328 MICROBIOLOGY OF SOIL 



of the true character of nitrification. The observations of Cavendish 

 in 1784 that potassium nitrate is formed when electric sparks are passed 

 through air confined over a solution of potassium hydrate formed the 

 starting point for various theories that attempted to account for nitrate 

 formation on the basis of purely chemical reactions. The formation of 

 nitric acid and of its salts was thus assumed to be due to electric dis- 

 charges in the atmosphere, to combustion processes in nature, or to the 

 oxidation of organic matter and of calcium, magnesium, iron and man- 

 ganese compounds in the soil. Much credence was given to the latter 

 explanation because of the almost universal occurrence of nitrates in 

 arable soils. 



The first indication that nitrate production in the soil and in de- 

 caying organic matter is due to biological activities was given by 

 Pasteur in 1862. A few years later Miiller expressed^his belief in the 

 biological origin of nitrates and nitrites in sewage and drinking water. 

 It was not, however, until 1877 that the true character of nitrification 

 was made clear. In that year Schloesing and Miintz demonstrated 

 that dilute solutions of ammonia could be changed into nitrate by being 

 passed slowly through long tubes filled with soil. The amounts of 

 nitrate nitrogen found in the leachings corresponded almost exactly 

 to the amount of ammonia nitrogen used up. When the soil in the 

 tubes was first sterilized by heating or by means of chloroform and other 

 germicides, the ammonia passed through unchanged. But when soils 

 sterilized by heat or chloroform were reinfected with small quantities 

 of fresh soils nitrification again proceeded in a normal manner. 



The biological nature of nitrification having been thus established 

 numerous investigators tried to isolate the specific organisms in pure 

 culture. A large amount of work in this direction was done by 

 Schloesing and Miintz, Celli and Marino-Zuco, Munro, Warington, the 

 Franklands and many others. A large number of bacteria, yeasts and 

 molds were tested with negative results. Warington, who gathered 

 a great mass of valuable information about nitrification, almost 

 succeeded in securing pure cultures of nitrifying bacteria. Finally,' 

 Winogradski showed in 1890 not only that nitrification is caused by 

 specific bacteria, but explained also why the others failed in securing 

 pure cultures. He proved that these organisms do not develop colonies 

 on the ordinary gelatin and other organic media, a fact whose recog- 

 nition was largely Responsible for his successful solution of the problem. 



