66 NITKOGEN-FIXATION IN SOILS 



Nitrogen-fixing Organisms in Soils. Winogradski 1 in 1893 

 obtained from soil a large bacillus which, when cultivated in a solution 

 containing suitable mineral ingredients and pure dextrose and supplied 

 with air purified by means of potash and sulphuric acid, caused the 

 destruction of the dextrose (CgHjgOg), the formation of butyric acid 

 (C 3 H 7 COOH), the evolution of carbon dioxide and free hydrogen, and 

 the formation of nitric acid. The nitrogen must have been obtained 

 from the air. Subsequent experiments of Winogradski showed that 

 the bacillus was anaerobic and if air were present could only assimilate 

 free nitrogen by the aid of other micro-organisms which may have 

 acted by removing the dissolved oxygen from the solution. The 

 amount of nitrogen assimilated seems to bear some relation to the 

 sugar consumed, but the action is greatly affected by the presence of 

 combined nitrogen. 



Cultures of an organism (known as Bacillus ellenbacliensis), said to 

 have similar properties, were made in Germany and sold under the 

 name of " alinit ". They have not been very successful in practice. 



A large number of organisms, capable of effecting fixation of 

 nitrogen from the air when supplied with mineral matter and carbohy- 

 drates, have been discovered. The most active of these appears to be 

 Azotobacter chroococcum discovered by Beyerinck, associated with 

 another organism known as Eadiobacter. At one time the nitrogen- 

 fixation was ascribed to the simultaneous life processes of the two or- 

 ganisms, but it is now considered that the former alone does most of 

 the work. 2 



In the presence of many carbonaceous compounds mannitol, 

 xylose, arabinose, dextrose, starch, sodium tartrate, calcium lactate, 

 calcium malate, or even humus Azotobacter can fix considerable 

 quantities of atmospheric nitrogen, provided it be supplied with 

 mineral nutrients and with calcium or magnesium carbonate. Ashby 8 

 found it to be most abundant in the soil near the surface, that it with- 

 stands drying well and can be carried about as dust by wind. 



It is apparently only active when the soil is well supplied with 

 carbonaceous food, is free from acid, contains a fair supply of calcium 

 or magnesium carbonate, and has a high temperature. Addition of 

 sugar or starch to soil has, in some cases, been found to diminish fer- 

 tility ; this is attributed to the carbohydrates, in cold weather, favour- 

 ing the increase of other bacteria and moulds, rather than Azotobacter. 

 Hutchinson and Man* 4 in one case noted that the application of starch 

 and sugar increased the number of micro-organisms from 6-4 to 14'9 

 millions per gram of soil, and still reduced the fertility. This 

 occurred when the carbohydrate was added in the spring, when, the 

 temperature being low, putrefactive bacteria were more active than 

 Azotobacter. In other experiments, when the carbohydrates were 

 added to the warm soil, in autumn, the Azotobacter was stimulated 

 and nitrogen fixation and consequently enhanced fertility followed. 



iCompt. Rend., 116, 1385 ; Jour. Chem. Soc., 1893, Abstracts, ii. 482. 

 | 2 Jour. Chem. Soc., 1908, Abstracts, ii. 975. 

 3 Jour. Agric. ScL, 1907, 2, 35. 

 i 4 7th Inter. Congr. App. Chem., 1909, VII, 37. 



