FIXATION OF ATMOSPHERIC NITROGEN 



583 



Az. chroococcum at a concentration of 2 per cent in mannite solution. 103 

 Magnesium carbonate is very toxic above 0.1 to 0.2 per cent. Calcium 

 exerts a protective influence against the injurious effect of magnesium. 

 Influence of moisture and temperature upon nitrogen- fixation. Nitro- 

 gen-fixing bacteria are able to resist drying for a long period of time, 

 depending upon the nature of the medium. The soil contains sub- 

 stances which exert a protective influence upon bacteria subjected to 

 desiccation; the bacteria will resist desiccation longer in a rich clay soil 

 than in sand, probably because of the colloidal content of the clay. 104 



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'$* £5 £6 £7 £d £9 6.0 6/ 62 63 6.4 65 6.6 67 68 6.9 7.0 7./ 



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7.2 



Fig. 41 . Influence of reaction of medium upon nitrogen-fixation by Azotobacter 

 (from Gainey and Batchelor). 



Soils air-dried for five to twenty years and kept in stoppered museum 

 bottles were found to contain 103 viable nitrogen-fixing bacteria. Nitro- 

 gen-fixation takes place in the soil when its moisture content is only 2 



103 Lipman, C. B., and Burgess, P. S. The protective action against magnesium 

 carbonate of calcium carbonate for Azotobacter chroococcum. Jour. Agr. Sci. 

 6: 484-494. 1914; Gainey, P. L. On the use of calcium carbonate in nitrogen 

 fixation experiments. Jour. Agr. Res., 24: 185-190. 1923. 



104 Giltner, W., and Langworth, H. V. Some factors influencing the longevity 

 of soil microorganisms subjected to desiccation with special reference to soil 

 solution. Jour. Agr. Res., 5: 927-942. 1916. 



105 Lipman, C. B., and Burgess, P. S. Studies on nitrogen-fixation and Azoto- 

 bacter forms in soils of foreign countries. Centrbl. Bakt. II, 44: 481-511. 1915. 



