292 
Journal of Agricultural Research 
Vol. XXIV, No. 4 
A further examination of the two tables will show that, when cultured 
immediately after mixing, all mixtures of A and B and E and B, except 
I part of E to 199 parts of B, contained Azotobacter. Subsequent cul¬ 
tures, made after varying periods of incubation, show a gradual disap¬ 
pearance of Azotobacter in the more acid mixtures until only those 
samples contain them in which the acidity approaches very close to or 
is less than Ph 6.0. Numerous experiments have shown that the more 
acid a soil is the quicker will Azotobacter disappear therefrom when 
introduced, and that they can exist for some time in soils only slightly 
more acid than Ph 6.0. All the evidence accumulated so far, however, 
indicates that they can not remain indefinitely in a soil very much more 
acid than Ph 6.0. 
ADDITION OF CALCIUM CARBONATE 
In these experiments the aim was to find out whether lowering the 
hydrogen-ion concentration of a soil more acid than Ph 6.0, and not 
containing Azotobacter, would render the soil capable of supporting such 
a flora when introduced. The method followed was to add increasing 
quantities of various basic materials, particularly calcium carbonate, to 
acid soils containing no Azotobacter, inoculate the soil with Azotobacter, 
and after varying incubation periods culture to see whether or not the 
introduced organisms survived. 
^AB^nlll.—Egect oj CaCO^ upon the longevity of Azotobacter in an acid soil(B)a 
Percent¬ 
age of 
CaCOa. 
Pounds 
CaCOa 
per acre. 6 
Azotobacter cultures. 
Sample No. 
Sept. I. 
Oct. 21 
Jan. 4. 
Feb. 18. 
0. 0 
0 
+ 
4 - 
__ 
_ 
. I 
3,000 
+ 
+ 
— 
— 
. 2 
6, 000 
+ 
4 - 
— 
— 
. 3 
9 , 000 
12, 000 
+ 
4 - 
— 
— 
• 4 
+ 
4 - 
— 
— 
. 5 
15,000 
+ 
4 - 
-- 
— 
.6 
18, 000 
+ 
4 - 
4 - 
+ 
8 . 
. 7 
21, 000 
4 - 
4 - 
4 - 
4 - 
.8 
24, 000 
+ 
4 - 
+ 
4 - 
• 9 
27, 000 
+ 
+ 
+ 
4 - 
I. 0 
30, 000 
33) 000 
4 - 
4 - 
+ 
4 - 
I. I 
4 - 
4 - 
+ 
4 - 
I. 2 
36, 000 
4 - 
+ 
4 - 
4 - 
I- 3 
39,000 
42, 000 
4 - 
4 - 
+ 
4 - 
I. 4 
4 - 
4 - 
+ 
+ 
I- 5 
45 ) 000 
4 - 
4 - 
4 - 
+ 
o 4-= Presence of Azotobacter. -=Absence of Azotobacter. Experiment set up Aug. 20. Moisture 
content optimum. 
6 3,oco,ooo pounds soil. 
In Table III it will be observed that a minimum of 18,000 pounds per 
acre (3,000,000 pounds), or 0.6 per cent calcium carbonate, were required 
to render soil B capable of supporting an Azotobacter flora. Unfortu¬ 
nately the hydrogen-ion concentration of the samples of soil of these 
experiments was not determined. 
