178 RECAPITULATION 



water; the same formula may and does represent them 

 all. 



Water consists of hydrogen and oxygen. 



The atmosphere consists of nitrogen and oxygen. 



CHAPTER V. 



Composition of the soil. 



We find here also an organic, and an inorganic 

 part; the inorganic part largest, contrary to what was 

 observed in plants.. 



The organic part is derived from the decay of ani- 

 mals an I vegetables; the inorganic part from the 

 decomposition of rocks, 



The inorganic part consists of the same substances 

 as the inorganic part of plants, with the addition of 

 alumina. This is a white substance, which gives 

 stiiFness to clays. 



A very fertile soil contains all of these substances, 

 and that in considerable quantity. 



One which is fertile only with the addition of ma- 

 nure, has deficiencies of some substances which the 

 manures added supply. 



One which is barren, has nearly every thing that is 

 valuable wanting. 



The three principal varieties of rocks, are limestones, 

 sandstones, and clays. 



Soils may be named, as one or other of these pre- 

 dominate. 



CHAPTER VL 



m 



Mechanical improvement of the soil. 



Nature of the connection between the soil and the 

 plant. Benefit of mixing clay with sand, and sand 

 with chy. 



