ORGANISMS IN THE SOIL 431 



where animal manure has been applied or where a carcass 

 has been buried, the number becomes very large, as 

 many as 100,000,000 to a gram of soil having been found ; 

 while in soil of ordinary fertility and tilth the numbers 

 range from 1 ,000,000 to 5,000,000 to a gram. The extreme 

 rapidity with which reproduction occurs makes it possible 

 for the number to increase enormously when conditions 

 are favorable for their growth. 



The table on page 432 shows the number of bacteria 

 to a gram of soil found in different parts of the United 

 States during some portion of the growing season. 



The figures showing the number of bacteria in each 

 gram of soil that are presented in this table cannot be 

 used for a comparison of the relative numbers of bacteria 

 in soils of different regions of this country, because dif- 

 ferent methods were used by the experimenters in making 

 the estimations. They are, however, an indication of 

 what may be considered the ordinary range in arable 

 soils. • 



353. Numbers as influenced by season. — It might be 

 supposed that, like most plants, bacteria would develop 

 most rapidly in summer months and that they would be 

 found in largest numbers at that season, at least in regions 

 of low temperatures during the winter months. That 

 this is not always the case has been shown by Conn, 1 

 who found as the result of periodical enumeration of bac- 

 teria throughout a term of two years that the highest 

 counts were obtained during the winter months, when 

 the soil was frozen. This does not mean that all classes 

 of bacteria are present in largest numbers at that season, 

 but, as explained by Conn, it seems likely that certain 



1 Conn, H. J. Bacteria in Frozen Soils II. Centrlb. f. 

 Bakt., II, Band 32, Seite 70-97. 1912. 



