June i6.1923 Azotobacter Flora and Nitrogen Fixing Ability of Soils 925 
Summary of Tables IX and X 
Number of soils examined. 114 
Number of soils containing Azotobacter.. . 60 
Number of soils not containing Azotobacter. 54 
Average mgm. nitrogen fixed, 76 soils. ... 6. 95 
Average mgm. nitrogen fixed, 44 soils containing Azotobacter. 8. 28 
Average mgm. nitrogen fixed, 32 soils not containing Azotobacter. 5. 10 
Number of soils electrometric Pr 6.0 or above. 63 
Number of soils electrometric Ph below 6.0. 51 
Number of soils colorimetric Ph 6.0 or above. 60 
Number of soils colorimetric Ph below 6.0. 54 
Number of soils electrometric Ph 6.0 or above containing Azotobacter. 53 
Number of soils electrometric Ph 6.0 or above not containing Azotobacter.... 10 
Number of soils electrometric Ph below 6.0 containing Azotobacter. 7 
Number of soils electrometric Ph below 6.0 not containing Azotobacter. 44 
Number of soils colorimetric Ph 6.0 or above containing Azotobacter. 49 
Number of soils colorimetric Ph 6.0 or above not containing Azotobacter. ii 
Number of soils colorimetric Ph below 6.0 containing Azotobacter. ii 
Number of soils colorimetric Ph below 6.0 not containing Azotobacter. 43 
Average electrgmetric Ph, 114 soils. 6. 32 
Average colorimetric Ph, 114 soils. 6. 21 
Average electrometric Ph, 60 soils containing Azotobacter. 6. 94 
Average electrometric Ph, 54 soils not containing Azotobacter. 5. 63 
Average colorimetric Ph, 60 soils containing Azotobacter. 6. 62 
Average colorimetric Ph, 54 soils not containing Azotobacter. 5. 77 
Association coefficient based on electrometric Ph determinations... o. 942 
Association coefficient based on colorimetric Ph determinations.o. 891 
The data here summarized vary but slightly from those for the two 
former series. There were 10 soils with an electrometric and ii with a 
colorimetric Ph of 6.0 or above that failed to show Azotobacter. Of 
these 11,8 were among those that remained in the laboratory for several 
months before being examined. One of the remaining 3 reacted more 
acid and i only slightly less acid than Ph 6.0 by one of the methods. 
There were 7 soils that reacted electrometrically and 11 colorimetrically 
more acid than Ph 6.0, recorded as containing Azotobacter. Six of 
these reacted less acid than Ph 6.0 and 2 of the remaining less acid than 
Ph 5.9 by one of the methods. Only i soil more acid than Ph 5.9 
produced a typical Azotobacter growth accompanied by good fixation 
of nitrogen. 
SERIES IV 
In Table XI and XII data collected from the soils of Series IV are 
presented. These soils were collected in a large number of instances 
from experimental plots where the effect of lime upon acid soils was 
being studied. Such plots are usually located upon areas of acid soil. 
From observation such soils do not contain Azotobacter. The data here 
presented show that many of these limed soils should be, as far as reac¬ 
tion is concerned, favorable to the growth of Azotobacter, yet no Azoto¬ 
bacter were found. In other words, the addition of lime has changed 
the hydrogen-ion from an unfavorable to a favorable concentration for 
Aizotobacter without apparently influencing the Azotobacter flora. There 
are many limed soils, however, to which the quantity of lime added appar¬ 
ently has not been sufficient to change appreciably the hydrogen-ion 
concentration. 
