580 PRINCIPLES OP SOIL MICROBIOLOGY 



posed, the fixation of nitrogen was even greater. By increasing the 

 amount of stable manure added up to three per cent, there was 90 a 

 great increase in nitrogen-fixation. The stimulating effect is probably 

 caused by the increase in available energy, due to the introduction of 

 straw and its derivatives, and by the colloidal content of the material. 

 A further increase in organic content may bring about a depression un- 

 less the soil is well aerated and contains sufficient CaC0 3 . The various 

 plant residues, such as roots, leaves and stems will react similarly. 

 Manure is also a carrier of various nitrogen-fixing organisms. 91 



The possible favorable influence of growth-promoting substances 

 (vitamins, auximones) upon Azotobacter is still a subject of discussion. 92 



The beneficial action of humus is frequently ascribed to its inorganic 

 constituents, particularly aluminum and silicic acid. This is confirmed 

 by the fact that the so-called artificial humus has no such effect, while 

 the source of the natural humus influences the degree of its beneficial 

 action. The claim that the action of the humus is due to its inorganic 

 constituents has been further substantiated by the fact that purified 

 humates do not possess the stimulating effect. 93 The role of the col- 

 loid is probably chiefly due to its catalytic action and its protective 

 action against poisons; 94 the protective action of the colloid has also 

 been ascribed to the distribution of the phosphorus and to the buffer- 

 ing effect upon the reaction of the medium. 



90 Hanzawa, 1913 (p. 575). 



91 Tottingham, W. E. The increase of nitrogen in fermenting manures. 

 Jour. Biol. Chem., 24: 221-225. 1916; Fulmer, H. L., and Fred, E. B. Nitrogen- 

 assimilating organisms in manure. Jour. Bact., 2: 422-434. 1917. 



92 Bottomley, W. B. Some accessory factors in plant growth and nutrition. 

 Proc. Roy. Soc. B., 82: 627. 1910; 85: 466. 1912;88:237-247. 1914; 89: 102. 

 1915; The significance of certain food substances for plant growth. Ann. Bot., 

 28: 531-540. 1914 ; Mockeridge, F. A. Some effects of organic growth promoting 

 substances (auximones) on the soil organisms concerned in the nitrogen cycle. 

 Proc. Roy. Soc. B., 89: 508-533. 1915; Biochem. Jour., 19: 272-283. 1915; 

 Allen, E. R. Some conditions affecting the growth and activities of Azotobacter 

 chroococcum. Ann. Mo. Bot. Gard., 6: 1-AA. 1919; Itano, A. Physiological 

 study of Azotobacter chroococcum. I. Influence of vitamine B (?) and nucleic 

 acid on Azotobacter. Jour. Bact., 8: 483-486. 1923; Hunter, O. W. Production 

 of a growth promoting substance by Azotobacter. Jour. Agr. Res., 23: 825- 

 830. 1923. 



93 Kaserer, H. Uber die biologische Reizwirkung naturlicher Humusstoffe. 

 Centrbl. Bakt. II, 31: 577-578. 1912. 



94 Voicu, J. Influence de l'humus sur la sensibilite de V Azotobacter chroococ- 

 cum vis-a-vis du bore. Compt. Rend. Acad. Sci., 175: 317-319. 1922. 



