58 SOILS AND FERTILIZERS 



disintegrated, particularly by glacial action, form very 

 fertile soils. 



63. Granite is composed of quartz, feldspar, and 

 mica. It is a very hard rock and is slow to disinte- 

 grate. The different shades of granite depend upon 

 the proportion in which the various minerals are pres- 

 ent. Inasmuch as granite contains so many minerals 

 it nsnallv follows that thorotisrhlv disintegrated granite 

 soil is very fertile. Pure powdered granite before un- 

 dergoing disintegration furnishes no plant food. After 

 weathering, the plant food gradually becomes availa- 

 ble. Gneiss belongs to the granite series but differs 

 from true granite by containing a larger amount of 

 mica. Mica schist contains a larger amount of mica 

 than gneiss. 



64. Apatite or Phosphate Rock. — Apatite is com- 

 posed mainly of phosphate of lime, Ca (PO ) 2 , together 

 with small amounts of other compounds as fluorides 

 and chlorides. This mineral is generally of a green 

 or yellow color. It is present in many soils and is 

 of little value as plant food unless associated with or- 

 ganic matter or some soluble salts. 



65. Kaolin is chemically pure clay and is formed by 

 the disintegration of feldspar. When feldspar is de- 

 composed and is acted upon by water the potash is re- 

 moved and water of hydration is taken np, forming 

 the product kaolin, which is hydrated silicate of alu- 

 mina, Al (SiO ) .H 2 0. Impure varieties of clay are col- 



