76 SOILS: PROPERTIES AND MANAGEMENT 



carbon dioxide. As 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 may exist, due to its solubility ; 

 yet from the oxidation that it undergoes, it tends to 

 persist and accumulate in soils. A corollary might be 

 added to the law of mineral resistance, to the effect that 

 " the more iron a mineral contains, the more susceptible 

 it is to the weathering agencies." 



Therefore, from the geochemical standpoint, the de- 

 velopment of the red and yellow colors in soils has been 

 the subject of considerable dispute from time to time. 

 The red and yellow soils of the Cotton States frequently 

 excite comment, especially as a difference in fertility is 

 popularly recognized ; the red surface soil with a red 

 subsoil being considered more fertile than a similar soil 

 with a yellow subsoil. Crosby 1 believes that the differ- 

 ence in color is due to a difference in hydration of the 

 iron oxides. The soil temperatures, particularly in 

 tropical and subtropical regions, have first tended to 

 fully 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. The ultimate product of both oxidation and 

 hydration would be limnite, a yellow mineral; while 

 if only oxidation were active, hematite, which imparts 

 a red color, would result as a final product. A dehydra- 

 tion of the limnite Avould cause the formation of hematite 



1 Crosby, W. O. Colors of Soils. Proc. Boston Soc. Nat. Hist., 

 Vol. 23, pp. 219-222. 1875. 



