72 PHYSIOLOGY OF NUTRITION 



The presence of calcium carbonate prevents the accumulation of free acid in 

 both cases. 



Such experiments with natural soils cannot answer the question regarding 

 the direct assimilation of ammonia. Sterilized soils must be used, in which the 

 nitrifying process is eliminated. The experiments of Pitsch, 1 Breal, 2 and Kos- 

 sovich, 3 who used sterilized soils, gave positive results. 



§4. Circulation of Nitrogen in Nature. — The investigations of Boussingault 

 and of Schlosing and Müntz [see note a, page 64, and notes c, d, e, page 65] 

 established the view that higher plants can assimilate only combined nitrogen. 

 Free nitrogen should thus have absolutely no value for green plants, in spite of 

 the enormous amount of it present in the air. Schlosing pictures the circula- 

 tion of nitrogen in nature in the following way. Nitric acid (HNO3) formed in 

 the soil is taken up by plants and transformed into proteins and other organic 

 compounds, which, in their turn, serve for the nutrition of animals. These 

 compounds of nitrogen finally return to the soil as decomposition products of 

 plants and animals, and are there again oxidized to nitrates. The nitrate of the 

 soil, that is not assimilated by plants, is washed into the deep soil layers by pre- 

 cipitation water and finally reaches the sea, where it is changed back into am- 

 monium salts by the life activities of marine organisms. Ammonia evaporates, 

 with water vapor, from the surface of the sea, and is again taken up from the 

 atmosphere by plant leaves or by the soil, and in this way re-enters the general 

 circulation. All these transformations of combined nitrogen have no effect 

 upon the total amount of it occurring in nature. Natural processes are known, 

 however, which lead to the decomposition of nitrogenous compounds and to the 

 liberation of molecular nitrogen. Thus, in the complete combustion of nitroge- 

 nous organic compounds the total nitrogen is eliminated as nitrogen gas. 

 The decomposition of organic compounds in the soil is also accompanied by 

 the liberation of free nitrogen. 



The total amount of combined nitrogen in nature is diminished by these 

 processes and, for this reason, many authors have sought some natural process 

 that might lead to fixation of free nitrogen. Nitrogen is one of the elements that 

 form only weak combinations with other elements. Until recently chemistry 

 could name but three kinds of nitrogen fixation that might be of importance in 

 nature: (1) An electric spark discharge effects the union of nitrogen and oxygen 

 (Cavendish). (2) A silent electrical discharge causes the union of nitrogen 

 with organic substances (Berthelot). (3) During the evaporation of water a 

 small amount of nitrogen combines with hydrogen from the water and produces 

 ammonium nitrate (Schönbein). Only the first of these three possibilities has 

 real significance in nature, namely the fixation of atmospheric nitrogen during 

 thunderstorms. 



1 Pitsch, Otto, Versuche zur Entscheidung der Frage ob saltpetersäure Salze für die Entwicklung der 

 landw. Kulturgewächse unentbehrlich sind. II. Landw. Versuchsst. 42: 1-95. 1893. Pitsch, O., and 

 Haarst, J. Van, same title as above, III. /6i'i. 46: 357-382. 1896. 



2 Breal, E., Contribution ä l'etude de l'alimentation azotee des vegetaux. Ann. agron. 19: 274-293. 

 1893. 



3 Kossowitch, P., Ammoniaksalze als unmittelbare Stickstoff Quelle für pflanzen. [Abstract in German, 

 p. 637-638. Text in Russian.] Jour exp. Landw. 2 : 623-638. 1901. 



