NITROGEN ASSIMILATION; THE NITROGEN CYCLE 93 



their oxygen from these nitrogen compounds that contain it. It 

 is evident that the conditions for the maximum activity of these 

 organisms are the reverse of those for nitrification, — poor aeration 

 of the soil, too much water, low soil temperature, and an acid soil 

 with much organic matter. These are just the reverse conditions 

 for good fertility, and might be found after saturating the soil 

 with water following a heavy application of manure. In this case, 

 although adding organic matter for fertilizer, one is actually 

 diminishing the amount of nitrogen and producing exactly the 

 opposite results from those desired. 



Nitrogen Fixation. — As we have seen, nitrogen is being con- 

 stantly lost to the air through the processes of denitrification, and 

 much nitrogen is also leached out of the soil and carried down to 

 the ocean. If there were no way of tapping the vast reserves in 

 the atmosphere, which is 80% nitrogen, the available supply of 

 this element would ultimately be lost forever from the organic 

 cycle once it had escaped to the atmosphere in the free form. As 

 long as the nitrogen is in the form of nitrates or ammonium com- 

 pounds, it is still in the organic cycle, because these compounds 

 may be used by plants or may be easily put into an available 

 form. The denitrification processes and the burning of organic 

 compounds are the most serious causes of loss of organic nitrogen,, 

 since in both these cases the nitrogen is freed as nitrogen gas or 

 as a gaseous nitrogen compound. The decomposition of organisms 

 in the soil also results in the escape of some nitrogen. 



To counteract this gradual loss of the nitrogen from the organic 

 cycle, natural processes have been sought which lead to the con- 

 version of free nitrogen into some compound easily used by plants. 

 Three such fixation processes have been determined to date. 

 Cavendish pointed out that an electric spark effects the union of 

 nitrogen and oxygen, with the result that during thunderstorms 

 the amount of nitrate is somewhat increased; Berthelot showed 

 that the silent discharge of electricity might bring about similar 

 fixations of nitrogen with organic compounds; and Schonbein has 

 called attention to the previously mentioned fact that during 

 evaporation small amounts of nitrogen are combined with hydro- 

 gen to produce ammonium nitrate. Of these three, only the first 

 can be considered as of any real significance, the importance of 

 which can be realized from the figures of Arrhenius, who has 

 estimated that rainfall brings to earth each year one and a half 



