Ch. 2] TIME 45 



Examination of numerous time functions indicates that the soil 

 properties s have rates of change with time, ds/dt which become 

 smaller as the age of the soil increases. Soils which have become rela- 

 tively stabilized in relation to time are often designated as mature 

 soils. Soil maturity does not imply complete arrest of soil develop- 

 ment; it merely indicates relatively slow reaction rates. In climates 

 which are not extremely dry (deserts) or cold (arctics), mature soils 

 have well-developed profile features. The time necessary to develop 



Fig. 3. 



20 30 40 



Depth in inches 



Chronosequences of soil profiles (Harradine). The age of the soil in- 

 creases as follows : Yolo <C Zamora < Hillgate. 



mature soils varies with the constellation of the soil-forming factors. 

 In soft, porous, parent materials stabilized humus profiles may evolve 

 within a few centuries. To produce claypan soils, which have B hori- 

 zons rich in clay, probably tens of thousands of years are required. 

 Soils are sometimes studied in accordance with the equation 



E{s) =/(time) 



(3) 



Here the subscripts are missing. The soil- forming factors are not kept 

 constant. This formulation represents the historic approach. Soils 

 are studied in relation to time, irrespective of time changes of climate, 

 and the biotic factor. The historic approach is sometimes designated 

 as the study of soil evolution, as it may occur during cycles involving 

 long geological periods. In contrast, the chronofunctions deal only 

 with those soil changes which take place under relatively constant 

 cl, o, r, and p. 



