1916]. WILLIAMS—NITROGEN FIXATION 313 
itself and regain something of its original status, but similar quali- 
tative tests at the end of 3 weeks showed A zolobacter present in only 
3 soils, and from one alone was the organism successfully isolated. 
It appeared from these tests that one at least of the free nitrogen 
fixing organisms had materially deteriorated through the processes 
of drying attendant upon ro months’ storage of the soil in the 
laboratory. 
To determine to just what extent drying affected the fixing 
power of the soils as a whole was the object of the next series of 
tests. Four soils were selected which had exhibited rather extraor- 
dinary fixing powers when first brought from the field, but which 
_ had been in the laboratory for 15 months. Fixation tests were 
made with them, the solution method being employed, which uses 
ro gm. of soil in 100 cc. of Ashby’s medium, incubated for 21 days, 
with total nitrogen analysis for increase at the end of that period. 
The films which formed on two of the flasks were good and gave 
evidence of the presence of a vigorous culture of Azotobacter, but 
the two remaining ones did not exhibit this characteristic evidence. 
In table I the efficiency of the soils in question for fixing nitrogen 
in solution is given. 
TABLE I 
EFFECT OF DRYING ON THE NITROGEN FIXING POWER OF CERTAIN SOILS 
MG. NITROGEN FIXED PER LOO CC. 
ASHBY’S MEDIUM 
No. MolsTuRE CONTENT 
AFTER I5 MONTHS 
November January Percentage of 
1913 19t5 decrease 
be re is 10.6 75 3° 
Woy vets t3 13.4 8.1 40 
ee ee 1.0 15.4 11.6 24 
Ba ae eth 3.8 13.9 7.8 43 
The soils lost 24-43 per cent of their original efficiency for fixing 
nitrogen during the period of 15 months’ storage. Whether this 
falling off in nitrogen fixing power is due particularly to attenua- 
tion of aerobic or anaerobic forms is difficult to say, but from 
evidence previously obtained on the persistence of A sotobacter under 
drying, as previously noted, it is probable that the vitality of this 
