RECLAIMING LOST NITROGEN. 147 



many instances in nature where two quite different organisms 

 are associated with each other, and, when combined, thrive 

 excellently, each showing much more vigorous growth than 

 when growing alone. This phenomenon of association for 

 mutual benefit is known by biologists as symbiosis. It has 

 been the usual belief that the phenomenon we are considering 

 is one of symbiosis ; that neither the legume nor the bacterium 

 by itself is able to absorb and fix atmospheric nitrogen, but 

 that the two together accomplish the purpose, each benefiting 

 by the presence of the other. 



Still another suggestion has been advanced in recent times. 

 As shown on a previous page certain soil bacteria can fix 

 atmospheric nitrogen, causing it to be deposited in an insoluble 

 form. It has been suggested that the first fixation is of this 

 character by the soil bacteria (Clostridium pasteurianum f), and 

 that the tubercle bacterium uses these insoluble nitrogen com- 

 pounds, rendering them soluble for the use of the legume. 

 Such an interpretation would involve three organisms in the 

 phenomenon, two species of bacteria and the legume. 



It is impossible at the present time to determine positively 

 which of these suggestions is correct. This much is certain, 

 the legume and the bacterium together are able to fix atmos- 

 pheric nitrogen in quite large amounts. Neither of them can 

 do this alone in the ordinary conditions of the soil. The pea 

 plant without the bacterium cannot fix nitrogen, and the bac- 

 terium without the legume does not, at least in the ordinary 

 soil or in ordinary cultures, fix atmospheric nitrogen. But 

 when the bacterium grows in the root of the legume and stimu- 

 lates the production of nodules, atmospheric nitrogen is fixed, 

 either by the one or the other, or by the two plants acting 

 together. 



This power of fixing nitrogen is in considerable degree 

 dependent upon the amount of nitrogenous food already in the 

 soil. In most of the experiments, where a large amount of 



