NUMBERS OF MICROORGANISMS 5 



tance in the physiological processes in the soil, namely the organisms 

 which oxidize ammonium salts to nitrites, nitrites to nitrates, sulfur 

 and sulfur compounds to sulfates, and a few other less important groups. 



The heterotrophic bacteria are further subdivided on the basis of 

 their nitrogen utilization: (1) Those bacteria that are able to fix 

 atmospheric nitrogen in the presence of sufficient carbohydrates as 

 sources of energy. This division is again only secondary in numbers, 

 but its three representative groups play an important part in the soil 

 economy, namely in the increase of the combined nitrogen of the soil. 

 They are the symbiotic nitrogen-fixing, or nodule bacteria; the non- 

 symbiotic aerobic nitrogen-fixing bacteria and the non-symbiotic 

 anaerobic nitrogen-fixing bacteria. (2) Those bacteria which depend, 

 for their metabolism, upon the nitrogen of the soil, in organic or inor- 

 ganic forms. The heterotrophic non-nitrogen-fixing bacteria can be 

 further subdivided, using as a basis either the need of free or com- 

 bined oxygen or spore formation. The heterotrophic, non-nitrogen- 

 fixing, aerobic bacteria are usually the organisms which are found on 

 the plates, when an analysis of numbers of bacteria in the soil is made 

 by the common agar or gelatin plate method. 



In addition to the microscopic forms, ultramicroscopic microorganisms 

 capable of passing through bacterial filters have been reported 1 as 

 present in the soil. These have been only insufficiently studied. We 

 may be dealing here with certain stages of other organisms, as sug- 

 gested by Lohnis for gonidia. A certain relation was observed between 

 the ultrafilterable microbes and microbial enzymes and other cell 

 constituents. Attention may be called here to the extensive literature 

 concerning the nature of the bacteriophage; investigators do not agree 

 as yet whether these are ultramicroscopic organisms or are of the nature 

 of enzymes. An attempt to study the physiological activities of the 

 invisible soil microorganisms has been made 2 but without any success. 



Proof of microbial activities in the soil. The food requirements of the 

 various groups of soil microorganisms are so distinctly different that 

 no single artificial culture medium could be devised on which all of them 

 could be studied. A large number of microorganisms, to which some 

 of the most important soil forms belong, will grow only under very 

 special conditions, such as selective media or selective environments. 



1 Melin, E. Ultramikroskopische Mikroben im Waldboden. Ber. deut. Bot. 

 Gesell. 40: 21-25. 1922. See also Miehe, H. Biol. Centrbl. 43: 1-15. 1923. 



2 Rossi, G. Preliminary note on the microbiology of the soil and the possible 

 existence therein of invisible germs. Soil Sci. 12: 409-412. 1921. 



