CHAPTER V.

THE MINOR MINERALINGREDIENTS OF SOILS; MINERAL FERTILIZERS; MINERALS INJURIOUS TO AGRICULTURE.

(A.) MINERALS USED AS FERTILIZERS.

Of minerals important in soil-formation, not usually present in large amounts in rocks, but extensively used in fertilization, the following require mention:

Apatite; phosphate of lime containing more or less of the chlorids and fluorids of the same metal; the mineral from which the phosphoric acid of the soil is mostly derived. In the crystallized condition when perfectly pure it is colorless; but it is mostly of a greenish tint (hence “asparagus stone”). The pure crystalline mineral rarely occurs in large masses (as in Canada); but small to minute crystals are found widely disseminated in many rocks (granites, “basalts” of the Pacific Northwest), thus passing into the soils formed from these rocks. These crystals are readily recognized, being regular six-sided prisms with a flat or obtusely pyramidal termination (distinction from quartz), and do not effervesce with acids (distinction from calcite). By far the largest deposits of this mineral occur in connection with carbonate of lime, in the rock materials known as phosphorites. Lime phosphate being, like the carbonate, soluble in carbonated water, the two naturally frequently pass into solution, and are subsequently deposited together. Most limestones contain a small proportion of lime phosphate, being, as already stated, formed from the shells and the framework of animal organisms usually containing also phosphates. But the content of phosphates in limestones is not readily apparent to the eye, and the richest deposits, save such as contain animal bones, have long passed unsuspected as to their being anything else but limestone. Systematic search has now revealed the presence of phosphate rock in numerous localities, chiefly where limestone formations occur. In the United States, in South Carolina, Florida, Alabama, Tennessee, Kentucky, Nevada; in South America, on Curaçoa island, Venezuela; in the Antilles on Sombrero, St. Martins and Navassa islands. In Africa, in Algiers and Tunisia; in Europe, in Spain (Estremadura, one of the first deposits known), France, Belgium and the adjacent parts of Germany; in Bohemia and Galicia in Austria; and very extendedly in European Russia. Many islands of Oceanica supply phosphorites derived from the decomposition of bird guano by the coral limestone.

Unfortunately the percentage of phosphate in a large proportion of these materials is not sufficiently high to make their conversion into water-soluble superphosphate economically possible at the present time; since all the calcic carbonate present must also be converted into comparatively worthless sulphate (gypsum) by the use of sulfuric acid; and as yet no practicable method for avoiding this difficulty has been found.

“Thomas Slag.”—Probably the nearest approach to such a method is indicated by the fact that a compound containing four instead of three molecules of lime to one of P₂O₅, such as is contained in the “Thomas slag” of the basic process of steel manufacture, is nearly or in some cases (“sour” soils) quite as effective for the nutrition of plants as the water-soluble superphosphate. This discovery has rendered available for agricultural use the phosphoric acid contained in the enormous deposits of limonite iron ore known as bog ore, which contains a large proportion of ferric phosphate and from that cause has until lately been excluded from the manufacture of wrought iron and steel. It is reasonable to hope that by some analogous process the low-grade phosphorites, such as those of Nevada and the plains of Russia, will also in the course of time become available for agricultural use. Extremely fine grinding and washing (producing “floats”) has been resorted to for the purpose of rendering the raw phosphorites effective in fertilization. But while this is successful on some soils, on others the “floats” remain almost inert; so that this method has found only limited acceptance.

Animal bones, which consist of from 24 to 30% of animal substance and 70 to 76% of “bone earth,” (or when fossil are free from the former), are largely used for the manufacture of superphosphate. The bone-earth consists in the main of tri-calcic phosphate with from one to two per cent of calcium fluorid (much as in natural apatite), a small amount of magnesic phosphate, and about 4 to 6% of calcic carbonate. Bone meal can therefore supply to plants both phosphoric acid and nitrogen, and the presence of the latter has been largely the cause of a material overestimate of its efficacy as a fertilizer in the past. Wagner’s and Maerker’s experiments have shown that at least in sandy soils poor in humus, it cannot be considered an adequate source of phosphoric acid for annual crops, and that in these soils its immediate effects are almost wholly due to its nitrogen-content. The slow availability of the phosphoric acid renders it unprofitable as a source of the latter, outside of the heavier lands with abundance of humus; in “sour” lands (notably on meadows) bone meal produces its best results. In soils naturally calcareous, or in such as have received heavy dressings of lime either as carbonate or in the caustic condition, the manurial effects of bone meal are seriously diminished. Nagaoka (Bull. Coll. Agr. Tokyo, Vol. 6, No. 3) shows that the crop of rice fertilized with bone meal was reduced to less than half when limed, and that the phosphoric acid taken up by the crop was reduced to one-sixth. In any case it is most important that bone meal should be as finely ground as possible, as in the case of the phosphorites; and this can best be done when it has first been freed from fats by boiling with water, and then steamed under pressure. It can then also be most readily converted into superphosphate.

The phosphate minerals and the fertilizers manufactured therefrom are of primary importance to agriculture. The phosphoric-acid content of soils is mostly very small, and only a fraction of it is usually in an immediately available form. Hence for permanent productiveness, and especially for intensive farming or gardening, a cheap supply of phosphate fertilizers is of first importance in all soils and climates.

Other phosphate minerals occur frequently, but as a rule only in small amounts, in connection with the ores of most metals. The only ones of these of interest to agriculture are Vivianite and Dufrenite, the phosphates respectively of the protoxid and peroxid of iron. The former occurs in mineral deposits as small blue crystals, or more frequently as blue earthy masses or streaks, in the substrata of rich alluvial ground (Louisiana, California). Dufrenite sometimes results directly from the oxidation of the protoxid mineral, which then turns greenish and finally brown. Unfortunately these minerals, rich as they are in phosphoric acid, cannot readily be utilized as sources of phosphate fertilizers, because of the difficulty of getting rid of the iron. Their occurrence usually suggests the presence of abundance of phosphoric acid in the soil. But that which is actually combined with the iron oxids is practically unavailable to plants; especially so in the case of the peroxid compound, the formation of which is a common source of loss of phosphoric acid when soils rich in iron are submerged for any length of time; a point which is discussed below (chapt. 13).

Among the iron phosphate minerals, may also be mentioned “bog ore,” which results from the reductive maceration of swamped ferruginous soils, and accumulates in the subsoils and in the bottom of swamps or moors, forming “moorbedpan”; a dark brown, rather soft mass, which is sometimes used as an iron ore, especially since the invention of the “basic process” of iron smelting, one of the products of which is the phosphate or Thomas slag. (See above).

Nitrate of Soda or Chile saltpeter.—This mineral being (like all nitrates) easily soluble in water, can only occur in regions nearly or quite destitute of rainfall. Such is the case in the Plateau of Tarapacà in Northern Chile, where it occurs in large quantities; it is likewise found, but to much smaller extent, in Nevada, southern California, Egypt and India. By far its most extended occurrence is that in Chile, where, together with common salt, it fills cavities and crevices in a gravelly clay that forms the surface of a plateau from three to six thousand feet above the sea. It is never pure, but always mingled with a large proportion (up to 50% and over) of common salt; also some Glauber’s salt (sulfate of soda) and some sodic perchlorate and iodid; hence it forms an important commercial source of iodine.

The mixed mineral mass, called “Caliche,” when taken out of the ground is dissolved in water; and the solution boiled down, during which process the common salt is first deposited and is raked out of the pans; the nitrate is afterward farther purified by crystallization. As brought into commerce for agricultural purposes it constitutes a moist gray saline mass, somewhat resembling common salt, of which substance it usually contains a few per cent; occasionally also a small amount of sodic perchlorate (which acts injuriously on vegetation). Aside from its use as a fertilizer, Chile saltpeter serves for the manufacture of nitric acid; and either directly, or after previous transformation into potassic nitrate, for that of gunpowder.

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.

Kainit.—Of the products derived from the Stassfurt salt industry for agricultural use, the two requiring special consideration are “kainit,” a natural mixture of the several chlorid minerals in varying proportions; and “high-grade sulfate.” Being a natural product, “kainit” is the cheapest source of potash available to the farmer; but on account of its variability in composition it must be sold and purchased on guaranteed assay. On account of its large content of chlorin it is not desirable in the production of certain crops, especially in the arid region, where alkali soils, and even those not visibly alkaline, often contain already large amounts of chlorin. Moreover, kainit usually contains a considerable proportion of common salt. For the arid region therefore the sulfate is generally preferable, although it is somewhat higher in price for the same amount of potash. The potash content of commercial kainit (calculated as K₂O) ranges from 16 to 35%, while the sulphate frequently ranges from 80 up to 95% of the pure sulfate; thus costing materially less in freight charges than the lower-grade kainit. Its potash content ranges from 43 to over 50% of K₂O.

Potash Salts in Alkali Soils.—The sulfates and chlorids of potassium, however, occur not only in connection with rock-salt deposits, but are also found in the alkali soils of the arid region. They are, in fact, never absent where such salts occur at all, and their percentage in the total of salts ranges all the way from about 4 to as much as 20% of potash sulphate. In numerous cases it has been found that the content of this salt to the depth of four feet amounts to from 1200 to 1500 pounds per acre. In such lands, of course, additional fertilization with potash salts is totally uncalled for, the more as such soils invariably contain, besides the water-soluble potash, an unusually large percentage of the same in the form of easily decomposable silicates, or zeolites.

Farmyard or Stable Manure.—In connection with the subject of mineral fertilizers, it will be proper to discuss briefly the uses and special merits of stable manure, composts, etc. Up to within the last century, these were practically the only fertilizers known and used, and the exclusive use of this manure might have continued indefinitely but for the discovery that as time progressed, stable manure and with it grain crops, for the production of which it was necessary, became less and less in amount, so as to threaten bread famines. The cause of this diminution was, of course, the incompleteness of the return of the soil-ingredients taken off by the crops, when these were exported to feed the cities or foreign countries. Thus the attention of chemists, and notably that of Liebig, was attracted to the solution of the problem of keeping up production even with an insufficient supply of stable manure; and the discovery of the use of mineral fertilizers was the result of their activity.

The chemical composition of stable manure does not, alone, suffice to explain its remarkable efficacy and the difficulty of replacing it by any other material. The composition of manure of course differs not only with different animals but also with the different feeds consumed by them; but the average composition of farmyard manure is approximately given thus by Wolff and others:

ANALYSES OF VARIOUS FARMYARD MANURES.

========================================================== | 1. | 2. | 3. | 4. | 5. | | | | | Water | 71.00 | 75.00 | 79.00 | 79.95 | 72.33 Dry Matter | 29.00 | 25.00 | 21.00 | 20.05 | 27.67 Ash ingredients | 4.40 | 5.80 | 6.50 | | 5.87 Potash | 0.52 | 0.63 | 0.50 | 0.84| 0.69 Lime | 0.57 | 0.70 | 0.88 | | 0.85 Magnesia | 0.14 | 0.18 | 0.18 | | 0.14 Phosphoric acid | 0.21 | 0.26 | 0.30 | 0.40 | 0.30 Ammonia | | | | | 0.02 Total Nitrogen | 0.45 | 0.50 | 0.58 | 0.78 | 0.46 ----------------+-------+-------+-------+----------+-------

1. Average composition of fresh farm manure (Wolff).

2. Average composition of moderately rotted farm manure (Wolff).

3. Average composition of very thoroughly rotted farm manure (Wolff).

4. Mixed cow and horse manure from a bed two feet thick, accumulated during the winter in a large covered yard, and packed solid by the tramping of cattle (The analysis by F. E. Furry).

5. “Box Manure,” consisting of mixed manure of bullocks, horses, and pigs (Way, Royal Agric. Soc. Journ., 1850, II., 769).

And soda.

It is thus seen that the percentage of the important plant-foods in stable manure are minute when compared with those commonly found in “commercial” fertilizers. Nor are they so much more available for plant absorption than the latter; a very large proportion is not utilized at all the first year, and unless the amount applied is very large it hardly carries the supply needed for the usual crops.

It is now well understood that its efficacy is largely due to the important physical effects it produces in the soil. It helps directly to render heavy clay soils more loose and readily tillable. If well “rotted” or cured it also serves to render sandy, leachy soils more retentive of moisture; and the humus formed in its progressive decay imparts to all soils the highly important qualities discussed later on (chapt. 8). More than this, the later researches have shown that stable manure acts perhaps most immediately upon the bacterial activity in the soil, greatly increasing it not only directly by the vast numbers of these organisms it brings with it, but also in supplying appropriate food for those normally existing in the soil (see chapt. 9). In so doing it serves indirectly to render the soil ingredients more available, and to impart to the soil the loose condition required in a good seed-bed—a “tilth” which cannot be brought about by the operations of tillage alone.

The only possible substitute for the use of stable manure is found in green-manuring with leguminous crops conjointly with the use of commercial or mineral fertilizers. Unless this is done the use of the latter, alone, ultimately leads to a depletion of humus substances, which renders the acquisition of proper tilth by the seed-bed impossible, and causes a compacting of the surface soil which no tillage can remedy.

Proper method of using stable manure in humid and arid climates.—In the humid region it is a common practice to spread the stable manure on the surface of the fields and leave it there without any special operation to put it into the soil; trusting to the rains, earthworms and subsequent tillage for its being brought into adequate contact with the roots; it is rarely plowed in. In the arid region this mode of using it is impracticable; it would remain on the surface indefinitely without advancing in its decay because of the dryness, and unless plowed in very deep the ordinary, strawy manure would ruin the seed-bed by rendering it too pervious to the dry air, thus preventing germination. Much of this valuable material has therefore been, and to some extent is still being burnt, thus causing a severe depletion of the land, both of humus and of mineral plant-food. The best way to deal with stable manure in the arid regions is to thoroughly rot or cure it before putting it on the land, and then plowing it in. To do this of course it must be put in piles and wetted regularly; a procedure which at the high prices of labor is thought to be too expensive, but which in the end would be found eminently profitable, unless green-manuring is regularly done. The very small proportion of humus generally present in arid soils renders this precaution indispensable, if production and proper tilth is to be maintained. The saving of stable manure and of all composting material, even if less needful as a means of supplying plant-food in the rich soils of the arid regions, is fully as essential in order to maintain the humus supply.

(B.) MINERALS UNESSENTIAL OR INJURIOUS TO SOILS.

The minerals heretofore mentioned contribute to soil formation either one or several ingredients, important to plant growth either by their mechanical or chemical action. It remains to consider some not intrinsically desirable, but frequently present in certain soils, which should be known to the farmer in order that he may be enabled to counteract or remove their injurious effects. Leaving aside such as are of only casual or rare occurrence, the following may be mentioned as among those which not unfrequently affect soils desirable for culture to such extent as to make them unavailable for general farming purposes:

Iron Pyrite; sulphid of iron containing two molecules of sulphur to one of iron; a mineral exceedingly common in deposits of metallic ores, and whose deceptive gold-like color has caused it to be mistaken for gold so often as to cause it to be designated as “fool’s gold” among miners. While it frequently does contain some gold and is often associated with valuable ores, it is practically valueless when occurring outside of mineral veins, in rock-masses; and more especially in sedimentary rocks, such as sandstones, limestones, shales and clays.

When present in soils it sometimes becomes a source of trouble to the farmer, because in contact with air it is soon transformed into ferrous sulfate or copperas, which, like the carbonate referred to above, is injurious to plants. Sometimes indeed iron pyrite is actually formed in badly-drained soils alongside of the carbonate of iron, when much sulfate (such as gypsum) is present; and then its injurious effects subside more slowly than do those of the carbonate (see above, p. 46).

Recognition of Iron pyrite.—The mineral is easily recognized by its golden or brass-yellow tint; the latter color being the one most commonly shown in the “sulphur balls” occurring in marls or soft limestones. A very easy test is to pulverize it and then heat it on a shovel over a fire, when it will soon itself take fire, burning with a blue sulphur flame, and upon more complete roasting, leaving behind a red powder, viz., “Venetian red” or red ochre; that is, ferric oxid. In clays it commonly occurs in large, well-defined cubes, which do not readily form copperas but rather become covered with a crust of limonite or brown iron ore.

When a subsoil is found to contain pyrite, or when “sulfur balls” have been accidentally introduced with dressings of marl, the remedy is thorough and persistent aeration of the material. In the case of marls nothing more need be done; but in that of ill-drained subsoils it is best to add lime in moderate dressings, to accelerate the transformation into ferric hydrate or iron rust, and gypsum; whereby the copperas becomes not only innocuous but adds two beneficial ingredients to the soil. The same policy will render available manure or other materials which have been disinfected by means of solution of copperas.

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.

It is hardly necessary to say that saline waters containing any of the above salts in notable amounts must be used for irrigation very cautiously. The measures to be observed in this respect will be discussed later.

PART SECOND.

PHYSICS OF SOILS.