62 



nitrous acid in the solution. At any given time the actual amount of nitrie 

 acid present can be found by taking the difference bet ween the continuous 

 and dotted lines. Thus, at the end of the fifth week it is seen that there 

 were nearly four parts of nitric acid present per million The diagram 

 shows that no ;.ction took place during the first two weeks after seeding. 

 During the third week there was a vigorous evolution of nitrous acid, with 

 only a trace of niiric rcid During the fourth week, attending a depres- 

 sion of temperature, the bacterial action was less active. During th« 

 fifth week both the nitrous and nitric organisms were active, attending a 

 considerable rise of temperature. After the fifth week, the nitrous acid 

 began rapidly to disappear, being converted into nitric acid. The hori- 

 zontal position, however, of the continuous line shows that no additional 

 nitrons acid was formed from the ammonia during the sixth week. Dur- 

 ing the seventh week there was no activity either of the nitrous or the 

 nitric ferment. During the eight and ninth weeks both ferments were 

 again active, the nitrous acid being converted into nitric acid as soon as 

 formed. 



The second diagram gives the variations in temperature of the 

 closet where nitrification took place during the whole time of observa- 

 tion. The upper line represents the maximum and the lower the mini- 

 mum temperatures at the time mentioned. It will be seen by comparing 

 the two diagrams that there is in general quite a marked agreement be- 

 tween the rate of nitrification and the degree of temperature This is 

 shown by the slow rate of nitrification during the third and fourth week* 

 and the rapid rate during the fifth week. 



It is evident that many conditions beyond the control of the operator 

 may serve to render the observations upon the rate of nitrification some- 

 what unreliable, but in general the data of nitrification properly ascer- 

 tained will give an unerring insight into the character of a soil as affect- 

 ing its ability to furnish nitrogen to the growing plant, and hence to that 

 extent to the degree of its fertility. 



PREPARATION OF PURE CULTURES. 



It is evident from an inspection of the processes mentioned above that 

 the ferments which are obtained in the culture solution are not confined 

 to the nitrous and nitric orgaaisms. All the ferments which the sample 

 of soil may have contained of every description suited to grow in the 

 culture solution employed will be developed. Ihe solution, therefore, 

 after the nitrification is complete, contains not only the nitrous and nitric 

 micro-organisms, but also all the other bacteria contained in the original 

 sample capable of growing in the environment provided. It is probable 

 that in different parts of the country and at different latitudes the species 

 of nitrifying ferment may vary, and, therefore, it is of great importance 

 to continue the examination of these bacteria until pure cultures are 

 obtained. The methods of securing these are so technical and of so 

 purely a bacteriological nature as to exelude them from description here. 

 It will be sufficient to say that these pure cultures are obtained by seed- 

 ing new cultures directly from the solutions obtained in the nitrifications 

 produced by the soils as described. This work is continued until all the 

 disturbing bacteria are eliminated, and there are left only those which 

 will produce under favourable circumstances the nitrous and nitric fer- 

 mentations alone. 



