434 SOILS: PROPERTIES AND MANAGEMENT 



bacteria. This is an important distinction, because those 

 bacteria that are of the greatest benefit to the soil are, in 

 the main, aerobes, and those that are injurious in their 

 action are chiefly anaerobes. However, it seems likely 

 that an aerobic bacterium may gradually accommodate 

 itself within certain limits to an environment containing 

 less oxygen, and an anaerobic bacterium may accommo- 

 date itself to the presence of a larger amount of oxygen. 

 Thus a bacterium may be most active in the presence of 

 an abundant supply of oxygen, but, when subjected to 

 conditions in which the supply is small, growth continues 

 but with lessened vigor. The term facultative bacteria 

 has been used to designate those bacteria that are able 

 to adapt themselves to considerable variation in oxygen 

 supply. The structure, tilth, and drainage of the soil 

 consequently determine largely whether aerobic or an- 

 aerobic bacteria shall be more active. 



356. Moisture. Bacteria require some moisture for 

 their growth. A notable decrease in the moisture con- 

 tent of the soil may temporarily decrease the number of 

 bacteria by limiting their development to the films of 

 moisture surrounding the particles. With a decrease in 

 the moisture content of a soil, there occurs an increase 

 in the oxygen in the interstitial spaces. Those bacteria 

 that thrive in the presence of oxygen are thereby favored, 

 and the character of the bacterial flora is correspondingly 

 changed. When the soil remains saturated, or nearly 

 so, for any considerable period, the anaerobic forms 

 assert themselves, and the usually beneficial activities 

 of the aerobic bacteria are temporarily suspended. 

 The most favorable moisture condition for the activity 

 of the most desirable bacteria is that found in a well- 

 drained soil. 



