82 Occurrence of Microorganisms in Soil 



tensive application in human and in animal therapy. Their function 

 in soil processes is still unknown. 



According to the system of classifying actinomycetes proposed by 

 Waksman and Henrici and adopted in Bergey's manual, four genera 

 are now recognized: Actinomyces, Nocardia, Streptomyces, and 



Fig. 38. Effect of actinophage upon Streptomyces griseus ( from Reilly, Harris, 



and Waksman). 



Micromonospora. The first genus includes the animal pathogens; 

 the second comprises both parasites and saprophytes; the third em- 

 braces most of the soil forms, including the plant-pathogenic scab 

 producer; and the fourth is the most abundant in lake bottoms and 

 in high-temperature composts. 



Soil Fungi 



Although fungi are not represented in the soil by so many physio- 

 logical groups as are the bacteria, many thousands of species find in 

 the soil a temporary or permanent habitat. Of the various genera of 

 fungi found in the soil, the most common, both in the number of 



