Sódio em Braun-Blanquet (1932)

Sódio em Braun-Blanquet (1932)
Osmotic Concentration of the Soil Water.—The osmotic concentration of the cell sap of plants is opposed to the osmotic concentration of the soil water. High osmotic values in the soil prevent the free intake of water by the roots of plants and tend to make the habitat barren of vegetation. Stocker (1930) found that in the sodium soils of Hungary an osmotic concentration equivalent to 28 atmospheres made the acquisition of water by plants impossible. At concentrations equivalent to 28 to 12 atmospheres only steppe plants were able to grow; and only habitats with concentrations of less than 12 atmospheres were open to colonization by non-steppe plants. The osmotic concentration in roots usually exceeds that of the soil water by 2.7 atmospheres or more. (Braun-Blanquet 1932:131)

The floristic distinctions between sodium chloride, sodium sulphate, and sodium carbonate soils are yet to be ascertained. No doubt, there [192] are distinct differences, even though it is customary to lump the vegetations of all three together under the term “halophyte vegetation” because of their similar external appearance. (Braun-Blanquet 1932:191-2)

The experiments of Lesage (1890) have shown that sodium chloride soils induce a certain degree of succulence in many species. Numerous observations on the conduct of non-halophytic, inland species in shore regions agree with this. Upon saline soils quite a few non-halophytic species develop special, more or less succulent varieties: Tetragonolobus siliquosus var. maritima, Plantago major var. carnosa, P. coronopus var. maritima, etc. Crossing between genotypes of a population of salt favoring species, with extinction of unfit descendants, seems to have [194] led to the genotypical fixation of the ecologically advantageous structure of succulents, favored by the salt soil. Succulence, however, need not necessarily be xeromorphic, even though it actually is so in numerous cases, as was demonstrated by Duval-Jouve (1868) in the case of S. macrostachya. (Braun-Blanquet 1932:193-4)

The experiments of Paris with Atriplex halimus (Bequinot, 1913, p. 101) indicate that there are species which thrive normally and continuously only upon sodium chloride soils. Such species have not only a high suction force but require also the specific ionic effects of the components of the salt. From the investigations of Iljin (1925) this fact may now be accepted for Na ions as well as for Ca ions. (Braun-Blanquet 1932:194)

Vegetation of Sodium Chloride Soils.—Few reliable facts are known in regard to the limitation of plant communities to certain types of saline soils, but the vegetation of sodium chloride soils has received most attention. High concentrations of pure sodium chloride are deadly to all plants. In mixed solutions the harmful effects of the Na and Cl ions are counteracted by the antagonism of the Ca ions, and along with Na and Cl the soil usually contains a considerable quantity of CaCO₃ and MgCO₃ together with varying amounts of sulphates in dry areas. In places the sodium chloride soils of the Mediterranean are rich in ferric oxide. A sample from the lagoon of Venice near Mestre, according to Beguinot (1913, p. 46), gave these figures: NaCl 1.10 per cent; CaCO₃ 6.08 per cent; MgCO₃ 6.87 per cent; Fe₂O₃ + Al₂O₃ 11.25 per cent; organic matter 3.00 per cent; and insoluble residue 69.62 per cent. (Braun-Blanquet 1932:195)

The average sodium chloride content of the Mediterranean Sea ranges around 3.8 per cent and is subject to only minor variations. But the salt concentration of lagoon water and of soils which are occasionally flooded will vary greatly. During the summer drought sodium chloride concentrations of 8 to 10 per cent in the uppermost layers of the soil occur frequently, while after the autumn rains the soil [196] is almost completely leached out (0.15 per cent of NaCl). The sodium chloride content of soils which are not continuously overflowed with sea water depends upon the amount of atmospheric precipitation. The effect of this is greatest in the uppermost layers of the soil, that is, in the region of the roots of the halophytes. Investigations on the seasonal fluctuations of salt concentration in the soils of southern France were made by Lagatu and Sicard (1911). They have reference to dry and moist soil as well as to the aqueous soil solution. (Braun-Blanquet 1932:195-6)

The Salicornia vegetation of these soils can stand sodium chloride concentrations of 8 to 10 per cent; the seasonal fluctuations in the topmost soil layers approximate 8 to 9 per cent. Below depths of 50 cm. they are rather insignificant (Fig. 104). The sodium chloride concentration of the ground water at 1 to 2 m. shows only small changes in the course of a year. This soil water is the permanent storage reservoir which constantly gives off salt in solution to the upper soil layers as water evaporates. (Braun-Blanquet 1932:196)

Similar belt formations are also to be observed near the lagoons of the Mediterranean Sea, where, however, the decrease of sodium content inland runs parallel to and simultaneous with declining moisture of the soil (Fig. 105). (Braun-Blanquet 1932:197)

The following may be cited as extremely perhaloid-anastatic associations of the sodium chloride soils of southern Europe: the Suaeda maritima-Kochia hirsuta association of the small lagoons in coastal dunes; Salicornietum radicantis (resists the longest flooding); Salicornietum fruticosae, which, with Atriplex portulacoides, covers many square miles (Fig. 106); and the Salicornietum macrostachyae [198] (see p. 231). This last characteristic association endures the greatest and most continuous salt concentration. It covers the otherwise plantless salt pans, which are flooded in winter and in summer crack into characteristic polygonal columnar structures (Fig. 118). (Braun-Blanquet 1932:197-8)

The mere notation of the sodium chloride content, on the basis of a dry soil, is not very significant ecologically, without the simultaneous determination of the moisture in the soil. (Braun-Blanquet 1932:201)

Soda Soils.—In less arid regions, in the northern part of the blackearth zone of Russia, in Rumania, Hungary, also in the United States (California) and Central Asia, soda soils rich in carbonic acid, take the place of sulphate soils. The principal soluble salt is sodium carbonate, Na₂CO₃. (Braun-Blanquet 1932:204)

This salt migration is responsible for the fact that the soda soil is free from sodium in the spring and rich in sodium sulphate in the deeper layers, while in the fall sodium is abundant in the upper stratum and but Uttle sodium sulphate remains deeper in the soil. Soda soils (Hungarian szek) contain the halides in the unflocculated form and are therefore finely divided, densely packed, and often crustlike. Owing to the dispersion of organic material the soil solution is colored black. In North America these soils are called “black-alkali” land, in contrast to the “white-alkali” land (chloride and sodium sulphate soils) . Thus soda soils are distinguished not only chemically but also physically from the other saHne soils which have a coarser and more granular structure. (Braun-Blanquet 1932:204)

Determination of Soda.—A 50-g. sample of the soil is boiled with 500 cc. distilled water, filtered, and diluted to 500 cc. This solution is titrated with tenth-normal HCl, with methyl orange as indicator (Na₂CO₃ + 2HC1 = 2NaCl + H₂CO₃). The sodium content may be calculated from the amount of tenth-normal HCl used in the displacement of the carbonic acid. (Braun-Blanquet 1932:205)

Vegetation of Soda Soils.—In his studies of the halophytic vegetation of sodium soils in the Hungarian lowlands Bernatsky gives an account of the plant communities. Kerner in his work on the plant life of the Danube countries (1863) had already outlined the salt vegetation of the great Hungarian Alfold. In large areas Statice gmelini dominates. Moist sandy saline soils are characterized by great masses of Achillea crustata along with Aster pannonicus, Scorzonera parviflora, Erythraea linarifolia, Carex divisa, and other salt-favoring species. Deeper depressions in the soil with high sodium content, often plantless in the center, are surrounded by a belt of dark reddish-brown Kochias and Salicornias, adjacent to the ashy-gray Atriplex and Artemisia salina belt. The companion floras of these communities are characteristic and richly varied, containing Crypsis aculeata, C. schoenoides, C. alopecuroides, Pholiurus pannonicus, Cyperus pannonicus, Lepidium cartilagineum, L. ruderale, and L. perfoliatum. The osmotic concentration of the soil solution in the Hungarian sodium soils has been examined by Stocker (1930). All sodium plants are able to put out new absorbing roots in a few hours, as soon as the soil is sufficiently moistened, but these roots dry up again as soon as the osmotic concentration goes above 28 atmospheres. (Braun-Blanquet 1932:205)

Gypsum swamps of large extent occur in Australia, according to Osborn (1925). In floristic composition they resemble those of sodium chloride and sodium sulphate soils. The identical halophytic genera which are spread over the European, north African, and central Asiatic salt regions are also found in Australia. The Mesembryanthemum australe association of the gypsum swamps of Flinders island are surrounded by the grass Lepturus incurvatus, which is also widely distributed upon the Mediterranean saline soils. (Braun-Blanquet 1932:205)

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