DENITEIFICATION 219 



state the organisms respectively carrying on these reactions, but 

 his experiments proved that the ordinary process of nitrification 

 in soil, etc., must be due to two distinct organisms, the nitrous 

 and nitric, each incapable of doing the work of the other. 

 Soon afterwards Warington's conclusions were confirmed by 

 Winogradsky, who succeeded by a new method in preparing 

 pure cultures of the two organisms. 



The position in which Warington's investigations left the 

 question of nitrification has not been materially advanced since ; 

 the process is carried out by two distinct organisms present in 

 enormous numbers in all cultivated soils, being only absent from 

 soils possessing an acid reaction like peat. The action of these 

 organisms is dependent on certain conditions of temperature 

 and aeration, on a supply of inorganic food like phosphates, 

 on the presence of a base and on the absence of any excess of 

 soluble organic matter. 



II. DENITRIFICATION. 



During the progress of the investigations on nitrification 

 Warington found that under certain conditions soils possessed 

 the power of destroying any nitrates which had been formed 

 previously. This had been observed before, and shown to be 

 due to the action of sundry living organisms which are univer- 

 sally distributed in natural waters, sewage, and soil. The main 

 conditions necessary for the development of this reducing action 

 are the absence of oxygen and the presence of a sufficiency of 

 readily oxidisable organic matter ; it will then depend on the 

 conditions to which the soil is subjected whether the nitrate- 

 making or the nitrate-destroying organisms become active. 

 Warington, for example, showed that if an ordinary soil were 

 deprived of air by keeping it in a water-logged condition, 

 any nitrates added to it would be rapidly destroyed with the 

 evolution of nitrogen gas. 



The action of a number of organisms was studied by intro- 

 ducing them into beef broth containing some nitrate and 

 protected from the access of oxygen by a layer of paraffin oil. 



