SOIL BACTERIA, CONDITIONS FOR GROWTH 399 



268. Conditions affecting growth. Many conditions 

 of the soil affect the growth of bacteria. Among the 

 most important of these are the supply of oxygen 

 and moisture, the temperature, the presence of organic 

 matter, and the acidity or basicity of the soil. 



269. Oxygen. All soil bacteria require for their 

 growth a certain quantity of oxygen. Some bacteria, 

 however, can continue their activities with much less 

 oxygen than can others. Those requiring an abundant 

 supply of oxygen have been called aerobic bacteria, 

 while those preferring little or no air are designated 

 anaerobic bacteria. This is an important distinction, 

 because those bacteria which are of the greatest benefit 

 to the soil are, in the main, aerobes, and those bac- 

 teria 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 accommodate 

 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 1 the soil consequently determine largely 

 whether aerobic or anaerobic bacteria shall be most 

 active. 



270. Moisture. Bacteria require some moisture 



