250 AGEICULTUEAL AND INDUSTRIAL BACTERIOLOGY 



formed and may be taken up and utilized directly by the 

 plant. 



Nitrogen Fixation. — A study of the cycle of nitrogen 

 makes it evident that the amounts of living material upon 

 the earth, that is, the number of plants and animals, must 

 depend quite directly upon the amount of nitrogen in the 

 nitrogen cycle. There are several ways in vsrhich nitrogen 

 may be lost. First, there is the occasional process of 

 denitrification and evolution of free nitrogen gas. Then 

 explosives give off free nitrogen when fired. Fuel, when 

 consumed, gives off some free nitrogen. It is evident that 

 unless there is some way in which this nitrogen can be 

 replaced in compounds, the number of living creatures on 

 earth must gradually diminish. Until methods of fixation 

 of atmospheric nitrogen were discovered this fact was a 

 matter of grave concern to chemists. Laboratory studies 

 have shown that nitrogen may be fixed, that is, atmospheric 

 nitrogen may be combined with the other elements, by the 

 use of high temperatures or the electric current. It is noted 

 that there is some such combination of nitrogen during the 

 electric discharges of a thunder storm. The amount of 

 nitrogen fixed in this way, however, is very small in com- 

 parison to the amount fixed by the activity of microorgan- 

 isms, particularly of bacteria and perhaps of some of the 

 molds. 



Nitrogen as an element is exceedingly inert. The nitro- 

 gen molecule is apparently in a stable condition and in 

 order to force it to combine with other elements there is 

 usually necessary an expenditure of energy. This is evident 

 in the laboratory, for, as has been stated, high temperatures 

 or the electric current must be utilized. These forces are 

 not available to microorganisms. It is necessary, therefore, 

 in all cases that some easily oxidizable food material be 

 present so that energy may be secured for fixation. In most 

 cases tho nitrogen-fixing bacteria utilize carbohydrates. 



