June i6,1923 Azotobacter Flora and Nitrogen Fixing A bility of Soils 931 
It could not be expected that the mere correcting of the reaction of 
an acid, Azotobacter-free soil would bring about lie establishment of 
an Azotobacter flora, unless accompanied by natural or artificial inocu¬ 
lation, Artificial inoculation with. Azotobacter has been practiced to a 
very limited extent and it is probable that none of the soils examined 
have received such treatment. Natural inoculation would probably 
eventually occur in all cases. The rapidity with which it would take 
place would depend primarily upon the proximity of soils containing 
Azotobacter and upon the activity of the agencies operating in the 
transfer of soil from one to the other. Among these agencies would 
be wind, drainage, animals, insects, birds, etc., and the more or less 
artificial transfer upon agricultural tools and machinery. If soils con¬ 
taining Azotobacter are in close proximity, and the means of transfer 
are active, inoculation may occur very rapidly. For, in an effort to 
study the effect of inoculation upon adjacent plots, the writer found it 
impossible to prevent inoculation under certain conditions, while under 
other conditions such inoculation has not appeared during a period of 
three years. Some of the limed Azotobacter-free soils here recorded 
have been receiving lime for many years. As previously mentioned, 
many of the plots to which lime has been applied are located on rather 
large areas of strongly acid soils. Such plots may be some distance 
from areas that contain Azotobacter and, hence, chances for natural 
inoculation are poor. On the other hand, the writer repeatedly cul¬ 
tured the fertility plots of the Missouri Agricultural Experiment Station 
for Azotobacter during the years 1912-1914 with negative results. 
During 1914-15 certain of these plots received an addition of lime. 
When cultured for Azotobacter again in 1919 a vigorous flora was found 
to be present. At this last examination the reaction of the soil from a 
plot that had been limed was found to be neutral (soil No. 267, Table 
XI). Soil from adjacent unlimed plots was found to be strongly acid 
(soils Nos. 265 and 266, Table XI). In other words, decreasing the 
hydrogen-ion concentration of this soil to a point below i X io~® has 
resulted in the establishment of a vigorous Azotobacter flora in a com¬ 
paratively short time. Other soils not far from these plots have been 
found to contain Azotobacter and it may be assumed that natural 
inoculation readily took place. It is not uncommon to find adjacent 
plots or areas of natural soil one of which contains Azotobacter while 
the other does not. 
Summary of Tables XI and XII 
Number of soils examined. 154 
Number of soils containing Azotobacter. 42 
Number of soils not containing Azotobacter. 112 
Average mgm. nitrogen fixed, 148 soils. 5. 30 
Average mgm. nitrogen fixed, 40 soils containing Azotobacter. 7. 35 
Average mgm. nitrogen fixed, 108 soils not containing Azotobacter. 4. 53 
Number of soils electrometric Ph 6.0 or above. 60 
Number of soils electrometric Ph below 6.0. 94 
Number of soils colorimetric Ph 6.0 or above. . 57 
Number of soils colorimetric Ph below 6.0. 97 
Number of soils electrometric Ph 6.0 or above containing Azotobacter. 32 
Number of soils electrometric Ph 6.0 or above not containing Azotobacter. 28 
Number of limed soils electrometric Ph 6.0 or above not containing Azotobacter. 24 
Number of normal soils electrometric Ph 6.0 or above not containing Azoto¬ 
bacter . 4 
Number of soils electrometric Ph below 6.0 containing Azotobacter. 10 
Number of soils electrometric Ph below 6.0 not containing Azotobacter. 84 
