62 Occurrence of Microorganisms in Soil 



process of nitrification engaged the attention of some of the most 

 brilHant minds in the fields of agronomy, soil science, and micro- 

 biology. These studies cuhninated in the isolation by Winogradsky 

 in 1891 of the organisms concerned. 



Various purely chemical theories were at first suggested to explain 

 the formation of nitrates in nature. Pasteur was the first to suggest 

 that the oxidation of ammonia to nitrate is accomplished by the 

 agency of microorganisms. This view was confirmed in 1877 by 

 Schloesing and Miintz. When a soil capable of transforming am- 

 monia to nitrates was heated to 100 °C or treated with antiseptics, 

 such as chloroform, the process of nitrification was prevented. When 

 a fresh portion of soil was added to the soil that had been heated 

 or chloroformed, its power to transform ammonia to nitrate was 

 restored. 



Aeration was found to be essential to nitrification. Proper aera- 

 tion could be obtained either by bubbling air through the medium 

 or by placing the medium as a thin layer over the bottom of the 

 container. It was soon established that the quantity of oxygen con- 

 sumed during nitrification bore a definite ratio to the amount of 

 nitrogen nitrified. The addition of calcium carbonate or alkaline 

 carbonates in low concentrations (0.2-0.5 per cent) had a favorable 

 effect. 



The conditions commonly utilized in saltpeter heaps were thus 

 found to correspond to the essential factors favorable for the activi- 

 ties of the nitrifying organisms. These are: (a) presence of ni- 

 trogenous compounds; (b) thorough aeration; (c) proper moisture 

 content; (d) presence of bases, like calcium or magnesium car- 

 bonate. Nitrate formation was noticeable at 5°, became prominent 

 at 12°, and reached a maximum at 37°C. Higher temperatures, such 

 as 45 °C, exerted an injurious eflFect, and at 55 °C the process came to 

 a standstill. 



These observations of the French investigators concerning the 

 biological nature of the nitrification process were confirmed by 

 Warington. A dilute aqueous solution of ammonia, containing 

 chalk and sodium-potassium tartrate, proved to be a favorable 

 medium; addition of sugar to replace the tartrate exerted an injuri- 

 ous effect upon the process of nitrification. Upon inoculation with 

 soil, the ammonia was first oxidized to nitrite, and the latter to 

 nitrate. When organic nitrogenous compounds or nitrogen-rich 

 materials, such as urine, milk, and asparagine, were added to the 

 medium, they could be nitrified only after they were first converted 



