INFLUENCE OF ENVIRONMENTAL CONDITIONS 777 



73). Nitrate reduction takes place at 15° to 40°, with an optimum at 

 25° to 30°; the same is true of aerobic and anaerobic nitrogen-fixation. 25 



Ordinary soil bacteria can withstand very low temperatures, even 

 40°C. below zero. 26 The action of frost upon the soil flora may be 

 compared to that of partial sterilization. Prucha, 27 for example, 

 demonstrated that Bad. typhosum added to ice cream, kept constantly 

 frozen, at first increases in numbers (due to the breaking up of the 

 bacterial lumps), then gradually diminishes, but will no tbe completely 

 destroyed, even when the ice cream is frozen for over two years; when 

 the ice cream is allowed to thaw out, an increase in numbers takes 

 place. Freezing of soil or even very low temperatures may bring 

 about a change in the balance of soil microorganisms; this may be 

 followed by a great increase in numbers, especially of those organisms 

 which develop on the plate. This increase may be due both to the 

 breaking up of the clumps of bacteria and to an actual multiplication 

 when the soil is warmed up again. Freezing of soil causes the coagula- 

 tion of soil colloids; 28 - 29 heating of soil also modifies its physical condi- 

 tions. Grinding 30 of soil particles also exerts an important influence 

 upon the numbers and activities of soil microorganisms. 31 



One may compare the soil flora found in very cold climates with that 

 of temperate zones. Omeliansky 32 examined the soil obtained from 

 the frozen ground of Northern Siberia (72° 15' latitude and 142° longi- 

 tude East) and found the microflora to consist of spore forming and 



25 Panganiban, E. H. Temperature as a factor in nitrogen changes in the 

 soil. Jour. Amer. Soc. Agron., 17: 1-31. 1925. 



26 Harder, 1916 (p. 33). 



27 Prucha, M. J., and Brannon, J. M. Viability of Bacterium typhosum in ice 

 cream. Jour. Bact., 11: 27-30. 1926. 



28 Czermak, W. Ein Beitrag zur Erkenntnis der Veranderungen der sog. 

 physikalischen Bodeneigenschaften durch Frost, Hitze, und die Beigabe einiger 

 Salze. Landw. Vers. Sta., 76: 75-116. 1912. 



29 Nolte, O., and Hahn, E. Die Wirkung des Frostes auf den Boden. Jour. 

 Landw., 65: 75-81. 1917. 



30 Fred, E. B. Effect of grinding soil on the number of microorganisms. 

 Science, 44: 282. 1916. 



31 Further information on the influence of low temperatures upon soil bac- 

 teria is given by Smith, E. F. Das Verhalten von Mikroorganismen gegen 

 niedere Temperaturen. Centrbl. Bakt. II, 33: 335. 1912; Keith, S. C. Factors 

 influencing the survival of bacteria at temperatures in the vicinity of the freez- 

 ing point of water. Science, 37: 877. 1913. 



32 Omeliansky, V. L. Etude bacteriologique du mammouth de Sanga Jourah 

 et du sol adjacent. Arch. Sci. Biol. St. Petersburg, 16: No. 4. 1911. 



