Page:Popular Science Monthly Volume 85.djvu/142

138 is not confined to nitrogen fixation in solution, but is manifested in soils as well. The principal nitrogen fixing organisms in the soil were identified 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 who made a study covering a period of eight years at the Utah Experiment Station of the influence of irrigating 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 recalculation 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 accumulation of the former took place during a given period, the latter also increased 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, 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 concentration in nitrates in brown spots in which nothing will grow is not due to the accumulation of preexisting nitrates, but to the action of microorganisms which are able to bring about the fixation in the soil of atmospheric 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 commonly 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