Aug. i3,1918 Soil Reaction and the Growth of Azotobacter 
269 
solutions a more vigorous development of Azotobacter than do soils 
deficient in lime. There are, however, too many exceptions to this rule 
to regard the presence or absence of calcium carbonate alone as the con¬ 
trolling factor The available evidence regarding the influence of soil 
reaction upon the presence therein of Azotobacter has been obtained 
by methods which permit such wide discrepancy in results that they can 
be regarded only as indicative and not conclusive. 
Christensen 1 has carried out by far the most carefully executed ex¬ 
periments along this line. In the work here referred to, 145 Danish 
soils from varying conditions were examined for Azotobacter, of which 
53 per cent gave negative results. The reactions to litmus of 142 of 
these samples were recorded. The methods used are certainly not free 
from objections. Of 22 recorded as acid, only one gave Azotobacter. 
Fifty were recorded as neutral, and of these, 14 per cent gave Azoto¬ 
bacter. Eighty-seven per cent of the 70 recorded as alkaline gave posi¬ 
tive cultural results. 
The calcium-carbonate content of 136 of the same samples was deter¬ 
mined by decomposing with concentrated hydrochloric acid and meas¬ 
uring the carbondioxid evolved. Forty-seven samples contained no 
calcium carbonate. Christensen states that all samples recorded as 
0.05 per cent or less should be regarded as containing no carbonates. Of 
these 47, 32 per cent gave positive evidence of Azotobacter. Of 102 
containing less than 0.10 per cent of calcium carbonate, 33 per cent gave 
Azotobacter cultures. There were 34 samples giving more than 0.10 
per cent of calcium carbonate, and of these, 88 per cent gave Azotobacter, 
while all of the 23 samples containing more than 0.20 per cent gave 
positive results. None of the soils recorded as acid or neutral contained 
sufficient carbonates to replace the calcium carbonate of the cultural 
solution. Weis and Bomebusch, 2 however, studying the same problem 
in Danish forest soils found Azotobacter in only 2 out of 64 samples; 
nevertheless, 60 per cent of these soils contained sufficient carbonates 
to replace the calcium carbonate of Beijerinck’s cultural solution. The 
last-named authors state that none of the soils examined by them could 
be regarded as requiring lime. 
It would seem from the available experimental data that Azotobacter 
are capable of existing in many soils which contain none or only traces 
of calcium carbonate, and also in some soils reacting acid as ordinarily 
tested. The reaction, however, apparently plays a much more important 
r 61 e than the presence of calcium carbonate. 
1 Christensen, H. R. studien uber den einfeuss der bodenbeschaffenheit auf das baxteribn- 
LEben und den stoffumsatz im erdboden. In Centbl. Bakt. [etc.], Abt. a, Bd. 43, No. 1/7, p. 1-166, 
ai fig., 1 pi. I 4 teratur, p. 163-165. 1915. 
s Weis, Fr., and Bornebusch, C. H. om azotobacters forekomst i danske skove, samt om azo- 
TOBACTERPR0VENS BETYDNXNG FOR BESTRHMRESEN AF SKOVJORDBRS KAEKTRANG. (Abstract.) In 
Mo. Bui. Agr. Intel, and Plant Diseases, year 6, no. 4, p. 546-548. 1915. (Original article in Forstl. 
Fors^gsvaesen Danmark, Bd. 4, Haefte 4, p. 3x9-337* 1914* Not seen.) 
