ABSORPTION OF NITROGEN FROM SOIL AND AIR 57 



while the average amount of nitrogen contained in soils is between 

 0.1 and 0.5 per cent, by weight; but, because of a high mobility 

 of the atmosphere, the plant has at its disposal practically all of the 

 enormous amount of CO2 surrounding the earth. Besides, the 

 total amount of carbon dioxide in the air is continuously augmented 

 not only through the respiration of animals, plants, and micro- 

 organisms, but also by the processes of combustion and by volcanic 

 eruption. Conditions appear to be much less favorable in regard 

 to the presence of nitrogenous substances. The amount contained 

 in the soil is limited, and is increased only through the disintegra- 

 tion of such plants and animals as have existed within the same 

 regions of the earth's surface. Moreover, a constant transforma- 

 tion of fixed nitrogen into free molecular nitrogen takes place in 

 nature. This happens, for instance, when organic matter is 

 burned; likewise, in the process of denitrification or decomposition 

 of salts of nitric acid in the soil. Formation of free nitrogen in the 

 soil usually occurs as a result of a superabundant accumulation of 

 nitrogen-bearing matter. If the processes resulting in the fixation 

 of molecular nitrogen were not present, the nitrogen balance of 

 the earth naturally would show an ever-increasing deficit, and the 

 organic world would be in danger of perishing from nitrogen 

 starvation. This does not occur, owing to the fact that among the 

 living organisms populating the earth's surface there are many that 

 possess the capacity of carrying out the extremely difficult syn- 

 thesis of organic nitrogen compounds from non-nitrogenous sub- 

 stances and free molecular nitrogen. These are the so-called 

 "nitrogen-fixing" bacteria. 



The majority of these nitrogen-fixing bacteria live in the soil. 

 It has been known for a long time that fallow land becomes richer 

 in nitrogen and that the accumulation of fixed nitrogen occurring 

 in it is a biological process, since it is interrupted by the addition 

 of antiseptics to the soil. Winogradsky (1893) was the first to 

 succeed in isolating from the soil and in studying the physiology 

 of one group of these bacteria. It proved to be a sporogenous 

 bacillus, causing butyric-acid fermentation in sugary liquids and 

 simultaneously fixing molecular nitrogen (Fig. 23). The discov- 

 erer has named it Clostridium pasteurianum. This organism 

 belongs to the type of anaerobic bacteria. They are able to live 

 and develop only in the absence of oxygen, and therefore always 

 exist in soil, together with the ordinary saprophytic bacteria which 



