36 FERTILISERS CONTAINING NITROGEN [chap. 



cultivated soils where the crop is removed the action is 

 almost brought to a standstill, as may be seen in the 

 steady loss of nitrogen from the arable soils at 

 Rothamsted during the fifty years they have been 

 cropped without any extraneous nitrogen supply. Only 

 when land is laid down to grass is there a sufficient 

 amount of carbohydrate debris returned to the soil to 

 enable the Azotobacter to fix a measurable quantity of 

 nitrogen. A good example of the natural accumulation 

 of combined nitrogen may be seen in two pieces of 

 land at Rothamsted, which for the last twenty-five 

 years have been allowed to run wild and assume a 

 natural prairie condition of self-sown weeds and grasses, 

 that are never taken away but left to rot where they die 

 down. Samples of the soil had been taken at the 

 beginning of the period, and by comparing them with 

 more recently taken samples it has been possible to 

 detect a very considerable fixation of nitrogen, 

 amounting in the most favourable case to nearly fifty 

 pounds of nitrogen per acre per annum. The second 

 similar piece of land shows a much lower result, and this 

 is correlated with the lack of carbonate of lime in the 

 soil of this plot and a corresponding absence of the 

 Azotobacter organism. 



It is too early yet to speculate freely on the 

 work of the various nitrogen-fixing bacteria; we may, 

 however, confidently attribute to their action both the 

 current stock of combined nitrogen in the world and 

 the main source of its renewal in the future. 



Attempts have already been made to raise the 

 nitrogen-fixing bacteria artificially, particularly those 

 associated with leguminous plants, and by introducing 

 them into soil that is lacking or poorly supplied with 

 them, to render it capable of self-enrichment in this 

 most natural manner. Such cultures are, in fact, sold 



