30 The Colorado Experiment Station. 
surface foot, contained nitrates corresponding to 113,480 pounds, 
or 56.74 tons per acre foot; in another sample, taken to a depth 
of five inches, the area involved being about eight acres, sodic 
nitrate corresponding to 344,000 pounds or 172 tons was found in 
the surface five inches; in the top four inches of another eight acre 
tract, the equivalent of 189,971 pounds or 95 tons was found. 
With such quantities of niter in the soil as these figures in¬ 
dicate, it seems hardly necessary to look elsewhere for an explana¬ 
tion of the death of our trees and crops. 
Our laboratory studies and field observations have led us to 
believe that a number of factors are concerned in bringing about 
this sad condition of certain Colorado soils. There seems to be 
little doubt that in many cases the trouble has been aggravated by 
the too liberal use of irrigating water. To over-irrigate seems to 
be a common fault with many of our agriculturists. Azotobacter 
thrives best when the soil contains from 15 to 18 per cent, of 
moisture, practically the same as that required by growing crops. 
It does not succeed well where the ground is too wet or badly 
seeped; however, a constant supply of moisture favors its develop¬ 
ment. Continued, clean cultivation stimulates the activity of the 
organisms both by aerating the soil, and by raising the soil tem¬ 
perature through the free access of the sun's rays. The neutral 
or alkaline reaction of Colorado soils, due to the large ‘amounts of 
carbonate, is unquestionably the limiting factor which makes our 
soils superior to those of the East and Middle West in promoting 
the growth of Azotobacter. 
The one objection that has been raised to the Azotobacter 
theory, as an explanation of the excessive nitrogen, is the small 
amount of organic matter in our soils from which to derive the 
energy necessary for such a large amount of fixation. In a/ 
measure, this is true, if we understand the term organic matter in 
the ordinarily accepted sense; our soils are not rich in this con¬ 
stituent. It has been demonstrated that some of the simpler green 
plants that grow in the soil and in water, known as algae, can 
furnish Azotobacter with this energy. Following this clue, we 
have undertaken a systematic study of the algae which occur in 
:he niter soils, and I am glad to be able to state at this time that 
our search has been richly rewarded. Professor Robbins, who has 
had charge of this phase of the investigation*, has found some 
twenty-four different species of blue-green algae, occurring in 
*Bul. 184, Part II, Colo. Exp. Station, “Algae in Some Colorado Soils,” June, 1912, 
