32 Effective Farming 



subsoil usually contains a relatively small percentage of humus ; 

 consequently it is somewhat light in color. 



13. Formation of soil. Geology teaches that the earth 

 was once a molten mass, that it gradually cooled and formed 

 solid rock. From this solid rock the soil has been formed by 

 various weathering agents that have been at work for ages. 

 The time element is a very important factor and should be kept 

 in mind constantly when considering soil formation. The chief 

 weathering agents responsible for rock decay are the atmos- 

 phere, heat and cold, intermittent freezing and thawing, gla- 

 ciers, water, and vegetation. 



14. Classification of soil according to mode of formation. 

 The materials of which a soil has been formed are either seden- 

 tary or transported. Sedentary materials have not been shifted 

 far from the place of origin. Most transported materials have 

 been moved a considerable distance by such agencies as gravity, 

 water, wind, and glaciers. Consequently according to their 

 mode of formation, soils are either sedentary or transported. 

 Each of these groups is divided as follows : 



Sedentary Soils: Transported Soils: 



Residual Colluvial 



Cumulose Alluvial 



Marine 



Lacustrine 



Glacial 



J^olian 



Residual soils. Those soils that were formed in their pres- 

 ent locality from the disintegration of rocks are residual. The 

 rocks found underneath them are like those from which the 

 soil has been made. These are the oldest agricultural soils in 

 the United States. They are found in the eastern and central 

 parts and include the Piedmont Plateau, the Appalachian Moun- 

 tains, the Limestone Valleys, and the Great Plains regions. 



Cumulose soils. Peat bogs and muck soil areas are cumu- 



