4 THE SOIL AND THE PLANT 



the chemical changes involved in the weathering of orthoclase 

 and olivine, two rock-forming minerals: 



2KAlSi308 + 2H2O + CO2 = H4Al2Si209 + K2CO3 + 4Si02 



Orthoclase Water Carbon Kaolinite Potas- Silica 



dioxide sium 



carbonate 



12MgFeSiOi + 26H2O + 3O2 



Olivine 



= 4H4Mg3Si209 + 4Si02 + 6Fe203-3H20 



Serpentine Silica Limonite 



The soils thus owe their composition largely to the rocks from 

 which they are formed. Rocks are not homogeneous substances, 

 but aggregates of minerals which themselves are chemical entities 

 and which vary in complexity, from the elements, as graphite (C) 

 and free iron (Fe), to complex molecules like muscovite mica 

 (Al3KH2Si30i2). The relative abundance of any one or groups 

 of these minerals in the particular rock and the degree of their 

 consolidation determine not only the nature of the rocks but also 

 the soils which are formed from them. 



TABLE 2 



« 

 The Relative Abundance of Chemical Elements Existing in Greatest 

 Quantities in the Earth's Crust (from Clarke) 



Element Per Cent 



Oxygen 47 . 33 



Silicon 27.74 



Aluminum 7 . 85 



Iron 4 . 50 



Calcium 3 . 47 



Magnesium 2 . 24 



Sodium 2.46 



Potassium 2 . 46 



All others 1 . 95 



Total 100.00 



The common soil minerals contain only 21 of the known 

 chemical elements. Eight of these elements compose 98 per 

 cent of the total mineral matter of the earth's crust, as shown in 

 Table 2. The five elements, hydrogen, sulfur, carbon, titanium, 

 and phosphorus occur in many important minerals, and each 

 comprises from 0.1 to 0.5 per cent of the inorganic part of the soil. 

 The remaining eight (fluorine, chlorine, zirconium, boron, nitro- 



