160 NITRIFYING BACTERIA 



bacteria. They build up substances instead of breaking them 

 down. This work of combining two or more elements is termed 

 synthesis and we may term these forms the synthetic bacteria. 

 These bacteria are of great economic importance and those kinds 

 that have the power of combining nitrogen into a form in which 

 it can be used by plants are of first importance. Nitrogen, as it 

 appears in the process of decay is in a form that the plant can uti- 

 lize either not at all or only to a limited extent. There are several 

 bacteria that have the power of combining nitrogen with other 

 elements and so constructing a nitrogen compound, termed a 

 nitrate, that is the most valuable crude food product of plants. 

 Among these forms may first be mentioned the nitrifying bacteria. 

 They have the power of adding another element to nitrogen 

 compounds. They are able to add oxygen (O) to ammonia 

 (NH 3 ), that is to oxidize it, and thus transform it into a nitrate. 

 This synthesis is accomplished in a truly remarkable manner; 

 no less than two forms working together are required to accom- 

 plish the work. The first form adds two atoms of oxygen to 

 the NH compound, thus changing it to a nitrous or nitrite state 

 while the second form adds to the nitrite an additional atom 

 of oxygen, thus completing the formation of the nitric or nitrate 

 condition. The reactions caused by these two forms may be 

 expressed as follows: 

 2NH 3 + 3O 2 = 2H 2 O -f- 2HNO 2 (nitrous form of nitrogen), 



2HNO 2 + O 2 = 2HNO 3 (nitric form of nitrogen). 

 These two organisms work together, each doing its special work, 

 and they are so efficient that a solution of ammonia poured upon 

 soil will show no trace of the ammonia in the solution after it has 

 percolated through the soil. Fully 65 per cent, of the nitri- 

 fication is effected in the upper twelve inches of the soil and very 

 little below a few feet. It is probable that the Chilian saltpeter 

 beds, previously referred to, are the remains of inland seas 

 where great growths of seaweeds accumulated. Owing to the 

 drying out of these bodies of water and the decay of the vegeta- 

 tion ammonia was formed which became changed to nitrates as 

 outlined above. Were it not for these organisms the ammonia 

 compounds would quickly escape from the soil as a gas and thus 



