4 Historical 



or under aerobic conditions, a carbohydrate will be completely de- 

 stroyed with the production of carbon dioxide. In the absence of 

 oxygen, or under anaerobic conditions, the carbohydrate will be 

 only incompletely attacked, with the formation of alcohols, organic 

 acids, and certain gases, such as hydrogen, methane, and carbon 

 dioxide. The latter process, resulting from "life without air," came to 

 be known as "fermentation," although this term has often been 

 applied, quite incorrectly, to microbial life as a whole. 



The Concept of Soil Microbiology 



With the growing recognition of the numerous processes carried 

 out by the bacteria, fungi, and other microbes in the soil, there 

 gradually emerged a branch of microbiology which came to be 

 known as "soil microbiology." This deals with the microscopic 

 population of the soil, its role in the various transformations taking 

 place in the soil, and its importance in plant nutrition and crop pro- 

 duction. It concerns itself not only with the enumeration and classi- 

 fication of soil-inhabiting microorganisms, but also with the measure- 

 ment of their activities in the soil, notably the decomposition of or- 

 ganic substances that are present in the soil or that find their way 

 into the soil, with the production of ammonia and nitrates, with 

 the fixation of nitrogen, and with numerous other transformations. 

 The soil microbiologist is thus concerned with the isolation, identi- 

 fication, and description of the important groups of microbes occur- 

 ring in the soil, as well as with the part they play in the physical and 

 chemical changes that are brought about in that complex natural 

 substrate. 



Four distinct phases of soil microbiology have gradually emerged: 



1. The ecological phase, which comprises the study of the quanti- 

 tative and qualitative composition of the microscopic and ultra- 

 microscopic soil population. 



2. The experimental or physiological phase, which includes the 

 study of the physiology and the biochemistry of the organisms, their 

 role in the cycle of life in nature, and their utilization for the forma- 

 tion of valuable metabolic products. 



3. The agronomical phase, or the application of microbiological 

 activities to soil fertility and crop production. 



4. The pedological phase, or the importance of microorganisms in 

 soil formation and soil structure. 



