128 



SOILS AND FERTILIZERS 



157. Conditions affecting bacterial growth. — The en- 

 vironment is a controlling influence in the development of 

 bacteria as it is of all organisms. Among the important 

 environmental influences are the supply of air and moisture, 

 the temperature, the presence of organic matter, and the 



presence or absence of 

 acidity in the soil. 



158. Air supply. — 

 While all bacteria require 

 some air for their growth, 

 certain of them are able 

 to get along with much 

 less than others. Those 

 requiring an abundant 

 supply of air have been 

 called aerobic bacteria 

 and those that thrive 

 better on a small air sup- 

 ply are termed anaerobic. 

 The 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. 

 Bacteria, however, have more or less ability to adapt them- 

 selves to a larger or smaller air supply. The fact that struc- 

 ture, texture and drainage are so largely instrumental in 

 regulating the quantity of air in the soil makes them im- 

 portant factors in determining the kinds of bacterial processes 

 that take place in a soil. 



159. Moisture. — Like other forms of plant life, bacteria 

 require moisture for their growth. A soil may become so 

 dry that the number of bacteria is decreased, but owing to 

 their rapid multiplication the number soon increases with 

 a replenished moisture supply. An excess of water may 

 decrease the number or change the character of the flora 



C 



Fig. 23. — Diagram showing the relative 

 sizes of bacteria and some soil particles. 

 (A) a fine sand particle, (B) a large clay 

 particle, (C) a few soil bacteria. All are 

 magnified at the same rate. 



