Bacteriological Studies oe the Fixation of Nitrogen. 29 
While it may be early to state any conclusions, even tentatively 
held, the foregoing work suggests the following: 
1. Excessively high nitrates in the soil will kill the Azotobacter 
flora. 
2. A limited amount of soil nitrate does not seriously affect 
the nitrogen fixing power of a soil. 
3. Our adobe shale soils, both in the raw state and when newly 
cultivated, possess little if any nitrogen fixing power. 
4. The nitrogen fixing power of our soils is not limited to any 
geographical locality or class of soils, however, the degree of activity 
may vary. 
5. The power to fix atmospheric nitrogen is a property common 
to many cultivated Colorado soils. 
6. Azotobacter chroococcum appears to be the dominant nitrogen 
fixing agent. 
The Nitrogen Fixing Power of Soils in Situ. 
Having satisfied ourselves that certain Colorado soils possessed 
the power of fixing atmospheric nitrogen in solutions, the next point 
on which we wished to inform ourselves was whether these same soils 
had the power of fixing nitrogen in situ. If that could be demonstrat¬ 
ed, it would be a relatively simple matter to explain the high nitrates, 
for, given the proteid nitrogen from which to make the nitrates, we 
felt reasonably certain that the ammonifying and nitrifying flora would 
take care of the conversion. 
jFor this part of the investigation two samples of soil were selected 
at random, one from the central part of the state, and the other from the 
northern. Both were from localities where either the nitre trouble 
or the brown stain had been observed. The nitrogen fixing power of 
these soils was determined independently by two different workers, 
the northern Colorado sample by Dr. Headden, and the central by the 
writer. The soils were not handled in the same way by the two of us, 
so it may be well to discuss our respective manipulations. Other 
soils have been studied but these two are of particular interest 
since the results were obtained independently in different laboratories. 
Northern Sample. 
This soil was collected by Dr. Headden, December 12, 1910, from 
the young orchard designated as No. 10 in the preceding series. 
It was screened in a moist condition through a twenty-five mesh wire 
screen and 1200 grams of the moist soil, with no further treatment, 
were placed in a deep culture basin (10 in. x 2 in.) and pressed down 
firmly. The soil moisture was determined and sufficient boiled dis¬ 
tilled water was added to make eighteen to twenty percent moisture. 
This was maintained throughout the experiment. The soil was incu¬ 
bated for twenty-seven days in the dark at 28°C. to 3Q°C. at the end 
