70 DENITRIFICATION 



Breal in 1892 1 showed that straw always contains micro-organ- 

 isms which, in the absence of air, can produce free nitrogen, and to a 

 small extent organic nitrogenous compounds, from nitrates. Wagner 

 in 1895 2 by numerous experiments showed that the application of 

 large quantities of organic manures, e.g., farm-yard manure or 

 cow-dung, actually diminished the crop yielded by a soil, and to a 

 great extent interfered with the increase otherwise produced by nitrate 

 of soda. Maercker and other experimenters found similar results 

 attended the use of farm-yard manure when used with nitrates ; the 

 manure not only did no good, but actually interfered with the action 

 of the nitrate. The crop was both smaller and poorer in nitrogen. 

 These results are apparently due to denitrification produced by the 

 addition of the farm-yard manure, thus leading to the destruction of 

 nitrates. By the German investigators, the denitrifying organisms in- 

 troduced by the manure are credited with the phenomenon, but it has 

 been pointed out by Warington 3 and others that the farm-yard 

 manure introduces into the soil another factor of importance, viz., a 

 large increase in easily oxidisable organic matter, and this must 

 greatly favour denitrification, both by lessening the gaseous oxygen in 

 the soil and by actually tending to rob the nitrates of their oxygen. 

 Nevertheless, it can hardly be denied that the micro-organisms are es- 

 sential to the process, though it is not proved that there are not 

 abundance of these actually present in soils, only waiting for favour- 

 able circumstances to perform their destructive work. Such favour- 

 able circumstances are a diminished supply of oxygen, even by 

 consolidation of the soil, 4 and an increased quantity of oxidisable car- 

 bonaceous matter. 



In 1886 Gayon and Dupetit described two micro-organisms which 

 they named Bacterium denitrificans, a and b, which in the absence of 

 air, effect the oxidation of organic carbonaceous matter by reducing 

 any nitrates which may be present, nitrogen, or in some cases nitrous 

 oxide, being evolved. In presence of air, nitrates are not reduced, but 

 the oxidation is effected by the oxygen of the air. 



Other varieties of this Bacterium denitrificans have been dis- 

 covered, some obtained from soil, some found floating in the air. 5 

 The authors just alluded to ascribe denitrification to a reaction which 

 they thus formulate : 



5C 6 H 12 6 + 24KN0 3 = 24KHC0 3 + 6C0 2 + 18H 2 0+ 12N 2 . 

 Sugar. 



This reaction is quite possible and would be attended by the evolution 

 of heat. 



Deherain found that the addition of starch to a soil resulted in 

 the almost complete destruction of nitrates, but that when straw was 



. Rend., 114, 681; Jour. Chem. Soc., 1892, Abstracts, 1259. 



2 Jour. Agric. Prat., 1895, Aug. 26 ; also Jour. Chem. Soc., 1897, Abstracts, ii. 

 428. 



3 Jour. Roy. Agric. Soc., 1898. 



4 Breal, Ann. Agron., 1896, 32; Jour. Chem. Soc., 1896, Abstracts, ii. 444. 



5 Ampola & Ulpiani, Gazzetta, 1898, i. 410. 6 Ann. Agron., 1898, 130. 



