312 



PLANT SOCIOLOGY 



calcareous soils rich in humus. It withstands the wear of much rain 

 and snow water. The abundant carbon dioxide of the adsorptively 

 saturated humus leads to the formation of bicarbonates. Calcium and 

 magnesium go into solution and are the more completely leached out 

 as the humus is increased by additions of acid-forming litter from such 

 tufted plants as Elyna myosuroides and Agrostis alpina. There results 

 a deep, highly acid rendzina soil (pH 6.5 to 5.5). 



The corresponding association in the Central Alps is the Elynetum 

 on places briefly covered by snow or the Festuca violacea-Trifolium 

 thalii association on areas with prolonged snow cover. 



A decisive turning point is marked by the appearance of the 

 decidedly acidophilous tufted C. curvula, which may occur at pH 6 but 



p/fof fhe rg o7^/o/er7 





T^aw calcareous so/ J 



Rendzina 



Podsol 



Humus soil 

 climax 



Fig. 



157. — Soil formation and development of the vegetation upon limestone in high 

 mountains (schematic). {After Braun-Blanquct and Jenny.) 



is sure to be present at pH 5.5 to 5.2. The humus-forming activity of 

 this aggressive sedge increases the preponderance of hydrogen ions. 

 Important changes of soil follow. The exchange of ions restricts the 

 nutrient salts. The effect of humus upon dispersion and its action as a 

 protective colloid become more prominent with increasing acidity. 

 The sesquioxides and silicic acid, Si02, become soluble and sink to 

 greater depths. With this begins the transformation of the rendzina 

 into podsol. The acidophilous species now have a decided advantage 

 over the neutrophiles and acidophiles. Their expansion can be 

 followed step by step with the fall of the pH (p. 315). Whether the 

 podsol is permanent or the soil development goes a step farther to 

 a climax alpine humus, the development of vegetation always ends with 



