250 ORGANIC AGRICULTURAL CHEMISTRY 



other urine constituents, are first decomposed into ammonia. 

 This ammonia and other ammonium salts of whatever origin 

 are probably in greater part then oxidized to nitric acid. This 

 oxidation of ammonia to nitric acid takes place in the presence 

 of atmospheric oxygen in the soil through the action of aerobic 

 bacteria. The reaction may be represented in its simple form 

 as follows : 



NH3 + 2 O2 + bacteria -» HNO3 + H2O 



The nitric acid thus formed unites with bases in the soil, form- 

 ing nitrates, e.g. potassium, sodium, ammonium, calcium, 

 magnesium nitrates. In the form of these compounds, the 

 nitrate nitrogen is taken up by the plant and metabolized into 

 protein as above explained. It has been definitely proved, 

 however, that plants can utilize ammonia nitrogen directly and 

 also nitrogen in the form of urea or other amino compounds. 

 To just what extent such nitrogen is utilized has not been 

 established. 



One other source of nitrogen is of especial importance, but it 

 applies only to certain plants, so far as is known only legumes 

 such as clover, pea, bean, etc. In these plants there are present 

 on the roots nodules containing bacteria which possess the 

 characteristic property of assimilating free nitrogen from the 

 air and converting it into protein nitrogen. 



Nitrogen Cycle. — The cycle of changes through which the 

 element nitrogen passes in its existence in the atmosphere, soil, 

 plants and animals is exceedingly interesting. Nitrogen, phos- 

 phorus and sulphur are the only ones of the essential constituents 

 of the soil food of plants which are converted into organic or 

 energy food of the plant. The other more important plant 

 food constituents, obtained from the soil and present also in 

 fertilizers, are potassium and some others such as calcium, 

 magnesium and iron. While compounds of these elements must 

 be considered as true plant food, they are not energy food and 

 should always be termed the soil food of plants as distinguished 

 from their cell food or energy food. Their action is not fully 



