The Fixation of Nitrogen in Colorado Soils 45 
to the soil. The most probable agency having the power to accomplish 
this seemed to be the Azotobacter. Examination of these soils proved 
these organisms to be present in great abundance, except in areas too 
rich in soluble salts, but they were abundant at the edges of such areas, 
and below the surface. The Azotobacter form a brown to an almost black 
pigment. The formation of this pigment has. been shown by Professor 
Sackett to be conditioned by the presence of the nitrates. The “brown 
spots are rich in Azotobacter, either throughout or at their edges and 
below the surface, the nitrates are present in exceptional quahtities and 
the spots are recognized by their brown color. Our soils when incubated 
without the addition of any carbohydrate, show a marked fixation of 
nitrogen. My own results showed a fixation of 10.54 milligrams for each 
100 grams of soil in 2 7 days and the moist soil kept at the room tem¬ 
perature showed an increase of 4.82 milligrams in the same time for each 
100 grams of soil. It is usual to add glucose or mannite to furnish 
energy but an ordinary cultivated soil fixed these quantities without any 
addition of any kind except boiled, distilled water. That energy was 
necessary there can be no doubt, that it was not added in the form of 
glucose or mannite is also certain. If it was used, as we agree it must 
have been, it must have been derived from the soil itself, but our soils 
are not remarkably rich in organic matter. The total nitrogen was found 
to be 0.1075 percent at the beginning of the experiment. Other analyses 
of this soil gave the total nitrogen as 0.147, humus 0.426 and ignition 5.072 
percent. These are facts which we have recorded in Bulletin 178. My 
experiments do not stand uncorroborated. Professor Sackett obtained 
very similar results with other soils. I do not know the conditions in our 
soils which may possibly limit this ability to fix the atmospheric nitrogen. 
The conditions of my experiments seem to give rise to a strong develop¬ 
ment of Azotobacter. This soil at the beginning of the experiment gave 
a very moderate culture of Azotobacter, but after thirteen days another 
sample gave a remarkably strong one which developed the brown pig¬ 
ment within nine days. This soil, as are all of our soils, with almost no 
exceptions, is alkaline in reaction. The nitrification, too, in these samples 
without the addition of anything except distilled water which had been 
boiled to expel any traces of ammonia that might be present, was very 
marked. We found a maximum gain equal to 138 percent of the nitric 
nitrogen present at the beginning of the experiment which was 3 5.0 
p. p. m. Professor Sackett has studied this subject more thoroughly and 
systematcally with interesting results, one of which, with his permission, 
I use, i. e. some of our soils show a nitrifying efficiency 173 times greater 
than an Iowa soil which I understand was a typical one. The increase in 
the nitric nitrogen present in this soil in six weeks was 1040 fold. I 
have personally done nothing with the ammonifying efficiency of our 
soils, but Professor Sackett has presented the results of his investigations 
of this subject in Bulletin 184 of this Station, from which it appears that 
the ammonifying efficiency of our soils is from two to three times 
greater than average soils from other localities for which we have com¬ 
parable data. Two of Professor Sackett’s conclusions are as follows: 
“Soils in the incipient stage of the nitre trouble appear to surpass 
