Halite (rock-salt), or common salt, has already been mentioned as to its occurrence in connection with the Stassfurt potash salts (see above, page 71); but as rock-salt it rarely exerts any injurious influence upon lands. It is, however, a common ingredient of seashore lands, and is also present to a certain extent in the alkali lands of the arid countries. While it is true that occasionally small quantities of common salt are used as an ingredient in fertilization, its usefulness in that direction is exceedingly subordinate; and it is far more generally to be considered as an injurious ingredient of all cultivatable soils whenever present to a larger extent than a few hundredths of one per cent It is usually considered that one-fourth of one per cent of common salt renders lands unfit for most culture plants. Only a few, such as asparagus, the beet, the saltbushes and some others, succeed when it is present in this or in larger amounts. In the case of sea water it is usually accompanied by a still more injurious ingredient, magnesic chlorid or bittern; which is detrimental to plant growth in much smaller quantities than the common salt itself.
Recognition of Common Salt.—The presence of common salt may, as a rule, be detected by the taste, well-known to every one; when this taste is very intense or somewhat bitterish, it indicates the presence of bittern. The presence of salt, however, is easily verified without the use of chemical reagents, by slowly evaporating some of the clear water leached from the soil in a clean silver spoon. If the last few drops are allowed to evaporate spontaneously, it will be easy to distinguish, even with the unaided eye, the square, cubical crystals, sometimes combined into cross-shape, which are characteristic of common salt. It is always an unwelcome addition to the land, and as its action cannot be neutralized in any way, it can be gotten rid of only by leaching-out. This process is usually accomplished in seashore lands by the action of rain, or by the overflow of fresh-water streams, after the tide has been excluded by means of drains provided with check-valves to prevent the inflow of tidewater; or else by underdrainage, and flooding when possible.
Mirabilite, (Glauber’s salt) or sulfate of soda, exists not unfrequently in the soils of the arid region and sometimes encrusts extended areas of lowlands during the dry season. When present in the soil it will commonly be seen blooming out on the surface after a rain, in light, feathery, needle-shaped crystals, sometimes to such an extent that it can be collected by the handful. Subsequently, when wafted by the wind, it is reduced to a fine white dust, which constitutes a goodly proportion and sometimes the entire mass of the “alkali dust” that is so annoying on the plains of Nevada, and in the desert regions generally, during the hot summer. Near White Plains, Nevada, it forms a thick layer of “white sand,” in which the foot sinks deeply, and which is carried about by the wind with great ease.
Glauber’s salt is never a desirable soil-ingredient. It is largely produced as a by-product in several industries, but cannot be utilized for agricultural purposes to any extent. It is, however, much less injurious to plant growth than common salt; according to experience in California it may be considered about three times less so. It constitutes the major portion of what is commonly known as “white alkali,” which is well known to be much less injurious to crops than the “black” kind, which contains carbonate of soda.
Trona and Urao are natural forms of carbonate of soda or salsoda. Like Glauber’s salt, it commonly occurs as a surface efflorescence or crust in dry or desert regions; either from the evaporation of standing water, as in the case of the soda lakes of Nevada, Hungary and Egypt, or as an efflorescence on the surface of the soil, as in the western United States, Mexico (“urao”), North Africa (“trona”), and at many points in the Old Continent. In the United States it is commonly known as “black alkali,” because of the black spots formed on the surface by evaporation; practically the same name (“kara”) is given it in Arabia and Asia Minor, whence impure soda has long been imported into Europe; while in north India it forms part of the “reh” salts that incrust large areas (usar lands) in the Indo-Gangetic plain.
The natural mineral always contains an excess of carbonic acid over the “normal” salt, nearly in the proportion of four parts of carbonic dioxid to three of soda; it is sometimes designated as sesqui-carbonate. In hot sunshine it may lose most of this excess for a time; while within the soil itself it may, in presence of abundant carbonic acid, become temporarily converted wholly into hydrocarbonate or “bicarbonate,” which is less corrosive than the monocarbonate or common salsoda.
Injury caused in soils.—Like common and Glauber’s salt, carbonate of soda is always an unwelcome soil ingredient; more so, in fact, than either of the other two, since less than a tenth of one per cent is sufficient to render certain soils wholly untillable, by the deflocculation or puddling of the clay; at the same time rendering it impervious to water. It is by far the most injurious ingredient that ordinarily occurs in otherwise good, arable soils; for in addition to the physical effect just mentioned, it dissolves the humus-substance of the soil, forming an inky-black solution which, especially when evaporating on the surface and forming black spots, has given rise to the popular name of “black alkali.” As will be more fully explained hereafter, wherever such is the case, the first step necessary toward reclamation is the transformation of the carbonate of soda, at least in part, into the relatively innocuous sulfate, by means of gypsum in the presence of water; while carbonate of lime remains in the soil.
In its direct action on the plants themselves, soda is also most injurious; as when accumulated to any extent near the surface by evaporation it will corrode the root-crown or stem, and sometimes completely girdle the same, destroying the bark. Farther details on this subject are given in [chapter 22].
Epsomite, or Epsom salt, or sulfate of magnesia, is another one of the water-soluble minerals frequently found efflorescent on the surface of the ground; more commonly in saline seashore lands than in the alkali region proper, although it is rather common in the northeastern portion of the arid region of the United States. Whether on the soil surface or in the crevices of rocks, its needle-shaped, feathery crystals greatly resemble those of Glauber’s salt, but are readily distinguished by the more intensely bitter taste. Epsom salt is frequently the last remnant of sea-salts left in the soil after reclamation. Though probably somewhat more injurious to plant growth than Glauber’s salt, the mineral Kieserite, one of the Stassfurt salts and consisting essentially of Epsom salt, is sometimes used as an application to calcareous lands instead of gypsum, and with good results. Yet gypsum is usually the safer, and equally effective.
Borax (bi-borate of soda) occurs much more rarely than the salts just described; most frequently in certain portions of California, forming part of the “alkali” in the soil. It is injurious to plant growth, but is as readily dealt with as is the carbonate of soda, by dressings of gypsum, whereby inert borate of lime is produced.