172 



PLANT SOCIOLOGY 



samples are taken from the disintegrated rock itself or from the highly 

 organic cover. In places around springs and especially near running 

 water, soil acidity decreases. Hence, in such places weakly acido- 

 philous and even neutrophilous or basophilous species may appear, in 

 the midst of a strongly acidophilous community (see also Szafer et at , 

 1927). 



Lines of pH Values. — In order to determine the average pH value 

 and the pH variations within an individual stand, soil samples must be 

 taken in several places. The concept of the soil line used by Jenny 

 (1925) is useful in this case. The soil line gives a graphic representa- 

 tion of the horizontal variations of soil properties within a selected 

 area. Through a homogeneous plant community a straight line is 



pH.S - 



p/f.e 



pH.? 



Curvuletum 

 Humus soil 



Elynefum 

 Rendz'inasoil 



Firmeium 

 Calcareous soU 



to It 13 >3 t<f 



Fig. 97. — Lines of pH values in stands of Curvuletum, Elynetum, and Firmetum 

 in the Lower Engadine; spacial distribution of the pH values. (^After Braun-Blanquet 

 and Jenny.) 



laid out, and along it at certain intervals (2 m. in large communities) 

 samples of soil are taken and tested. The pH soil lines of the Firme- 

 tum, Elynetum, and Curvuletum are shown in Fig. 97. These soil 

 lines reflect the pH variation within an individual stand and also serve 

 as indicators of the pH range of their respective associations. The 

 great variations in the Elynetum are worthy of note. They are due 

 to different degrees of enrichment by humus and the resulting acidifi- 

 cation of the originally strongly alkaline virgin soil. The pH lines 

 of the Curvuletum and Elynetum were run very close together upon the 

 same substratum of Rhetic marl at the Murterjoch (lower Engadine 

 valley). Physiognomically, the two communities look very much 

 alike, but a floristic analysis reveals fundamental differences which 

 receive an adequate ecological explanation by reference to the soil lines. 

 Profiles of pH Values. — With increasing depth the soil reaction 

 usually changes rapidly. Deep-rooting species, particularly trees, 

 therefore often draw their nourishment from soils of very different H 

 ion concentrations. Nevertheless, it must be remembered that 



