BACTERIA IN THE SOIL 1 53 



amsted), it was later established (i) that the power of nitri- 

 fication could be communicated to substances which did not 

 hitherto nitrify by simply seeding them with a nitrified sub- 

 stance, and (2) that the process of nitrification in garden 

 soil was entirely suspended by the vapour of chloroform or 

 carbon disulphide. The conditions for nitrification, the 

 limit of temperature, and the necessity of plant food, have 

 furnished additional proof that the process is due to a living 

 organism. These conditions are briefly as follows: 



1. Food (of which phosphates are essential constituents). 

 " The nitrifying organism can apparently feed upon organic 

 matter, but it can also, apparently with equal ease, develop 

 and exercise all its functions with purely inorganic food *' 

 (Warington). 



Winogradsky prepared vessels and solutions carefully puri- 

 fied from organic matter, and these solutions he sowed with 

 the nitrifying organism, and found that they flourished. 

 Professor Warington has employed the acid carbonates of 

 sodium and calcium with distinct success as ingredients of an 

 ammoniacal solution undergoing nitrification. 



2. The next condition of nitrification is the presence of 

 oxygen. Without it the reverse process, denitrification, 

 occurs, and instead of a building up we get a breaking 

 down, with an evolution of nitrogen gas. The amount of 

 oxygen present has an intimate proportion to the amount 

 of nitrification, and with 16 to 21 per cent, of oxygen present 

 the nitrates are more than four times as much as when the 

 smallest quantity of oxygen is supplied. The use of tillage 

 in promoting nitrification is doubtless in part due to the 

 aeration of the soil thus obtained. 



3. A third condition is the presence of a base with which 

 nitric acid when formed may combine. Nitrification can 

 take place only in a feebly alkaline medium, but an excess 

 of alkilinity will retard the process. 



4. The last essential requirement is a favourable temper- 



