66 THE EEACTIONS OCCURRING IN SOILS. IV. 



hydrogen or nitrogen is evolved in the free state. It requires 

 the presence of oxygen, or if nitrates be present they are 

 reduced to nitrites or even ammonia. Its action nearly 

 ceases at 5 C., is greatest about 30, and stops at 42. It 

 acts upon other nitrogenous organic compounds, e.g., leucine, 

 CH ; ,(CH 2 ),.CH(NH 2 ).COOH,tyrosine,HO.C (i H 4 .CH. 2 .CH(NH 2 ). 

 COOH, creatine, C 4 H 7 N 4 O 8 , or asparagine, C 4 H 8 N 2 M , but not 

 upon urea, CO(NH 2 ). 2 . 



The ammonium salts thus formed from the nitrogenous 

 organic matter of the soil usually quickly oxidises again under 

 the influence of a micro-organism, with the production of 

 nitrous acid, or rather a nitrite, and finally, by additional 

 oxidation, a nitrate is formed. The chemical reaction is 

 simple : 



(i.) (NH 4 ) 2 CO 8 + 30. 2 CO 2 +2HNO 2 +3H 2 



(ii.) 2HNO 2 +0 2 2HN0 8 



These changes, effected by the oxygen of the air, were shown 

 in 1877 by Schloesing and Miintz* to be produced only under 

 the influence of micro-organisms. Since then the subject has 

 been carefully studied by Winogradski, Deherain, Warington, 

 Frankland, and many others. 



The general conclusions arrived at may be stated as fol- 

 lows : 



1. The nitrogen of organic matter and humus is converted 

 into nitrates by passing through the intermediate stages 

 of ammonium compounds and nitrites, the successive 

 steps being the work of different organisms. The pro- 

 duction of ammonium compounds may be brought about 

 by quite a number of bacteria and moulds, but the 

 changes of nitrogen from the state of ammonia to a 

 nitrite and from a nitrite to a nitrate are each appa- 

 rently the work of only one particular organism called by 

 Winogradski Nitroso-monas and Nitroso-coccus, capable 

 only of effecting the oxidation of ammonia to a nitrite, 

 and Nitro-bacter, capable only of converting nitrites 

 into nitrates. The simpler terms nitrons organism (of 

 which possibly two or more species exist) and nitric 

 organism, used by Warington, are equally distinctive. 



* Compt. Rend. 84, 301 ; J.C.S. 1877, 215. 



