Aug. is, 1918 Soil Reaction and the Growth of Azotobacter 
271 
except three, gave a Ph of 6.0 or above. The average P H of soils showing 
no Azotobacter growth was 5.71 and the nitrogen fixed 3.88 mgm. per 
culture. The average P H of those soils showing Azotobacter growth was 
6.78 and the average nitrogen fixed was 8.11 mgm. per culture. Of the 
exceptions to the above rule, soil 38 gave very few isolated colonies, and 
in the case of No. 76 only one culture gave Azotobacter. In these two 
instances the Azotobacter growth was probably due to contamination. 
All these exceptions are being studied further. 
It should be remembered that the acidity analyses were made on 
samples of soil that had been stored from 7 to 10 months. Gillespie has 
called attention to the possibility of determinations made under such 
conditions varying slightly from actual soil conditions. 
The writer does not, therefore, wish to leave the impression that the 
the maximum acidity tolerated by Azotobacter is necessarily represented 
exactly by a Ph of 6.0, or that the limits are necessarily so definite as 
these experiments would indicate. It is believed, however, that the results 
herein reported are very significant. Also that when investigations now 
under way are completed, the writer will be in a position to say that 
active Azotobacter will not exist in soils in which, other factors not inter¬ 
fering, the hydrogen-ion concentration exceeds a fairly definite limit. 
He hopes also to give much more accurate data as to what that limit is. 
The same phenomenaare being studied in the case of other soil organisms. 
