CLIMATIC AND GEOCHEMICAL RELATIONSHIPS 79 



Red colors in soil often show low or medium organic 

 content. Besides this, the presence of oxidized iron is 

 always an indication of age. The residual soils of the 

 Piedmont Plateau are especially characterized in this 

 way. Age gives opportunities for leaching, and conse- 

 quently a lack of the soluble bases may be expected in 

 such soils. The reds and the yellows are characteristic 

 of residual soils, or of soils that have arisen from them 

 by erosion or glaciation. A red color is almost as efficient 

 in the absorption of heat as is black; so that the early- 

 growth and quick-maturing tendencies of crops on a 

 red soil, other things being equal, are about the same as 

 on a dark soil. Hilgard, 1 who lays great stress on the 

 agricultural significance of color, considers the mottled 

 yellows and reds as indications of poor drainage, since 

 such a condition shows that oxidation has been both 

 unequal and insufficient. A soil that has a heavy blue 

 or mottled blue clay as a subsoil will in most cases be 

 greatly benefited by some form of drainage. 



60. Soil and subsoil. — A common distinction is made 

 between the surface soil and that which is some distance 

 below the surface. This is natural, as the forces of soil 

 formation have served to bring about certain distinctions, 

 especially in humid regions, which are of importance in 

 any consideration of soil fertility and crop growth. Cli- 

 matic agencies acting on soil after it has been formed have 

 served to intensify these distinctions. The term soil 

 is used to designate the top layer of earth, which usually 

 extends to the plow line or even deeper. The soil below 

 is spoken of as the subsoil, and may be rather variable in its 

 depth (Fig. 10) . Often the subsoil is divided into the upper 



1 Hilgard, E. W. Soils, pp. 283-285. New York. 1906. 



