56 SOILS AND FERTILIZERS 



silicic anhydride (Si0 2 ). White sand is nearly pure 

 quartz. 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 re- 

 place 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 feld- 

 spar or orthoclase. 



The members of the feldspar group are insoluble in 

 acids and before disintegration takes place are not ca- 

 pable 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 disin- 

 tegration it produces clay. A soil formed from feld- 

 spar is usually well stocked with potash. 



Orthoclase, AlKSi 3 O s Potash Feldspar. 



Albite, AlNaSi 3 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 

 members of the alkali family in hornblende. The au- 

 gites are double silicates of iron, manganese, calcium, 

 and magnesium. Quite frequently phosphoric acid is 

 present in chemical combination with the iron. The 

 members of this group are readily distinguished by 

 their color wmich is black, browm, or brownish green. 

 The hornblendes are insoluble in acids, hence unavail- 



