PHYSIOGRAPHIC FACTORS 



181 



ample, kaolinite has a very low capacity compared to clays of the 

 montmorillonite group, which have relatively high capacities. 



Since soils are constantly losing some of their adsorbed bases 

 due to replacement by H ions, the soil is rarely, if ever, saturated 

 with bases to its capacity. The degree of saturation at any given 

 time is known as the percentage of base saturation of the soil. The 

 base exchange capacity of a soil minus the percentage of base sat- 

 uration is theoretically equivalent to the percentage hydrogen 

 saturation of the soil, since hydrogen is the replacing ion. 



Both climate and vegetation have great effects upon the amounts 

 of exchangeable bases present in soils. On soils derived from the 

 same parent material, sugar maple-beech-yellow birch forest 

 maintains a soil at a higher percentage of base saturation than that 

 under a red spruce forest. 53 This seems to be due largely to the 

 ability of the hardwoods to absorb calcium from the subsoil and to 

 add it to the surface soil by leaf fall. 



Ca Ca K Ca K 



Ca Mg Ca K 



H 

 H 

 H 



K Mg Ca Na Ca 



Ca Na 



Ca H Ca 



Arid region 



Desert soils 



Arid brown soils 



Chestnut soils 



Transition zone 

 Chernozems 



Humid region 



Gray-brown- 



podsolic soils 



Podsols 



Many investigators have shown the relation between precipita- 

 tion, percentage base saturation, and pH. In brief, it may be stated 

 that, in regions of high precipitation, the bases are readily replaced 

 by hydrogen ions and then leached from the soil. The excess of 

 hydrogen ions results in lowering the pH and creating an acid soil. 

 Such conditions prevail in the cool, moist, coniferous forests of 

 the north. Just the opposite conditions prevail in the soils of arid 

 regions where low precipitation and scanty vegetation combine to 

 allow the bases to remain on the colloids, thus maintaining a hio-h 



