920 
Journal of Agricultural Research 
Vol. XXIV, No. II 
In the following summary it may be observed that there were no 
soils more alkaline than Ph 6.0 that did not contain Azotobacter. 
There were 13 soils with an electrometric and 10 with a colorimetric Ph 
below 6.0 that were recorded as containing Azotobacter. Only 5 of 
these produced typical Azotobacter film and 10 of the 13 gave a Ph of 
5.8 or above by one of the methods. It would appear from the data of 
this series of soils that Ph 5.8 was nearer the absolute limiting hydro- 
gen-ion concentration than Ph 6.0. 
Summary of Tables VII and VIII 
Number of soils examined. 60 
Number of soils containing Azotobacter. 46 
Number of soils not containing Azotobacter.. 14 
Average mgm. nitrogen fixed, 59 soils. 8. 41 
Average mgm. nitrogen fixed, 45 soils containing Azotobacter. 9. 16 
Average mgm. nitrogen fixed, 14 soils not containing Azotobacter. 6. 03 
Number of soils electrometric Ph 6.0 or above. 33 
Number of soils electrometric Ph below 6.0. 27 
Number of soils colorimetric Ph 6.0 or above. 36 
Number of soils colorimetric Ph below 6.0. 24 
Number of soils electrometric Ph 6.0 or above containing Azotobacter. 33 
Number of soils electrometric Ph 6.0 or above not containing Azotobacter. o 
Number of soils electrometric Ph below 6.0 containing Azotobacter .. 13 
Number of soils electrometric Ph below 6.0 not containing Azotobacter. 14 
Number of soils colorimetric Ph 6.0 or above containing Azotobacter. 36 
Number of soils colorimetric Ph 6.0 or above not containing Azotobacter. o 
Number of soils colorimetric Ph below 6.0 containing Azotobacter. 10 
Number of soils colorimetric Ph below 6.0 not containing Azotobacter. 14 
Average electrometric Ph, 59 soils.. 6. 37 
Average colorimetric Ph, 60 soils. 6. 39 
Average electrometric Ph, 45 soils containing Azotobacter. 6. 73 
Average electrometric Ph, 14 soils not containing Azotobacter. 5. 21 
Average colorimetric Ph, 46 soils containing Azotobacter. 6. 65 
Average colorimetric Ph, 14 soils not containing Azotobacter. 5. 55 
Association coefficient based on electrometric Ph determinations. i. 000 
Association coefficient based on colorimetric Ph determinations. i. 000 
The above summary with a few minor exceptions is very similar to 
that for Series I. The most marked difference is in the average nitrogen 
fixed by the soils not containing Azotobacter, this being 3.86 mgm. for 
Series I and 6.03 mgm. for Series II. This difference is due to the much 
higher fixation of nitrogen by soils not containing Azotobacter in media 
containing calcium carbonate than in the media without calcium car¬ 
bonate. This fact has been emphasized in a former article (4). No 
calcium carbonate was added to the media employed in the examination 
of the soils of Series I, while an excess was added to the media in the 
examination of all subsequent soils. 
SERi:^s III 
The soils of Series III were secured through the county agents of Kansas 
and did not reach the laboratory in as fresh and possibly as uncon¬ 
taminated condition as did those collected locally. Samples 138 to 180, 
inclusive, reached the laboratory while the writer was in the Army and 
remained several months before being examined. It was believed that 
the quantity of nitrogen fixed by these soils would not be comparable 
to that of fresh soils, hence no nitrogen determinations were made. 
Possibly these soils should be disregarded entirely. The data secured 
from the examination of the soils of this series are recorded in Tables IX 
and X, and summarized below. 
