6 
COLORADO EXPERIMENT STATION 
calcic chlorid and other salts and we are likewise quite as sure that land 
which up to six years ago was good, and this assumed period is from 
three to six times as long as the facts indicate, has not been two hun¬ 
dred and sixty-five years in going to the bad.’’ 
That these spots are the remains of great herds of extinct animals 
which perished from some unknown cause is highly improbable: 
First, because the areas involved are too great; second, as mentioned 
before, the present spots are increasing in size; and, third, spots are 
appearing today in localities where the trouble has never been reported 
before. * * ■ 
For the same reasons, there is no ground for believing that these 
areas are niter beds related to any established geological horizon. 
Having failed to find a satisfactory explanation for these brown 
areas in any of the foregoing hypotheses, we ventured the assumption 
that the nitrates were and are being formed in situ and proceeded with 
the hypothesis that they owe their origin to the activity of microorgan¬ 
isms. 
To assume that ammonifying and nitrifying bacteria alone were 
responsible for the formation of the nitrates was to assume the ex¬ 
istence of large quantities of organic matter in the soil, which, as a 
matter of fact, is quite deficient. This condition obtaining, we were 
obliged to begin our investigation one step farther back and to find some 
adequate supply of organic nitrogen which could furnish protein for 
the ammonifying organisms. 
Fixation oe Nitrogen 
In Bulletin 179, we have shown that our niter soils are abundanth 
stocked with Azotobacter chroococcum by virtue of which they are 
able to build up large quantities of proteid nitrogen from atmospheric 
nitrogen. Not only have we demonstrated the almost universal oc¬ 
currence of the nitrogen fixing organisms, but we have also been able 
to obtain a positive increase in the total nitrogen content of the soil 
itself by merely keeping the soil moistened and holding it at 28 degrees 
C. Without any inoculation other than that which the soil already 
contained and without the addition of any carbohydrate to supply 
energy, we were able to secure in thirty days an increase in the total 
nitrogen of 11.12 per cent, with one sample and 9.83 per cent, with 
another. 
So far as a supply of carbon in the soil with which to furnish Azo¬ 
tobacter with the necessary energy for nitrogen fixation is concerned, 
I may say that Professor Robbins (1) has reported to date the occur¬ 
rence of some twenty-four species of algae. All but two of these belong 
to the blue-green algae (Cyanophyceae), the family Nostocaceae being 
best represented. 
Brown Color 
The brown color of the niter soils has been discussed quite fully in 
(1) Algae in Some Colorado Soils, W. W. Robbins, Bui. 184, Part II, Colo. Exp. Sta., 
1912. 
