ACTIVITIES OF SOIL BACTERIA 



31 



Some of the soil bacteria are strictly aerobic in nature, devel- 

 oping only under conditions where there is free access of air; 

 some exist in the complete absence of oxygen, under so-called 

 anaerobic conditions. Most of the soil bacteria develop best at 

 reactions close to neutraUty, where there is no appreciable excess 

 acid or base. Some, however, can exist at a much higher acidity 

 than others. Thermophihc bacteria, or those which are favored 

 by high temperature, such as 50 to 60° C, also occur in soils, but 

 the great majority of the soil bacteria grow best at reactions 

 between 20 and 30° C. All the bacteria require a considerable 

 supply of water to permit their active 

 development. Under dry conditions, they 

 either change to resistant spores, become 

 dormant vegetative cells in the moist films 

 which cover the soil particles, or die from 

 the effects of desiccation. In the presence 

 of excessive moisture where rapid circula- 

 tion of oxygen is prevented, the aerobic 

 microorganisms are depressed and the 

 anaerobic cells find conditions more con- 

 ducive to their growth. 



As a result of microbial attack upon 

 proteins, there are formed carbon dioxide, 

 ammonia, and various incompletely de- 

 composed organic substances. From the 

 decomposition of carbohydrates, carbon 

 dioxide, organic acids, and alcohols are 

 produced. The oxidation of ammonia 

 leads to the formation of nitrites which are 

 oxidized to nitrates. These various sub- 

 stances may be further acted upon by soil organisms. The same 

 organism may also produce different end products from different 

 initial compounds, or from the same compound under different 

 conditions. The numerous processes brought about by soil 

 organisms are usually interdependent and follow one another. 

 Irrespective of their source, these substances are waste products 

 of the metabolism of the microbial cells, and many of the 

 nutrients obtained by higher plants as a result of the activities of 

 bacteria in the soil are actually waste products of the nutrition of 

 these bacteria. 



Fig. 19. — Rhizopus sp. 

 (from Jensen). 



