932 
Journal of Agricultural Research 
Vol. XXIV, No. XI 
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 limed soils colorimetric Ph 6.0 or above not containing Azotobacter. 
Number of normal 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, 154 soils. 
Average colorimetric Ph, 154 soils. 
Average electrometric Ph, 42 soils containing Azotobacter. 
Average electrometric Ph, 112 soils not containing Azotobacter. 
Average colorimetric Ph, 42 soils containing Azotobacter. 
Average colorimetric Ph, 112 soils not containing Azotobacter. 
Association coefficient based on electrometric Ph determinations. 
Association coefficient based on colorimetric Ph determinations. 
31 
26 
22 
4 
II 
86 
5 - 72 
5 - 70 
6. 70 
S -35 
6. 46 
5 - 42 
o. 971 
o. 968 
Liming is an artificial method by which the reaction of soils can sud¬ 
denly be changed from an unfavorable to a favorable condition for the 
growth of Azotobacter. Unless accompanied by inoculation with Azo¬ 
tobacter, soils that have thus been rendered favorable and do not contain 
Azotobacter should not be classed with soils that normally have a favor¬ 
able reaction and do not contain Azotobacter. For these reasons the 
limed soils with reactions of Ph 6.0 or above and not containing Azo¬ 
tobacter have been separated from the others and are not considered in 
the association coefficient caluclations. 
In Series IV there were 28 soils tested by the electrometric and 26 by 
the colorimetric method (32 different soils) with a reaction of Ph 6.0 
or above that did not contain Azotobacter. Twenty-four of the former 
and 22 of the latter are known to have been limed more or less recently. 
There were, therefore, only 6 unlimed soils with Ph 6.0 or above by one 
of the methods that did not contain Azotobacter. Five of the 6 reacted 
more acid than Ph 6.0 by one of the methods, leaving only i unlimed 
soil definitely more acid than Ph 6.0 and not containing Azotobacter. It 
is probable that lime has been applied to some of the soils of which no 
record is available. In a few instances the data indicate that some 
unknown factor has been influencing the reaction. (Compare soils 413 
with 414, 438 with 439, 399 with 400, and 363 with 364.) It is believed 
that the data on history were incorrectly recorded in a few instances. 
There were 10 soils that reacted electrometrically and ii colorimetrically 
(13 different soils) more acid than Ph 6.0 in which no Azotobacter were 
recorded. Five of these were less acid than Ph 6.0 by one of the methods; 
not one of the remaining 8 had a typical Azotobacter growth accom¬ 
panied by good fixation of nitrogen in both samples. 
GENERAL SUMMARY AND DISCUSSION 
Number of soils examined. 418 
Number of soils containing Azotobacter. 199 
Number of soils not containing Azotobacter. 219 
Average mgm. nitrogen fixed, 367 soils. 6. 36 
Average mgm. nitrogen fixed, 174 soils containing Azotobacter. 8. 30 
Average mgm. nitrogen fixed, 193 soils not containing Azotobacter. 4. 61 
Number of soils electrometric Ph 6.0 or above. 207 
Number of soils electrometric Ph below 6.0. 211 
Number of soils colorimetric Ph 6.0 or above. 206 
Number of soils colorimetric Ph below 6.0. 212 
Number of soils electrometric Ph 6.0 or above containing Azotobacter. 165 
Number of soils electrometric Ph 6.0 or above not containing Azotobacter.... 42 
Number of limed soils electrometric Ph 6.0 or above not containing Azotobacter. 24 
