June i6,1923 A zotobacter Flora and Nitrogen Fixing A bility 0f Soils 917 
The following summary of Tables V and VI needs little explanation. 
Summary of Tables V and VI 
Number of soils examined. 
Number of soils containing Azotobacter. 
Number of soils not containing Azotobacter. 
Average mgm. nitrogen fixed, 84 soils. 
Average mgm. nitrogen fixed, 45 soils containing Azotobacter. 
Average mgm. nitrogen fixed, 39 soils not containing Azotobacter. 
Number of soils electrometric Ph 6.0 or above. 
Number of soils electrometric Ph below 6.0. 
Number of soils colorimetric Ph 6.0 or above. 
Number of soils colorimetric Ph below 6.0. 
Number of soils electrometric Ph 6.0 or above containing Azotobacter ... 
Number of soils electrometric Ph 6.0 or above not containing Azotobacter 
Number of soils elcterometric Ph below 6.0 containing Azotobacter. 
Number of soils electrometric Ph below 6.0 not containing Azotobacter .., 
Number of soils colorimetric Ph 6.0 or above containing Azotobacter_ 
Number of soils colorimetric Ph 6.0 or above not containing Azotobacter . 
Number of soils colorimetric Ph below 6.0 containing Azotobacter. 
Number of soils colorimetric Ph below 6.0 not containing Azotobacter ... 
Average electrometric Ph, 71 soils. 
Average colorimetric Ph, 90 soils. 
Average electrometric Ph> 46 soils containing Azotobacter. 
Average electrometric Ph, 2 5 soils not containing Azotobacter. 
Average colorimetric Ph, 51 soils containing Azotobacter.. 
Average colorimetric Ph, 39 soils not containing Azotobacter. 
Association coefficient based on electrometric Pn determinations. 
Association coefficient based on colorimetric Ph determinations.. 
90 
SI 
6.25 
8.32 
3.86 
51 
39 
53 
37 
47 
4 
4 
35 
50 
3 
I 
36 
6.57 
6.50 
7.12 
5-57 
7.12 
5*70 
0.981 
O.Q97 
It will be observed that the number of soils more alkaline than Ph 
6.0 by either method and not containing Azotobacter and also the 
number more acid than Ph 6.0 containing Azotobacter are few. There 
were only four soils by the electrometric and three by the colorimetric 
method with a Ph of 6.0 or above that did not contain Azotobacter. 
One of the electrometric Ph 6.0 or above soils (No. 46) not containing 
Azotobacter was more acid than Ph 6.0 colorimetrically, while soil 
No. 37 was only slightly more alkaline than Ph 6.0 by both methods. 
There were four soils with an electrometric and one with a colorimetric 
reaction more acid than Ph 6.0 recorded as containing Azotobacter. 
Three of these gave a reaction less acid than Ph 6.0 by one of the methods 
and the remaining samples did not give a typical Azotobacter growth. 
S^Rms II 
In Table VII are recorded the data collected from the examination 
of the soils of Series II. Many of these soils were collected near the 
point where certain soils of Series I were taken and may thus be regarded 
as controls on the earlier examinations. In Table XIII are recorded in 
parallel columns the data secured from the two examinations. It will 
be noted that the data are quite similar even though two years inter¬ 
vened between the two analyses. Part of the data of Table VII are 
rearranged in Table VIII to show the possible association existing be¬ 
tween tie presence of Azotobacter and the hydrogen-ion concentration 
of the soil solution. 
