36 NATURE AND PROPERTIES OF SOILS 



23. Red and yellow colors of soil. 1 — The presence of 

 iron, as already noted, is a very important factor in rock 

 weathering, and the discoloration due to its presence is an 

 unfailing indication of chemical decay. The iron in minerals 

 occurs usually as ferrous oxide, which is soluble, especially 

 if the water circulating among the rock fragments carries 

 carbon dioxide. When this water comes in contact with the 

 air its excess of carbon dioxide is discharged and the oxides 

 and carbonates of iron are deposited. Under this condition 

 oxidation goes on rapidly, and the iron passes to the ferric 

 state and becomes insoluble. Thus it may be seen that iron 

 imparts a fatal weakness to rocks and minerals in which it 

 exists, due to its solubility; yet from the oxidation that it 

 undergoes it tends to persist and accumulate in soils. The 

 more iron a mineral or rock contains the more susceptible it 

 is to weathering. 



The red and yellow soils of the southern states frequently 

 excite comment, especially as a difference in fertility is popu- 

 larly recognized, the red surface soil with a red subsoil being 

 considered more fertile than a similar soil with a yellow sub- 

 soil. This is probably due to differences in hydration of 

 the iron oxides. 2 



The soil temperatures, particularly in tropical and sub- 

 tropical regions, have first tended fully to oxidize and hydrate 

 the iron, and then to dehydrate the soil at the surface into 

 the deep red color, leaving the subsoil yellow and causing 

 the contrasts so markedly evident. Soils having a yellow 

 surface soil are generally considered to be older and more 

 weathered than those where the red is well developed. When 



1 Eobinson, W. O., and McCaughey, W. J., The Color of Soils; U. S. 

 Dept. Agr., Bur. Soils, Bui. 79, p. 21. 1911. 



2 Crosby, W. O., Colors of Soils; Proc. Boston Soc. Nat. Hist., Vol. 23, 

 pp. 219-222. 1875. Merrill, G. P., Boclcs, Rock Weathering and Soils; 

 p. 375. New York, 1906. Van Bemmelen, J. M., Beitrage zur Eenntnis 

 der VerwitterungsproduTcte der Silicate in Ton-, VulJcanischen-, und 

 Laterite-Boden; Zeit. Anorg. Chem., Bd. 42, Steite 290-298. 1904. 



