EDAPHIC OR SOIL FACTORS: CHEMICAL 



171 



The optimum in the Curvuletum, however, hes within 0.6 pH unit and 

 in the Elynetum within 0.9 pH unit (Fig. 96). 



In order to interpret the changes in vegetation caused by changes 

 in the H ion concentration of the soil, the pH curve is indispensable. 

 The points of intersection of the pH curves of the different associations 

 which lie adjacent to one another are of especial ecological importance. 

 They mark the region of sharpest struggle between the established 

 community and the invading species which are favored by the pH 

 change. From such curves (Fig. 96) it may be seen that this keenness 

 of competition between the Elynetum and the Firmetum occurs in soils 

 of pH 7.0 to 6.5: and between the Curvuletum and the Elynetum, in 



pM 8 7.5 7.0 6.5 6ja 5.5 5.0 V.J t^.OpH 



Fig. 96. — Development of vegetation and soil reaction in the succession of Firmetum 

 to Elynetum to Curvuletum. (After Braun-Blanquet and Jenny.) 



soils of pH 5.5 to 5.1. In these transition regions of the Elynetum 

 the pH value of the soil is the deciding factor for the existence of the 

 association. 



In grassland communities, influenced by man's activities, the 

 importance of the H ion concentration is often obscured by other 

 factors. Klapp (1930), however, insists that the presence of a certain 

 association is a better indicator of the soil reaction than the occurrence 

 of any individual species. 



The opinion has been expressed that associations of strongly acid 

 soUs have a narrower pH amplitude than those of neutral or alkaline 

 soils (Wlodek and Strzemiensky, 1924, p. 811; Christophersen, 1925, 

 p. 566). This generaUzation, however, is hardly tenable. It is true 

 that in young calcareous soils in humid climates the enrichment of the 

 soU with humus and the leaching process lead quickly to the acidifi- 

 cation of the uppermost soil horizon; thus considerable variations in 

 the pH value of the surface soils arise, depending upon whether the 



