The Soil. 37 



the iron, potassium and calcium it contains. Its amount in the 

 earth's crust has been estimated at 8 per cent. 



Silicates of magnesia are also very abundant. Talc and 

 steatite are representatives of this class and are compounds of 

 magnesium, silicon and oxygen, designated by the chemist "sil- 

 icates of magnesia. ' ' They also contain water. When the mag- 

 nesium is partly replaced by other elements, as calcium, iron or 

 manganese, we have the distinct minerals known as hornblende 

 and augite. All the minerals of this class are easily acted upon 

 by water and air and often yield brightly colored clays due to 

 the presence of iron. 



Calcium carbonate occurs in a great many crystalline forms, 

 the principal variety being called calcite, and in the massive form 

 is known as chalk, limestone and marble. These are all essen- 

 tially made of calcium, carbon and oxygen, but in certain local- 

 ities the calcium is more or less* replaced by magnesium which 

 then gives us the mineral known as dolomite. This is true of 

 many of the "limestones" found in Wisconsin. Most calcium 

 carbonates contain notable quantities of phosphoric acid. Cal- 

 cium and magnesium carbonates, though only slightly soluble in 

 pure water, are readily soluble in water containing, as in the case 

 of nearly all forms of natural water, carbon dioxide. Rocks con- 

 taining these substances therefore are quickly eroded by exposure 

 to the atmosphere. Calcium carbonate 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 a hydrated silicate of aluminum and 

 is therefore devoid of plant food. By the term hydrated we 

 mean that the compound of silicon, aluminum and oxygen (sili- 

 cate of aluminum) is joined to a certain amount of water. Or- 

 dinary clay, however, contains iron and potassium, the latter re- 

 maining from the feldspar, from which most clays have been 

 formed. It therefore supplies potassium to plants. Its physical 

 properties are very- important and greatly influence soils in 

 which it is abundant. 



