II.] NITROGEN FIXED BY LEGUMINOUS PLANTS 33 



specialised and has not been transferred to other non- 

 leguminous plants ; only with some difficulty has it also 

 been made to grow and to fix nitrogen when living 

 alone and no longer in association with its host. But 

 with increasing knowledge of the methods of handling 

 this organism, it seems probable that by cultivation 

 we shall be able to obtain races showing variations in 

 their power of fixing nitrogen, though how long they 

 will retain this greater or lesser virulence after inocula- 

 tion back to the leguminous plant is still uncertain. 



The leguminous plants form, then, by their associa- 

 tion with nitrogen-fixing bacteria, one considerable 

 natural source of combined nitrogen, and how effective 

 they can be in accumulating fertilising matter in the 

 soil may be judged from the accompanying table 

 (VIII.) showing the results of one of the Rothamsted 

 experiments upon crops grown in rotation. 



Table VIII.— Effect of Clover on Succeeding Crops. 



On this field (Agdell) the rotation begins with a crop 

 of Swede turnips, which is manured, in one case with 

 mineral manures, in the other with a complete fertiliser. 

 Following the Swedes comes barley without manure, 

 then the field is divided, and on one portion clover is 

 grown, while the other is bare fallowed and carries no 



C 



