CHAPTER XXIII 



NATURE, DISTRIBUTION, AND FUNCTIONS OF SOIL 

 MICRO-ORGANISMS 



SELMAN A. WAKSMAN 

 New Jersey Agricultural Experiment Station, New Brunswick, N.J. 



THE SOIL POPULATION 



Practically every lump of soil, however small in size, contains numerous micro- 

 organisms, varying both in kind and activities. The microscopic plant world is repre- 

 sented in the soil by (i) the algae, including blue greens, grass greens, and diatoms; 

 (2) the fungi, including the actinomyces and yeasts; and (3) the bacteria, including 

 cocci, bacilli, and spirilla. The microsporic animal world is represented in the soil by 

 the numerous protozoa, including various amebae, flagellates, and ciliates, by the 

 nematodes, the rotifers, insects, myriapods, etc. 



These various micro-organisms are found in greatest abundance in the upper soil 

 layers, the numbers diminishing with depth of soil. Well-aerated soils, such as culti- 

 vated field and garden soils, may contain organisms which are quite different from 

 those found in water-logged soils, such as the peat bogs and acid forest soils. The dif- 

 ferences in the flora and fauna of various soils are frequently both qualitative and 

 quantitative in nature. The reaction of the soil, the amount of organic matter, soil 

 temperature, and moisture, are among the factors which modify considerably the 

 nature and distribution of the soil population. 



Methods have been developed which enable us not only to count the different 

 organisms present in the soil but also to isolate them from the soil, separate them from 

 one another and study, under controUed laboratory conditions, their physiological 

 activities. This is of great importance, since it enables us to determine the probable 

 role of these various organisms in soil processes. One must keep in mind, however, 

 the fact that these numerous micro-organisms act in the soil not in pure culture but 

 in associations. The activities taking place in the test tube or flask, under artificial 

 laboratory conditions, especially in the case of cultures long kept in cultivation, may 

 or may not take place in the soil, in the presence of numerous antagonistic and associa- 

 tive influences from other organisms. In the soil, the products of metabolism of one 

 group of organisms frequently serve as nutrients for other organisms. The substances 

 formed as a result of these activities under natural conditions may be altogether dif- 

 ferent from those produced in pure culture in the laboratory. 



NUMBERS OF MICRO-ORGANISMS IN SOIL 

 METHODS OF STUDY 



The methods used in the study of soil micro-organisms vary considerably, depending 

 chiefly upon the nature of the organisms under consiileration. For the determination of the 

 numbers of bacteria and fungi, direct microscopic and cultural methods are available. For 



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