290 
Journal of Agricultural Research 
Vol. XXIV, No. 4 
signifies the absence of Azotobacter. Where a question mark has been 
inserted it was impossible to tell whether Azotobacter were present. 
The hydrogen-ion concentration determinations were made either 
colorimetrically after Gillespies method,^ or electrometrically upon sus¬ 
pension of the soil. For the latter purpose a Leeds and Northrup potenio- 
meter outfit was used in connection with saturated K Cl—calomel and 
hydrogen electrodes similar to that described by Hildebrand.^ In gen¬ 
eral the two methods agreed very closely. 
The mixtures of soils, or soils with various additions, were placed in 
500-CC. wide-mouthed bottles plugged with cotton, and the moisture 
content was brought to a favorable point. Incubation was at room 
temperature. At frequent intervals the moisture lost through evapora¬ 
tion was restored. A more detailed description of the methods employed 
will be presented when the data are published in full. 
results 
MIXING SOILS 
In the two experiments reported in Tables I and II, two soils (A and E), 
containing a vigorous Azotobacter flora, and one soil (B), in which 
Azotobacter have never been found, were studied. Soil A is approxi¬ 
mately neutral, Ph 6.94. Soil E contains a high percentage of lime¬ 
stone and is alkaline, Ph 7.73. Soil B is strongly acid, Ph 3*65. Soils 
A and B would, under normal conditions, be expected to have approxi¬ 
mately the same reaction, since the two are from similar locations and 
were taken only a few yards apart. However, the soil where B was taken 
was planted to pine trees a number of years ago and the high acidity 
is undoubtedly due to the decomposition of the highly acid pine needles. 
Extensive use has been made of this soil because it is the only strongly 
acid soil yet located in this immediate vicinity. 
Previously reported experiments,^ indicated that the maximum 
hydrogen-ion concentration tolerated by Azotobacter in soils is near 
I X io“®, or Ph 6.0. If Ph 6.0 represents the maximum acidity tolerated 
by this group of organisms, then all mixtures of soils A and B or E and B 
less acid than Ph 6.0 should, upon subsequent analyses, show Azotobacter, 
while all mixtures in which the acidity was very much greater than Ph 
6,0 should not show their presence. It is probable that Azotobacter can 
exist for some time in a hydrogen-ion concentration that would not permit 
growth. This being true, some of the less acid samples in which Azoto¬ 
bacter can not grow might be able to initiate the growth of Azotobacter 
in a cultural solution. It would be expected that the higher the hydrogen- 
ion concentration the more rapidly the Azotobacter would be destroyed. 
From the relative reactions of the three soils it would also be expected 
that the quantity of soil A necessary to add to soil B in order to reduce 
the acidity of the mixture to Ph 6.0 would be larger than would be 
required in soil E. A glance at the second, third, and fourth columns 
of Tables I and II will show that in mixtures of soils A and B the ratio 
of A to B required to give an acidity less than Ph 6.0 lies somewhat 
between 9 to i and 4 to i. In mixtures of soils E and B the ratio of E 
to B required to give the same reaction is i to 3. 
8 Gll/I,BSPI 0 , L. J. THE REACTION OP SOU, AND MEASUREMENTS OE HYDROGEN-ION CONCENTRATION. 
In Jour. Wash. Acad. Sci., v. 6, p. 7-16, 2 fig. 1916. 
^ HII,DEBRAND, J. H. some applications of the hydrogen electrode in ANALYSIS, RESEARCH, AND 
TEACHING. In Jour. Amer. Chem. Soc., v. 35, p. 847-871, 15 fig. 1913. 
» Gainey, P. L. op. aT. 
