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- 
Fic. 167. Bacterial tubercles 
on the roots of the cowpea 
ail, 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 
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. 
