BDAPHIC OR SOIL FACTORS: CHEMICAL 



175 



Table 15. — ^Variations in pH Values in Red-earth Profiles in the 

 Mediterranean Region 



this fact rests his distinction of anastatic and eustatic soils. Our 

 question, then, is: Is the acidity of a soil constant, or does it change 

 from time to time? 



Soil studies of recent years have brought out the important fact that 

 most soils have the capacity of maintaining a very nearly constant H 

 ion concentration. Each soil has its characteristic pH value, its 

 "reaction constancy," which normally is little changed by external 

 conditions. This is analogous to the capacity to maintain its physical 

 condition to a large extent regardless of the surrounding temperature. 

 This property of tenaciously maintaining a "reaction constancy" is 

 called buffering (see Baver, 1931), 



The buffering substances in the soil are weak acids, salts of weak 

 acids with strong bases, amphoteric electrolytes, clay and humus 

 compounds capable of base exchange, and colloidal hydroxides of iron 

 and aluminum. Humus soils are especially highly buffered against 

 additions of acid and even more so against additions of alkali. Raw 

 calcareous soils are very highly buffered against acids. Thus these soil 

 types have a very persistent reaction constancy. Sandy soils, on the 

 other hand, are very poorly buffered. Their pH value changes rapidly 

 on the addition of acids or bases; the reaction constancy is negligible; 

 these soils are "flexible" or non-resistant. In podsol soils, therefore, 

 the leached fine-grained middle horizon is much more feebly buffered 

 than the organic and accumulation layers. A clear idea of the per- 



