04 SOILS AND FERTILIZERS 



In some localities volcanic soils are found. They are 

 extremely varied in texture and composition; some are 

 very fertile and contain liberal amounts of alkaline salts 

 and phosphates, while others contain so little plant food 

 that they are sterile. 



ROCKS AND MINERALS FROM WHICH SOILS ARE 



FORMED 



66. Composition of Rocks. Rocks are composed of 

 either a single mineral or of a combination of minerals. 

 Most of the common minerals are definite chemical 

 compounds and have a varied range in 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 derived contain minerals, as 

 feldspar, mica, hornblende, and quartz. 



67. Quartz. Quartz is the principal constituent of 

 many rock formations. Pure quartz is silicic anhydride, 

 SiO 2 , and a soil formed from pure quartz alone would 

 be sterile. White sand is nearly pure quartz or silica. 

 Silica enters into combination with many elements, 

 forming a large number of minerals. Particles of quartz 

 when cemented with iron compounds form sandstone 

 rock. Sand is derived mainly from the decay of rocks 

 containing quartz. 



68. Feldspar is composed of silica, alumina, and 

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



