1916] WILLIAMS—NITROGEN FIXATION 315 
connected with the humus content of a soil that this latter factor is 
by far the most important influence in connection with the process. 
The possibility that certain other factors lend a depressing effect 
has been eliminated to a degree from consideration in this laboratory 
by results of certain work with reference to toxic conditions of the 
soil. Whatever may be the inimical effects on higher plants of a 
toxic condition, if such in reality often exists, it is extremely doubt- 
ful whether similar effects are exerted on the bacterial flora. At 
least a number of compounds which are supposed to be responsible 
for soil toxicity have been shown to exhibit little deleterious effect 
on the growth of Azotobacter. Evidence from further tests goes 
to show that the soil extract itself probably does not carry sub- 
stances which would retard fixation. 
Three soils which possessed conspicuously low fixation powers 
were selected and extracts made therefrom. The extracts were 
made into Ashby’s medium and fixation tests with pure cultures 
of Azotobacter were carried out. Along with the flasks containing 
the extracts were controls using Ashby’s solution made with dis- 
tilled water. The same strain of pure culture was used for inocu- 
lating all the flasks. After 21 days the contents of the flasks were 
analyzed for total nitrogen. The nitrogen fixed by the culture 
grown on the extracts from the 3 soils was 6.5, 7.6, and 6.7 mg. 
nitrogen respectively. Those grown on the control flasks gave 
6.8 mg. There was certainly no depressing effect registered here 
from using extract from soils deficient in nitrogen fixing power in 
substitution for distilled water in the culture medium. 
The studies on nitrification (/oc. cit.), which up to this time had 
included an examination of some 93 soils, were continued along 
with the work on fixation. Among the soils examined were a 
number which had exhibited signal evidence of poor nitrifying 
power. These in the majority of cases would be classed as poor 
soils from a practical agricultural standpoint, and their bacterial 
activity was closely correlated with this condition. Previous tests 
with lime had emphasized the stimulus which it exerts on nitrifica- 
tion, and to measure its efficiency on these soils, particularly deficient 
4Reep, H. S., and WittiaMs, Bruce, The effect of certain organic soil constitu- 
ents upon nitrogen fixation by Asofobacter. Va. Agric. Exper. Sta. Tech. Bull. 4. 1915. 
