142 PRINCIPLES OF SOIL MICROBIOLOGY 



anaerobic bacteria, especially the obligate forms, will be treated sepa- 

 rately because of their peculiar physiology. Of the two groups of aero- 

 bic rod-shaped bacteria, the non-spore formers are more numerous than 

 the spore-formers. The latter usually become very active when fresh 

 organic matter, rich in proteins, is added to the soil but they soon 

 sporulate and generally remain in the soil in that condition until another 

 favorable period arrives. The non-spore forming bacteria and cocci, 

 living upon the colloidal film surrounding the inorganic soil particles, 

 make up the bulk of the numbers of the soil population. Most of these 

 organisms have not yet been described at all or only very insufficiently. 

 Their physiological activities are also insufficiently studied and their 

 role in soil processes is little understood. 



Spore-forming bacteria. The spore-forming, aerobic, heterotrophic 

 bacteria have been studied more completely than the non-spore formers 

 or the anaerobic bacteria. This is due to the fact that they readily 

 develop on the common gelatin and agar media, forming large charac- 

 teristic colonies. When a short period of incubation is used, they are 

 found to be among the most numerous organisms developing on the 

 plate. Houston 1 found in 1898 four common spore-forming bacteria 

 in the soil: Bac. mycoides, Bac. subtilis (which was, according to Conn, 1 

 Bac. cereus), a "granular bacillus," equivalent to Bac. megatherium, and 

 Bac. mesentericus representing a group composed of a number of ill- 

 defined, small spore-forming organisms. 2 Houston states that Bac. 

 mycoides is present in the vegetative stage and as spores. Others 3 

 found the spore-forming bacteria to be present in the soil only in the 



1 Houston, 1898 (p. 14). 



2 Conn, H. J. Soil flora studies. III. Spore-forming bacteria in soil. N. Y. 

 Agr. Exp. Sta. Tech. Bui. 58. 1917. 



3 Conn, H. J. Are spore-forming bacteria of any significance in soil under 

 normal conditions? N. Y. Agr. Exp. Sta. Tech. Bui. 51. 1916. 



PLATE VIII 



43. Bacterium phlel: A, colony of organism on washed agar containing inor- 

 ganic salts, with petroleum vapor as the only source of energy; B, colony on 

 agar with inorganic salts and 1 per cent glycerol (after Sohngen and de Rossi). 



44. Deep agar colonies of anaerobic bacteria: colonies of Bac. perfringens in 

 nitrate glucose agar (from Weinberg and Seguin). 



45. Deep agar colonies of anaerobic bacteria: Bac. pulrificus in glucose agar, 

 48 hours old (from Weinberg and Seguin). 



46. Ash-agar plate showing the organism forming nodules on the roots of 

 a, Genista tincloria, 25 days old; b, Pisurn sativum, 7 days old (from Burrill and 

 Hansen). 



