204 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1923 



Through the action of electrical discharges of thunderstorms, 

 which continually go on in the atmosphere, appreciable amounts of 

 free nitrogen in the air are converted into oxides of nitrogen which 

 are absorbed by falling rain water and enter the soil in the form 

 of nitric acid. This nitric acid then combines with the bases in the 

 soil, such as potassium, calcium, etc., and forms the corresponding 

 nitrates in which form they are taken up by the plants and 

 metabolized into protein. 



Through the action of bacteria in their root nodules, certain 

 legumes, such as peas, beans, and clover, also possess the capacity of 



directly absorbing free nitro- 

 gen from the air during 

 their growth and converting 

 it into protein. 



It has thus been shown 

 how the nitrogen is supplied 



ATMOSPHERIC NITROGEN 



ELECTRIC DISCHARGE. 

 I 

 NITRIC OXI0C, 



ORGANIC NITROGCN. 

 -(UREA, DEAD PROTEIN) 

 BACTERIA 

 AMH0NIA ." 

 OXYGEN.BACTERIA. 



POTASSIUM.CAICII 



RAIN. 



I 

 NITRIC ACIO 



IITRITCi NITRATE ■ 



I — DEAD PROTEIN - 



SOIL 



rOTASSUM. CALCIUM 



PLANT 

 PROTEIN. 



ixglimes_ 

 "bacteria? 



ANIMAL 

 PROTEIN. 



to the plants and protein 

 metabolized which serves 

 not only as their own food 

 supply but also as reserve 

 food for the plants' off- 

 spring. This now becomes 

 animal food and, for vege- 

 table eating animals, it is the 

 sole source of the nitrogen 

 for animal protein. 



Part of the nitrogen which 

 is not used for maintaining 

 the body substance of ani- 

 mals is eliminated as urea 

 and hippuric acid, and from 

 the urea much nitrogen is set free by the action of nitrous acid. 

 This urea and hippuric acid together with decayed vegetable and 

 animal matter — that is, dead protein — is then with the aid of 

 bacteria converted into ammonia. Part of this is by means of 

 oxidizing bacteria converted into nitric acid and nitrate in which 

 form it again is partly assimilated by the plants. Part of this 

 nitrate is, however, dinitrified by bacteria, one portion reverting 

 into free nitrogen which goes back to the atmosphere, the other 

 being converted into ammonia which fails to oxidize and is volatil- 

 ized as a gas, which is absorbed by rain and again returned to the 

 soil. Part of the ammonia is also taken up directly by the plants 

 from the soil. 



Fig. 1. 



-Elementary diagram of nature's nitro- 

 gen cycle 



