SALTS 41 



of mineral particles containing the necessary mineral ele- 

 ments explains why they are generally deficient in potassium 

 and phosphorus. 



Muck soils usually contain more potassium and phosphorus 

 than peats, because these soils contain considerable mineral 

 matter (Chapter II). 



MORE ABOUT WEATHERIN-G 



Products of Rocks Weathering. — We have learned that the 

 framework of mineral soils ^ consists of sand, silt and clay — the 

 common products of rock weathering. 



When a granite, for example, is transformed by weathering 

 into soil, many chemical changes occur during the transformation. 

 A residual soil from granite is not merely a powdered form of that 

 rock, represented by a combination of sand, silt and clay. Many 

 of the soil particles, it is true, are the same kind of mineral particles 

 as are found in the granite; others, however, are quite different 

 from any that ever occurred in the parent rock. The formation 

 of the new kinds, or secondary minerals, is a result of the chemical 

 changes, or decay. In the decay, or chemical changes, many of 

 the complex rock minerals are split up, chemically, and their ele- 

 ments recombine |n different ways, or unite with other elements, 

 water and gases. 



In addition, therefore, to the common products of rock weather- 

 ing — sand, silt and clay, there are formed other products, among 

 which are true clay (kaolin), carbonate of lime, and salts. 



True clay (kaolin) is a definite, fine material originating mainly 

 through the decay of feldspars and micas. 



Carbonate of Lime, or lime carbonate, is hme combined with 

 carbon dioxide gas. (CaO + CO2). It is formed in rock decay 

 when the calcium (Ca) in the rock enters into a new combination 

 with and through the action of carbon dioxide and water. 



The mineral calcite is a pure form of carbonate of lime. Lime- 

 stone, shells, coral, marble, dried lime mortar, air-slaked lime and 

 marl are other forms of material containing lime carbonate. 



Salts. — ^Among the many salts formed duiing the process of 

 rock decay are: 



Chlorides. — Common table salt is an example. 



Sulfates, — Glauber's salt is an example. 



3 Soils made up of materials derived from rocks are commonly referred to 

 as '* mineral soils." 



