Ferro em Braun-Blanquet (1932)

Protective Colloids.—Adsorptively unsaturated, highly disperse humus is, because of its lack of sensitiveness toward electrolytic effects, an efficient protective colloid. Iron hydroxide, aluminum hydroxide, silicic acid, etc., are much more sensitive toward coagulating electrolytes. When the particles are protected by the enveloping colloid particles of the less sensitive protecting colloid of the highly dispersed humus, they themselves are no longer sensitive toward electrolytic effects, so that changes of dispersion are retarded and precipitation may be stopped completely, under certain circumstances. Even very small quantities of protective colloids may prevent changes in the dispersity of the disperse phase. Unsaturated humus, for example, keeps various soil colloids (Al(OH)₃, Fe(OH)₃, etc.) in soluble, highly dispersed condition. It protects them from precipitation, and renders them highly mobile, susceptible to leaching out. The dark waters of regions of crystalline rocks and of moors owe their color to the protective colloid action of the acid humus. The color is due to leached humus colloids. (Braun-Blanquet 1932:163)

3. Iron.—The presence of iron oxide in the soil is indicated by a reddish or brownish-yellow coloration. It is iron sesquioxide, Fe₂O₃, which gives the characteristic red or red-brown color to the so-called [190] “red soils” of southern Europe, southern United States, and climatically similar regions. The deep-red bauxite concretions embedded in the upper layers of limestone are especially rich in iron (bauxite is considered to be chiefly amorphous Al(OH)₃ – gel). In the large deposits of southern Europe bauxite appears to be in no wise detrimental to plant growth. Upon exposure of these deposits the species of the immediate environment soon establish themselves, lime plants as well as indifferent species. Among the abundant species are Spartium junceum, Dorycnium suffruticosum, Euphorbia nicaeensis, Helichrysum stoechas, and Inula viscosa. The vegetation of the immediate environment of the Jurassic iron pits is composed solely of indifferent and lime-favoring species (Gontejean, 1881). The vegetation is quite different near the deposits of iron blende and pyrites of upper Italy. Large areas are entirely barren. The first pioneers of this soil, heavy with oxides of iron, are calciphobous plants, such as Calluna, Agrostis canina, Silene rupestris, and the indifferent Molinia coerulea (Gola, 1910), all somewhat dwarfed and chlorotic. (Braun-Blanquet 1932:189-90)

Mowing, Fertilization, Irrigation, Reclamation.—With the use of sickle and scythe in the bronze and iron ages we see the beginnings of the cultivation of food plants, the extension of cultivated and semi cultivated land. Mowing has about the same effect upon the [285] vegetation as moderate pasturing. Usually, however, mowed land is also fertilized and, often, irrigated and seeded. This brings about the anthropogenous rich meadows, which have such a surprisingly uniform floristic appearance over large areas. All over central and a large part of western Europe man has for a very long time controlled the grassland according to uniform and widely accepted practice. The result is a very uniform rich type of meadow with numerous species: the Arrhenatheretum elatioris. (Braun-Blanquet 1932:284-5)

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