6 MICROORGANISMS IN SOILS AND WATER BASINS 



todes that are pathogenic to the plants, such as the causative agents of 

 root rots, damping-off diseases, root-galls, and various others. Some of 

 the pathogens may become well established in the soil and may persist 

 there long after the specific host plants have been removed. They may 

 even be able to attack other hosts. Plant life thus exerts a variety of in- 

 fluences upon the nature and abundance of the soil-inhabiting micro- 

 organisms. 



Higher animals also influence the soil microbiological population. 

 Cattle and horses on pastures contribute, through their droppings, 

 energy sources and various other essential nutrients for the develop- 

 ment of microorganisms. After death, the bodies of animals, from the 

 smallest insects to man, the lord of creation, also offer available nutri- 

 ents for the growth of numerous microorganisms. Many animals living 

 in the soil, such as insects and rodents, become carriers of certain bac- 

 teria and fungi that are destructive to their hosts j this phenomenon is 

 often utilized for combating injurious animals. Finally, the numerous 

 animals living on the surface of the soil leave waste products rich in bac- 

 teria, fungi, and invertebrate animals, some of which are capable of 

 causing serious animal diseases. 



NATURE AND COMPOSITION OF THE SOIL 

 MICROBIOLOGICAL POPULATION 



The microorganisms inhabiting the soil can be divided, on the basis 

 of their systematic position in the biological kingdom, into the following 

 eight groups: bacteria, actinomycetes, fungi, algae, protozoa, worms, 

 insects and other near-microscopic animals, and ultramicroscopic forms. 

 The last group comprises bodies that range from living systems to 

 products of living organisms j they possess the property of activating 

 similar substances and imparting to them their specific activities, as in the 

 case of phages and viruses. 



Five methods are commonly employed for determining the abun- 

 dance of the various groups of microorganisms inhabiting the soilj 

 namely, plate culture, selective culture, direct microscopic methods, 

 Contact slide, and mechanical separation. Each of these has certain ad- 



