36 CHEMICAL COMPOSITION OF SOILS 



How Plants Sectire Their Nitrogen and Carbon. — Plants do 

 not and cannot use nitrogen in its gaseous form, hence they do not 

 take in any nitrogen directly from the air through their leaves. 

 AH nitrogen used by plants is taken in through their roots.^ 



Practically all the carbon contained in plants is taken directly 

 from the air through the leaves in the form of carbon dioxide (CO2). 



Much Carbon is Used by Plants. — ^A bushel of shelled corn 

 contains about 25 pounds of carbon and one pound of ash. A ton 

 of clover hay contains 950 pounds of carbon and about 140 pounds 



Air(G 



arbon dioxide) 



5 

 'I 



I 



PLA N T5 fi"^"'^^ f°°''' f"- Ammo 15 



^j. (Man and beosfj 



<^ O I I ( Wafer, nHro(^en on J mineral ^lennenis/ 



Fig. 16. — The relation of plants to the soil and air on the one hand and to animals 



on the other. 



of mineral matter (ash). These two facts show the importance 

 of both carbon and mineral elements in the production of crops. 

 Chemical Composition of an Animal Body is Similar to That of 

 Plants and Soil, — Plants are consumed by animals, and this means 

 that the elements composing the animal body are secured mainly 

 from plants. Thus it is not surprising that the chemical composi- 

 tion of an animal body should be similar to that of plants. The 

 ash of an animal body contains the same kinds of mineral elements 

 as are found in the ash of plants and in soils. In reaUty, therefore, 

 the ash of an animal body is nothing more than the "dust" of the 



sThis seems contrary to general opinion especially as regards clover, 

 alfalfa and other legumes. The nitrogen gathered by the bacteria within the 

 nodules on the roots of legumes is absorbed by the roots. 



