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 CaCO3. 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. (Braun-Blanquet 1932:181)
BRAUN-BLANQUET, Josias. 1932. Plant sociology: the study of plant communities. (Trans.: George D. Fuller; Henry S. Conard) New York: McGraw-Hill Book Company.