218 



ANALYSIS OF THE ENVIRONMENT 



from the same parental material, or some 

 may have been transported and deposited 

 after a partial evolution elsewhere. The soil 

 horizons in a given profile are in continual 

 interrelations with each other; even the 

 petrified hardpans, almost impervious to 

 water and to animals, are a fossifized end 

 product of soil dynamics. 



The different horizons are in direct con- 

 tact with their immediate neighbors and are 

 easily influenced by them. They are affect- 

 ed by those at a greater distance (a) by the 

 processes associated with growth and decay 



loess may settle on water-deposited sand or 

 gravel that in turn rests on unstratified gla- 

 cial till, and this finally may have been 

 superimposed on soil that evolved in situ 

 or on bedrock. These different materials 

 have certain "inherited" values, as con- 

 trasted with others acquired as a result of 

 pedogenic processes. 



The most concentrated human popula- 

 tions of the earth five on fertile deltas near 

 the mouths of the larger rivers. The soils 

 that support these dense populations were 

 first and in the main air-deposited collec- 



Undulating Flat 



Hilly 



Swamp 



PLANOSOL 



NORMAL (CLAYPAN) "'"'^"^ SOLUM HALF BOG BOG 



Fig. 55. Soil profiles from similar parental materials, but developed in regions of diflFerent 

 surface relief (see legends in Figure 54). (Redrawn from Byers, Kellogg, Anderson, and 

 Thorp. ) 



of penetrating root systems, (b) by the bur- 

 rowing of animals, and, most significantly, 

 (c) by water transport of dissolved or sus- 

 pended matter. This last process is called 

 eluviation, especially as concerns the trans- 

 port of colloids. Eluvial horizons have lost 

 material; illuvial ones have gained it. The 

 water transport may be downward or side- 

 wise, depending on the direction of water 

 movement through the soil. 



The soil profile in part reflects features of 

 surface rehef as well as parental material. 

 As Figure 55 shows, shallow soils develop 

 in hilly regions with accompanying exces- 

 sive run-off and erosion. Flat land has little 

 or no erosion and favors the development 

 of leached upper soil and a dense claypan. 

 Low-lying regions with poor drainage favor 

 accumulations of humus. 



The soil profile evolves from different 

 layers of stratified or unstratified material 

 mechanically superimposed one on another, 

 as well as from underlying rock. Air-borne 



tions of loess. They were then eroded away 

 by running water and carried along until 

 they became water-deposited deltas. Other 

 rich soils hke those of Iowa and the pam- 

 pas of northern Argentina are air-deposited 

 collections of loess formed from water 

 deposited materials. Much of the rich soil 

 in the 'granaries of the world' has been 

 transported and deposited by wind (Hobbs, 

 1943). 



Soil has solid, aqueous, and gaseous 

 phases. Soil solids form the skeletal frame- 

 work composed of bits of rocks and of 

 minerals and their decomposition products; 

 they range in size down to ultramicroscopic 

 colloidal particles. The solid phase has 

 much to do with determining soil texture, 

 which, in turn, is closely related to poros- 

 ity, a factor that is structurally determined 

 by the ratio of pores to soil solids. Soil 

 fluids, both aqueous solutions and gases, 

 flow through the interconnected pores, 

 whether these are relatively large or capil- 



