June i6 ,1923 Azotobacter Flora and Nitrogen Fixing Ability of Soils 933 
Number of normal soils electrometric Ph 6.0 or above not containing Azotobac¬ 
ter... 18 
Number of soils electrometric Ph below 6.0 containing Azotobacter. 34 
Number of soils electrometric Ph below 6.0 not containing Azotobacter. 177 
Number of soils colorimetric Ph 6.0 or above containing Azotobacter. 166 
Number of soils colorimetric Ph 6.0 or above not containing Azotobacter. 40 
Number of limed soils colorimetric Ph 6.0 or above not containing Azotobacter. 22 
Number of normal soils colorimetric Ph 6.0 or above not containing Azotobacter, 18 
Number of soils colorimetric Ph below 6.0 containing Azotobacter. 33 
Number of soils colorimetric Ph belor* ^.o not containing Azotobacter. 179 
Average electrometric Ph, 398 soils. 6. 09 
Average colorimetric Ph, 418 soils. 6. ii 
Average electrometric Ph, 193 soils containing Azotobacter. 6. 88 
Average electrometric Ph, 205 soils not containing Azotobacter. 5. 44 
Average colorimetric Ph, 199 soils containing Azotobacter. 6. 72 
Average colorimetric Ph, 219 soils not containing Azotobacter. 5. 56 
Association coefficient based on electrometric Ph determinations. o. 959 
Association coefficient based on colorimetric Ph determinations. o. 961 
In this summary there are a few points to which the writer wishes to 
call special attention. Little is known of the practical significance of 
Azotobacter in the nitrogen economy of soils. After carefully reviewing 
all the available literature and data on the subject Greaves (8) says: 
In conclusion, it may be stated that, although the part played by Azotobacter in 
maintaining the nitrogen of the soil has not been definitely measured, it is nevertheless 
an important factor. It is, therefore, conservative to state that these organisms imder 
favorable conditions add from 15 to 40 pounds of available nitrogen to each acre of 
soil yearly. 
If these figures are a conservative estimate of the quantity of nitrogen 
fixed by these organisms, the presence or absence of such a flora in soils 
is of enormous economic importance. Yet, more than half the soils 
examined during the past few years apparently do not contain Azotobac¬ 
ter. There are organisms present in practically all soils capable of fixing 
some nitrogen under laboratory conditions. The quantity of nitrogen 
fixed under the conditions of these experiments is only approximately 
half as great in the absence of Azotobacter as when they are present. 
It is believed that under actual soil conditions the differences are much 
more marked than these figures indicate. If this important group of 
organisms is absent from such a large per cent of soils, what are the 
factors controlling their presence and absence and how can the unfavor¬ 
able conditions be remedied ? 
In a former publication (2) an apparent close correlation between the 
absolute reaction of the soil solution and the presence of Azotobacter 
in soils was shown. The present data demonstrate the existence of 
such a correlation. By using Yule's association formula the very high 
coefficient of 0.96 is obtained, indicating an unmistakable correlation. 
Very few soils in which the hydrogemion concentrations exceed i X io~® 
contain Azotobacter, while practically 11 soils with a hydrogen-ion con¬ 
centration less than i X lo”® contain an active Azotobacter flora. The 
per cent of soils of different hydrogen-ion concentrations containing 
Azotobacter is found in Table XIV and shown graphically in figure i. 
