Hidrogênio em Braun-Blanquet (1932)

In the absence of bases, in high moors and upon sandy forest soils, the humus colloids remain adsorptively unsaturated and contain exchangeable hydrogen ions. Then the soil is densely packed, poorly aerated, poor in food substances and in electrolytes, and, because of its [163] lack of sensiliveness toward electrolytic effects, it gives an acid reaction. (Braun-Blanquet 1932:162-3)

The degree of acidity of any solution depends upon its content of positively charged hydrogen ions (H+). The preponderance of these H+ ions gives acids their common characteristic properties, while in bases the negatively charged hydroxyl ions (OH-) always predominate. In every solution, however, an enduring equilibrium exists between the H+ and OH- ions in such a manner that an increase of the H+ ions is always accompanied by a corresponding decrease of the OH- ions, or vice versa. The more H+ ions there are present the more strongly acid, the more OH- ions, the more basic, is the solution. When H+ and OH- are present in equal numbers the solution is neutral. This is the case in chemically pure H₂O. (Braun-Blanquet 1932:166)

Expression of hydrogen Ion Concentration.—The inconvenience of writing the actual H ion concentration of a solution that is exactly neutral as 1/0000000 gram-ion of H or as 0.0000001 gram-ion of H led Sorensen (1909) to propose the use of the negative logarithm of the H ion concentration as the expression of the degree of acidity, preceded by the sign “pH.” When the H ion concentration of a solution is 10^-6, by the use of the reciprocal the negative exponent is avoided and it is expressed, according to Sörensen, as pH 6. The pH value of a solution is therefore the logarithm of the reciprocal of the H ion concentration. For the sake of brevity, the pH value is called the hydrogen number. (Braun-Blanquet 1932:166)

It must always be remembered that large hydrogen numbers (pH 7 to 14) correspond to a low H ion concentration; small hydrogen numbers, to a high concentration of H ions. (Braun-Blanquet 1932:166)

In the language of the plant ecologist, the hydrogen numbers with reference to the behavior of individual species and communities have the following significance: (Braun-Blanquet 1932:166)

Significance of hydrogen Ions.—The H ion, a simply constructed and most active ion, affects the physiological processes within plants to a high degree. It greatly influences enzyme actions so important in the life cycle of a plant. The concentration of the cell sap of a plant must maintain a certain relation to the ionic concentration of the soil solution. When this relation is interfered with, by change of the concentration of the solution, morbid phenomena soon appear in the plant, and even death may occur. Certain pathogenic bacteria can grow only upon media of very definite H ion concentration. Ulehla (1923) proved that the apical cells of certain algae (Cladophora, Basidioholus) burst when they are transferred into an acid medium. Apparently adsorptive effects of H ions upon the colloidal material of the cell membrane are involved, and not osmotic phenomena. (Braun-Blanquet 1932:169)

Alpine Humus.—Ramann, Leiningen, and others call alpine humus the mild, adsorptively saturated humus of alpine and especially of subalpine calcareous soils. But Braun-Blanquet and Jenny would limit this term to the acid humus, which is to be considered the climax soil of the alpine zone of the Alps, of the summits of the high Cevennes and probably of other mountains with similar climatic conditions. It differs from the alpine humus of Ramann by its high dispersion and [252] high concentration of hydrogen ions. The organic remains are thoroughly decomposed and frequently well mixed with mineral loam. The organic matter ranges around 30 per cent. (Braun-Blanquet 1932:251-2)

A decisive turning point is marked by the appearance of the decidedly acidophilous tufted C. curvula, which may occur at pH 6 but is sure to be present at pH 5.5 to 5.2. The humus-forming activity of this aggressive sedge increases the preponderance of hydrogen ions. Important changes of soil follow. The exchange of ions restricts the nutrient salts. The effect of humus upon dispersion and its action as a protective colloid become more prominent with increasing acidity. The sesquioxides and silicic acid, SiO₂, become soluble and sink to greater depths. With this begins the transformation of the rendzina into podsol. The acidophilous species now have a decided advantage over the neutrophiles and acidophiles. Their expansion can be followed step by step with the fall of the pH (p. 315). Whether the podsol is permanent or the soil development goes a step farther to a climax alpine humus, the development of vegetation always ends with [313] the Caricion curvulae, the climax community of the alpine areas of the Central Alps (cf. Braun-Blanquet and Jenny., 1926, pp. 226-240). (Braun-Blanquet 1932:312-3)

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