42 Agricultural Chemistry. 



grains together, is dissolved away and the residual fragments 

 then readily crumble. 



A soil produced by mere mechanical pulverization of the rocks 

 would not furnish proper food for the higher plants. This can 

 readily be imagined if one thinks how unsuitable crushed granite 

 would be for plant production. The essential elements locked 

 up in these insoluble soil-forming materials must be changed into 

 materials that the plant can assimilate and water is an important 

 factor in bringing about such chemical changes. The minerals 

 forming our igneous rocks are, however, very slightly soluble in 

 pure water; but the water that enters the ground has dissolved 

 in it small amounts of carbon dioxide, derived from the air, and 

 water containing this gas will dissolve these minerals in appre- 

 ciable quantities. 



Another important agent in soil formation is the air, which 

 acts in several ways. 



Mechanically. Wind actually detaches large projecting pieces 

 of rock in mountainous districts and sends them crashing down 

 onto the rocks below. In addition, by hurling sand and small 

 peebles against the surface of rocks it brings about the erosion of 

 the latter. In most cases the effects of this form of erosion are 

 masked and hidden by those of other denuding agencies. 



Chemically. In many rocks are minerals capable of taking up 

 oxygen. On exposure to air, oxidation occurs and the mineral 

 swells up and often crumbles to powder, thus loosening the other 

 minerals in the rocks. This oxidation is in many cases accom- 

 panied by a change in color, from green or gray to yellow or 

 red. The carbon dioxide of the air also acts corrosively on car- 

 bonates in the presence of water. 



Animals are also important agencies in soil formation. Bur- 

 rowing animals, as for example, rabbits, moles, etc., admit air 

 into soil or sand and thus favor the changes which air produces. 

 The part played by the humbler creatures, earth worms, is prob- 

 ably much more important. They bring portions of the subsoil 

 to the sin f;icc. they draw dead leaves and other vegetable refuse 



