134 AGRICULTURAL BACTERIOLOGY. 



atmospheric nitrogen is due to the growth of microorganisms. 

 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 nitro- 

 gen, 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 reclaiming 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. To some problematical microorganisms in 

 the soil, then, must we attribute this first process of reclaiming 

 for the soil the lost nitrogen. 



If such a microorganism exists we ought to be able to dis- 

 cover it and isolate it for experiment. Certainly this ought to 

 be possible if it should be a bacterium. But the isolation from 

 the soil of an organism having this power has not proved to 

 be an easy task. The power of assimilating free nitrogen is 

 not common among microorganisms. Winogradsky was the 

 first to search systematically for such a bacterium and he tested 

 a large number of soil bacteria. Only one of those studied by 

 him proved to possess this power in any considerable degree. 

 This organism he named Clostridiiim pastcurianuui. From its 

 growth there results, not only nitrogen, fixed in the soil by 

 the organism in an insoluble form, but such other by-products 

 as hydrogen, carbon dioxide, butyric and acetic acids. Like 

 nearly all of the nitrogen-fixing organisms, this power of assimi- 

 lating nitrogen is best shown when the organism is growing in a 

 nitrogen-free medium. Since Winogradsky's first discovery of 

 this bacterium, a few others have been found with the same 



