218 THE LIVING ORGANISMS OF THE SOIL [chap. 



pasture must be due to the accumulation of nitrogen by 

 the same agency, because we know that land under grass 

 accumulates carbon compounds from the roots and 

 stubble that is not removed during grazing. 



Nitrification. 



It has long been known that when any organic com- 

 pound of nitrogen is applied to the soil it becomes event- 

 ually oxidised to a nitrate, which is, as a rule, the 

 . compound of nitrogen taken up by cultivated plants, the 

 Leguminosse excepted. The potassium nitrate collected 

 from Indian soils, the calcium nitrate made artificially in 

 nitre beds in Europe, owe their origin to this oxidation 

 of organic compounds of nitrogen. That the process 

 was a biological one was first indicated by Miiller in 

 1873, but any widespread recognition of the fact did not 

 take place before the work of Schloesing and Muntz in 

 1877. These investigators showed that the formation 

 of nitrates in the soil ceased at temperatures below 5 

 and above 55 C, that it could be stopped by chloroform 

 vapour and similar antiseptics, and that the soil lost 

 entirely its power of nitrification if it were heated to the 

 temperature of boiling water. The investigations of 

 Warington confirmed these results, and brought to light 

 the further fact that there were two stages in the oxida- 

 tion process, one being the formation of a nitrite, 

 followed by the conversion of this nitrite into the com- 

 pletely oxidised product. It was found possible to obtain 

 cultures which would only push the oxidation to the 

 nitrite stage, thus indicating that there must be at least 

 two organisms concerned in the complete nitrification 

 process. The further study of the organisms was for 

 a long time hindered by the fact that they could not be 

 got to grow upon the gelatinous media employed in the 

 ordinary methods of isolating specific bacteria ; and 



