268 
Journal of Agricultural Research 
Vd. XIV, No. 7 
Table I .—Relation between soil type , condition of soil , growth of Azotobacter , nitrogen 
fixed per 50 cc. of culture , and reaction of soil solution —Continued 
Soil 
No. 
Date. 
Soil type. 
Condition of 
ground. 
Type of 
growth. 
Nitrogen- 
fixed. 
Reaction 
expressed 
as Ph. 
75 . • 
76.... 
June 28 
.. .do.... 
Osage silt loam. 
Cultivated.... 
.do. 
Nontypical.. . 
.do. 
Mgm. 
6.08 
3 - 4 i 
4.20 
5 -So 
7 - 59 
9.24 
9.62 
7-97 
9.90 
9 - 5 i 
(a) 
(0) 
(a) 
(a) 
(a) 
(a) 
7-5 
5-5 
5-6 
7-7 
7-7 
7.6 
7-4 
7-7 
7-6 
7 * 5 
7-5 
7 -S 
7-8 
6.9 
7 -S 
7-3 
.do. 
...do .. 
... do... 
Alfalfa. 
None. 
78... 
... do.... 
Summit stony loam. 
Forest. 
Nontypical.... 
.do. 
79 --.. 
80.... 
June 30 
... do.... 
Laurel medium sand. 
Sand bar. 
Laurel very fine sandy loam..... 
River bank.. . 
.do. 
81.... 
. do.... 
Laurel fine sandy loam. 
Cultivated.... 
Forest. 
Typical. 
82.... 
...do.... 
(?) 
(Yilluvifd Stimniit 8’U loam. 
.do. 
83... . 
. do. 
Cultivated.... 
.do. 
84 . 
.. do.. . 
Daurel finp sandy loam . 
Weedgrowtli-. 
85.. .. 
86.. .. 
July 11 
...do.... 
Osage fine sandy loam. 
Cultivated.... 
.do. 
Osage silt loam. 
.do. 
Nontypical.... 
Typical. 
87.... 
... do.... 
Summit stony loam. 
Creek bank... 
Alfalfa. 
88. .. 
. do.. 
Osage silt loam. 
..... do... 
89.... 
...do.. . 
fYsftfre fine sandv loam . 
Creek bank.. . 
..... do. 
.,. do.... 
Osage silt loam... 
Cultivated.... 
.do. 
a Quantitative nitrogen determination -was not made. 
DISCUSSION OF RESULTS 
In order to show that the observed differences in Azotobacter growth 
and nitrogen fixation were not due to faulty technic, a soil known to 
possess a high nitrogen-fixing power was cultured in parallel as a con¬ 
trol. In every instance this soil gave a typical film. The nitrogen 
fixed varied from 10.12 to 12.05 mgm., with an average of 10.50 mgm. 
per culture. There is evidently, therefore, a wide variation in the nitro¬ 
gen-fixing power and in the Azotobacter development from the soils 
examined. 
Efforts to correlate this variation with soil type, moisture content of 
soils, condition of soil with respect to cultivation, fertility, etc., gave 
negative results. In some instances soils of a similar type and collected 
very close to one another gave, on the one hand, good growth and nitro¬ 
gen fixation, and, on the other, no surface growth. Many soils in high 
state of fertility gave no Azotobacter, while other almost barren or non- 
cultivated soils gave excellent growth and high nitrogen fixation. 
The only gross factor that the presence or absence of Azotobacter 
could in any way be associated with was the elevation from which the 
samples were taken. As a rule those soils coming from the higher ele¬ 
vations gave no Azotobacter growth, while those from the lower levels 
gave growth. There were, however, a number of marked exceptions to 
these rules. For example, soil 35 was from the top qf a barren hill, 
while 37 was from low bottom land; No. 35 gave Azotobacter growth, 
while No. 37 did not. 
The presence of Azotobacter in soils has frequently been associated 
both with the presence of calcium carbonate and with the reaction. 
From available evidence there seems to be no doubt that soils well sup¬ 
plied with calcium carbonate and necessarily alkaline give in cultural 
