NUMBERS OF MICROORGANISMS 33 



theory was suggested that surface tension exerted by the soil particles 

 on the films of water, as well as the presence of salts in the water and the 

 concentration of salts, which may occur when the main body of water 

 begins to freeze, all cause the hygroscopic water in soils to remain un- 

 congealed, and consequently bacteria may live in it and multiply to a 

 comparatively large extent. Other results 66 also tended to indicate that 

 low temperatures may greatly increase the numbers of bacteria. To 

 explain these phenomena, the mechanical transportation of the bacteria 

 by the moisture coming up from below during heavy frost was sug- 

 gested. 67 It was later found, 68 however, that a slightly frozen condition 

 of the soil allowed bacterial development, but severe frosts produced a 

 checking action, the decrease being parallel with the depression of 

 temperature. No change in crop and plant remains took place in 

 Canada during the winter since the temperature of the soil goes down too 

 low. The occurrence of two maximum counts of bacteria observed 69 

 in Iowa soils during the year, on February 12 and June 19, with inter- 

 vening minimum counts, were used to prove the theory that tempera- 

 tures much below zero are necessary before the hygroscopic moisture 

 freezes and, until that occurred, a development of bacteria might be 

 expected. It may also be suggested that a slight freezing of the soil, 

 in modifying the colloidal condition of the soil may have the same stim- 

 ulating action as air drying, treating with disinfectants, etc., in other 

 words, shifting the soil equilibrium, so that a more rapid multiplication 

 of the bacteria may take place. 



A more recent careful study of the influence of freezing upon soil 

 bacteria demonstrated that the increase recorded previously is not 

 due to an actual multiplication of the bacteria but rather to a breaking 

 up of the clumps of bacteria in the soil resulting in a larger number of 

 colonies developing on the plate. 70 The moisture content and rate of 



66 Weber, G. G. A. Die Einwirkung der Kalte auf die Mikroorganismen und 

 ihre Tatigkeit im Boden. Diss. Jena, 88. 1912. (Centrbl. Bakt., II, 37: 113. 

 1912.) 



67 Harder, E. G. The occurrence of bacteria in frozen soil. Bot. Gaz. 61: 

 363. 1916. 



68 Vanderleck, J. Bacteria of frozen soils in Quebec. I and II. Trans. Roy. 

 Soc. Canada (Ser. Ill, Sec. IV), 11: 15. 1918; 12: 1. 1918. 



69 Brown, P. E., and Halversen, W. V. Effect of seasonal conditions and 

 soil treatment on bacteria and molds in the soil. Iowa Agr. Exp. Sta., Res. Bui. 

 56: 251-275. 1919. 



70 Vass, A. F. The influence of low temperature on soil bacteria. Cornell 

 Univ. Agr. Exp. Sta. Memoir 27. 1919. 



