92 AGRICULTURAL BACTERIOLOGY. 



are acted upon by common putrefactive bacteria, and this is 

 doubtless one of the means for rendering nitrogen directly 

 available. It must be noticed, however, that it is somewhat 

 doubtful whether the ammonia products are directly used by 

 plants. In sterilized soils plants make much less use of 

 these salts than they do in soils which are not sterilized, a fact 

 which indicates that possibly all of the ammonia compounds 

 must be first nitrified by bacteria into nitrates before they are 

 directly available. 



But, however this may be, it is certain that ammonia salts 

 constitute only a small part of the available nitrogen in ordi- 

 nary soils. Much of the nitrogen in the soil eventually 

 reaches other forms, and in the humus there is a variety of 

 compounds which demand change in their chemical nature 

 before they can be utilized by plants. It is certain that these 

 compounds can be rendered available for vegetation by a proc- 

 ess of oxidation. It is only necessary to oxidize the lower 

 compounds of nitrogen to convert them into nitrates. For 

 example : ammonia (or its salts) may be united with oxygen to 

 form nitrous acid as follows : 



2NH 3 -f 2O = 2HNO 2 -j- 3H 2 O 



By replacing the hydrogen with potassium this becomes KNO 2 , 

 or nitrite of potassium. By a further oxidation the nitrous 

 acid or nitrite becomes nitric acid or its salts, nitrates ; 



or later KNO 3 which is nitrate of potassium. In this form the 

 nitrogen is in its most useful form for plant food, nitrate of 

 potassium being one of the best fertilizers. 



This process of the oxidation of nitrogenous matter, which 

 was known long before it was understood, is called nitrifica- 

 tion. It was at first supposed to be a simple chemical process 

 taking place under the direct oxidizing power of atmospheric 

 oxygen. Such an oxidation would be simple and was shown 



