i 3 8 THE POPULAR SCIENCE MONTHLY 



is not confined to nitrogen fixation in solution, but is manifested in soils 

 as well. The principal nitrogen fixing organisms in the soil were identi- 

 fied as Azotobacter chroococcum. These have the power of developing 

 a dark brown color in cultures containing organic matter and a nitrate, 

 but give no color when the nitrate is replaced by organic nitrogen. The 

 rate at which the fixation took place was considered sufficient to account 

 for the formation of the nitrates found in the soil. 



Exception has been taken to this view regarding the origin of these 

 nitrate beds by Stewart and Greaves 2 who made a study covering a period 

 of eight years at the Utah Experiment Station of the influence of irri- 

 gating water upon the production and movement of nitric nitrogen in 

 the soil. Although the soils upon which the investigations were made 

 were ideally adapted both chemically and biologically to support a rapid 

 biological action, no unusual amounts of nitrates were found. A recalcu- 

 lation of the results reported by Headden showed that in the samples 

 which were taken from the surface of the soil at different points in the 

 same locality and from the same point at different depths, the nitrates 

 and chlorides varied in the same ratio, and that whenever an accumula- 

 tion of the former took place during a given period, the latter also in- 

 creased during the same time in the same general proportion. It was 

 therefore concluded that the excessive quantities of nitrates formed in 

 the soils of Colorado were not formed in situ, but owe their origin to the 

 same source as the other water-soluble salts. 



Further investigations by Headden, 3 however, showed that while 

 large amounts of chlorine generally occur with excessive nitrates, this 

 is accidental rather than necessary, and that on the whole there is no 

 ralation between the amount of nitrates and that of any other class of 

 salts present. Additional evidence is given to show* that the concentra- 

 tion in nitrates in brown spots in which nothing will grow is not due to 

 the accumulation of preexisting nitrates, but to the action of micro- 

 organisms which are able to bring about the fixation in the soil of atmos- 

 pheric nitrogen, and that the dark-brown color which is characteristic of 

 the spots is due, not to black alkali, but to the development of pigment by 

 the organisms. 



The occurrence of nitrate deposits in caves has long been known. 

 During the War of 1812 and again at the time of the Civil War, the 

 "saltpeter" deposits in the Mammouth Cave, and in other caves in 

 Alabama and Georgia, formed an important source of nitrates required 

 in the manufacture of gunpowder. The origin of these deposits is com- 

 monly ascribed to the decomposition of animal remains, and particularly 

 to the excrements of bats. In the southwest small deposits of guano are 

 still to be found in some caves, but almost all deposits which are suffi- 



2 Utah Agri. Expt. Sta., Bull. No. 114. 



3 Col. Agri. Expt. Sta., Bull. No. 186. 



