642 PRINCIPLES OF SOIL MICROBIOLOGY 



esses which depend upon the activities of 'other microorganisms: the 

 nitrite-forming bacteria depend upon the heterotrophic bacteria and 

 fungi for ammonia, the nitrate bacteria depend upon the nitrite formers 

 for nitrite, the sulfur bacteria depend upon the heterotrophic organisms 

 for the decomposition of the proteins and liberation of the H 2 S. 



The soil population. In the complex medium of the soil and under the 

 influence of various physical, chemical and environmental conditions, 

 soil microorganisms carry on their activities, not as individual forms or 

 even as groups, but as a soil population. The most common soil bacteria, 

 including the heterotrophic spore-formers (Bac. cereus, Bac. mycoides, 

 Bac. mesentericus, etc.) and the non-spore formers (Bad. fluorescens, 

 Bad. caudatum, Bad. radiobacter, etc.), the nitrogen-fixing Azotobacter 

 and CI. pastorianum, the nitrifying bacteria, etc., are of universal occur- 

 rence, limited only by specific soil conditions, such as reaction. They 

 are reported from all soils, from East and West, North 70 and South. 

 The most common soil protozoa, including the amoebae, flagellates 

 and ciliates, are also universally distributed in the soil. 71 The common 

 soil fungi, including species of Zygorhynchus, Trichoderma, Penicil- 

 lium, etc., have also been isolated from various soils coming from dif- 

 ferent parts of the world. 72 The same is true of algae and other soil 

 microorganisms. 



As a result of soil conditions this population shows quantitative, 

 rather than qualitative differences. Excessive moisture, for example, 

 stimulates the development of anaerobic bacteria 73 but inhibits the 

 development of aerobic forms like Azotobacter and fungi. Ex- 

 cessive acidity and a high content of organic matter rich in carbo- 

 hydrates greatly stimulate the development of fungi. Applications 

 of lime and the addition of organic matter rich in protein stimulate the 

 development of bacteria and actinomyces. 



We are thus fully justified in speaking of a soil population and may 

 even accept the idea of an Edaphon as suggested by France^ 74 although 

 his conclusion that the edaphon is an indicator of soil fertility may not 

 be fully justified. 75 The composition and activities of the soil popula- 



70 Barthel, Chr. Recherches bactcriologiques sur le sol et sur les matures 

 fecales des animaux polaircs du Groenland Septentrional. Saertr. Meddl. 

 Gronland., 64. 1922. 



71 Sandon, 1924-1927 (p. 329). 



72 Waksman, 1917 (p. 237). 



73 Winogradsky, 1924 (p. 542). 



74 France, R. H. Das Edaphon, Stuttgart. 1921. 



75 Fischer, H. Gibt es ein Edaphon? Int. Mitt. Bodenk., 13: 192-200. 1923. 



