INFLUENCE OF ENVIRONMENTAL CONDITIONS 769 



followed by a decrease and in some cases there were no more bacteria 

 in the treated soils than in the control soils after two and one-half 

 months. The addition of glucose to the soil increases the number of 

 bacteria but diminishes the number of fungi, both in the presence and 

 absence of available nitrogen salts. 3 It was suggested that this may 

 be due to the formation of a dry pellicle of sugar on the surface of the 

 soil preventing the admission of oxygen. Cellulose, however, greatly 

 favors the development of fungi and also favorably influences the 

 development of bacteria, especially in the presence of ammonium salts. 

 By direct microscopic examination, it can be demonstrated 4 that the 

 addition of glucose or mannite to the soil brings about a rapid develop- 

 ment of nitrogen-fixing bacteria; starch stimulates the development of 

 actinomyces, the addition of cellulose brings about an extensive 

 development of fungi, the addition of dried blood leads to an abundant 

 growth of spore forming bacteria. These results indicate that bacteria 

 are favored primarily by lower carbohydrates and protein-rich sub- 

 stances, while the fungi and actinomyces are increased to a greater 

 extent by celluloses and other polysaccharides and natural organic 

 substances, especially those of a non-protein nature. The greater 

 stimulative effect of sugars upon bacteria than upon fungi and actino- 

 myces is due to several factors: 



1. The majority of bacteria prefer glucose to higher carbohydrates and their 

 derivatives, while many fungi readily decompose celluloses, pentosans, etc.; 

 many actinomyces are capable of attacking the lignins in the natural organic 

 materials. 



2. Bacteria generally require much less nitrogen for the synthesis of their 

 cells per unit of glucose decomposed than do the fungi, which produce an abun- 

 dant mycelium requiring considerable nitrogen. 



3. Among the bacteria, the nitrogen-fixing forms readily utilize sugars as 

 sources of energy without requiring any combined nitrogen. 



The addition of glucose, in the presence of even a small amount 

 of available nitrogen will, therefore, greatly stimulate the develop- 

 ment of rapidly growing bacteria and may not affect at all the develop- 

 ment of fungi, which require a large amount of available nitrogen, or 

 of actinomyces, which develop only very slowly. 



The stimulative effect of celluloses upon the development of fungi 

 in the soil is especially marked under aerobic conditions and in the 

 presence of available nitrogen. An extensive bacterial development 



a Bazarevski, 1916 (p. 653). 

 4 Winogradsky, 1924 (p. 10). 



