June 2, 1923 
Nitrogen Fixation by Azotobacter 
761 > 
nitrogen in the control flasks and in the cultures where no growth was 
visible was very small and, as will be observed in Table II, the experi¬ 
mental error for the controls was large. In examining the data relative 
to the quantity of nitrogen fixed this experimental error must be taken 
into consideration, otherwise erroneous conclusions are likely to be drawn. 
In the data recorded in Tables II and IV the quantity of nitrogen 
present in controls has been deducted from that present in the inoculated 
flasks and only the net gain recorded. Nitrogen determinations were 
made on at least four control flasks to determine the probable error 
where only small quantities of nitrogen were present. 
INFLUENCE OF REACTION UPON GROWTH 
In Table I data are recorded relative to the influence of the hydrogen- 
ion concentration of the culture medium upon the growth of several 
strains of Azotobacter. A study of these data will show that the maxi¬ 
mum acidity permitting appreciable growth to take place is very close to 
Th 5-9- There is only one instance where definite growth is recorded in 
a higher degree of acidity (experiment 15, Ph 5.8) and the dupHcate 
of this culture failed to show growth. Another strain, 12-323-6, not 
cultured at Ph 5.9 in the experiment here recorded grew at Ph 5.9 in 
an unrecorded experiment. All cultures showed some growth at Ph 
5.9-6.0. The amount of growth that took place at Ph 5.9 was in most 
instances very slight, and growth increased as the acidity decreased 
until Ph 6.1 to 6.3 was reached. At this reaction growth appeared 
in most instances to be as vigorous as at lower hydrogen-ion concen¬ 
trations. Cultures 407-5 and 449 did not grow as abundantly as did the 
other cultures, even when the reaction was favorable. These two cul¬ 
tures failed to produce a surface film, and when growth is compared with 
nitrogen fixation it will be noted that they also fixed small quantities of 
nitrogen. 
