NITRATE DEPOSITS 137 



crease in the quantity of nitrogen combined, but simply brings about a 

 transformation from one form to another. 



Shortly after the discovery that the bacteria which are associated 

 with the roots of leguminous plants are able to fix nitrogen, a great many 

 experiments were made to determine if other bacteria have a similar 

 function. Very conflicting results were obtained. Some investigators 

 reported that certain bacteria are able to fix nitrogen, while others 

 arrived at an opposite conclusion with respect to the same bacteria. Ex- 

 periments along this line are still being made. It seems that most of the 

 bacteria in the soil do not bring about any fixation of nitrogen, but there 

 is good evidence that there are two or three kinds in addition 

 to the Bacillus radicicola which are able to do this. Unlike the latter, 

 these bacteria do not require the medium of leguminous plants, but may 

 be active in soils which are devoid of growing vegetation. The most 

 important of these bacteria have been called Azotobacter diroococcum. 

 It has also been shown that various fungi, alga? and other organisms 

 possess greater or less power of fixing atmospheric nitrogen. 



A special study of the amount of fixation which may take place 

 through the activities of these organisms, particularly the Azotobacter 

 diroococcum, has been made by Headden 1 and others at the Colorado 

 Agricultural Experiment Station. It was observed that in certain parts 

 of Colorado dark-brown spots occur in which nothing will grow. From 

 the color of the spots it was natural to assume that the cause of the spots 

 was due to the presence of black alkali, or sodium carbonate, which is so 

 commonly met with in the soils of arid and semi-arid countries. An 

 analysis of the soils, however, showed little or no sodium carbonate pres- 

 ent, but instead surprisingly large quantities of sodium, calcium and 

 magnesium nitrates. The spots observed were not fixed, but spread 

 rapidly and sometimes covered an area of several acres in extent. More- 

 over, new spots were noticed to appear in old and new localities. In 

 some cases the amount of nitrogen accumulated, as shown by analysis, 

 amounted to as much as one hundred tons to the acre foot. Arguments 

 were given to show that the nitrates could not have originated with the 

 irrigating water nor from concentration of ground water in the surface 

 layers of the soil. That the spots are the remains of great herds of ex- 

 tinct animals which perished from some unknown cause, was likewise 

 considered improbable for the reason that the areas involved are too 

 large/and that the spots are increasing in size and appearing in localities 

 where they were never before noted. The theory was therefore advanced 

 that the excessive quantities of nitrates were formed in situ, through the 

 nitrogen-fixing activity of micro-organisms in the soil. 



It was actually shown that the power to fix atmospheric nitrogen is a 

 property common to many cultivated Colorado soils, and that this power 

 i Col. Agri. Expt. Sta., Bull. Nos. 155, 178 and 179. 



VOL. LXXXV. — 10. 



