CHAPTER XII. 



THE FIXATION OR ASSIMILATION OF 

 NITROGEN ( Continued}. 



i. The Fixation of Nitrogen by Bacteria in Symbiosis 

 with Leguminous Plants. It has been known among 

 agriculturists from the earliest times that the growth of 

 a leguminous crop, such as beans or clover, leaves the 

 land in a high condition for the growth of a subsequent 

 crop of wheat or other cereal. It was recognized that 

 in some way or another a leguminous crop differed very 

 much from any other class in its effects upon the land, 

 and its cultivation was to a large extent found to be 

 equivalent to the application of a considerable amount 

 of dung or other manure. Experiments carried out in the 

 open field at Rothamsted have shown that the growth 

 of a leguminous plant leaves the ground richer in nitrogen 

 than it was before, and this in spite of the fact that the 

 crop itself is highly nitrogenous in composition. Below 

 are given the average annual amounts of nitrogen per acre 

 obtained in the several crops mentioned, grown on the 

 land every season during a period of about twenty-four 

 years : the soil was manured with phosphate, potash, and 

 other mineral manures, but no nitrogen was applied : 



Average amount of Nitrogen 

 per Acre per Annum. 



Wheat . . _ ._. . 22.1 Ibs. 



Barley . . ... 22.4 Ibs. 

 Beans . . . V 45.5 Ibs. 



Clover (22 years only) . 39.8 Ibs. 



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