[vanderleck] bacteria OF FROZEN SOILS 37 



The foregoing discussion might explain why Harden found the 

 highest bacterial counts always connected with a high moisture 

 percentage. In this work similar facts were observed. The difficulty 

 remains, however, that the moisture content of the soil in winter is very 

 variable and that as a moderate estimate any bacterial increase 

 caused by a moisture increase could not be evident for at least 7 to 10 

 days. Bacteria multiply very slowly at a temperature of 32°F. 

 But in a period of 10 days the soil moisture may have varied at least 

 10% for that same spot and still more for another spot. In winter it is 

 practically impossible to take all samples in one and the same place 

 when the soil is frozen. In order to ensure normal conditions test pits 

 should be dug at least 3 feet away from the last one, bringing other 

 factors in play. From the different experiments so far executed the 

 writer draws the following conclusions, expecting to give each a 

 thorough trial in next winter's experiments. 



Conclusions and Summary. 



1. Bacteria increased rapidly in January in all soils where raw material 



was available for bacterial decomposition. This increase took 

 place in frozen and unfrozen soils. 



2. When raw material was absent this increase did not materialise. 



3. During March in frozen soils a moderate increase in the numbers 



of bacteria was observed. This increase reached only from 2 

 to 4 times the original numbers. 



4. Severe frost would check any bacterial development in frozen soil. 



5. A high water content of the soil would counteract the frost action; 



a low water content would assist in depressing bacterial develop- 

 ment. 



6. A sudden severe frost killed most of the bacteria in the exposed 



soil. 



7. As soon as the soil thawed a decrease in the numbers of bacteria 



w^as observed . 



8. An increase in soil moisture was usually followed by an increase in 



bacteria. 



