48 The Colorado Experiment Station 
may have reached a depth of two inches, but the area involved 
was small. The other sample may have reached a depth of an inch 
but the area involved was large, some eight or ten acres. In the 
case of another sample taken to the depth of five inches we find 
the nitrates equal to 2.571 per cent of the air dried soil. This sam¬ 
ple was taken to a medium depth and the area involved is at least 
eight acres and we would have 344,000 pounds or 172 tons of sodic 
nitrate in the top five inches of this land. In another sample taken 
to a depth of four inches we find that 1.706 per cent of the air 
dried sample is sodic nitrate. The area involved represented by 
this sample is certainly as large, i. e., eight acres, and we would 
have 189,971 pounds or practically 95 tons in the top four inches 
of this land. The largest amount of nitrates indicated by the analy¬ 
sis of any sample taken to a depth greater than four inches was 
in the case of a sample taken to the depth of one foot which con¬ 
tained 2.837 P er cent °f sodic nitrate corresponding to 113,480 
pounds or 56.74 tons per acre in the surface foot. These figures 
are given to show the large aggregate amount of nitrogen which 
is being taken from the air and converted into nitrates in these 
semi-arid soils usually fairly rich in sulfate of soda and containing 
large amounts of calcic carbonate. 
The aggregate area involved in this active fixation of nitrogen 
would be difficult to estimate accurately but as stated elsewhere in 
this bulletin it is present in widely separated sections of the state. 
In some places only a small area may be involved, in others it is 
almost continuous for miles. 
SUMMARY. 
The cause of the barren spots in some sections of Colorado which is 
popularly, though incorrectly, called “black alkali” is the presence of 
excessive quantities of nitrates. 
These nitrates do not come from the soil nor from the shale as fre¬ 
quently assumed but are formed in the soil. 
The death of many apple trees, some poplars and other shade trees 
during the season of 1909 was caused by excessive amounts of nitrates 
in the soil. 
These nitrates were carried down within the feeding area of the 
roots by the spring rains and irrigation. 
The amount of these nitrates accumulated in some of these soils is 
already very large, amounting to many tons per acre foot of soil, 100 tons 
per acre foot having been indicated by some samples. 
The agency by which the nitrogen of the air is converted into these 
nitrates in the soil is a group of micro-organisms possessing the power 
of converting the nitrogen of the air into nitric acid. 
These organisms have a very wide distribution in our soils and are not 
always hurtful, but when the conditions of the soil, including moisture, 
temperature, and the presence of much alkaline earth carbonate, become 
very favorable they develop so vigorously that they produce the effects 
recorded in this bulletin. 
These organisms thrive in some of our best cultivated lands, and 
some of the anomalies of our agriculture are probably due to them. 
The very considerable amounts of nitrates found in some of our 
soils, together with the large areas so enriched, and their wide distribution 
suggest the probability that the formation of the nitrates of Chile and 
Peru may have been due to the agency of these organisms. 
