June i6, 1923 Azotobacter Flora and Nitrogen Fixing A bility of Soils 93 5 
The existence in soils of an excess of hydrogen over hydroxyl-ions is 
no longer questioned. In 75 per cent of the 418 soils tested an excess 
of hydrogen over hydroxyl-ions was found whether the concentration 
was measured electrometrically or colorimetrically. A large number of 
examples are on record showing the toxic effect of a high concentration 
of hydrogen-ions upon bacteria. The writer has recently shown (< 5 ) that 
pure cultures of Azotobacter will not grow in laboratory media with a 
hydrogen-ion concentration appreciably in excess of i X 10"®. 
It is quite generally accepted that soil acidity is indirectly responsible 
for certain plant root diseases, and it is possible that the influence of the 
absolute reaction of the soil solution upon Azotobacter here indicated 
is also indirect. However, since the same identical limiting hydrogen-ion 
concentration has been found with pure cultures, in laboratory media { 6 ) 
as in soil solutions, it is believed that the recorded absence of this group 
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Fig. I. —Percentage of soils of different reactions containing Azotobacter 
of organisms in the soils examined is in most instances correct and is 
directly due to the toxic effect of the high concentration of hydrogen-ions 
existing in the soil solution. Furthermore, it is believed that the maxi¬ 
mum hydrogen-ion concentration tolerated by this group of organisms 
is very near i X io~® in soils as well as in laboratory media. Whether or 
not the hydrogen-ions are the ultimate limiting agent, the fact has been 
clearly established that the two are definitely associated. It is also 
evident from the data accumulated in this laboratory and shown else¬ 
where (j) that the same factors that will raise the hydrogen-ion concen¬ 
tration of a soil solution appreciably above, or lower it below, i X 10”® 
will also render the soil capable or incapable, as the case may be, of sup¬ 
porting an Azotobacter flora. 
How can the unfavorable conditions for the existence and growth of 
Azotobacter in soils be corrected? Attention has been called to the 
fact that a reduction in the hydrogen-ion concentration is probably the 
