NITRIFICATION. 833 



work has been done in this direction both by this investigator and by- 

 others. As a result, the ferment theory of nitrification has been very 

 thoroughly established, the organism producing it has been isolated, 

 and considerable progress made in the study of the conditions affecting 

 nitrification, particularly in fluid media.* 



That nitrification is due to the action of a living organism is shown 

 in various ways. Sterilized solutions, otherwise suitable for nitrifica- 

 tion, have been preserved for as long as three years unchanged. But, 

 if to such a solution a small amount of a solution or a soil in which 

 nitrification has recently taken place be added, the solution nitrifies 

 within a short time. 



Nitrification is strictly confined to the range of temperature within 

 which the action of low organisms is possible. It does not take place 

 unless all the nutritive materials necessary for such organisms are pres- 

 ent, absence of phosphoric acid, for example,- completely preventing 

 it. Antiseptics, as already illustrated, inhibit nitrification. The action 

 of heat likewise confirms the ferment theory. The temperature of 

 boiling water at once stops nitrification, and it is not resumed until 

 the medium is seeded again from some external source. 



Some of the more important conditions affecting nitrification in 

 liquids (and presumably also in porous solids, such as soil) are : 1. 

 Alkalinity of the solution ; 2. Concentration of the solution ; 3. Char- 

 acter and amount of the ferment ; 4. Temperature. 



1. While nitrification does not take place in the absence of a salifi- 

 able base, any considerable degree of alkalinity greatly retards it, and, 

 if it exceeds the equivalent of about three hundred and fifty parts of 

 nitrogen per million, stops it. 



2. Under like circumstances, nitrification begins more promptly the 

 more dilute the solution. No definite limit of concentration can be 

 stated, beyond which nitrification can not take place on account of the 

 great differences caused by differences in the 



3. Character and amount of the ferment. The character of the fer- 

 ment is determined by its previous history. A strong ferment, pro- 

 ducing prompt and rapid nitrification, is obtained by repeated culti- 

 vations in moderately strong solutions well supplied with nutritive 

 matter, while the opposite course produces a weak ferment. The 

 stronger the ferment, and the greater the amount of it used for seed- 

 ing, the sooner the nitrification begins, and the greater is the admis- 

 sible concentration of the solution. 



4. Nitrification has been observed to take place at a mean temper- 

 ature of 3*2 C. The superior limit seems to be 40 to 50 C, the 

 optimum 35 to 37 C. 



A variety of nitrogenous substances have proved susceptible to nit- 

 rification in solution. The weight of evidence, however, appears to 



* Compare especially "Warrington, " Transactions of the Chemical Society," 1884, 

 p. 637. 



vol. xxix. 53 



