Some Conditions influencing the Fixation of Nitrogen 
by Azotobacter and the Growth of the Organism. 
BY 
FLORENCE A. MOCKERIDGE, B.Sc., A.K.C. 
Layton Research Scholar at King's College, London . 
URING the progress of some research upon the utilization of various 
organic substances as food material by Azotobacter , of which an 
account will appear in due course, considerable difficulty was experienced in 
obtaining and keeping pure liquid cultures of the organism. Other investi- 
gators appear to have encountered a similar difficulty, for many different 
nutrient media and methods of culture are recommended by various authors. 
Most of the solutions advocated by them were tested in turn, with such un- 
satisfactory results that it was at last deemed advisable to make a thorough 
examination of the behaviour of the organism in these different culture 
solutions, both in respect of the rapidity and purity of the growth, and the 
extent of fixation of nitrogen obtained per unit of the carbohydrate con- 
sumed. The work was undertaken in the first place solely for the author’s 
convenience, in order to clear away these preliminary difficulties as a 
preparation for further research, but the results appear to be of sufficient 
importance to warrant publication, and are embodied in the following 
account, in the hope that they may be of use to other workers on the 
subject. 
Undoubtedly the best method of obtaining a luxuriant growth in 
a short space of time is by the use of plate cultures in the manner advocated 
by Hoffmann and Hammer, in which the surface of a mannite-agar plate is 
covered with 5 or 10 c.c. of a suspension of Azotobacter in 0-9 per cent, 
sodium-chloride solution. By this means a prolific growth is obtained 
uniformly over the whole surface of the plate, and, if the medium is sterile 
in the first place, no difficulty is experienced in keeping pure vigorous 
cultures on these agar plates. The trouble arose when liquid cultures were 
required, and it was necessary, in the first place, to arrive at some conclusion 
as to what are the optimum conditions with regard to alkalinity ; and, 
in the second place, to decide which are the inorganic constituents of the 
nutrient medium which are necessary and beneficial to the organism, the 
[Annals of Botany, Vol. XXVI. No. CIII. July, 1912.] 
