32 



THE MICROBE AND ITS ACTIVITIES 



Among the soil processes in which bacteria are important and 

 frequently play a predominant role, the following need only be 

 enumerated here in order to suggest the 

 diversity of the changes: the fixation of 

 atmospheric nitrogen symbiotically by bac- 

 teria in association with leguminous plants, 

 and non-symbiotically by the bacteria in a 

 free-living condition in the soil ; the decom- 

 position of various organic constituents of 

 plant and animal residues, including cellu- 

 lose, hemicelluloses, sugars, proteins, amino 

 acids, fats, and waxes, both under anerobic 

 and aerobic conditions; the liberation of 

 ammonia as a result of the decomposition of 

 various proteins, protein derivatives, and 

 other organic nitrogenous substances; the 

 formation of nitrites and nitrates from am- 

 monia; the reduction of nitrates to nitrites, 

 to ammonia and to atmospheric nitrogen; 

 the oxidation of sulfur and of hydrogen, as 

 well as of incompletely oxidized compounds 

 of sulfur and of iron, and the oxidation of various other simple 

 inorganic and complex 

 organic substances. 



Truly, bacteria are 

 active in all phases of 

 transformation of inor- 

 ganic complexes and of 

 organic matter in the 

 soil, this fact partly jus- 

 tifying the early concep- 

 tion that bacteria are 

 the only significant soil 

 organisms. However, as 

 the important role of 

 other groups of soil mi- 

 crobes is understood, it 

 becomes evident why 

 the general conception of the soil population was changed to 

 include a consideration of many other soil microbes. The science 



Fig. 20. — Trichoderma 

 sp. : (a) arrangement of 

 spores on sporulating 

 hyphae; (6) higher 

 magnification of one of 

 the branches (from 

 Henrici). 



Fig. 21. — Acrosfalagmus sp. : (a) arrangement of 

 spores on sporulating branches, (6) higher mag- 

 nification (after Goddard). 



