Soils, their formation, properties, composition, and relations to climate and plant growth in the humid and arid regions

by Eugene W. Hilgard

The Chilean locality is the only one from which the commercial article is derived; the deposits elsewhere are too limited in extent to compete commercially with the South American product. Caliche ranging as high as 80% of nitrate of soda has been sent to the writer from the Colorado Desert in Southern California, but the exact locality of occurrence has not been divulged. Extended areas of clay hills impregnated with nitrates exist in the Death Valley region of California, but in the absence or extreme scarcity of water in that region, it is doubtful whether these impregnations can be made practically available. Another locality is that near White Plains, Nevada, where Caliche averaging about 50% purity is found in cavities and crevices of a reddish volcanic rock. The rainfall in this region is so slight that the greater part of the dust or sand blown about by the wind consists of Glauber’s salt. Here also, as in Chile, the niter deposits appear to be restricted to within a short distance from the surface, and the total amount thus far observed appears to be insufficient to encourage large-scale exploitation.

Origin of Nitrate Deposits.—The probable origin of these niter deposits has given rise to a great deal of discussion, and a wide difference of opinion exists as to the source from which the nitrogen may reasonably be supposed to have been derived. According to the present state of our knowledge, it must be presumed that its sources have been organic, and that the niter has been produced by the activity of the same bacteria which now produce nitrates in our soils, rendering the nitrogen of humus available to plants. But it is by no means clear what that organic material could have been; for at the present time the plateau of Tarapacà is almost wholly destitute of vegetation, if not of animal life. The latest and apparently most reasonable suggestion is that of Kuntze, who calls attention to the fact that the vicuñas and llamas which are at home in this portion of the Andes, and are known to have roamed over that region in countless herds, have the curious habit of always depositing their manure in one and the same place whenever at liberty. Each herd of these animals has its definite dunging place at some convenient point. That such herds have existed in the region from time immemorial is obvious from historical as well as collateral evidence; and as their manure accumulated, its nitrification would progress rapidly under the prevailing arid conditions. The common salt would naturally be derived from the urine and excrements, and the alkaline salts which exist throughout this region as the products of soil decomposition, would be quite sufficient to account for the alkaline bases in the caliche. On the other hand, the presence of iodine points to seaweeds as the organic source.

Intensity of Nitrification in Arid Climates.—Of the efficacy of nitrification under arid conditions abundant evidence may be found within the State of California. In the alkali lands of southern California the nitrates of soda, lime and magnesia are almost universally present; they form at times as much as one-fifth and even more of the entire mass of alkali salts, and in one case the total amount in the soil has been found to reach two tons per acre, with an average of twelve hundred pounds over ten acres. In the plains of the San Joaquin Valley, spots strongly impregnated with niter are found, especially under the shadows of isolated oak trees, where the cattle have been in the habit of congregating for a long time; a case quite analogous to that supposed by Kuntze to exist in the Chilean locality. Of course it is only in arid climates that the accumulation of nitrates can usually occur; for in the region of summer rains the nitrates formed during the warm season will inevitably be washed into the subdrainage, unless restrained by absorption by the roots of vegetation. The heavy losses occasionally occurring from this cause in the course of a rainy winter on summer-fallowed land have been amply demonstrated by many investigations.

Potash Minerals.—By far the most abundant occurrence of potash in the earth’s crust is that in silicates and notably in orthoclase or potash feldspar, which contributes so largely to soil-formation. But in the absence of any economically successful artificial method for producing potash compounds from feldspars on a commercial scale, almost the entire supply of potash salts was, until a comparatively late period, derived from plant ashes, viz., the “potashes” of commerce. At the same time, almost the entire demand for alkalies for industrial uses bore upon the same product, until the invention, toward the end of the last century, of LeBlanc’s process for the manufacture of soda from common salt; for until that time, soda in the various forms in which it was imported from the Orient or prepared from seaweed ashes, was a comparatively costly product. LeBlanc’s invention was most timely in that it very quickly diminished materially the production of potashes which, in view of the increased demand for alkalies for industrial uses, seriously threatened the depletion of agricultural lands, and of woodlands as well, of one of its most essential ingredients. Yet as there are many industrial uses in which soda cannot replace potash, the manufacture of potashes continued to a greater or less extent, as no other available source except the ashes of land plants, was then known. The production of potassic chlorid from the mother-waters of sea salt in the spontaneous evaporation of sea water for the manufacture of common salt, was on too small a scale to influence materially the manufacture of potashes.

Discovery of Stassfurt Salts.—The depletion of potash had become so serious a matter in the agricultural lands of Europe, that for a time much research was bestowed, and prizes offered for an economical method of producing potash salts from feldspar, on a commercial scale. But the problem had not been satisfactory solved when, in the year 1860, attention was called to the fact that the saline deposits overlying certain large rock-salt beds that had been developed by borings near Stassfurt in Prussia, contained so large a proportion of potash salts, as to render their purification and conversion into fairly pure sulphate and chlorid technically feasible. The impulse having been given, the potash industry developed rapidly in that region as well as in the adjacent portions of Saxony, where the same formation underlies; the production of “Stassfurt Salts” rapidly assumed a greater development than that of the rock-salt which had originally prompted the enterprise, and numerous additional boreholes demonstrated an unexpectedly wide extension of the same beds. At the present time, in consequence of such development, the manufacture of potashes from plant ash has almost ceased, outside of Canada and Hungary; and the production of potash salts in the Stassfurt region now supplies the demand of the entire world, both for industrial and agricultural purposes.

The cheapening of potash as a fertilizer has rendered possible the profitable cultivation of large areas of land which were naturally too poor in that substance for ordinary cultures; and has likewise rendered possible the restoration to general culture of lands that had ceased to produce adequately, on account of the depletion caused by long-continued cropping. It has likewise served to intensify agricultural production wherever desired; and between this supply and that of phosphoric acid from the phosphorites (see above), and the discovery of the nitrogen-absorbing power of leguminous plants, which can be used for green-manuring, farmers have been enabled to dispense, in many regions, with the production and use of stable-manure, which until then had been considered an indispensable adjunct to agriculture everywhere. Even within the last fifty years it was proclaimed by high authority in Germany that stable-manure constituted, as it were, the farmer’s raw material, from which he manufactured the various products of the field through the intervention of the plant-producing power of the soil.

Origin of the Potash Deposits.—The manner in which this accumulation of potash salts has been formed deserves explanation. It is abundantly evident that nearly all deposits of rock-salt thus far known have been formed by the evaporation of sea-water at times when bays or arms of the sea were cut off from open communication with the ocean. The composition of sea-water has already been given and discussed ([chap. 2, p. 26]); and by the slow evaporation of sea-water on a small scale we can quite successfully imitate the phenomena observed in natural rock-salt deposits. When sea-water is heated a slight deposit of lime carbonate (usually containing a little ferric oxid and silica) is soon formed; and a corresponding thin deposit of ferruginous limestone is commonly found at the base of rock-salt-bearing deposits. Next above this we almost invariably find a deposit of gypsum, sometimes of great thickness; in the artificial evaporation of sea-water the same thing occurs so soon as the brine has reached a certain degree of concentration. It constitutes the major portion of the “panstone” of salt-boilers. Next above follows a deposit of rock-salt, at base somewhat mixed with gypsum; its thickness varies greatly according to circumstances. Above it lie the potash salts.

In the manufacture of sea-salt by evaporation in shore lagoons or “saltpans,” the solution remaining after the salt has been deposited (known as “mother-waters,” or “bittern”), of course remains on the surface of the salt unless allowed to drain off, as is done in the process of manufacture. When not drained off, the water gradually evaporates, and there remains a saline crust of a composition exactly resembling that of the upper layers at Stassfurt, containing a large proportion of potash salts.

If it be asked why the Stassfurt salts are not found overlying every rock-salt deposit in the world, the answer is that in a great many cases the concentrated mother-waters have had an opportunity to flow off from the surface of the rock-salt by the action of tides, the inflow of fresh water from the land or from other causes. Their presence therefore depends upon the fulfilment of accidental conditions not nearly always realized in the natural evaporation of sea-water, but which happened to occur on a very large scale in that portion of the North-European continent.

Nature of the Salts.—The potash is present in the Stassfurt salts in the form of complex sulfates and chlorids containing, besides, sodium, calcium and magnesium in various proportions and modes of combination. The most abundant of the potassic chlorid minerals is carnallite, a hydrous chlorid of potassium and magnesium. The chlorids characterize chiefly the upper portions of the deposit, the sulfates the lower.