Composition of Soil Microbiological Population 37 



al)ilit\ to produce chloropln 11 makes their life in the soil, especially 

 oil its surface, independent of the presence of organic matter. 



5. Protozoa. These comprise amoebae, flagellates, and ciliates. 

 The \egetati\e \s. cyst condition of the protozoa in the soil has at- 

 tracted considerable attention. This is true also of their relation to 

 the bacteria, since it was suggested at one time that protozoa func- 

 tion in the soil as the natural enemies of the bacteria. By feeding 

 upon bacteria, protozoa exert, it was believed, a controlling effect 

 upon the abundance of bacteria, thus affecting adversely a variety 

 of soil processes. 



6. Higher Animal Forms. These include nematodes, rotifers, 

 earthworms, and lar\ae of insects. These organisms have a variety 

 of functions in the soil. The ability of some of the soil-inhabiting 

 insects to attack certain higher plants frequently makes them of 

 great economic importance. The action of earthworms as "soil cul- 

 tivators" places them in an important category. The fact that some 

 of the injurious insects spend part of their life cycle in the soil 

 suggests certain methods of control. 



7. Filterable Organisms. These include phages and other 

 viruses. Although our knowledge of the importance of these forms 

 in soil processes is still very limited, their ability to attack both lower 

 and higher forms of life, ranging from the bacteria and actinomycetes 

 to cultivated and wild plants and animals, makes them of great 

 potential importance. 



8. Higher Plant Forms. In addition to the microscopic and ultra- 

 microscopic organisms, the soil also harbors the root systems of 

 higher plants. The activities of these are frequently dovetailed with 

 those of the microorganisms. 



Although the soil-inhabiting organisms form only a very small part 

 of the total soil mass, they are responsible for the major changes that 

 take place in the soil. These organisms are disti-ibuted throughout 

 the soil, primarily in the upper layers, where the plants send down 

 their roots and where they obtain their necessary nutrients. When 

 the roots die, they are rapidly decomposed by the fungi, bacteria, 

 and other groups of organisms. The constituent chemical elements, 

 notably the carbon, nitrogen, and phosphorus, are thereby returned 

 to circulation and again made available for the growth of new roots 

 and new plants. In these processes of decomposition, microorgan- 

 isms build up extensive cell substance, which contributes to the 

 organic matter of the soil or the soil humus. The microbial cells 



