174 



PLANT SOCIOLOGY 



In podsolic and podsol soils of the Alps, of northern Europe, and of 

 the northern United States the raw humus layer (horizon Ai) always 

 shows a high H ion concentration; the leached, light earth or sandy 

 soil layer (horizon ^2), a somewhat lower; and the accumulating layer 

 (horizon B), the lowest H ion concentration. Examples are shown 

 in Table 14. 1 



Table 14. — Variations in pH Values in Northern and Alpine Podsol 



Profiles 

 (After Braun-Blanquet and Jenny, and Christophersen) 



Vegetation 



Horizon Ai 

 T&w humus 



pH 



Depth, 

 centimeter 



Horizon A2 

 leached 



pH 



Depth, 

 centimeter 



Horizon B 

 enriched 



pH 



Depth, 

 centimeter 



Horizon C 

 disintegrating 

 soil material 



pH 



Depth, 

 centimeter 



Curvuletum on rend zina-pod« 



sol; Central Alps 



Curvuletum onhumus-podsol ; 



Central Alps 



Empetrum-Vaccinium uligin- 



osum heath ; Central Alps . . 

 Larix-Pinus forest; Central 



Alps 



CaUuna-Cladonia heath; Sy- 



lene, Norway 



Vaccinium myrtillus-Betula 



pubescens forest, Sylene . . . 



5.0 

 5.5 

 4.2 

 6.0 

 4.3 

 4.0 



to 35 

 to 3 

 2 to 5 

 to 15 



5.7 

 5.4 

 5.0 

 6.4 

 4.4 

 4.0 



35 to 38 



3 to 13 



15 



15 to 30 



6.5 



5.4 



6.8 

 4.7 

 4.7 



38 to 45 

 13 to 18 



30 to 50 



7.8 

 6.3 



7.2 



45 

 18 



55 



Only sparse data now exist regarding the pH changes in the soil 

 profiles of definite plant communities of other climatic regions. In the 

 arid portions of central Bohemia, Firbas and Sigmond (1928) found 

 the upper 2 cm. of the soil always less acid than the 4 to 10 cm. imme- 

 diately below. This also appears to be the situation in the red soils of 

 the Mediterranean region (Table 15). Under the dry climate of 

 Bohemia and in the hot, dry Mediterranean region it appears that the 

 acidity of the soil increases with depth or remains constant. The 

 litter of the woody evergreen plants is, on the contrary, somewhat acid 

 {Pinus halpensis litter pH 4.5; Quercus ilex pH 6; Myrtus pH 6.4), but 

 these organic acids are very quickly neutralized. 



Buffering of Soils. — As far back as Gola (1910) it was known that 

 the constant or rapidly changing concentration of soil solutions affects 

 the composition of the vegetation in greatly varying degrees. Upon 



^Soil profile studies on podsols of the Black Forest by Frank (1927) and on 

 soils in Michigan by McCool et al. (1923) give similar pH data. 



