NITROGEN GATHERING OR NITROGEN-FIXING BACTERIA. 93 



NITROGEN-GATHERING OR NITROGEN-FIXING 

 BACTERIA. 



It was soon demonstrated that nitrogen fixation in the soil is not 

 due to purely chemical processes, but rather to the growth of micro- 

 organisms. That it is due to the action of living organisms is proved 

 by the effect of sterilizing such soil. Two vessels may be filled with 

 similar soil, one of which is sterilized by heating, while the other 

 serves as a control. The former fails to gain nitrogen, no matter 

 how long it is kept in contact with the air; the latter slowly but 

 surely increases its store of fixed nitrogen in the form of nitrates. 

 This proves that some living organisms are concerned, and the fact 

 that no visible plants are growing in the soil shows that the higher 

 plants do not produce the result. The only conclusion that can be 

 drawn, therefore, is that microorganisms are the agents for reclaim- 

 ing free nitrogen from the atmosphere and fixing it in the earth in 

 some form of nitrogen compounds, which eventually become nitrates 

 and, thus, plant foods. 



Such facts plainly pointed toward bacteria or allied organisms as 

 the real agents for fixing nitrogen from the air, and this suggestion 

 once made was quickly followed by the isolation of the bacteria con- 

 cerned. It is now a demonstrated fact that the power of gathering 

 atmospheric nitrogen and fixing it in the soil belongs to bacteria, and 

 during the last fifteen years much study has been devoted to the 

 microorganisms concerned. It appears that there are two general 

 types of such nitrogen fixations associated with different classes of 

 bacteria: i. Nitrogen fixation by bacteria alone (non-symbiotic). 

 2. Nitrogen fixation by bacteria in connection with legumes 

 (symbiotic). 



NON-SYMBIOTIC FIXTURES. 



These are soil bacteria that are able to produce an increase of 

 nitrogen in ordinary soil without the aid of other organisms. Of 

 these there are two types, the first acting anaerobically, and the 

 second aerobically. i. The first one that was found with this 

 power was isolated from soil and named Clostridium pasteurianum 

 (Fig. 21). It is an anaerobic bacterium, and in culture media will not 



