THE SOIL. 35 



the soil proper may be said to be composed of the fol- 

 lowing substances: 

 Phosphorus — (Phosphor- Sodium — (Soda. ) 



ic acid.) Magnesium — (Magnesia.) 



Potassium — (Potash.) Chlorine. 



Lime. Silicon — (Silica.) 



Iron — (Oxide of Iron.) Aluminum — (Alumina.) 

 Sulphur — (Sulphuric acid.) 



These substances make up 90 or 95 per cent, of 

 most fertile soils, the remainder being humus, or de- 

 cayed vegetable matter. In peaty soils this last may 

 be a fourth or even a half of the soil, but some fertile 

 soils have not more than two or three per cent of hu- 

 mus. It will be remembered that the mineral or ash 

 constituents of plants usually constitute not more than 

 five per cent, of the whole plant. All the substances 

 named above, except alumina, are always found in 

 plants, and alumina is always found in good soils. 

 In one sense it may be said all are essential; hence 

 equally important. Some, however, are always found 

 in abundance, while others are frequently in insuffi- 

 cient supply for the production of good crops. This 

 is especially true of phosphoric acid and potash. 

 Hence these substances are often spoken of as the 

 most important mineral elements in the soil. 



If sand be thought of as silica, and alumina as the 

 base or foundation of clay, the other element being 

 silica, it will be evident there is no lack of these two 

 substances. Lime and iron are found in most soils in 

 great abundance, so far as their use as plant food is 

 concerned. The other substances are not found in so 

 large quantities, but almost any soil contains more of 

 them than crops need. A like condition is found as 



