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cal i _ _ , , . ! IK- rock materials brings 



absorptiuu of carbon dioxide and the formation of 

 soluble bicarbonates. Hydration softens and increases 

 the mass of some minerals, so that physical weather- 

 ing of the rock bearing them is facilitated. Oxidation 

 discoloring many rocks, especially those containing 

 iron, is symptomatic of chemical changes in progress ; 

 binding materials are weakened and crumbling oc- 

 curs easily. Finally, many substances simply go into 

 solution and are carried away. Where precipitation 

 is frequent, water percolating through the soil carries 

 soil nutrients to greater depths than where precipi- 

 tation is light. In hot dry climates, organic matter 

 may oxidize completely and so quickly its nutrients 

 are lost to plants and microfauna. 



The cbniate prevailing there is a determinant of 

 the kinds and prosperity of plant and animal life in 

 an area. The biota has much less effect on soil for- 

 mation in arid climates than in humid climates. By 

 so much, desert vegetation is usually quite as locally 

 distinctive as local soils are distinctive, but in humid 

 regions, where many plant stages succeed one an- 

 other, climax vegetations may be essentially the same 

 regardless of whether the sere originally started on 

 limestone, sandstone, or in a pond. Because of the 

 interactions of parental rock, biota, and climate, dif- 

 ferent soil profiles are formed, each characteristic of 

 a specific climatic region and type of climax vegeta- 

 tion. An understanding of soil profiles is prerequisite 

 to understanding vagaries of animal distribution. 



Reactions, soil formation, and cycles 169 



