CHEMICAL COMPOSITION OF THE SOIL 



179 



tinued leaching of the soils in humid regions washes out the plant 

 food. It may also be partly due to the difference in origin, as 

 many of the humid soils are coastal deposits worn by the water, 

 while many soils in the arid region are comparatively new soils 

 from igneous rocks. 



Soils and Subsoils. If we compare the composition of soils 

 with the corresponding subsoil, we find, almost always, that the 

 subsoil contains less nitrogen than the surface soil. It often con- 

 tains more potash, more oxide of iron and alumina, and less in- 

 soluble material, than the surface soil. This difference is largely 

 due to the percolating water carrying the finer particles of the 

 soil (clay) into the subsoil. There is, however, a good deal of 

 difference in soils in this respect. 



Relation of Composition to Type. Soils of different type differ 

 to some extent in chemical composition, though such is not in- 

 variably the case. There is also some variation in individual 

 members of the type. Differences in composition are usually 

 accompanied by differences in properties, productiveness or value 

 of the soil. 



AVERAGE PERCENTAGE COMPOSITION OF SOME TYPICAL TEXAS SOILS. l 



Detailed analyses and discussion of the soils of the United 

 States are to be found in Bulletin 57 of the Bureau of Soils, and 

 in "Soil Fertility and Permanent Agriculture" by C. G. Hopkins. 

 1 Bulletin 126, Texas Station. 



