FIXATION OF ATMOSPHERIC NITROGEN 585 



lies at 28°C. (25° to 30°), with limits between 9° and 33°C. 109 Koch 110 

 obtained fixation of 3, 11, and 15.5 mgm. of nitrogen in 100 grams of 

 soil at 7°, 15,° and 24° respectively. The maximum temperature for 

 Azotobacter is 111 55° to 60° and the minimum near zero. The organism 

 can withstand heating at 45° to 50°C. for 15 minutes, but is destroyed in 

 30 minutes. The optimum temperature for the growth of CI. pastoria- 

 num is 20° to 25°, according to Winogradsky, and 28° to 30°, according 

 to Bredemann. 112 At 30° to 35°, the action of the organism is retarded. m 

 The organism can withstand a temperature of 75° for even 5 hours or 

 more; the spores can be preserved in a dry state for 20 years without 

 losing their power of germination and nitrogen fixation. 



Soil cultivation and nitrogen-fixation. It has been generally observed 114 

 that fallowing leads to an increase in nitrogen-fixation, probably due to 

 better aeration and moisture conditions. According to Hiltner, 115 

 non-symbiotic nitrogen-fixation is stimulated by growing plant roots; 

 the higher plants use up the available nitrogen in the soil and thus create 

 a nitrogen-hunger for the non-symbiotic nitrogen-fixing bacteria. The 

 plants supply the bacteria with available energy, in the form of rotting 

 roots hairs, root tips, etc. Plant roots may also create a better physical 

 environment for the nitrogen fixing organisms. In view of the fact that 

 different cultural methods are used for the growth of different crops, 

 the influence of the crops upon nitrogen-fixation will vary. A much 

 higher nitrogen-fixing power was found 116 in cultivated than in virgin 

 soils; the fallowed soils show more nitrogen fixed than the cropped soils. 



Importance of non-symbiotic nitrogen- fixation in the soil. It should 



109 Krzemieniewski, 1908 (p. 579); Lohnis and Westermann, 1908 (p. 115). 



110 Koch, A. Ernahrung der Pflanzen durch frei im Boden lebende stick- 

 stoffsammelnde Bakterien. Ber. deut. landw. Gesell., 22: 117-121. 1907. 



111 Jones, 1913 (p. 114). 



112 Bredemann, 1909 (p. 111). 



113 Omeliansky, W. L. Sur la physiologie et la biologie des bacteries fixant 

 l'azote. Arch. Sci. Biol. Petrograd, 19: 209-228. 1915. 



114 Heinze, B. tlber die Stickstoffassimilation durch niedere Organismen 

 Landw. Jahrb., 35: 889-910 1906. 



116 Hiltner, L. Ueber neuere Erfahrungen und Probleme auf dem Gebiete 

 der Bodenbakteriologie und unter besonderer Berticksichtigung der Grundting- 

 ung und Brache. Arb. deut. landw. Ges. H. 98: 59-78. 1904 (Centrbl. Bakt. 

 II, 14: 46-48. 1904). 



116 Greaves, J. E. A study of the bacterial activities of virgin and cultivated 

 soils. Centrbl. Bakt. II, 41: 444-459. 1914; Reed, H. S., and Williams, B. 

 Nitrogen-fixation and nitrification in various soil types. Va. Agr. Exp. Sta. 

 Tech. Bui., 3: 59-80. 1915. 



