THE BACTERIA 



205 



preceding section the relation of decay to soils was suggested. 

 Although the relation of plants and soils is more fully dis- 

 cussed in a later chapter, it is advisable at this point to discuss 

 briefly the relation of bacteria to soils. Their methods of living 

 bear peculiar relations to the nitro- 

 gen supply of the soil. 



There are at least four groups 

 of soil bacteria which are of interest 

 in this connection. First, there are 

 saprophytic forms which in their 

 processes of nutrition make a com- 

 pound of nitrogen and hydrogen 

 which is known as ammonia. This 

 bacterial action is known as ammon- 

 ification, which means " ammonia- 

 making," and the bacteria which are 

 responsible for the action are called 

 the ammonification bacteria. Sec- 

 ond, there are the so-called nitrite 

 bacteria, which in their process 

 of nutrition change ammonia into 

 other compounds in which there is 

 one part of nitrogen to every two of 

 oxygen. Such compounds are called 

 nitrites. Third, there are the ni- 

 trate bacteria, which change nitrites 

 into compounds in which there is 

 one part of nitrogen to every three 

 of oxygen. Such compounds are 

 known as nitrates. These last two 

 processes are spoken of as nitrifica- 

 tion. Fourth, there is a still different and most important 

 group of bacteria which enter the roots of certain kinds of 

 plants, as clover, soy beans, peas, and alfalfa. When some of 

 these bacteria have entered the roots, they become surrounded 



1 From " Nitrogen Bacteria and Legumes," Bulletin 94, S.C. Exp. Sta. 



FIG. 167. Bacterial tubercles 

 on the roots of the cowpea l 



A , leaves of the cowpea ; B, part 

 of the root system of the cow- 

 pea, with numerous tubercles, 

 the tubercles being induced by 

 bacteria which gather nitrogen 

 from the air of the soil. One 

 third natural size 



