140 BACTERIA AND SOIL FERTILITY 



generations that uncropped soils increase in fertility. Less an- 

 cient, however, is the knowledge that this increase may be due 

 to a gain of nitrogen in the abandoned soils. Even more recent 

 than this is the knowledge that it may be due to bacteriological 

 action. 



In the middle of the nineteenth century Boussingault wrote: 

 "Vegetable earth contains living organisms — germs — the vitality 

 of which is suspended by drying and reestablished under favor- 

 able conditions as to moisture and temperature." He also hinted 

 at the fact that these microorganisms take part in the process of 

 nitrogen fixation. It was not until thirty years later when Hell- 

 riegel and Wilfarth made their discovery of nitrogen fixation by 

 symbiotic organisms. At that time the laboratory technique of 

 modern bacteriology was still undeveloped. Since then, however, 

 we have learned much concerning the relationship of plants to 

 free and combined nitrogen of the air and of the soil. We know 

 that soil gains in nitrogen are often due to microorganisms either 

 living free in the soil or in company with the higher plants. The 

 production of nitrogen compounds from atmospheric nitrogen by 

 bacteria in the absence of higher plants is designated non-symhiotic 

 nitrogen fixation or a%ofication. When fixation is accomplished 

 by bacteria living in connection with and receiving benefit from 

 higher plants it is called symbiotic nitrogen fixation. 



As early as 1883 Berthelot undertook the study of soils with 

 regard their relation to atmospheric and combined nitrogen, and 

 as a result of these studies he was the first to definitely recognize 

 that gains which occur in bare, unsterilized soils are due to micro- 

 scopic plants. 



Berthelot's discovery interested Winogradski who commenced 

 work which eventually bridged the chasm. He employed, as a 

 medium, a nutritive solution free from combined nitrogen but 

 containing mineral salts and dextrose. Fifteen separate species 

 of soil bacteria were isolated, but only one — a long spore-bearing 

 bacillus which developed normally in the absence of combined 

 nitrogen and seemed to produce butyric fermentation — fixed 

 nitrogen in an appreciable degree. Quantitative tests showed 



