﻿i 9 i6] HARDER—BACTERIA 509 



the growth of other varieties which at ordinary temperatures 

 interfere with the growth of these prolific kinds. 



An analogous hypothesis proposed by Russell 3 for increases 

 in the number of bacteria after partial sterilization by heat, frost, 

 or other means is that by such partial sterilization the protozoa 

 are killed, thus permitting the unhindered development of bacteria 

 which under ordinary conditions is held in check by protozoa. 



Brown and Smith (loc. cit.) in their investigations dealt mainly 

 with the physiological activities of bacteria under conditions of 

 low temperature and frost, although they also made some deter- 

 minations of the number of bacteria in frozen soil. Their principal 

 conclusions regarding the ammonifying, nitrifying, denitrifying, and 

 nitrogen fixing powers of frozen soils are as follows: (1) that "frozen 

 soils possess a much greater ammonifying power than unfrozen 

 soils"; (2) that "during the fall season, the, ammonifying power 

 of the soil increases until the temperature of the soil almost reaches 

 zero, when a decrease occurs, and this is followed by a gradual 

 increase and the ammonifying power of the soil reaches a maximum 

 at the end of the frozen period"; (3) that "the nitrifying power of 

 frozen soils is weak and shows no tendency to increase with exten- 

 sion of the frozen period"; (4) that "frozen soils possess a decided 

 denitrifying power which seems to diminish with the continuance 

 of the frozen period"; (5) that "during the fall season, the denitri- 

 fying power of the soil increases until the soil freezes, after which a 

 decrease occurs"; (6) that "frozen soils possess a nitrogen fixing 

 power which increases with the continuance of the frozen period, 

 being independent of moderate changes in the moisture conditions, 

 but restricted by large decreases in moisture"; and (7) that "in 

 the fall, the nitrogen fixing power of the soil increases until the soil 

 becomes frozen, when it almost ceases, after which a smaller nitrogen 

 fixing power is established." 



The experiments on which these conclusions are based were 

 conducted with air-dried soil of uniform texture and composition 

 as a medium. This soil was enriched with suitable materials for 

 ammonification, nitrification, denitrincation, or nitrogen fixation. 



* Russell, E. J., The effect of partial sterilization of soil on the production of 

 plant food. Jour. Agric. Sci. 5:152-221. 1913. 



