68 SOILS AND FERTILIZERS 



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

 moved and water of hydration is taken up, forming the 

 product kaolin, which is hydrated silicate of alumina, 

 Al 4 (SiO 4 ) 3 . H 2 O. Impure varieties of clay are colored 

 red and yellow owing to the presence of iron and 

 other impurities. Pure kaolin is white, is insoluble in 

 acids, and is incapable of supplying any nourishment to 

 plants. Clay soils are fertile on account of the other 

 minerals and organic matter mixed with the clay and are 

 usually well stocked with potash because of its incom- 

 plete removal from the disintegrated feldspar. It is to 

 be observed that the term ' clay ' used chemically means 

 aluminum silicate, while physically it is any substance 

 the particles of which are less than 0.005 mm - m diameter. 



75. Limestone. Limestone is present in many sec- 

 ondary rocks. It is composed of calcium carbonate and 

 is slowly soluble in water containing carbon dioxide. 

 Extensive deposits of calcium carbonate, as limestone, 

 marble, and chalk, occur in nature. It is widely dif- 

 fused in soils, and is a constituent that imparts fertility. 

 Many soils contain appreciable amounts of disintegrated 

 limestone. 



76. Disintegration of Rocks and Minerals. In ad- 

 dition to the rocks and minerals which have been 

 mentioned, there are many others that contribute to soil 

 formation, as glauconite, a hydrated silicate of iron ; 

 alumina and potash; limonite, a hydrated oxide of 



