GEOLOGICAL CLASSIFICATION OF SOILS 41 



The following analyses^ show the general chemical char- 

 acter of surface residual soils and the variations that may 

 be expected: 



Table V 



Constituents 



1 



2 



3 



4 



SiOs 



66.49 



76.71 



74.33 



70.99 



AI2O3 



17.11 



12.85 



11.00 



11.39 



Fe^Os 



7.43 



2.81 



4.64 



4.23 



TiO^ 



1.02 



.41 



1.04 



1.28 



CaO 



.36 



.08 



1.13 



.93 



MgO 



.31 



.29 



.69 



1.08 



K,0 



.62 



3.26 



1.57 



2.71 



Na^O 



.16 



.89 



1.53 



.82 



P2O. 



.17 



.05 



.16 



.19 



SO3 



.07 



.12 



.15 



.34 



Organic matter 



1.26 



1.78 



1.99 



.93 



The organic matter of residual soils largely depends, in 

 amount and condition, on climatic factors. If rainfall and 

 temperature, for example, are favorable for the rapid and 

 continued development of a natural vegetation the soil will 

 be rich in humus, so rich at times as to mask to a certain 

 extent the red color so characteristic of such soils. If plants 

 do not grow well on this soil, however, it will be low in organic 

 matter and probably in poor physical condition. Eesidual 

 soils vary greatly in their general characteristics, especially as 

 to crop productivity. 



Eesidual soils are of wide distribution in the United States, 

 particularly in the eastern and central parts, although great 



* Robinson, W*. 0., The InorgaTiic Composition of Some Important 

 American Soils; U. S. Dept. Agr., Bui. 122, Aug. 1914. 



1. Cecil clay, from granite and gneiss. Charlotte, N. C. 



2. York silt loam, from schists. Bethany, S. C. 



3. Penn silt loam, from sandstone. Morristown, Pa. 



4. Hagerstown loam, from limestone. Conshohocken, Pa. 



