 soils and in the same soil at different seasons. It is nevertheless 

 possible, considering the crudity of present methods for classifying 

 bacteria, that differences exist which have not been detected. 



10 See footnote 6. 



11 See footnote 1 1 . 



12 Conn, H. J. The Distribution of Bacteria in Various Soil Types. Journ. Amer. 

 Soc. Agron. 5 : 218-221. 1913. 



