39 2 THE FOOD OF PLANTS 



Animals and such plants as are unable to synthesize proteids from simple nitrogen- 

 compounds are dependent for their supply of combined nitrogen upon those 

 plants which are able to assimilate such ultimate products of decomposition as 

 nitric acid or ammonia. Both plants and animals decompose large quantities 

 of proteids, and the organisms which grow upon animal and vegetable remains 

 produce simpler nitrogen-compounds, including ammonia : this again may serve 

 as a source of energy to nitrite and nitrate bacteria, which co-operate to pro- 

 duce nitrates in a well aerated soil '. The green plants which grow upon 

 such soils as a rule prefer their nitrogen in the form of nitrates, and these are 

 continually produced in small amount by the nitrifying bacteria, so that the 

 danger of loss by the solvent action of water upon the soil is largely avoided. 

 Most nitrates are highly soluble, and are moreover not retained by humus, 

 and hence any large accumulation of nitrates may be almost entirely washed 

 away by the first heavy shower of rain. Where the rainfall is slight or absent, 

 however, the decomposition of masses of organic material may result in the 

 production of large deposits of nitrates, as in the nitrate beds of South America. 



Certain micro-organisms decompose nitrogen-compounds with a liberation of 

 free nitrogen (Sect. 102), and the latter may also be liberated by special chemical 

 actions. Hence a fixation of nitrogen must occur if the balance of nature is to 

 be maintained, and certain plants are actually able to assimilate free nitrogen. 

 This is perhaps the most important means by which nitrogen is brought into 

 circulation again in the organic world, but other processes leading to the same 

 end are also continually active 2 , such, for example, as the formation of oxides of 

 nitrogen by electrical discharges passing through moist air during thunderstorms, 

 or even by the passage of weak electric currents through the soil. Schonbein 

 erroneously supposed that the evaporation of water induces the combination of 

 nitrogen and oxygen, but it is not impossible that other natural agencies may 

 produce this effect, for in the process of combustion or directly by the aid of 

 heat, nitrogen may be caused to enter into combination with other elements, as 

 takes place for example when phosphorus slowly oxidizes in contact with air 

 and induces a production of oxides of nitrogen and ultimately of nitric acid. 

 During volcanic outbursts, a fixation of nitrogen may occur in some such manner, 

 and it is possible that the chemist may ultimately succeed in producing nitrogen- 

 compounds from the air with such readiness as to form a cheap source of artificial 



manure 3 . 



The removal of large quantities of nitrogen from the soil by the annual 

 harvest sooner or later causes the crops to become enfeebled, unless the loss is 



1 Cf. Sect. 63. Schlosing and Muntz (1877) first showed that nitrification was due to the action 

 of micro-organisms, and all the more recent researches have confirmed their discovery. See Sachsse, 

 Agr.-Chem., iSSS, p. 139; Wortmann, Landw. Jahrb , 1891, Bd. XX, p. 175; Pitsch, Versuchsst., 

 1893, Bd. XLII, p. 87 ; Burri, Centralbl. f. Bact., 1895, Abth. ii, Bd. I, p. 22. 



2 Literature on fixation of nitrogen : E. Schulze, Landw. Jahrb., 1877, Bd. vi, p. 695 ; Baumann, 

 Versuchsst., 1888, Bd. xxxv, p. 243; Frank, Unters. liber d. Ernahrung d. Pflanze mit Stickstoff, 

 1888, p. 66 (Sep.-abdr. a. Landw. Jahrb.) ; Ilsova, Ber. d. Chem. Ges., 1890, ref. p. 85 ; Loew, ibid., 

 1890, p. 1443 ; F. v. Lepel, ibid., 1897, p. 1027. 



3 See Report Brit. Assoc. 1898 (Presidential Address). 



