Bacteriological Studies of the Fixation of Nitrogen. 41 
Solubility of the Pigment. 
In their extended studies on the formation of pigment by A. cliro- 
ococcunL Beijerinck 1 , Omeliansky and Ssewerowa 2 have found that the 
pigment is insoluble in the ordinary solvents. On this point, Beijerinck 
says, y Insoluble in water, alcohol, ether, chloroform, carbon disulphide, 
the pigment went into solution under the influence of alkalies, whereby 
it probably underwent a chemical change.” Omeliansky and Ssewerowa 
state that, “The pigment is insoluble in the usual solvents. Only under 
the action of alkalies does it go into solution, thereby, nevertheless, 
changing itself chemically.” 
The relation of alkalies to the solution of pigment, as described by 
these investigators, suggests a further explanation for the brown stain 
which we find on the ditch banks and irrigation furrows. May it not 
be possible that under the influence of the soil nitrates, Azotobacter 
chroococcum produces an intense pigment which is brought into solu¬ 
tion by the alkaline soil waters, and once in solution, the coloring mat¬ 
ter is carried to the surface where it becomes concentrated and pro¬ 
duces the characteristic appearance? While we have given but little 
consideration to this explanation of the color, we have reasons for 
believing that there is more to this hypothesis than mere speculation and 
idle fancy. 
A C K NO WLEDG M ENTS. 
I wish to acknowledge, with thanks, my indebtedness to Dr. M. W. 
Beijerinck of Delft, Holland, for the stock cultures of A. chroococcum, 
A. agilis and A. lactose, which he has so kindly sent me. To Dr. 
Headden, I am indebted for the problem itself as well as for many field 
notes and chemical data. To Professor Gillette, the Director, I wish 
to express my appreciation of the two colored plates, and Miss Palmer, 
I wish to thank for preparing the originals from which these were 
made. 
SUMMARY. 
The power to fix atmospheric nitrogen is a property common to 
many cultivated Colorado soils. 
This power is not confined to nitrogen fixation in solutions, but is 
manifested in soils as well. 
The rate of fixation of nitrogen obtained is sufficient to account 
for the nitrates found in the soil provided that it is nitrified. The rate 
of nitrification obtained is sufficient to account for the formation of the 
nitrates found, in most cases if not all of them.” 3 
The nitrogen fixing power is not limited to any geographical' 
locality or class of soils, howevpr, the adobe shale soils, both in a’raw 
Woe. cit., page 39. 
2 Loc. cit., page 39. 
3 Bulletin 178, p. 96, Colo. Exp. Station, 1911. 
