COMPOSITION OF SOILS. 29 



CHAPTEE III. 

 Composition of Soils. 



A SOIL, like a plant, consists of two distinct classes 

 of substances, — first, organic or vegetable, derived, as we 

 have seen, from decaying growths ; second, inorganic or 

 mineral constituents, derived from the rocks which form 

 the earth's surface. 



Organic Substances. — Organic substances are made 

 up of carbon, oxygen, hydrogen, and nitrogen. Of these 

 the nitrogen is of the most direct importance in the 

 growth of the plant, and is the valuable constituent of 

 the humus already described. 



Inorganic Substances. — The inorganic or mineral 

 substances of the soil are also identical with the sub- 

 stance of the ash of plants (with the addition of alu- 

 mina, which is not taken up by the latter), namely, 

 silica, alumina, lime, potash, magnesia, phosphoric acid, 

 soda, iron, chlorine, and sulphuric acid. The first three 

 of these, as has already been noted, are the principal 

 ingredients of soils, and give to them their distinctive 

 character; with the exception of lime, they do not aid 

 materially in furnishing food. The more important con- 

 stituents are phosphoric acid and potash. 



Phosphoric Acid. — Phosphoric acid is an ingredient 

 of all fertile soils, but is contained in very small quan- 



