912 
Journal of A gricultural Research voi. xxiv, No. n 
In converting volt readings into Ph use has been made of the tables 
prepared by Schmidt and Hoagland {12) adding 91 millivolts to the 
readings to convert them into N/io KCl-calomel electrode readings. 
Some investigators regard the difference in potential between saturated 
and N/io KCl-calomel electrodes to be of a value other than 91 millivolts. 
To convert the Pa values here recorded into those of any other difference 
in potential between saturated and N/io KCl-calomeT electrodes, it is 
only necessary to add or subtract, as the case may be, 0.017 from the 
figure here recorded for each millivolt above or below 91. All determina¬ 
tions were made at room temperature, and Schmidt and Hoagland 
temperature correction factors were used to convert room temperature 
readings into 25° C. readings. 
Hydrogen was purchased in cylinders and washed through a saturated 
solution of mercuric chlorid, alkaline potassium permanganate solution, 
alkaline pyrogallic acid solution, and distilled water before entering the 
hydrogen electrode cell. The connection between the calomel and 
hydrogen electrodes was made through a glass stopcock. The end of 
this immersed in the soil suspension was drawn out to a capillary opening 
and the cock was kept closed during the determination. Between 
successive determinations, however, the connection was refilled with 
fresh saturated potassium chlorid. 
RESULTS 
Data relative to the type of growth and the quantity of nitrogen fixed 
in culture media inoculated from soils, together with the absolute reaction 
of the soil extract determined colorimetrically and the soil suspension 
determined electrometrically of 418 soils, are recorded in Tables V, VII, 
IX, and XI. In Tables VI, VIII, X, and XII some of the data have 
been rearranged to show the possible association, or correlation, exist¬ 
ing between the hydrogen-ion concentration of the soil solution and 
the presence or absence of Azotobacter in the soils. In these various 
tables under the heading “Azotobactera + (plus) sign has been recorded 
to indicate the presence of Azotobacter and a — (minus) sign to indicate 
the absence of Azotobacter in the soils. These data are not absolute. 
Sometimes it was very difficult to differentiate between the presence or 
absence of Azotobacter, There are a few instances where an Azotobacter 
film developed in only one of duplicate samples, or where either 
the film or Azotobacter cells were questionable, accompanied by poor 
fixation of nitrogen, that Azotobacter have been recorded as absent. 
On the other hand, there are a few instances where similar conditions 
were accompanied by good fixation of nitrogen that have been recorded 
as containing Azotobacter. An effort has been made to weigh the 
evidence in questionable cases as carefully as possible and record 
-Azotobacter as present if it were believed that the data indicated the 
presence of an active Azotobacter flora, and as absent if the available 
data did not indicate the presence of such a flora. 
It should be borne in mind that the chance for contamination of 
Azotobacter free soils with Azotobacter was not entirely eliminated. 
Especially was this true in Series III and IV, for nearly all of the soils 
in these two series were collected by men inexperienced in bacteriological 
- technic and were shipped long distances. Sometimes one or more of 
the individual containers were broken open upon arrival at the laboratory. 
