A STUDY OF THE EFFECT OF CHANGING THE ABSOLUTE 
REACTION OF SOILS UPON THEIR AZOTOBACTER 
CONTENT‘ 
By P. L. Gainey 
Professor of Bacteriology, Kansas Agricultural Experiment Station 
INTRODUCTION 
In studying the apparent correlation between the absolute reaction 
of soils and the presence of Azotobacter it occurred to the writer that 
if such a correlation existed one ought to be able, by varying the reac¬ 
tion, to control the Azotobacter content of any soil. To test the correct¬ 
ness of this view three lines of investigation were suggested. 
(1) If two soils, one with a hydrogen-ion concentration favorable to 
the growth of Azotobacter, the other unfavorable, are mixed in varying 
proportions the hydrogen-ion concentration of some mixtures should be 
favorable and others unfavorable to the growth or existence of this group 
of organisms. All mixtures giving a hydrogen-ion concentration less 
than the maximum tolerated by Azotobacter should, upon subsequent 
analyses, show their presence. On the other hand, all mixtures giving 
a hydrogen-ion concentration greater than the maximum should not 
show the presence of Azotobacter, provided a sufficient incubation period 
elapsed to bring about their destruction. 
(2) If the absence of Azotobacter in any soil is due to its high hydrogen- 
ion concentration, one should be able to decrease the acidity by the 
addition of increasing quantities of nontoxic basic materials, to a point 
where, once introduced, Azotobacter would survive. 
(3) If a high hydrogen-ion concentration is inimical to the presence 
of Azotobacter one should be able, by the addition of acid, to increase 
the acidity of any soil containing Azotobacter to a point beyond the 
maximum tolerated by these organisms and thereby cause their dis¬ 
appearance. 
The purpose of this paper is to r^eport a few typical examples of a large 
munber of experiments that have been carried out along these lines. 
methods 
The methods employed in the experiments here reported were similar 
to those previously reported.^ Tests for Azotobacter were made by 
inoculating a mannite cultural solution with 10 per cent of soil, or the 
equivalent, as a suspension. Incubation was at room temperature. At 
frequent intervals during incubation the cultures were examined macro- 
scopically to ascertain whether or not a film characteristic of Azotobacter 
was present. If the film was not typical and there were any indications 
of Azotobacter being present a microscopic examination was made. If 
the evidence thus obtained showed the presence of Azotobacter, it has 
been indicated by a plus sign in the following tables. A minus sign 
* Accepted for publication June 29, 1922. Contribution No. 48, Department of Bacteriology, Xansas 
Agricultural Experiment Station. . „ , 
2 Gainey, P. D. soil reaction and the growth of azotobacter. In Jour. Agr. Research, v. 14, 
p. 265-271. 1918. 
Journal of Agricultural Research, 
Washington, D. C. 
acb 
(289) 
Vol. XXIV. No. 4 
Apr. 28, 1923 
Key No. Kans.-34 
