What Is a Soil Microbiological Population? 263 



torining bacteria utilize the nitrite produced by ammonia-oxidizing 

 forms. 



The next step in the development of a microbial population comes 

 when organic materials are made a\'ailablc. To simplify the reactions 

 in\olved, it is sufficient to consider the effect of the various chemical 

 constituents of plant life upon the growth of a complex population. 

 Since 80-99 per cent of the organic matter in plant materials is made 

 up of carbohydrates and lignins, the effect is a group of important 

 reactions. One ma\' take, for illustration, three groups of the non- 

 nitrogenous constituents, the glucose, the cellulose, and the lignin. 



The simple carbohydrate can be attacked by many types of organ- 

 isms. When the nitrogen supply is low, only organisms capable of 

 fixing nitrogen of the atmosphere will be able to grow and utilize 

 glucose. Under these conditions there is very little competition, 

 since the two groups of bacteria capable of bringing this process 

 about, in the absence of the green plant, are aerobic forms {Azoto- 

 hactcr) and anaerobic types {Clostridium). These can only supple- 

 ment one another, the first assisting the second by consuming the 

 oxygen, and the second helping the first by breaking down its waste 

 products. Thus it has been shown that collaboration of these two 

 groups of organisms leads to greater fixation of nitrogen. 



The cellulose leads to the development of totally different groups 

 of organisms, since it cannot be utilized directly by nitrogen-fixing 

 bacteria. Its decomposition is, therefore, controlled entirely by the 

 amount of available nitrogen. The abundance and nature of the 

 nitrogen and the nature of the environment influence greatly the 

 nature of the organisms developing at the expense of the cellulose. 

 A variety of associative and competitive phenomena may result. The 

 first is manifested when the cellulose is broken down by some organ- 

 isms to dextrin-like compounds or to simple carbohydrates. These 

 are transformed by other organisms to organic acids, which are finally 

 broken down by still other organisms to COo and water or to COo 

 and methane. Competitive processes result when the cellulose is 

 attacked by bacteria, lower or higher fungi, actinomycetes, or even 

 invertebrate animals. Whether one group or another becomes domi- 

 nant depends on the reaction of the soil, nature and amount of avail- 

 able nitrogen, oxygen supply, and temperature (Table 59). 



Finally, lignin presents a different problem in regard to microbial 

 development. This substance, the chemical nature of which is still 

 in dispute, is more resistant to decomposition than most other organic 

 compounds synthesized by plant or by animal life. Although it is 



