2. Magnesium.—Magnesium occurs in numerous compounds but usually in small quantities in rocks and soils. There are two important magnesian rocks. Dolomite, MgCO3 + CaCO3, with 54 per cent Ca and 46 per cent Mg, forms great white shining mountain chains (the Dolomites of southern Tirol). Serpentine, 2SiO2MgO 2H20, has a dark greenish luster, weathers into sheets and jagged pieces, and forms an unusually sterile residual soil. The serpentine ridges of the Alps, in their dark, deathly hardness, are among the most depressingly lonely phenomena of nature, and the popular reference tote Alp (dead Alp) is quite appropriate. (Braun-Blanquet 1932:186)
Saline soils of dry regions and large salt-water basins have their magnesium mostly in the form of chloride and sulphate. (Braun-Blanquet 1932:186)
Novak (1928) has studied the serpentine problem and added to the list of plants peculiar to serpentine and magnesium substrata. He cites among others certain paleoendemic species of the Balkan Peninsula, such as Halacsya sendtneri, Potentilla visiani, and Fumana honapartei together with various subspecies and varieties. He considers that the serpentine vegetation is controlled by the Mg:Ca ratio, which must be greater than unity; that is, there must be in the soil an excess of Mg ions. Most of the facultative serpentine plants have an extraordinarily developed root system, whereas the surface parts of the plants are rather meagerly developed, and frequently their flowers and fruits are quite sparse. Little is known about the plant communities of immature serpentine soil. In contrast to communities peculiar to the saline soils of arid regions, the serpentine vegetation presents a purely local, edaphically controlled phenomenon. (Braun-Blanquet 1932:189)
BRAUN-BLANQUET, Josias. 1932. Plant sociology: the study of plant communities. (Trans.: George D. Fuller; Henry S. Conard) New York: McGraw-Hill Book Company.