448 SOILS: I>B0PJSBTIE8 AND MANAGEMENT 



been determined for maize, rice, peas, barley, and po- 

 tatoes. Other plants^ such as beets, show a decided 

 preference for nitrogen in the form of nitrates. Whether 

 any of the common crops can thrive as well on ammo- 

 nium salts as on nitrates has not been finally demon- 

 strated. In most arable soils the transformation of nitro- 

 gen does not stop with its conversion into ammonia, but 

 goes on by an oxidation process to the formation of first 

 nitrous, and then nitric, acids (see Fig. 61). This may be 

 considered to proceed according to the following equa- 

 tions : — 



2 NH3 + 3 O2 = 2 HNO2 + 2 H2O 



2 HNO2 + O2 = 2 HNO3 



The acid in either case combines 'with one of the bases 

 of the soil, usually calcium, so that calcium nitrate 



Each of these steps is brought about by a distinct 

 bacterium, but the bacteria are closely, related. Collec- 

 tively they are called nitrobacteria. Nitrosomonas and 

 Nitrosococcus are the bacteria concerned in the conver- 

 sion of ammonia into nitrous acid or nitrites. The former 

 are supposed to be characteristic of European, and the 

 latter of American, soils. They are sometimes referred 

 to as nitrous ferments. 



Nitrobacter are those bacteria that convert nitrites 

 into nitrates. They are also designated nitric ferments. 

 There seem to be some differences in bacteria from dif- 

 ferent soils, but the differences are slight and the condi- 

 tions favoring the actions of the bacteria are similar. It 

 is also true that the conditions favoring the action of 

 Nitrosomonas and Nitrobacte^ are similar, and they 

 are generally found in the same soils, although some 



