216 THE LIVING ORGANISMS OF THE SOIL [chap. 



the small supply of nitrogen available to the crop. 

 Possibly, too, some of the products of their action on 

 the carbohydrate were injurious. But with an autumn 

 application in the still warm soil, nitrogen fixation goes 

 on actively, and some of the material thus absorbed from 

 the air becomes available for the crop in the following 

 spring. 



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 

 17 lbs. of nitrogen per acre per annum. Analyses of 

 the soil at the beginning and end of the period showed 

 a decline in the amount of nitrogen equivalent to a 

 removal of 12 lbs. per acre per annum, and the rainfall 

 is known to bring down between 4 and 5 lbs. per acre 



