THE NITRIFYING EFFICIENCY OF CERTAIN COLORADO SOILS 
7 
Bulletin 179 so that mere reference to it will suffice at this time. In the 
light of our experiments, we have little hesitancy in saying that this 
is due in a large measure to the pigment of Azotobacter chroococcum. 
Even the colorless varieties in the presence of .05 per cent, of sodium 
nitrate produce a dark chocolate brown pigment. This experiment has 
been carried out repeatedly on the ordinary solid media, ground quartz, 
sand and an assortment of soils. Given a source of energy, the nitrate 
appears to be the limiting factor in the formation of the brown color 
by the nitrogen fixing organisms. 
Ammonification 
In Part I of Bulletin 184, we have taken up a study of the am¬ 
monifying efficiency of this same class of soils, and we have been able 
to show that so far as the presence of an active, ammonifying microbial 
flora, as well as suitable medium for their development for convert¬ 
ing the protein of the azotobacter cells into ammonia, our 
soils are all that could be desired. The results of our experiments, 
based upon the change of the protein of alfalfa-meal, cotton-seed-meal, 
flaxseed-meal, and dried blood into ammonia led us to believe that 
this is a property common to many cultivated Colorado soils; that soils 
in the incipient stage of niter trouble surpass our normal soils in 
ammonifying efficiency; also that compared with the soils from other 
localities our niter soils excel in this function to a very marked degree. 
The maximum per cent, of ammonia produced in seven days by any 
soil from 100 m. g. of nitrogen as cotton-seed-meal was 51.98 per cent.; 
as dried blood 52.64 per cent.; as alfalfa-meal 34.85 per cent.; as flax¬ 
seed-meal 12.15 per cent. 
Nitrification 
Having shown that the niter soils could transform atmospheric nitro¬ 
gen into ammonia salts by the two processes just outlined, we had yet to 
demonstrate their ability to change the ammonia compounds into ni¬ 
trates before we could assert definitely that the atmosphere is in all 
probability contributing the nitrogen for the excessive nitrates. In the 
pages which follow, will be found a detailed discussion of our studies 
upon the nitrifying efficiency of these soils,—nitrification being the 
third and final step in the transformation of the atmospheric nitrogen 
into nitrates. I use the term nitrifying efficiency, rather than nitrify¬ 
ing power , in the sense in which it has been employed by Stevens and 
Withers (1) to denote not only the presence of the nitrifying organ¬ 
isms in the soil which are capable of exercising their specific function 
under favorable conditions (nitrifying power) but also the suitability of 
the soil as a medium in which the process of nitrification may pro¬ 
ceed advantageously (nitrifying capacity). 
Soil Samples 
We have not confined the investigation to Colorado soils alone, 
(1) Studies in Soil Bacteriology III. Concerning methods for the determination of 
nitrifying and ammonifying powers, Stevens, F. L. and Withers, W. A., Cent. f. Bakt., Abt. 
II., Bd. 25, No. 1-4, p. 64, 1909. 
