54 SOILS AND FERTILIZERS 



and belong to the group of compounds which are 

 capable, to a certain extent, of becoming available as 

 plant food. In color, they are white, gray, or red. 

 Zeolites are quite abundant in clay and are an import- 

 ant factor in soil fertility. It is this group of hydrated 

 silicates which takes such an important part in th e 

 process of fixation. The zeolites, when 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 slow to disintegrate. 

 The different shades of granite depend upon the pro- 

 portion in which the various minerals are present. 

 Inasmuch as granite contains so many minerals it 

 usually follows that thoroughly disintegrated granite 

 soil is very fertile. Pure powdered granite before 

 undergoing disintegration furnishes no plant food. 

 After weathering, the plant food gradually becomes 

 available. Gneiss belongs to the granite series but 

 differs from true granite in 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 4 ) 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- 



