THE FIXATION OF NITROGEN IN COLORADO SOILS 
By WM. P. HEADDEN 
Shortly after the publication of Bulletin 155, “The Fixation 
of Nitrogen in Some Colorado Soils,” my attention was called to 
the question of the transportation of the nitrates from other sources 
to where we found these exceptional quantities and also to the 
question of their distribution both laterally and vertically in the 
soil. It was distinctly stated that the occurrences of these salts, the 
nitrates, were confined to certain characteristic “brown spots.” 
The cause of the brown color was attributed to the Azotobacter 
films, i. e. pigmentation. The smallness of the areas when first 
observed, their erratic occurrence and wide distribution without 
regard to character of soil or geological horizon, practically pre¬ 
cluded the idea of their being the products of any general concen¬ 
tration process. These questions, however, had already received 
consideration, especially in connection with the water which we 
found seeping from certain shale banks, in which connection we 
made this statement, “One question has undoubtedly suggested 
itself in regard to the origin of these nitrates, i. e., whether the 
popular idea that the irrigation water brings them to the surface, 
may not be correct. Some at least will deem this question as de¬ 
serving a definite answer, especially as I have found that the waters 
issuing from the shales underlying the mesas carry significant 
quantities of nitrates. The shales themselves, when a sufficient 
quantity of them, 1,280 grams, was extracted with water, actually 
showed a trace of nitric acid. There are two samples of water and 
one of shale. They represent three different localities, two of them 
within three miles of one another, while the third is more than fifty 
miles from either of the other two.” It might be argued “that the 
shale area is very large and though it contains but a trace of nitric 
acid, it might suffice to furnish all of the nitric acid which has 
been found, especially as the water has been issuing from these 
shales and filling up the lower portions of the country for a very 
long period.” 
“The occurrence of nitrates in the waters and apparently in 
the shale, is susceptible of an easy explanation, i. e., the nitre spots, 
which are only exaggerated instances of a general condition, occur 
in the lands above these shales. The water that falls or is put upon 
these lands, washes the nitrates down into the shales. The soil has 
no power, or but a very small one, to retain these salts and this 
seepage water is simply washing the nitrates out of the land.” 
We considered the soil overlying these shales and not the shales 
themselves as the source of the nitrates. Those who believe in the 
