ACTION OF VEGETATION 49 



chemical reactions in the soil which result in render- 

 ing plant food available. 



53. Action of Air and Gases. In the disintegra- 

 tion of materials to form soil, air takes a prominent 

 but less important part than water. By the aid of 

 oxygen, carbon dioxide, and other gases and vapors 

 in the air, rock disintegration is hastened. The action 

 of oxygen changes the lower oxides to higher forms. 

 All rock contains more or less oxygen in chemical 

 combination. The carbon dioxide of the air under 

 some conditions favors the formation of carbonates. 

 The disintegrating action of air, moisture, and frost is 

 illustrated in the case of building stones which in 

 time crumble and form a powder. This is called 

 weathering. Many of the benefits of cultivation are 

 due to aeration of the soil. 



54. Action of Micro-organisms. Micro-organisms, 

 found on the surface and in the crevices of rocks, and 

 in decaying vegetable matter, are active agents in 

 bringing about rock decay. The nitrifying organisms 

 have taken an important part in rendering soils fer- 

 tile, and these with others have aided in soil forma- 

 tion. Some of the organisms found on the surface of 

 rocks are capable of producing carbonaceous matter 

 out of the carbon dioxide and other compounds of 

 the air. 21 This action results in adding vegetable 

 matter to the soil. 



55. Action of Vegetation. Some of the lower 

 forms of plants as lichens do not require soil for 

 growth, but are capable of living on the bare surface 



(4) 



