CHAPTER I 



SOILS AND WATER BASINS AS HABITATS \. tt 

 OF MICROORGANISMS 



Although microorganisms inhabit a variety of substrates, from the 

 dust in the atmosphere, the surface of living plants and plant residues, 

 and numerous foodstuffs to the living systems of plants and animals, 

 their natural habitations are soils and water basins. 



The soil is by no means an inert mass of organic and inorganic de- 

 bris. On the contrary, it fairly teems with life. The organisms inhabit- 

 ing the soil range from those of ultramicroscopic size to those readily 

 recognizable with the naked eye. Many thousands of species, capable 

 of a great variety of activities, are represented in the soil. The physical 

 nature and chemical composition of the soil, the climate, the plant vege- 

 tation, and the topography influence greatly both the composition of 

 the microbiological population of the soil and its relative abundance. 

 One gram of soil contains hundreds, even thousands, of millions of bac- 

 teria, fungi, actinomycetes, protozoa, and other groups of microorgan- 

 isms. Under certain conditions, especially when the supply of fresh or- 

 ganic matter in the form of plant and animal residues is increased, the 

 number may be much greater. This varied microbiological population 

 renders the soil capable of bringing about a great variety of chemical 

 and biological reactions. 



Through its diverse activities, the microscopic population inhabiting 

 soils and water basins forms one of the most important links in the chain 

 of life on earth. However, its great influence upon numerous phases of 

 human endeavor has been recognized only within recent years. All 

 plants and all animals, including man himself, are dependent upon 

 these organisms to bring about some of the processes essential to the 

 continuation of life. The growth of annual and perennial plants, the 

 supply of food for man and animals, and the provision of clothing and 

 shelter depend largely upon the activities of these microorganisms, 

 especially the transformations brought about in the state of such ele- 

 ments as carbon, nitrogen, sulfur, and phosphorus. 



