Vii.) NITROGEN FIXATION IN VIRGIN SOILS 1S9 



easily be rendered evident by an increased yie'd of 

 crop. Koch treated soil in pots with large quantities of 

 sugar, 2 per cent, 4 per cent., and even more of dextrose, 

 and then sowed oats, buckwheat, etc. At first the sugar 

 was injurious, and the first crop suffered in consequence ; 

 but the proportion of nitrogen in the soil increased, and 

 the second and third crops were far greater than those 

 in the check plots of untreated soil. When the soil, 

 after the application of the sugar, was placed in an 

 incubator for a month, in order to complete the oxidation 

 of the sugar, the increased yield due to nitrogen fixation 

 was also seen in the first crop. 



To the Azotobacter and kindred organisms must 

 certainly be ascribed a large part in preparing and 

 maintaining the world's stock of combined nitrogen. It 

 is customary to regard such virgin soils as the black soils 

 of the Russian Steppes, of Manitoba, and of the Argen- 

 tina, as rich in nitrogen because of the accumulation of the 

 vegetable debris of many epochs ; but since plants other 

 than the Leguminosae do not fix nitrogen themselves, 

 there could in this way be no addition to the original 

 stock, which would only circulate from the soil to the 

 plant and back to the soil again. Under such conditions, 

 however, there is a continual addition to the soil of the 

 carbon compounds which the plant derives from the 

 atmosphere, and this is material which the Azotobacter 

 can oxidise, and so derive the energy required for the 

 fixation of nitrogen. It is the constant return to the soil 

 of oxidisable organic matter which differentiates the 

 wild from the cultivated land, and renders possible the 

 long-continued storing up of nitrogen in the virgin soils. 



Interesting evidence on this point may be derived 

 from the Rothamsted experiments; on the Broadbalk 

 wheatfield the unmanured plot has, during the fifty 

 years 1844-93 yielded a crop containing on the average 



