84 SOILS: PE0PERTIE8 AND MANAGEMENT 



either disappeared or changed their combinations to more 

 stable fornas. The tendency of all soils is toward a con- 

 dition of equilibrium, and consequently they approach, 

 but never reach, a common composition. This does not 

 apply to their productivity, because many other factors 

 besides chemical composition go to determine cropping 

 power. Residual limestone soils, therefore, become poorer 

 and poorer in lime as their age increases. The organic 

 matter of residual soils largely depends, in amount and 

 condition, on climatic factors. If rainfall and tempera- 

 ture, for instance, are favorable for the rapid and con- 

 tinued development of a natural vegetation, the soil 

 will be rich in humus, so rich at times as to mask to a 

 certain extent the red color so characteristic of such soils. 

 If plants do not grow well on this soil, however, it will be 

 low in organic matter and probably in poor physical con- 

 dition, so vital is humus to a proper foothold for plant 

 life. Two residual soils coming from the same kind of 

 rocks may vary rather widely in their general character- 

 istics, especially as to crop productivity. 



26. Distribution of residual soils. — Residual soils are of 

 wide distribution in the United States,^ particularly in the 

 eastern and central parts. A glance at the soil map of this 

 country (See Pig. 4) shows four great provinces — the Pied- 

 mont Plateau, the Appalachian Mountains and Plateaus, 

 the Limestone Valleys and Uplands, and the Great Plains 

 Region. The age of these soils varies in the order named, 

 showing that while they are very old as compared with 

 other soils yet to be discussed, there may be vast periods 

 of geologic time between their beginnings. As a matter 



^ For a full discussion of tlie origin and characteristics of the 

 soils of the United States, see Marbot, C. F., and others. U. S. 

 D. A., Bur. Soils, Bui. 96. 1913. 



