70 THE EEACTIONS OCCURRING IN SOILS. IV. 



from nitrates. Wagner in 1895" by numerous experiments 

 showed that the application- of large quantities of organic 

 manures, e.g., farm-yard manure, cow-dung, &c., 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 introduced by the manure are credited with the 

 phenomenon, but it has been pointed out by Waringtonf 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 denitri- 

 fication, both by lessening the gaseous oxygen in the soil and 

 by actually tending to rob the nitrates of their oxygen. Never- 

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

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

 not abundance of these actually present in soils, only waiting 

 for favourable circumstances to perform their destructive work. 

 Such favourable circumstances are a diminished supply of 

 oxygen, even by consolidation of the soil,]: and an increased 

 quantity of oxidisable carbonaceous 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, nitro- 

 gen, 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 

 discovered, some obtained from soil, some found floating in the 



t J.R.A 



Journ. Agrie. Prat. 1895, Aug. 26; also.I.C.S. 1897, abst. ii., 428. 

 LS. 1898. I Breal, Ann. Agron. 1890, 32; J.t'.S. 189<3, abst. ii., 



