EDAFHIC OR SOIL FACTORS: CHEMICAL 181 



1. The simple, chemical effect of nutrition. 



2. The osmotic effect of higher concentrations of salts. 



3. The colloid-chemical effect of the ions. 



Of these three phases, we are best informed concerning the nutritive 

 eifects, thanks to investigations upon cultivated plants. The third 

 phase is almost untouched. For the chemical nutritive effect of salts 

 and their significance in building up the plant body, the "law of 

 relativity" (Mitscherlich-Lundegardh) is of value. It has replaced the 

 time-honored "law of the minimum" of Liebig. The latter is no 

 longer tenable in its original form (Romell, 1926). Lundegardh (1931) 

 has given the law the following ecological expression : 



The more nearly a factor is in minimum in relation to the other factors 

 acting upon the organism the greater is the relative influence of a change of 

 that factor upon the growth of the organism. As a factor increases in inten- 

 sity, its relative effect upon the organism decreases; and when the factor is 

 in the region of its maximum, the effect of a change upon the organism is nil. 



We have shown elsewhere that this law applies to the pH factor in 

 the Curvuletum of the Alps. In the minimum regions, or in the harm- 

 ful maximum, the pH is decisive, while in the optimum other factors 

 such as soil humidity, snow cover, and wind determine the survival of 

 the association. 



While the following pages are an attempt to illuminate the signifi- 

 cance of the most important nutritive substances of the soil in their 

 ecological effects upon the structure of vegetation, it must be admitted 

 that very few precise investigations are on record in this field. The 

 whole section, therefore, is only a general survey. 



1. Calcium. — In its effect upon the distribution and grouping of 

 plants the bivalent calcium ion (Ca++) is second only in ionic effect to 

 the hydrogen (H+). Calcium, whose manifold importance in plant 

 structure is well known, is found extensively in nature as humate in 

 organic combination, as a sulphuric salt (gypsum), as a silicate, but 

 most abundantly in the form of a carbonate. Certain limestone rocks, 

 such as marble, consist of as much as 99 per cent of CaCOs. Even 

 some siliceous rocks, as syenite, plagioclase gneiss, and diorite, contain 

 considerable quantities of calcium. The effectiveness of the calcium 

 content for plants is determined by its solubility rather than by the 

 absolute amount of calcium present. For, according to Klochmann, in 

 water saturated with CO2, calcium carbonates dissolve in the proportion 

 of 9 to 12 parts per 10,000, while the dolomitic carbonates dissolve in 

 the proportion of 3 to 10 per 10,000. 



