THE SOIL 35 



ferric oxide is usually present, replacing a portion of the 

 alumina, and the potash is often partially replaced by magnesia, 

 lime, or soda. Mica also suffers decomposition under the in- 

 fluence of the weather, but not so readily as felspar. It 

 furnishes plant food in the potash, iron and lime which it con- 

 tains. Its amount in the earth's crust has been estimated at 

 8 per cent. 



Silicates of Magnesia are also very abundant, the magnesia 

 generally being partially replaced by lime, ferrous oxide, or man- 

 ganese oxide. Talc and steatite may be taken as typical, their 

 composition being represented by the formula 6Mg0.4Si02.H,0 

 Hornblende and augite, (Mg.Ca.Fe.Mn)0Si02, are also very 

 abundant. They usually contain alumina and ferric oxide. 

 These minerals are easily acted upon by air and water, 

 and often yield brightly coloured (due to presence of iron) 

 clays. 



Calcium Carbonate occurs in a great many crystalline 

 forms in the varieties of calcite (rhombohedral),and arragonite 

 (rhombic), also in the massive form as chalky li'inestone and 

 marble. These are all essentially CaOO,, but the calcium is more 

 or less replaced by magnesium, and, moreover, most forms of 

 calcium carbonate contain notable quantities of phosphoric 

 acid. Calcium and magnesium carbonates, though only slightly 

 soluble in pure water,* are readily soluble in water containing, 

 as is the case with nearly all forms of natural water, carbon 

 dioxide. Rocks containing these substances, therefore, are 

 quickly eroded by exposure to the atmosphere. Calcium car- 

 bonate is of great importance in soils, both on account of its 

 providing plant food and because of its relationship to many 

 of the processes which go on in soils. 



Clay in its pure form is hydrated silicate of alumina, 

 AljO3.2SiOa.2H2O, and is therefore devoid of plant food. 

 Ordinary clay, however, contains iron oxide and potash, the 

 latter remaining from the felspar from which most clays have 



