THE SOIL AND THE PLANT 



face, is that horizon from which certain material has been removed 

 by mechanical or chemical means. B, is that horizon into which 



material has been car- 

 ried. C, designates the 

 parent material. These 

 horizons are frequently 

 subdivided into Ai, A2, 

 etc. The microbiologi- 

 cal processes in the soil 

 are carried out largely in 

 the A horizons, and it is 

 here that most of the 

 plant remains become in- 

 corporated. A consid- 

 eration of the composi- 

 tion of the various hori- 

 z o n s reveals marked 

 differences, especially in 

 the content of organic 

 matter, as shown in 

 Table 1. 



Weathering of 

 Rocks. — The surface of 

 the earth is modified in 

 ] physical appearance so 

 slowly or in such ways 

 that one is inclined to 

 create a mental picture 

 of the soil as a static or 

 fixed formation. Violent 

 natural changes are so 

 few or so seldom noticed that the transformations which we may 

 read in the rocks and soils as occurring through geologic ages 

 appear to be widely separated from the times we live in. Only 

 as our attention may become attracted by floods, earthquakes, 

 volcanoes, or glaciers do we begin to appreciate the fact that 

 physical forces are active in modifying the surface of the earth. 



Such phenomena and others of a less apparent nature have 

 been active in producing the earth's surface as we know it, and 

 are active continually in creating new changes. The layer of fine 



Fig. 1. — Profile of podsol soil with raw humus. 

 The light-colored leached layer (A2) so typical 

 of a podsol, is clearly apparent (from Tamm). 



