CHAPTER XXVI.. The Vegetation of Saline and Alkali Lands.

THE VEGETATION OF SALINE AND ALKALI LANDS.

Marine Saline Lands.—While the saline alluvial lands of the sea-coast differ both in their mode of origin and in their nature from the alkali soils or “terrestrial saline lands,” as they have been called in Europe, their vegetation has in many respects a common character. Not only is there much similarity, sometimes even identity, in the kinds of plants inhabiting these lands, but their saline ingredients induce certain changes of form and structure in plants not properly “saline” but more or less tolerant of soluble salts, by which the saline or alkali character of the lands may be recognized.

Just as in the case of lime we must distinguish between the plants definitely repelled by a large amount of this substance in the soil (calcifuge), while others prefer the soils in which lime is abundant (calciphile), and still others appear to be indifferent to its presence and are governed in their habitat by the physical conditions presented: so in the case of saline lands the salts may attract or repel certain plants. The latter class is much the largest; while there is also a number of plants which are more or less indifferent to the presence of salts, provided these be not in very great excess. Such plants constitute the next-largest class; while those attracted by salts, and whose welfare is conditioned upon their presence, are comparatively few in number, and still fewer among them are of economic importance. Hence the soluble salts have largely a negative importance for agriculture; the question usually being how to utilize the land until the undesirable surplus of salts can be got rid of, partially or wholly, as the case may be; the former usually in seashore lands, the latter in the alkali lands proper; in which a small remnant, not sufficient to injure crop plants, is usually desirable (see chap. 23, p. 462).

General Character of Saline Vegetation.—Those familiar with seashore marshes cannot fail to note the fleshiness and succulence of the characteristic plants. This “incrassation” belongs not only to the saline flora proper, but is acquired to a greater or less degree when plants not ordinarily at home on saline ground are transferred to it artificially, or by saline overflows; while at the same time the leaves usually become smaller, and the growth more compact. Correspondingly, when saline plants are transferred to non-saline ground, the leaves generally become thinner and larger, and the growth more slender. The well-known “Russian thistle” is a case in point, as is also its close relative, the soda saltwort (Salsola soda); although the latter does not often venture as far from the saline lands as does the former (Salsola kali tragus), which now seems to have become a world-wide weed, with only a shade of preference for alkali lands.

Structural and Functional Differences Caused by Saline Solutions.—It has been definitely shown by the investigations of Schimper, Brick, Hoffmann, Lesage, Rosenberg and others, that the peculiarities or changes of structure brought about by saline solutions are essentially those pertaining to xerophile (drought-enduring) vegetation; which in general tend to the diminution of evaporation from the plant surfaces. It may be said, roughly speaking, that the absorption of water by the roots begins to diminish so soon as the concentration of the saline solution approaches or exceeds one-half of one per cent; while when it rises as high as three per cent, water-absorption by the roots ceases even in the wettest soils, and the plant suffers from drought quite as much as from any directly injurious effects of the salts. Different plants of course differ in the measure of concentration which brings about these phenomena, which vary also with the character of the soluble salts. It is stated that injurious or useless salts like common salt act at lower concentrations than e. g., saltpeter, which is useful. The difference in external structure are: diminution of the size of leaves, assumption of cylindrical or spinous forms, sinking-in of the breathing pores below the outer surface, dense hairy covering, resinous exudations, etc. Internally we find that xerophile plants have developed on their upper or outer leaf-surfaces instead of one, several layers of “palisade” (long and erect, closely-packed) cells, through which transpiration is extremely slow, as is also the transmission of heat. When salt-tolerant plants are grown on saline soils, their palisade cells are relatively lengthened.

Coincident with these external means for the retardation of evaporation, the leaves of xerophiles are frequently supplied with special water-storage cells, which supply moisture for the physiological processes when the root supply falls short. The cactus tribe and similar-looking plants are examples of the latter provision, which causes even animals suffering from thirst to resort to them, although they eschew the saline vegetation.

Absorption of the Salts.—The true halophytes or exclusive salt plants, which refuse to grow on lands not containing a large proportions of salt, often absorb so much salt that on drying it blooms out on their surface; they usually have, even when green, a distinctly salty taste, and their ash is rich in chlorids, specially of sodium. Such is the case of the samphire, common in saline marshes everywhere. The total ash is usually very high, often varying with the salinity of the water or soil in which they have grown. Thus the salt-content of the ash of samphire may vary by several per cent. In other cases, as in that of one of the Australian saltbushes investigated at the California station, neither the ash content nor the composition of the ash varies materially whether the plant be grown on strong alkali land, or on uplands whose total saline content does not exceed (in four feet depth) .015% or 2500 pounds per acre.

The following table gives the composition of the ash of this saltbush alongside of that of two other prominent alkali-plants of the same relationship, occurring, one in the San Joaquin valley of California, in strongly saline lands, the other in the Great Basin region of the interior, on lands strongly impregnated with carbonate of soda. All these, it will be seen, take up very large amounts of sodium salts, notably the chlorid; the Australian plant most so, the “greasewood” of the Great Basin least so; a large proportion of the alkali salts being evidently, in the latter case, contained in the form of organic salts, which in the ash become carbonates.

ANALYSES OF ASHES OF SALINE AND ALKALI PLANTS.

(A) = Australian Saltbush, Atriplex semibaccata.† (B) = Bushy Samphire, Allenrolfea occidentalis.† (C) = Greasewood, Sarcobatus vermiculatus.† (D) = Saltgrass, Distichlis spicata.‡ (E) = Tussock grass, Sporobolus airodies.‡ (F) = Prickly Pear, Opuntia macrocentra.‡ ========================+======+======+======+======+======+====== | (A) | (B) | (C) | (D) | (E) | (F) ------------------------+------+------+------+------+------+------ Ash, air-dried plant, %.| 19.37| 12.03| 13.81| 11.61| 7.99| 24.18 ------------------------+------+------+------+------+------+------ Potash (K₂O) | 11.42| 18.53| 30.11| 3.30| 5.78| 1.61 Soda (Na₂O) | 35.39| 39.45| 32.58| 2.38| 5.15| 2.76 Lime (CaO) | 5.75| 1.36| 8.70| 5.25| 8.05| 65.66 Magnesia (MgO) | 3.23| 1.09| 1.09| 2.95| 4.15| 26.70 ------------------------+------+------+------+------+------+------ Br. ox. of Manganese | | | | | | (Mn₃O₄) | .22| | | .16| .25| ------------------------+------+------+------+------+------+------ Peroxid of Iron (Fe₂O₃) | 3.33| 7.06| not | 2.22| 2.39| 1.19 Alumina (Al₂O₃) | | |det’d.| | | ------------------------+------+------+------+------+------+------ Silica | 16.24| 11.81| 4.00| 78.73| 66.79| .81 Phosphoric acid (P₂O₅) | 2.80| 3.51| 5.60| .83| 1.25| .47 Sulfuric acid (SO₃) | 2.64| 4.93| 5.90| 3.20| 4.52| .64 Chlorin, percent | 24.33| 15.30| 11.00| 1.40| 2.13| .21 ------------------------+------+------+------+------+------+------ Totals |105.35|103.04| 99.79|100.31|100.46|100.05 Less excess, O: Cl | 5.35| 3.25| 2.50| .31| .46| .05 ------------------------+------+------+------+------+------+------ True totals |100.00| 99.79| 97.29|100.00|100.00|100.00 ------------------------+------+------+------+------+------+------

(G) = Shad scale, Atriplex canescens.‡ (H) = Alfalfa Hay. (Cal.)† (I) = Timothy Hay.

OF FORAGE CROPS. ============================+========+========+======= | (G) | (H) | (I) ----------------------------+--------+--------+------- Ash, air-dried plant, %. | 4.23 | 9.85 | 6.15 ----------------------------+--------+-------+------- Potash (K₂O) | 25.17 | 43.72 | 28.80 Soda (Na₂O) | 6.23 | 4.48 | 2.70 Lime (CaO) | 25.97 | 20.51 | 9.83 Magnesia (MgO) | 16.63 | 2.56 | 3.60 Br. ox. of Manganese (Mn₃O₄)| .51 | | ----------------------------+--------+--------+------- Peroxid of Iron (Fe₂O₃) | 5.89 | 2.95 | Alumina (Al₂O₃) | | | ----------------------------+--------+--------+------- Silica | 11.94 | 5.87 | 35.00 Phosphoric acid (P₂O₅) | 3.11 | 5.00 | 10.80 Sulfuric acid (SO₃) | 4.93 | 6.92 | 3.90 Chlorin, percent | 2.07 | 10.25 | 5.00 ----------------------------+--------+--------+------- Totals | 100.45 | 102.26 | 99.70 Less excess, O: Cl | .45 | 22.6 | 1.13 ----------------------------+--------+--------+------- True totals | 100.00 | 100.00 | 98.57 ----------------------------+--------+--------+------- † Jaffa, Cal. St’n. Rept. 1894-95, p. 169. ‡ Goss, New Mex. St’n. Bull. No. 44; recalculated.

It will be noted that the saltbush hay contains nearly one-fifth of its (air-dry) weight of ash, of which nearly 40% is common salt. It therefore has a distinctly salty taste, and is always moist to the touch, containing-ordinarily over 15% of moisture. It is therefore much liked by stock when fed intermixed with other hay, and thus supplies all the salt needed by cattle. The greasewood is much less liked by stock, and bushy samphire is wholly rejected by them. Comparing with these fleshy plants the ash of the two grasses, the first a world-wide “salt grass,” the other a common grass of the American arid region, we note that not only do they contain much less soluble ash than the saltbushes, but especially much smaller amounts of sodium salts; proving that even when growing in company with the saltbushes on strongly impregnated land, they can repel from absorption these to them useless or injurious salts. But in the case of the “shad scale,” also a “saltbush” of the Great Basin, the ash-content is remarkably low—only about one-fifth of that of its Australian relative—and it differs widely from the latter in having but a very low proportion of soda, and a very high one of lime and potash, approaching in these respects to our usual forage crops; and being also fairly rich in nitrogen, it forms acceptable browsing when other pasture plants are scarce. It therefore does not exert the laxative action produced by the exclusive feeding on the more saline herbages.

The exceptionally high ash-content of the cactus or prickly pear, also given in the table, arises, it will be noted, not from the soluble salts but from the absorption of extraordinarily high proportions of lime and magnesia. Owing probably to the latter substance, and also the oxalate form in which lime is usually found in the cactus tribe, this plant when used as forage is also somewhat laxative.

Altogether, this table offers remarkable examples of wide differences in the kind and amount of ash ingredients absorbed by plants growing upon similar soils and under identical climatic conditions; indicating a selective power which no merely physical theory of soil-action in plant growth can explain.

Injury to Plants from the Various Salts.—The early observers, especially Contejean, were predisposed from their observations of lime on vegetation to ascribe the action of salt upon marine vegetation to the sodium component. But the wide differences in the effects of different sodium compounds, notably of common salt and Glaubers salt, led some to the conclusion that the acidic ingredients are the chief determining factors. Moreover, it was soon found that a single salt is more injurious than a mixture of several, such as sea water. This also led to the inference that the varying degree of dissociation of these salts essentially influences the effects.

Kearney and Cameron have investigated these relations, and have by artificial cultures in solutions of varying concentration and composition studied the behavior of plant roots and the limits of their endurance. They found for the several salts occurring in alkali soils, taken separately, the following figures, in 100,000 parts of water:

Magnesium sulfate 7 “ chlorid 12 Sodium carbonate 26 “ sulfate 53 “ chlorid 116 “ bicarbonate 167 Calcium chlorid 1,377

Report No. 71, U. S. Dep’t. of Agriculture, 1902.

It will be noted that in many respects the results given in this table stand in marked contrast to the facts observed in alkali lands everywhere; and therefore while interesting physiologically, are not directly applicable to practice. Magnesium sulfate, which according to this table is the most injurious of all, is a common ingredient of alkali lands from Wyoming to New Mexico, as also is sodium sulfate; yet there, as well as in the Musselshell valley in Montana, and at many other points, it shows no specially deleterious action either upon native or cultivated plants, and in Europe as well as in New England the mineral kieserite is freely used as a fertilizer at many points. That sodium sulfate should be twice as harmful as sodium chlorid or common salt, and half as harmful as the carbonate or black alkali, is again wholly contrary to actual experience, which as shown elsewhere in this chapter, indicates that the majority of plants will tolerate between three and four times as much of sodium sulfate as of common salt; while the ratio of tolerance as against the carbonate seems sometimes to rise as high as ten to one.

It is clearly evident, however, that it is the metallic or basic ingredient that in the main determines the toxicity of these salts. The universal presence of lime in some form in all alkali lands doubtless explains the discrepancies mentioned, since lime is especially potent in counteracting the injurious effects; thus throwing additional light upon the importance of the lime-content of alkali soils proper, and also upon the causes of the narrow limitations of the littoral (marine saline) flora; inasmuch as, unlike alkali soils, marine alluvial lands are by no means always calcareous. Cameron goes so far as to attribute the favorable effects of gypsum upon black alkali not so much to the conversion of the latter into neutral sulfate, as to the effect of gypsum solution in counteracting the saline effects. This interpretation, however, seems rather far-fetched, since there can be no question about the double decomposition of gypsum with carbonate of soda; or the intense injuriousness of carbonate of soda in the actual corrosion of vegetable tissues. The corresponding protective influence of various salts, more especially of those of lime, against the injurious effects of pure common salt on marine animals, has already been mentioned (chapter 20, page 380), and later investigations by Osterhout on marine algæ, show the same relation to hold true for them also.

Reclamation of Marine Saline Lands for Culture.—The reclamation of sea-coast lands and marshes for agricultural use is based in general upon the same methods as those already outlined for alkali lands in