186 



THE SOIL: INORGANIC MATTER 



calcium, but which have lost a large part of this constituent 

 during decomposition. 



Of course, there may be particles of limestone and calcium- 

 containing silicates left in the soil, and these serve as sources 

 of calcium for a time, but nevertheless, limestone soils are 

 among the first to become acid, due to lack of calcium car- 

 bonate. 



Fig. 40. — Soil-forming rock: Limestone. 



Survey. 



United States Geological 



(d) Sandstones and Shales form sandy and clayey soils, 

 respectively, containing more or less plant food according 

 to their original composition and derivation, for they are 

 secondary rocks derived from decomposition products of 

 others. Sandstones are more apt to yield poor soils unless 

 it happens that the binding material is of a clayey 

 nature. 



144. Kinds of Soil. — The number of different kinds of 

 soil is necessarily very great, and any attempt to classify 

 them is a very considerable undertaking. Ordinarily they 

 are divided according to their geological origin, but they may 



