2C>8 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



vert nitrates into ammonia or bacterial substances, or ammonia 

 into bacterial substance. Under special conditions favorable to 

 the growth of the bacteria concerned, these reverse tendencies 

 may predominate, or develop to such an extent as to materially 

 modify the final results. 



The bacteria which affect the soil nitrogen are very important. 

 Stoklosa 1 divides the bacteria into seven groups with respect to 

 their action towards nitrogen : 



1 i ) Bacteria which decompose nitrogenous organic bodies and 

 produce ammonia. 



(2) Bacteria which oxidize ammonia to nitrites. 



(3) Bacteria which oxidize nitrites to nitrates. 



(4) Bacteria which reduce nitrates to nitritec and ammonia. 



(5) Bacteria which reduce nitrates to nitrites and the latter to 

 elementary nitrogen. 



(6) Bacteria which change ammonia, nitrites or nitrates into 

 protein or bacterial body substance. This includes members of 

 all the other groups. 



(7) Bacteria which fix atmospheric nitrogen and use it to 

 form compounds. 



Bacteria have been found in the soil which take oxygen from 

 sulphates, thus reducing them. Other bacteria are found which 

 oxidize hydrogen sulphide to sulphates. 



As regards organic carbon there are two great classes of 

 bacteria : 



(1) Those which oxidize organic carbon and produce carbon 

 dioxide. 



(2) Those which reduce organic carbon and form marsh gas 

 and solid products. Bacteria are also found in the soil which can 

 utilize carbon dioxide and hydrogen to form organic matter, and 

 there are also some which can use marsh gas. Bacteria which 

 require oxygen are called aerobic, those which do best without 

 oxygen, anaerobic. 



Ammonification. A large number of different bacteria and 

 molds are capable of converting organic nitrogen into ammonia. 

 1 Bulletin 94, p. 193, Office Exp. Sta. 



