BDAPHIC OR SOIL FACTORS: CHEMICAL 



173 



germinating seeds and young plants are always dependent upon the 

 topmost soil layer. Accordingly, the pH value of the uppermost 

 horizons of the soil is of particular importance. In this very top 

 layer (1 to 2 cm.) the H ion concentration is notably dependent upon 

 the predominant plant species or, more exactly, upon their decom- 

 position products (see p. 245). 



With increasing depth the influence of dead plant matter upon pH 

 values diminishes, in favor of the climatic influences of water and the 



Fig. 98. — Soil profile in the Curvuletum on dolomite in Central Alps, 2,500 m. 

 show-ing 2 cm. black humus (^i); 6 cm. fine soil with some humus (A2); light gray 

 disintegrating soil material (B); and dolomite fragments (C). (After Braun-Blanquet 

 and Jenny.) 



effect of the underlying rock. On consideration of a soil profile in its 

 entirety the climatic influence upon pH values stands out very clearly. 

 In the very humid climate of the Alps, the H ion concentration 

 usually decreases with increase of depth in soil. This applies to 

 podsol and podsolic soils, as well as to alpine humus and rcndzina soils. 

 The production of H ion is greatest in the acid raw humus horizon and 

 decreases slowly in the case of deep soils, more rapidly in shallow soils. 

 Above calcareous substrata a very sudden pH reversal often occurs 

 between the humus layer and the mineral soil, a fact which is brought 

 out in Fig. 98. The extent and layering of roots of plants of acid raw 

 humus reflect distinctly this acid reversal in the soil. 



