768 PRINCIPLES OF SOIL MICROBIOLOGY 



between the different members of the population and between the 

 latter and the environmental conditions. 



Influence of organic matter upon the soil population. The addition 

 of organic matter to the soil results in an increase in the numbers of 

 various groups of soil microorganisms, of which some are directly 

 concerned in the process of decomposition and some utilize the 

 products formed by the first or attack the living or dead cells of those 

 organisms themselves. Differences in the physical and chemical 

 nature of the soil lead to the development of different groups of micro- 

 organisms as a result of addition of the same organic substance. The 

 composition of the organic matter is of prime importance in this con- 

 nection, modifying largely the nature of the organisms which develop 

 in preference to others. The addition of soluble sugars to the soil 

 brings about an extensive development of bacteria, primarily nitrogen- 

 fixing forms, such as Azotobacter, under aerobic conditions, and Clos- 

 tridium under anaerobic conditions. The addition of substances rich 

 in celluloses and poor in proteins, like cereal straw, stimulates largely 

 the development of fungi; these decompose the celluloses and synthesize 

 an extensive mycelium. This mycelium is immediately attacked by a 

 large number of bacteria and actinomyces, leading to the formation 

 of various products in addition to the synthesized cells of these micro- 

 organisms, which in their turn along with the residual products of the 

 organic matter and the fungus mycelium, serve as food for protozoa 

 and other invertebrate animals. One group of organisms carries on a 

 certain process and then gives way to another group, which carries 

 on the process further; both may be active at the same time. The 

 activities of these organisms lead to a gradual increase in the carbon 

 content of the residual organic matter, which has become changed 

 into a colloidal mass more or less constant in composition and less 

 readily available as a source of energy. Finally those microorganisms 

 (largely the minute non-spore forming bacteria), which require only a 

 small amount of energy and nutrients, continue to act in the colloidal 

 film surrounding the soil particles, slowly breaking down the residual 

 lignins and the resistant synthesized materials of a protein nature. 



Engberding 2 found that the addition of 2 per cent cane sugar to a 

 soil brought about an increase of 1000 to 1500 per cent in the number of 

 bacteria developing upon Heyden agar; 0.5 per cent glucose brought 

 about an increase of only 300 to 400 per cent. This increase was soon 



2 Engberding, 1909 (p. 14). 



