82 



The Substratum 



The process involved in producing and maintaining a typical soil 

 profile are indicated schematically in Fig. 3.9. Here is shown one 



Horizons 

 r— - — I 



True soil 



^1 



Ao 



Litter of undecomposed organic debris 



Organic debris partly decomposed or matted 



High content of organic matter 

 mixed with minerals 



Zone of maximum leaching 

 Transitional 



Zone of concentration of transported 

 fine particles 



Weathered or unconsolidated parent material 



Rock or unmodified parent material 



Fig. 3.8. The soil profile with its principal subdivisions shown by a vertical sec- 

 tion through the soil. 



member of a stand of trees that has grown in the area for a long time 

 and that has produced in the soil the characteristic layering. Per- 

 haps the most important of the climatic agents is the moisture factor, 

 and this involves both rainfall and evaporation. Where the land is 

 sloping a portion of the rain runs off over the surface, sometimes wash- 

 ing away organic materials or even some of the soil itself. The rain 

 that enters the ground percolates through the pore spaces carrying 

 fine particles and dissolved salts with it. In some places the ground 

 water drains completely through the soil into the strata beneath, with 

 the result that the dissolved materials may be lost from the soil. 

 With excessive rainfall valuable constituents may thus be leached 

 from the upper horizons. Other portions of the ground water enter 

 the roots of the vegetation and are carried upward again. Water 

 vapor moves through the pore spaces and evaporates from the sur- 



