CLASSIFICATION OF SOILS 53 



The carbon dioxide of the air under some conditions 

 favors the formation of carbonates. The disinte- 

 grating action of air, moisture, and frost is illustrated 

 in the case of building stones which in time crumble 

 and form a powder. The combined action of air, 

 moisture, and frost is called weathering. 



54. 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 

 of rocks, obtaining food from the air, and leaving a 

 certain amount of vegetable matter which under- 

 goes decay and is incorporated with the rock parti- 

 cles, preparing the way for higher orders of plants 

 which take their food from the soil. When this 

 vegetable matter decays it enters into chemical com- 

 bination with the pulverized rock, forming humates. 17 

 The disintegrating action of plant roots and vegetable 

 matter upon rocks has been an important factor in 

 soil formation. 



55. Action of Micro-organisms. — Micro-organisms, 

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

 considered by many as active agents in bringing about 

 rock decav. The nitrifvino- organisms have taken 

 an important part in rendering soils fertile, and these 

 with others have without doubt- 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 



