THE GERM LIFE OF THE SOIL 129 



by cutting off the air supply. A well-drained soil in good 

 tilth affords the best moisture conditions for the develop- 

 ment of desirable bacteria. 



160. Temperature. — It is seldom that soil temperatures 

 become sufficiently high to interfere with bacterial activity, 

 and then it is only near the surface. Freezing does not 

 kill most soil bacteria, but it renders them inactive during 

 the frozen period. It is in the early spring that temperature 

 is an important factor so far as its effect on bacteria is con- 

 cerned. At that season it is desirable to warm the soil 

 as rapidly as possible. 



161. Organic matter. — Many forms of bacteria utilize 

 the organic matter of the soil as a source of food supply. 

 Others thrive without any organic matter. For the proper 

 functioning of a normal bacterial flora there should be a 

 good supply of organic matter in the soil. 



162. Soil acidity. — Most of the useful bacteria make 

 their best growth in a soil that shows no acidity. This is 

 notably true of those bacteria that assist in the process 

 of making organic nitrogenous matter suitable for use by 

 plants, and also the symbiotic bacteria of alfalfa and red 

 clover. One of the important effects of lime is the increased 

 activity of beneficial soil bacteria. 



163. Bacteria in relation to soil fertility. — We have now 

 discussed the conditions under which soil bacteria grow. 

 The next step will be to describe the various processes by 

 which they increase soil fertility and also, to some extent, 

 by which they unfavorably influence soil productiveness. To 

 do this they will be discussed in the order stated in § 155. 

 The reader must, however, bear in mind that there are doubt- 

 less many bacteriological processes in the soil regarding 

 which nothing is known. 



164. Action on mineral matter. — There are, without 

 doubt, microorganisms that act on mineral matter in soil, 



K 



