266 
Journal of Agricultural Research 
Vol. XIV, No. 7 
hydroxid. Flasks were inoculated in quadruplicate with io cc. of a 
suspension made by shaking ioo gm. of soil in 200 cc. of sterile water. 
Two of the flasks were immediately sterilized, and all were incubated 
at room temperature for three weeks. The remaining soil was spread 
out in a thin layer, allowed to dry thoroughly, and stored for future 
physical and chemical study. 
During incubation the growth was observed at frequent intervals, and 
microscopic examinations of the surface growth were made both at the 
end of one and at the end of three weeks. After incubation total- 
nitrogen determinations were made on all samples, and that present in 
the sterilized controls was deducted from that in the cultures. In all 
except a very few instances the growth in duplicate cultures was similar 
both macroscopically and microscopically. The quantity of nitrogen 
present in duplicates also checked within very narrow limits except in 
a few instances. 
EXPERIMENTAL RESULTS 
In Table I are given the sample number, date of collection, soil type, 
condition of ground, type of growth observed, average nitrogen fixed 
per culture, expressed in milligrams, and the reaction of the soil, ex¬ 
pressed in Ph* 
Under “Type of growth” the terms “typical,” nontypical,” and 
“none” have been used. Those designated as “typical” conform quite 
well with previously described cultures of Azotobacter chroococcum . 
The growth was a uniform brown to black film covering the entire sur¬ 
face and composed almost entirely of Azotobacter cells. The “non¬ 
typical” samples exhibited usually a heavy, more or less gelatinous, 
irregular, gray, yellowish, or even an irregularly brown spotted film. 
Under the microscope such a film was found to be composed of numerous 
types of bacteria, fungi, and protozoa. Always, however, there were 
large numbers of organisms similar to, if not identical with, Azotobacter. 
Those cultures designated as “none” gave very little, if any, surface 
growth and Azotobacter-like cells were never observed. In some 
instances such cultures exhibited a copious gas formation, while in others 
there was no visible evidence of growth. In most, if not all, cultures 
butyric acid was formed; especially was this true where gas formation 
took place. 
In 37 samples, or 41 per cent, no Azotobacter developed. The nitrogen 
fixed in such cultures varied from — 0.60 to 5.55 tngm. per culture, with an 
average of 3.88 mgm. In 28 samples, or 31 per cent, the growth was 
nontypical. The nitrogen fixed in these cultures varied from 3.41 to 
9.95 mgm., with an average of 7.09 mgm. per culture. In 25 samples, or 
28 per cent, the typical growth occurred. The quantity of nitrogen fixed 
in these cultures varied from 7.95 to 10.95 with an average per 
culture of 9.47 mgm. 
