COLOR OF SOILS 101 



the assistance of various organic acids, and are again 

 deposited where there is some change in physical con- 

 ditions. This form is most common in the unglaciated 

 section of the country in the older deposits. Some of the 

 red soils of the coastal plain region exhibit a strong 

 tendency to "case-harden," that is, become quite 

 hard at the surface upon drying, largely due to iron 

 compounds. The fourth cementing material is silica, 

 and is less prominent in soil practice than the other 

 three cementing materials mentioned. It is the binding 

 material in most sandstones and quartzite rock, as an 

 advanced stage of silica infiltration. 



All these cementing materials except the iron com- 

 pounds, which are red, yellow or brown, are light- 

 colored. 



40. Color. A great variety of colors are exhibited 

 by soils. These are not usually the result of the color 

 of the individual particles which make up the bulk 

 of the material. Rather, it is usually the result of 

 material which adheres to the particles. 



There are two chief coloring materials in soil. These 

 are iron compounds and organic matter. The first gives 

 rise to red, yellow, blue and gray colors. The latter 

 gives rise to some shade of black or brown color. When 

 these are combined, various intermediate tints are 

 obtained. For example, when a red soil is rich in decayed 

 organic matter humus it becomes of a rich brown 

 color. 



The color of soils, especially as regards iron com- 

 pounds, is not fully understood, but it is safe to say 

 that much color is the result of different forms of iron 



