176 



PLANT SOCIOLOGY 



manency of the reaction constancy in humus soils and the "flexibiUty" 

 or lack of resistance of fine loam may be gained from Table 16. 



Table 16. — Amount of Buffering in Some Alpine Soils 

 (After Braun-Blanquet and Jenny, 1926) 



Soil 



Remarks 



1. Humus: 



Curvuletum 



Elynetum 



Firmetum (with limestone) . . . . 



2. Light soil: 



Podsol under Curvuletum 



Podsol, upper Engadine 



Podsol under fir forest, Cluoza. 



Well buffered, rela- 

 tively constant pH 

 values 



Little buffering, easily 

 changed pH values 



Of the soils listed above, the most highly buffered is the calcareous 

 soil with abundant humus of the Carex firma association. The acid 

 organic soils of the Curvuletum also are well buffered against the 

 addition of alkali. Large amounts of alkali may be added without 

 effecting a change in their reaction constancy; yet on addition of 1 cc. of 

 iV/10 HCl the acidity rises from pH 5.3 to 4.8. By this experiment the 

 tendency of climax alpine soils to become acid is clearly demonstrated. 



Measurement of Buffering. — The flexibility of soils is determined by 

 observing the pH changes of the reaction constancy on the addition of 

 1 cc. of A^/10 HCl or 1 cc. of A^/10 NaOH to 10 g. of soil dissolved in 30 

 cc. of pure water. 



Buffer Effect and Plant Communities. — The ability of most soils to 

 maintain their reaction constancy favors the stability of plant com- 

 munities and retards changes in vegetation. In weakly buffered soils 

 with fluctuating H ion concentration the vegetation must be able to 

 withstand considerable pH changes. The pH tolerance of the invaders 

 of such soils is wide; the factor loses its selective importance. This 

 may be assumed to be the case in many sandy soils and in siliceous 

 gravel soils. 



How large an addition of alkali is borne by the humus soils of the 

 central alpine C. curvula association without effecting a change in its 

 reaction constancy is shown by researches in the Rhetic Alps. 

 Measurements of wind-borne dust have shown that in the region of the 

 Alp Murter (Val Cluoza) large amounts of calcium carbonate are 

 deposited annually. It must, therefore, be due to the strong buffer- 



