SOIL ORGANISMS 415 



229. Nitrification. — Some agricultural plants can utilize 

 ammonium salts as a source of nitrogen. 1 This has been shown 

 to be true for rice, maize, peas, barley, and potatoes (see par. 

 248). Most plants, however, except for rice, show a decided 

 preference for nitrogen in the nitrate form. Whether these 

 common crops can thrive as well on ammonium salts as on 

 nitrates has not been definitely demonstrated. In most arable 

 soils the transformation of nitrogen does not stop with its 

 conversion into ammonia, but goes on by an oxidation proc- 

 ess to the formation of nitrous acid. The nitrous acid, after 

 reaction with a base, is farther oxidized, a salt of nitric acid 

 resulting. This process of oxidation is generally spoken of 

 as nitrification. The reactions involved may be written as 

 follows : 



2NH 3 + 30 2 = 2HN0 2 + 2H 2 2 



2HN0 2 + CaH 2 (C0 3 ) 2 = Ca(NO,) 2 + 2H 2 + 2C0 2 3 



Ca(N0 2 ) 2 + 2 = Ca(N0 3 ) 2 



Each of these steps is brought about by a distinct bacteri- 

 um, but the groups are closely related. Collectively they are 

 called nitrobacteria. Nitrosomonas and Nitrosococcus are 

 the bacteria concerned in the conversion of ammonia into 

 nitrous acid or nitrites. The former are supposed to be char- 

 acteristic of European, and the latter of American, soils. 

 The organisms concerned in the oxidation of nitrites to ni- 



1 Kelley, W. P., The Assimilation of Nitrogen by Bice; Haw. Agr. 

 Exp. Sta., Bui. 24, 1911. 



Hutchinson, H. B., and Miller, N. H. J., The Direct Assimilation of 

 Inorganic and Organic Forms of Nitrogen by Higher Plants; Centrlb. 

 f. Bakt., II, Band 30, Seite 513-547, 1911. 



2 Loew states that the reaction is as follows: 



2NH 3 + 20 2 = 2HN0 2 + 4H 



Loew, O., Die Chemischen Verhaltnisse des BaJcterienlebens: II. 

 Centrlb. f. Bakt., II, Bd. 9, Seite 690-697, 1891. 



3 It has often been suggested that the acid produced by the nitrifying 

 process is of considerable importance in rendering mineral nutrients 

 available. While this may be true, the extent to which the solution 

 phenomenon takes place and its practical significance have never been 

 satisfactorily established by experimentation. 



