258 MICROBIOLOGY OF SOIL. 



were therefore thrown on their own resources and were forced to develop 

 the domestic production of saltpeter. The industry came under govern- 

 ment control and experts were appointed to study the so-called saltpeter 

 plantations and the conditions affecting the appearance and increase of 

 nitrates in compost heaps and in the soil. Much knowledge was thus 

 gained about nitrification even though it was not suspected that living 

 organisms were concerned in the process. 



With the rapid development of chemistry in the latter half of the 

 eighteenth century a nearer approach was made to the understanding 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 decaying 

 organic matter is due to biological activities was first 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 germi- 

 cides, the ammonia passed through unchanged. But when soils ster- 

 ilized 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 



