CHAPTER VI 

 PLANT FOOD AND MANURES 



REFERENCE has already been made to water and 

 carbon dioxide as two of the constituents of plant food. 

 These two substances are obtained from the atmo- 

 sphere, the former falling as rain and usually entering 

 plants through their roots, the latter being absorbed 

 and assimilated by the green leaves. In addition, 

 certain constituents of the food of plants are derived 

 from the soil. We may divide these into two classes : 

 nitrogenous and mineral matters. This division is 

 convenient, for the absorption of nitrogen is sufficiently 

 interesting to make it desirable to devote separate 

 attention to it. Moreover, when a plant or any other 

 vegetable substance is burned, the nitrogen disappears 

 in the gases or vapours, together with the carbon, 

 hydrogen and oxygen, while the mineral matter remains 

 behind in the form of ash. 



Nitrogenous Matter. 



As the air by which we are surrounded consists of 

 four parts of nitrogen and one part of oxygen, it would 



seem reasonable to suppose that its nitrogen would 

 in 



