182 PLANT SOCIOLOGY 



For a better understanding of the relations between lime content of 

 the soil and vegetation it will be necessary to distinguish between direct 

 and indirect action of calcium. 



Indirect Action of Calcium. — Calcium affects in large measure the 

 physicochemical conditions of the soil and thereby, indirectly, the 

 vegetation. Lime affects the coagulation and aggregation of the soil 

 colloids whereby coarse crumby structure, water conduction, tempera- 

 ture, and aeration of the soil are favorably influenced. 



Soils rich in lime nearly always have a neutral or alkaline reaction, 

 since CaCOs neutralizes acids. ^ This affords favorable living condi- 

 tions to such soil organisms as bacteria and worms, while the food 

 supply for higher plants increases and is converted into a more assimi- 

 lable form. 



Upon this indirect action of calcium ions depend a number of 

 phenomena which have not always been adequately distinguished from 

 the direct action of lime. 



Anyone from northern or central Europe who observes the soil 

 constancy (adherence of a species to a type of soil) of certain, 

 apparently obligate, lime plants in northern Europe will discover to his 

 astonishment that many a lime-constant species of the north grows in 

 the south upon soils with little or no lime. In such cases basophilous or 

 neutrophilous species may be concerned, which find their soil prefer- 

 ences in the north upon lime only but in the south grow upon limeless 

 soils. We are then dealing with a conditioning effect of H ions. If a 

 neutral or basic reaction prevails, many of these species thrive upon 

 practically limeless silicate soils. Similarly, the Sphagnaceae, 

 described as calcifugal, seem much more sensitive to 0H~ than to 

 Ca++ ions (in low concentration) (Paul, 1908; Mevius, 1921). Iljin 

 (1925, p. 349) was able to prove that Vincetoxicum officinale, a plant 

 which endures Ca++ ions very well, is extremely sensitive to 0H~ 

 ions. 



Plants which thrive in a dry climate are more and more confined to 

 calcareous soils as they approach the northern limits of their ranges, 

 since these soils alone can afford them favorable physical conditions of 

 temperature, water supply, and soil aeration. Frequent reference has 

 been made to this situation since Flahault called attention to it in 1893. 

 Only in this respect is it proper to speak of physical factors acting as a 

 substitute for lime. 



^ The alkalinity, however, does not rise proportionally with the amount of lime 

 present, as one might expect, the H2CO3 being a determining factor. In a soil 

 solution a mixture of CaCOs and CO2 ^^'ill have a pH value of 7.8 to 7.2 (Wiegner, 

 1926, p. 154). 



