﻿iqi6] HARDER— BACTERIA 511 



in numbers, however, seems to be due to mechanical transportation 

 by moisture coming up from below during heavy frost, and where 

 such transportation is not possible there is an actual retardation in 

 growth as compared with that in unfrozen soils. 



Experiments with field soil. — The investigations were 

 begun during the latter part of October, and the samples were 

 examined every week until the latter part of February, after which 

 time the samples were examined at greater intervals. 



A dark, medium rich, slightly sandy garden soil was studied. 

 It was obtained from the university campus near the College of 

 Agriculture, and care was taken to secure successive samples from 

 exactly the same place. The soil was sampled to a depth of about 

 6 inches each time, thus giving an average of a surface layer to this 

 depth. The temperature of the soil was taken roughly with an 

 ordinary thermometer. 



The entire sample was thawed, when necessary, and thoroughly 

 mixed in a mortar, previously washed out with 95 per cent of alco- 

 hol. Twenty gms. of soil were then mixed with 400 cc. of sterile 

 water, and from this 25 cc. were carried to a second 400 cc. sterile 

 water blank, and so on to the fourth dilution. From the fourth 

 or fifth dilution plates were poured and these were counted after 

 an incubation period of 8 days at 28 0 C. Heyden NahrstofT agar 

 was used as a culture medium. 



It appears that in order to obtain conclusive results regarding 

 the bacterial content of soils, samples should be investigated at 

 short intervals of time, perhaps every few days, or even every day, 

 as at times of heavy frost, or after rainfall. It was found that the 

 bacterial content of soils is closely dependent on atmospheric 

 conditions as regards temperature and precipitation. When 

 atmospheric conditions vary rapidly the bacterial content of soils 

 also may change rapidly. Table I shows the variations in the 

 bacterial content of the soil from October 1914 to April 1915. 



Table I shows the general close relation between the moisture 

 content and the number of bacteria in the soil. This is also shown 

 in graph 1. While slight discrepancies undoubtedly occur, the 

 general correspondence is very marked. Graph 2 gives the curves 

 of high and low daily temperature, as well as figures for the 



