June i6,1923 A zotobacter Flora and Nitrogen Fixing A bility of Soils 937 
The possibility of a biological means of determining the lime require¬ 
ments of soils is indicated by the data here presented. If a series of 
groups or of individual microorganisms with definite limiting hydrogen- 
ion concentrations could be isolated from soils it would only be necessary 
to determine the presence or absence of certain species in order to estab¬ 
lish the absolute reaction of the soil solution in situ. Furthermore, it 
would be comparatively simple to ascertain how much lime to add in 
order to render the soil capable of supporting the various species. It 
has been possible to determine with an accuracy of a few hundred pounds 
the quantity of lime necessary to add to soils more acid than Ph 6.0 in 
order to render them capable of supporting Azotobacter. It is believed 
that not until the plant physiologist has established the optimum and 
limiting hydrogen-ion concentrations of the soil solution for the various 
agricultural plants, and the bacteriologist has associated these limits 
with definite groups of microorganisms, will a true and accurate physio¬ 
logical basis for determining and correcting the reaction of soils be 
possible. Azotobacter can now be used as a biological means of separat¬ 
ing soils into two groups, depending upon reaction; and the quantity 
of lime necessary to change the reaction of the more acid group into the 
less acid group can be definitely determined. 
CONCLUSIONS 
(1) Seventy-five per cent of the 418 soils examined were found to be 
acid. Fifty per cent were more acid than Ph 6.0 and 50 per cent appar¬ 
ently did not contain Azotobacter. 
(2) In general the electrometric and colorimetric methods of determin¬ 
ing the hydrogen-ion concentration of the soil solution agreed. 
(3) A definite and very close correlation has been established between 
the absolute reaction of the soil solution and the presence or absence 
of Azotobacter in the soil. 
(4) Very few soils more acid than indicated by a hydrogen-ion con¬ 
centration of I X lo”*"’ contain Azotobacter, while this group of organisms 
is in most instances present in soils with a lower hydrogen-ion concen¬ 
tration. Since a very similar limiting hydrogen-ion concentration has 
been found for pure cultures of Azotobacter in laboratory media, it is 
believed that the very close correlation existing between the reaction 
and the presence of Azotobacter indicates that the absolute reaction is 
of paramount importance, if not the actual limiting factor, in controlling 
the presence of this group of organisms in soils. 
LITERATURE CITED 
(1) C1.ARK, William Mansfield, and Lubs, Herbert A. 
1917. THH COLORIMETRIC DETERMINATION OR HYDROGEN-ION CONCENTRATION 
AND ITS APPLICATION IN BACTERIOLOGY. In Jour. Bact., V. 2, p. I-34, 
109-136, 191-236, 9 fig. Refer^aces, p. 233-236. 
(2) Gainey, P. L. 
1918. soil reaction and the GROWTH OR AZOTOBACTER. (Preliminary paper.) 
In Jour. Agr. Research, v. 14, p. 265-271. 
(3) - 
1922. A STUDY OR SOME RACTORS CONTROLLING THE PRESENCE OR AZOTOBACTER 
IN SOILS. (Abstract.) In Abst. Bact., v. 6, p. 14. 
(4) - 
1922. THE USE OR CALCIUM CARBONATE IN NITROGEN RIXATION EXPERIMENTS. 
(Abstract.) In Abst. Bact., v. 6, p. 15. 
