804 



THE ROLE OF NITROGEN, ETC., I., 



The Nitrogen of the Soil. — The following table shows the 

 amount of nitrogen present in ordinary arable soil unmanured(l) 



First 9 inches of soil contain 3000 lbs. N per acre. 

 Second 9 ,, ,, ,, 1700 „ 



Third 9 ,, ,, ,, 1500 



27 



6200 



The average value is 0-1 % N. 



This nitrogen exists chiefly in organic combination in the 

 humus, which is an indefinite mass of decaying vegetation, and 

 out of which, by slow bacterial oxidation, nitrates are formed. We 

 find here all the intermediate stages of nitrogen-transformation — 

 proteins, (2) amides, amino-acids, amines, ammonium salts, nitrites 

 and nitrates. These are the materials available to plants for 

 food, and from which they synthesise their special proteins. 



The numerous stages in the conversion of the organic nitrogen 

 to nitrates is considerably hastened by the influence of micro- 

 organisms, and firstly, the ordinary putrefactive bacteria convert 

 the decaying vegetable material into ammonia and carbon dioxide; 

 secondly, the " nitrosomonas " are able only to change this 

 ammonia into nitrites; and lastly, the " nitrobacter " complete 

 the oxidation to nitrates. The two latter organisms are generally 

 known as the "nitrifying organisms" of the soil, and were first 

 isolated in pure cultures by Winogradsky in 1890.(3) This nitri- 

 fication process everywhere goes on in arable soil; the nitrates, 

 being the most highly oxidised form, are also the most stable, 

 and the greater part of the nitrogen in the soil exists in this 

 form. 



Relation of Protein to the Non-Nitrogenous Reserve 

 Material. 



Since proteins are composed of amino-acids, and the latter are 

 ammonia derivatives of the fatty acids, it is necessary here to 

 briefly summarise our views on the possible sources of the fatty 

 acids in plants. 



