Composition of Soil Microbiological Population 35 



tion. This is illustrated by the work of Russell and Hutchinson in 

 England; of J. G. Lipman, H. J. Conn, and E. B. Fred in this coun- 

 try; of Lohnis and others in Cermany. Little attention was paid to 

 the other groups of soil microorganisms. The protozoa were looked 

 upon merely as "injurious forms" or as "enemies of bacteria"; the 

 fungi were considered cither nuisances or "dust contaminants." 



Admittedly, it has been known since the time of Ehrenburg, in 

 1839, that the soil harbors numerous protozoa; since Darwin, in 1881, 

 that earthworms may play an important role in certain soil processes; 

 since Adametz, in 1886, that fungi are found in great abundance in 

 the soil; and since Hiltner and Stormer, in 1904, that actinomycetes 

 form an important constituent group of the soil population, as could 

 be measured by simple plating procedures. None of these organ- 

 isms, however, were given sufficient consideration in any systematic 

 study of the soil population; if they were considered at all, only 

 feeble attempts were made to coordinate their occurrence and activi- 

 ties with important soil processes. 



It has become definitely established only during the last three 

 and a half decades that the soil is characterized by a distinct micro- 

 biological population, which is made up of specific groups. These 

 exert a great variety of associative and antagonistic effects upon one 

 another. These actixities markedly influence the fertility of the soil 

 and the growth of cultivated and uncultivated plants. 



The following major groups make up the soil microbiological 

 population, or its flora and fauna: 



1. Bacteria. These include spore-forming and non-spore-forming 

 rods, cocci, vibrios, and spirilla. They vary considerably in size, 

 shape, oxygen requirements (aerobic and anaerobic), energy utili- 

 zation (autotrophic and heterotrophic), slime formation, and rela- 

 tion to plants and animals (saprophytic and parasitic). 



2. Actinomycetes. Three of the genera of actinomycetes are well 

 represented in the soil. Species of Nocardia are closely related to 

 some of the bacteria, especially the mycobacteria and corynebac- 

 teria. Species belonging to the genera Strepfomyces and Micro- 

 monospora are more closely related to the true fungi. Actinomy- 

 cetes vary greatly in their biochemical properties, in their relation to 

 higher plants and animals (saprophytic vs. parasitic), and in their 

 effect upon soil bacteria ( associative and antagonistic interrelations ) . 



3. Fungi. These include large groups of organisms, known as 

 Phycomycetes, Ascomycetes, Hyphomycetes or Fungi Imperfecti, 

 and Basidiomycetes. They produce extensive mycelium and spores 



