CLASSIFICATION OF SOILS 



75 



Although somewhat easily soluble, lime is a very abundant con- 

 stituent of soils of arid regions. The amount varies* from 0.(>9 to 

 5.G6 per cent. The table below gives the average for arid and humid 

 soils. 



Lime and Magnesia in Soils of Different Regions * 



These arid soils are generally gray or light in color, with no very 

 decided change in texture in the subsoil. The rainfall is not suffi- 

 cient to carry the fine material downward in any large amount. 



The organic-matter content of arid soils is generally low, 

 although it contains a larger percentage of nitrogen than the organic 

 matter of humid regions. 



(b) Humid Soils. In this group of soils the agencies of 

 decomposition have predominated over those of disintegration. The 

 feldspar and many other minerals have largely undergone chemical 

 change, producing the finer soil constituents. Hence the soils con- 

 tain large amounts of clay. The subsoil differs quite noticeably in 

 texture from the surface. Leaching has done very effective work 

 in removing soluble salts, as shown by the second table on page 74. 

 Limestone has been leached out, and most of the soils are acid and 

 in great need of this most important constituent. The colors are 

 more highly developed, due to the greater oxidation of iron and the 

 larger amount of organic matter. 



5. Vegetation. Xot only do bacteria, fungi, and alga 1 play a 

 very important part in soil formation and soil changes, but the 

 higher plants, especially grasses and trees, exert a most important 

 influence upon soil in several ways. They are responsible to a very 

 large extent for the amount of organic matter in soils. The char- 

 acter of the vegetation has given rise in humid and subhumid 

 regions to two great groups of upland soils, (a) prairie, and (b) 

 timber (Fig. 63). 



(a) Prairie Soils. Prairie soils are usually characterized by a 

 dark color, due to a large content of organic matter. The prairies 

 were covered with a rank growth of grasses which produced a dense 



