314 SOIL MICRO-ORGANISMS 



of combined nitrogen is inoculated with particles of soil, the development of the aer- 

 obic nitrogen-fixing organism Azotobacier will take place. This organism is frequently 

 accompanied by the anaerobic nitrogen-fixing BacL amylohacter. The isolation of the 

 nitrite- and nitrate-forming bacteria and a number of other organisms that are highly 

 selective in their nutrition can be accomplished by the use of this method. 



Those organisms, especially the fungi, actinomyces, yeasts, protozoa, and many het- 

 erotrophic bacteria that develop readily upon nutrient agar or gelatin plates can be 

 most readily iso'ated from the respective colonies upon the plates. The protozoa 

 have so far not been isolated in cultures free from bacteria (with certain exceptions, 

 which are still a matter of dispute); however, they can be isolated free from other 

 protozoa and can even be grown on media containing only dead bacterial cells. 

 There are indications that some protozoa at least may be grown on purely inorganic 

 media. The algae can be isolated on media (agar, siHca gel, sand) containing the nec- 

 essary minerals and a source of nitrogen; they are cultivated in the light, since they 

 utilize the energy of the sun. 



NUTRITION OF SOIL MICRO-ORGANISMS 



The soil micro-organisms can be generally divided, on the basis of their nutrition, 

 into two groups: (i) The autotrophic organisms, or those which obtain their energy 

 from the oxidation of inorganic elements (S2, H2), inorganic compounds (NH3+, NO2-, 

 S2O3 ~,H2S), simple compounds of carbon (CO, CHJ ; carbon dioxide of the atmosphere 

 is used as a source of carbon for structural purposes. The nitrite- and nitrate-forming, 

 the sulphur-oxidizing, the hydrogen, methane, and iron bacteria belong to this group. 

 All these organisms obtain their energy chemosynthetically. The algae, or the chlo- 

 orophyll-bearing microscopic plants, which obtain their energy photosynthetically, 

 are also autotrophic organisms. (2) The heterotrophic organisms, or those that ob- 

 tain both their energy and carbon from complex organic compounds. 



There are very few autotrophic bacteria that are obligate in nature; most of them 

 are facultative, being capable of obtaining their energy also from various organic 

 compounds.' 



The heterotrophic micro-organisms include a great many forms, which vary con- 

 siderably both in their morphology and physiology. A number of organisms which 

 are very specific in their nutrition are found among the bacteria. The fungi, actino- 

 myces, and protozoa are all heterotrophic. Some of the bacteria are aerobic; others 

 are anaerobic. Some are capable of utilizing the required nitrogen in the form of gas- 

 eous atmospheric nitrogen; others require combined nitrogen, either in an inorganic 

 or in an organic form. Some require celluloses as the only source of energy; others 

 can grow on a great variety of organic compounds. Some decompose paraffins, phe- 

 nols, fats, lignins; others prefer proteins and carbohydrates. 



The composition of the media, to be used for the cultivation of these numerous 

 micro-organisms, is so selected as to have it adapted to the food requirements of the 

 particular organisms. Liquid and solid, organic and inorganic, media are employed. 



' See chap, xxiv in this volume. 



