2 MICROORGANISMS IN SOILS AND WATER BASINS 



Soils and water basins may be regarded as the primary reservoirs for 

 all living systems inhabiting this planet. Whereas the great majority of 

 microorganisms are saprophytic in nature, living upon inorganic ele- 

 ments and compounds and upon the dead residues of plant and animal 

 life, others have become adapted to a parasitic form of existence and 

 have learned to thrive upon the living tissues of plants and animals. 

 Many of these parasites find their way into the soil and into water basins 

 and may be able to survive there for long periods of time or even in- 

 definitely. 



Although the following discussion is limited primarily to the micro- 

 biological population of the soil, it also applies, to a greater or lesser 

 extent, to the microorganisms that inhabit manures made up of animal 

 excreta, household wastes, and artificially prepared composts and to 

 those that inhabit water basins, including rivers, lakes, and seas. There 

 are, however, marked differences in the nature of the microbial popu- 

 lation of waters and of soils because of the physical and chemical differ- 

 ences in the composition of these two substrates. Nevertheless, some of 

 the underlying principles apply to all substrates. There are, for exam- 

 ple, marked differences in the nature and abundance of the populations 

 of soil and water and those of milk, sewage, and foodstuffs. Whereas 

 microorganisms multiply in the latter substrates at a very rapid rate, 

 those in the soil and in water basins are more nearly static, since the rate 

 of their multiplication is much slower except under very special condi- 

 tions, such as the addition of fresh, undecomposed plant and animal 

 residues or a change in the environment or in the chemical nature of 

 the substrate. 



PHYSICAL PROPERTIES OF THE SOIL 



The soil — the surface layer of the earth's crust — comprises three dis- 

 tinct phases, the gaseous, the liquid, and the solid. The last is largely 

 inorganic in nature, with varying concentrations of organic constituents 

 originating from plant and animal residues and found in the soil in dif- 

 ferent stages of decomposition. The organic substances together with 

 the living and dead cells of microorganisms that inhabit the soil make 

 up what is known as soil organic matter or, more often, soil humus. The 

 soil as a medium for the development of microorganisms is thus mark- 



