64 THE PRINCIPLES OP SOIL MANAGEMENT 



physical properties. They are also likely to be highly 

 silicious. But the loess formations are of great agricul- 

 tural importance, and in this country they constitute 

 some of the most important soil types. In some sections 

 its value has been greatly reduced by erosion. Some of 

 the bluff areas along the Mississippi river are thus 

 modified, and some of the loess of China is also deeply 

 eroded. 



But the physical properties, as well as the chemical 

 properties of loess, combine to give it in general a 

 high agricultural value. 



VI. HUMID AND ARID SOILS 



In discussing the process by which soil is derived 

 from rock, attention was directed to the fact that phys- 

 ical disintegration results in material having different 

 properties from those derived through chemical decom- 

 position, and that the relative prominence of these 

 two processes is dependent largely on climate. Aridity 

 is one of those phases of climate which markedly alters 

 the balance between these two processes, giving the 

 larger ascendancy to the physical. Soils formed under 

 arid conditions are less fine in texture than those formed 

 from the same rock in humid regions. A study of soils 

 in the two regions reveals a much greater prevalence of 

 the coarser soils the sandy and loamy soils in the 

 arid region. 



But chemical processes are not absent, for in every 

 arid region there is some precipitation which is able to 

 bring about changes in the minerals, although the 



