NITROGEN 



179 



teria, and second, the bacterial action which Hberates the 

 nitrogen of dead plants and animals. 



The various species of bacteria and their numbers, far 

 beyond the concept of man, must remain in a kind of balance 

 because of their dependence on each other. Without this 

 great group of microscopic organisms, agriculture, as we 

 know it, would be impossible. The fact is more startling 

 when we realize that so little is known about the various bac- 

 teria involved and that no general attempt is made to main- 

 tain the most desirable balance. 



The nitrogen-fixing bacteria that live in the soil have 

 been difficult to study but several groups are known to use 

 humus as a source of energy and are then able to fix atmos- 

 pheric nitrogen (use free nitrogen and convert it into a 

 usable molecule containing nitrogen). The soil must be 

 aerated since the one group must have oxygen for their 

 respiration. Azotobacter and Clostridium are two groups of 

 nitrogen-fixing bacteria, but detailed studies of their activi- 

 ties are difficult since the soil is so complex a medium that 

 the factors cannot be controlled and they probably do not 

 react the same in sterile condition in a test tube as they do in 

 the soil. It is believed that part of the value derived from 

 manure or other forms of humus is due to the increased activ- 

 ity of the nitrogen-fixing bacteria. It is quite possible that 

 these bacteria add more nitrogen to the soil than do the 

 legume bacteria in the ordinary farm rotation but in garden- 

 ing practice the legumes find little use. 



The bacterial action of decay and the preparation of that 

 nitrogen for plant use involves several groups of bacteria. 

 Bacteria which cause decay of other bacteria, plants, or ani- 

 mal excrement, change the proteins to amino acids and then 

 to ammonia. This forms ammonium compounds in the soil. 

 As explained above and shown in the chart the ammonium 

 compounds may be used by the plant, but if not, they are 

 acted on by other bacteria which oxidize them in order to get 



