52 SOILS AND FERTILIZERS 



compounds and have a variable range of composition, 

 due to the fact that one element or compound may be 

 partially or entirely replaced by another. Most rocks 

 from which soils have been produced contain minerals 

 as feldspar, mica, hornblende, and quartz. 



59. Quartz and Feldspar. Quartz is the principal 

 constituent of many rock formations. Pure quartz is 

 silicic anhydride (SiO 2 ). White sand is nearly pure 

 quartz or silica. Silica enters into combination with 

 many elements, forming a large number of minerals. 

 A soil formed from pure quartz would be sterile. 



Feldspar is composed of silica, alumina, and potash 

 or soda. Lime may also be present, and replace a 

 part or nearly all of the soda. If the mineral contains 

 soda as the alkaline constituent it is known as albite, 

 or if mainly potash it is called potash feldspar or 

 orthoclase. 



The members of the feldspar group are insoluble in 

 acids and before disintegration takes place are not 

 capable of supplying plant food. Potash feldspar 

 contains from 12 to 15 per cent, of potash, none of 

 which is of value as plant food. When feldspar 

 undergoes disintegration it produces kaolin or clay. 

 A soil formed from feldspar is usually well-stocked 

 with potash. 



Orthoclase, AlKSi 3 O 8 Potash feldspar. 



Albite, AlNaSi 3 O 8 Sodium feldspar. 



60. Hornblende. The hornblende and augite groups 

 are formed by the union of magnesium, calcium, iron, 

 and manganese, with silica. There are none of the 



