PHYSICAL PROPERTIES OF SOILS 177 



stone outcrops are indicated by dark soils. Coifey 1 speaks of being 

 able to trace an outcropping limestone stratum by the dark color 

 of the soil, and the same tiling has been observed in the southern 

 part of Illinois in the acid soils of that region. In soils of arid 

 regions limestone is frequently so abundant as to impart a light 

 color. 



Iron oxides give various colors to the soil, depending upon the 

 degree of oxidation. Ferric oxide (Fe.,0.,) imparts a bright reddish 

 color. Due to the presence of this oxide, many of the subsoils of the 

 Piedmont Plateau are decidedly red in color. Tbe hydrated ferric 

 oxide (2Fe.,0 3 .;3H..()) imparts a dull yellowish color to the soil, but 

 is 'sometimes mixed with the anhydrous ferric oxide, giving a red- 

 dish yellow or yellowish red color, depending upon which predomi- 

 nates. In some cases, deoxidation has occurred through the effect 

 of organic acids, or some other agency, and the higher oxides of 

 iron have been reduced to the lower form. This gives a bluish, 

 grayish or drab color to the soil. This is especially true in acid 

 soils, in poorly drained ones, and in subsoils beneath peat, peaty 

 loam or muck. In the latter case the iron has been deoxidized so 

 completely that the soil usually presents a uniformly light drab 

 color. The most striking effect of deoxidation is seen in the acid 

 soils where drainage is intercepted by an impervious clay stratum. 

 The iron is so completely deoxidized that the subsurface stratum 

 is frequently white. 



Soil constituents themselves in some cases may impart color 

 to the soil, as where an abundance of quartz sand is found, giving 

 the soil a grayish or whitish cast. Sometimes mica is sufficient! v 

 abundant to produce a glittering appearance in the soil. In sonic 

 parts of the Piedmont Plateau the mica formerly existed in granitic 

 rocks in large crystals from one-half to one and one-half inches in 

 diameter. When the rock decomposed, the mica remained as large 

 flakes, giving the soil a glittering appearance. The color of soils 

 may undergo some change, usually due to the loss of organic matter 

 through cropping, but mostly because of erosion, producing yel- 

 lowish brown or yellow color. 



5. Odor of Soils. As a general rule soils possess a distinct but 

 feeble odor, due to a very small amount of an organic compound of 

 the aromatic family and analogous to that of the camphorated 

 bodies. A very minute quantity is present, there being only a few 

 millionth* ~ of a per cent. 

 12 



