Saturated and Unsaturated Colloids.—With progressive division of the particles the effect of the surface forces increases. Colloidal particles are therefore capable, to a high degree, of attracting foreign substances, i.e., of adsorption. A distinction is made between saturated colloids, which have gathered a maximum of foreign substances, and unsaturated colloids, which have not reached their limit of adsorption. Important changes of properties occur when saturation is reached. In the case of soils, these changes are clearly reflected in the vegetation. The behavior of the humus colloids in the soil is of special importance. According to their more minute division they are characterized by greater capacity for adsorption. If soil-alkali cations are present (magnesium, calcium) in ample quantities, they bring about the coagulation of the humus substances; the product is a saturated humus (cf. p. 250). (Braun-Blanquet 1932:162)
Direct Action of Calcium.—The direct action of Ca ions, determined by the amount of soluble calcium, seems unaffected by acidity or alkalinity. According to Magnin (1886), Contejean (1881), and others, soils with at least 2 to 3 per cent of soluble calcium are to be considered lime soils. Under particularly favorable external circumstances, with no competition from better adapted species, many calciphobous species endure this amount of lime. On the other hand, mere traces of lime (0.2 to 0.3 per cent) enable calcicoles to survive. Large quantities of CaCO3 drive out the indifferent plants, and the calcicoles or lime-constant species gain more and more monopoly over an area. Magnesium carbonate, MgCO3, has a similar effect and may partially take the place of CaCO3. (Braun-Blanquet 1932:183)
Physiological Effects of Magnesium.—Willstätter and his students have discussed the importance of magnesium in the molecular structure of chlorophyll. Canals (1920, p. 33) has given a good digest of our knowledge of the physiological effects of magnesium. By a number of experiments he confirmed Andre’s view that the Mg ion, in small doses, favors the development of plants. Concentrations of less than 0.0001 mg. of Mg were beneficial to the plants used, while at higher concentrations a toxic effect appeared. The addition of calcium, as mentioned, reduces the toxic effect of magnesium, and considerable amounts of magnesium may accumulate in plant tissues. The ashes of the leaves of Ilex aquifolium contain 12.34 per cent Mg; those of Stellaria media, 13.08 per cent; those of Solanum tuberosum, 17.08 per cent. (Braun-Blanquet 1932:186)
Dolomite Vegetation.—Dolomite, a compound of soluble CaCo3 and rather insoluble MgCO3, supports, in general, the same type of vegetation as compact limestone. The effect of Ca ions is of supreme importance upon dolomite substrata. But whether the exclusively dolomite species (and without doubt there are such) need considerable quantities of magnesium in combination with calcium or are strictly confined to dolomite soils for other reasons cannot now be decided. Perhaps it is only in such places that favorable physical conditions of the soil are found. In future studies of vegetation closer attention will have to be paid to whether or not the soil is dolomitic. (Braun-Blanquet 1932:187)
BRAUN-BLANQUET, Josias. 1932. Plant sociology: the study of plant communities. (Trans.: George D. Fuller; Henry S. Conard) New York: McGraw-Hill Book Company.