CHAPTER III.. The Major Soil-Forming Minerals.

THE MAJOR SOIL-FORMING MINERALS.

Since the several stratified rocks, such as sandstones, shales, claystones, clays, limestones, etc., are themselves but the outcome of the same disintegrating and decomposing influences upon the crystalline rocks by which soils are now formed, we must study the action of these influences upon the minerals composing the latter rocks in order to gain a comprehensive understanding of the subject. While the number of different minerals known to science is very large, such study need not go beyond a small number of the chiefly important, rock-forming species which are so generally distributed as to require consideration in this connection. These minerals are the following: Quartz and its varieties; the several feldspars; hornblende and augite; the micas; talc and serpentine. Calcite, gypsum and dolomite, though not contained in the older rocks, must be considered because of their forming large rock deposits by themselves; and zeolites require mention because, though rarely forming a large proportion of rocks, they are of special importance as soil ingredients.

Quartz and the minerals allied to it consist essentially of dioxid of silicon, usually without (quartz proper) but partly also with water in combination (opal and its varieties). Silicon is next to oxygen the most abundant element found on the earth’s surface. It occurs largely in the various forms of quartz, alone, or as one ingredient of compound rock-masses; the rest, in combination (as silica) with various metallic oxids, forms the important group of silicate minerals, constituting the bulk of most rocks.

Quartz occurs frequently in crystals (rock crystal; six-sided prisms terminated by six-sided pyramids), clear or variously colored; but more abundantly as quartz rock or quartzite, readily known by its hardness, so as to strike fire with steel, and by its glass-like, irregular fracture. Besides the crystalline quartz rock we find close-grained and at least partly non-crystalline varieties, such as hornstone and flint. Sandstones most commonly consist of grains of quartz cemented by some other mineral, or by silica itself; in the latter case the siliceous sandstone frequently passes insensibly into true quartzite. The loose sand so well known to common life is prevalently composed of quartz grains, whose hardness and resistance to weathering enables them to survive longest the soil-forming agencies.

Quartz and its allied rocks—jasper, hornstone, siliceous schist, etc., are all, as already stated, acted on with difficulty by the “weathering” agencies. Crystalline quartz rock may be considered as practically refractory against all but the mechanical agencies, and hence remains in the form of sand and gravel, more or less rounded by attrition, as a prominent component of most soils; sometimes to the extent of over 92 per cent, even in soils highly esteemed in cultivation, especially in the arid region. Such soils are mostly the result of the disintegration of sandstones, the cement of which has been dissolved out in the course of weathering; or they may be derived directly from geological deposits of more or less loose and unconsolidated sand. Among crystalline rocks, granites, gneiss and mica-schists are those most usually concerned in the formation of sandy soils; since in common parlance, quartz is understood to be the substance of the sand unless otherwise stated. The exceptions are especially important in the regions of deficient rainfall.

But while crystalline quartz is practically insoluble in all natural solvents, the same is not true of the jaspers and hornstones. These consist of a mixture of crystalline and amorphous (non-crystalline) silica, which is more readily soluble than the crystalline, and is attacked by many natural waters, especially by those containing even very small amounts of the carbonates of potash or soda. We thus often find that hornstone and jasper pebbles buried in the soil, while still hard internally, have externally been converted into a friable, almost chalky substance, consisting of crystalline quartz from which the cementing amorphous silex has been removed by the soil water. In the course of time such pebbles may be completely destroyed by this process, so as to be light and chalky throughout, and readily crushed in tillage. The change is the more striking when, as frequently happens, the hornstone pebble is traversed by small veins of crystalline quartz, which remain as a skeleton.

Solubility of Silica in Water.—It is easily shown experimentally that the compound of silica with water (hydrate) is under certain conditions readily soluble not only in pure water, but also in such as contains carbonic acid. It thus occurs in nearly all spring and well waters; some hot springs deposit large masses of it (sinter); and geological evidence clearly demonstrates that quartz veins have as rule been formed from water-solutions of silica.

That silica in its soluble form circulates freely in the soil water, is abundantly evident from the large amounts of it which are secreted on the outside of the stems of grasses, horsetail rushes and other plants, imparting a gritty roughness to their outer surface. In the case of the giant bamboo grass of Asia, the silica accumulated on the outside of the joints forms a hard sheath of considerable thickness, known to commerce as tabashir.

That among the first products of rock decomposition we often find small amounts of the silicates of the alkalies (potash and soda) has already been mentioned. It cannot be doubted that the same continues to be formed in soil containing the proper minerals; and there they also take part in the formation of the easily decomposable hydrous silicates designated as zeolites, which are largely instrumental in retaining the “reserve” of mineral plant-food in soils.

SILICATE MINERALS.

Silica occurs in nature combined with the oxids of most metals, forming silicates; but most abundantly with the earths (lime, magnesia, alumina) and alkalies (potash and soda). These compounds are the most important in soil formation; and among them the following are the chief:

The Feldspars, which may be defined as compounds of silicates of potash, soda or lime (either or all) with silicate of alumina. They are prominent ingredients of most crystalline rocks; potash feldspar (orthoclase) with quartz and mica forms granite and gneiss; feldspars containing soda and lime (either or both) form part of many other crystalline rocks, such as basalt, diabase, diorite, gabbro and most lavas. The feldspars are decomposed by weathering rather readily, and are important in being the chief source of clays as well as of potash in soils. When acted upon by carbonated water, the bases potash, soda, and lime or carbonates, the silica being mostly displaced; while the silicate of alumina takes up water and forms kaolinite, the essential basis of clays, and one of the most important constituents of soils; imparting to them the necessary firmness and cohesion, together with other important physical properties, discussed more in detail hereafter.

While thus on the one hand feldspars are the source of clay, on the other they supply one of the most essential ingredients of plant food, viz. potash; which is first dissolved by the water in the forms of carbonate and silicate, but in most cases soon becomes fixed in the soil by forming more complex (zeolitic) combinations. The soda not being retained by the soil as strongly as is potash is washed through into the country drainage; while if lime is present, it mostly remains in the form of the carbonate.

Orthoclase feldspar contains nearly 17% of potash; Leucite, a related mineral occurring in some lavas, contains 21.5%. The other feldspars contain only a few per cent, sometimes none.

Other silicate minerals, so far as they contain the same bases, are acted upon similarly to the feldspars.

In the decomposition of the feldspars by carbonated water, the compounds of potash and soda so formed are soluble in water, those of lime and magnesia are insoluble or nearly so. Hence pure clays can be formed only in the decomposition of the potash-and soda-feldspars (orthoclase, albite) while in the case of lime feldspar (labradorite) and the mixed feldspars (plagioclase, anorthite) calcareous clays (marls) are the result. Lime feldspar resists decomposition more tenaciously than do those containing large proportions of the strong bases potash and soda; potash feldspar especially is attacked most readily, and is the main source of the formation of the valuable deposits of porcelain earth or kaolin, which is essentially a mixture of kaolinite with fine silex and more or less of undecomposed feldspar, and is of a chalky texture.

Formation of Clays.—When instead of remaining in place, this kaolin is washed away and triturated in the transportation by water, it is partially changed from its original chalky condition to that plastic and adhesive form which is the characteristic ingredient of all clays. The remarkable properties of this substance and the part it plays in the physical constitution of soils, will be discussed in another chapter. Its lightness and extreme fineness of grain (if grain it can be called) cause it to be carried farther on by the streams than any other portion of the products of rock decomposition save those actually in solution; it can therefore be deposited only in water that is almost or quite still (as in swamps) so long as the latter is fresh. So soon however as brackish or salt water is encountered, clay promptly gathers into floccules (“flocculates”), and thus enveloping the finest-grained silts that may have been carried along with it, it quickly settles down, forming the “mud banks” and heavy clay soils that are so characteristic of the lower deltas of rivers, as well as of swamps formed by the backwater or overflow of the same.

When instead of potash feldspar alone, the lime- or soda-lime feldspars are also concerned in the decomposition process, the resulting clay soils will be more or less calcareous, while the soda, as stated above, is for the greater part leached out permanently.

Hornblende (Amphibole) and Pyroxene (Augite). These are two very widely diffused minerals, differing but little in composition though somewhat differently crystallized, mostly in short columnar forms. The typical and most abundant varieties of these minerals appear black to the eye, though in thin sections they are bottle-green; they form the black ingredient of most rocks.

The color is due to ferroso-ferric (magnetic) oxid of iron; the mineral as a whole may be considered as a silicate of lime, magnesia, alumina and iron, varying greatly in their absolute proportions; alumina and iron being sometimes almost absent. When iron is lacking the mineral may be almost white (tremolite, asbestos), and its weathering is then much retarded, since the oxygen of the air cannot take part in the process of disintegration.

The black variety of hornblende is not only the most abundant as a rock-ingredient, but it also the one most easily decomposed and therefore most commonly concerned in soil formation. The black hornblende owes its easy decomposition under the atmospheric influences to two properties; one, its easy cleavage, whereby cracks are readily formed and extended by the agencies already mentioned (pp. 1-3). The other is its large content of ferrous silicate (silicate of iron protoxide), whereby it is liable to attack from atmospheric oxygen; the latter forms ferric hydrate (iron rust) out of the protoxide, thus causing an increase of bulk which tends to split the masses of the mineral in several directions, while the silex is set free. At the same time the carbonic acid of the air converts the silicate of lime and magnesia, which forms the rest of the mineral, into carbonates; and the alumina present forms kaolinite, as in the case of the feldspars. There is thus formed from this mineral, when alone, a strongly rust-colored, more or less calcareous and magnesian clay, constituting the material for rather light-textured “red” soils. In most cases however the hornblende is associated in the rock itself with the several feldspars, (mostly lime- and soda-lime feldspars) as well as with more or less quartz. The rust-colored soils are therefore most commonly the joint result of the weathering of these several minerals. This is well exemplified in the case of the “red” soils formed from the so-called granites and slates of the western slope of the Sierra Nevada of California.

Pyroxene or Augite so nearly resembles hornblende in its chemical composition and crystalline form, that what is said of the latter may be considered as applying to augite also. Owing however to the absence of any prominent tendency to cleavage, the smooth crystals of this mineral are attacked much less readily than is hornblende, so that we often find them as “black gravel” in the soils formed from rocks containing it. Such soils are particularly abundant and important in the region covered by the great sheet of eruptive rocks (basalts, so-called) in the Pacific Northwest, and on the plateau of South Central India (the Deccan), and result likewise from the decomposition of the black lavas of volcanoes; thus in the Hawaiian islands, and in the Andes of Peru and Chile.

Both hornblende and augite being either free from, or deficient in potash, of coarse the soils formed from them are apt to lack an adequate supply of this substance for plant use. This is markedly true of hornblende schist or amphibolite rocks.

Mica, commonly known as isinglass, is so conspicuous wherever it occurs that it is more readily recognized than any other mineral. It occurs in glittering scales in soils and sands, and in rocks it sometimes forms sheets of sufficient size to supply the small panes for the doors of stoves, lamp chimneys, etc., which being flexible are not liable to break, but only gradually scale into very thin films, into which it can also be split by hand. When white, (muscovite, phlogopite) its scales are sometimes mistaken for silver by mine prospectors; when yellow, for gold; but their extreme lightness should soon remove these delusions. The composition of mica is not widely different from that of the two preceding minerals; like these it sometimes contains much iron, and is then dark bottle-green (biotite); this variety in weathering becomes bright yellow, and soon disintegrates.

This mineral is so abundant an ingredient of many rocks and soils, that one naturally looks for it to play some definite or important part in soil formation. By its ready cleavage it favors the disintegration of rocks; but it seems that owing to the extremely slow weathering of its smooth, shining cleavage surfaces, it exerts no notable effect upon the chemical composition of the soil, although, owing to its peculiar character of fine scales, it sometimes adds not immaterially to the facility of tillage in otherwise somewhat intractable soils. So far as is known at present, its presence or absence does not constitute, in itself, any definite cause or indication of the quality of any soil. It may nevertheless be said that the rock in which it usually occurs most abundantly—mica-schist, a mixture of mica and quartz—is known to form, as a rule, lands of poor quality. On the other hand, the soils derived from granites and gneisses, even when rich in mica, are usually excellent, on account of their content of feldspars, and frequently of other associated minerals.

Hydromica differs from the preceding mainly in containing a larger proportion of combined water; but it hardly decomposes more readily, and the rocks in which it mainly occurs (hydromica schists) are refractory to weathering, and in any case do not yield soils of any fertility, the mineral being associated simply with quartz.

Chlorite, essentially a silicate of alumina and iron, somewhat resembles mica but is deep green or black, in small scales. It forms part of certain rocks (chlorite schists), which greatly resemble the hornblende schists, but are usually inferior to the latter as soil-formers, containing but little of any direct value to plant life.

Talc and Serpentine, Hydrous silicates of magnesia, are extensive rock-materials in some regions, and as such require mention as soil-formers also. Serpentine usually forms blackish-green rock-masses, that although soft disintegrate very slowly in the absence of definite structure, and are attacked with some energy only when charged—as is frequently the case—with ferrous oxide. The conversion of this into ferric hydrate, so common in nature, here also serves as the point of attack on a rock otherwise very stable; causing it to crumble, even though slowly.

Talc (the true “soapstone”) being usually free from iron, would be even more slow than serpentine to yield to weathering, but that its extreme softness and ready cleavage greatly facilitate its abrasion. Thus talc schist, which is usually a mixture of talc with more or less quartz, undergoes mechanical disintegration quite readily.

But the soils formed from either serpentine or talcose rocks are almost always very poor in plant food, and sometimes totally sterile. Magnesia, though an indispensable ingredient of plant food, is rarely deficient in soils and unlike lime does not influence in any sensible degree the process of soil formation. Magnesian rocks as a whole are practically found to be not specially desirable soil-formers, even in the form of magnesian limestones. They do not even, as a rule, contain as many useful accessory minerals as are commonly found in limestones. Moreover, an excess of magnesia over lime is injurious to most crops, as is shown later (chapt. 18).

The Zeolites.—Zeolites may be defined as hydro-silicates containing as bases chiefly lime and alumina, commonly together with more or less of potash and soda, more rarely magnesia and baryta. The water is easily expelled by heating, but is present in the basic form, not merely as water of crystallization. All zeolites are readily decomposed by chlorhydric and other stronger acids.

The zeolites proper are not original rock ingredients, but are formed in the course of rock decomposition by atmospheric agencies, heated water, and other processes not fully understood. They are therefore usually found in the cavities and crevices of rocks that have been subject to the influence of atmospheric or thermal waters, most frequently in eruptive rocks, particularly in the vesicular cavities characterizing what is known as amygdaloids. They are also found in the crevices of sandstones and shales percolated by water, as well as in nodules of infiltration (geodes), in which they are frequently associated with quartz. Those found in the cavities of rocks are usually well crystallized wherever room is afforded, and are readily recognized by their crystalline form; they are mostly colorless, sometimes yellow or reddish.

Exchange of bases in Zeolites.—Although zeolites rarely form a large proportion of rock-masses and therefore do not enter directly into the soil minerals to any great extent, their interest in connection with soil-formation is very great, because of the continuation, within the soil, of the same processes that bring about their formation in rocks. Under the conditions existing in soils they will naturally rarely form crystals, but will appear in the pulverulent or gelatinous form, leaving the zeolitic nature of the material to be inferred from its chemical behavior. Among these characters the ready decomposability by acids has already been mentioned; another of special importance in the economy of soils is the fact that when a pulverized zeolite is subjected to the action of a solution containing either of the stronger bases usually present (potash, soda or lime), such base or bases will be partially or wholly taken up by the zeolitic powder, while corresponding amounts of the bases originally present will pass into solution.

Thus when a hydrosilicate of soda and alumina is digested with a solution of potassic chlorid or sulphate, the soda may be partially or wholly replaced by potash, while the corresponding sodium salt passes into solution. In the case of zeolites containing lime or magnesia or both, the action of potassic or sodic chlorid will be to partially replace the lime, while calcic and magnesic chlorids pass into solution, resulting in the partial or complete replacement of the lime by one or the other, or by both bases. It is important to note that, other things being equal, potash is usually absorbed in greater amounts and is held more tenaciously than soda. The process may frequently be partially or wholly reversed again, by subsequent treatment with large amounts of solutions of the displaced base or bases. Thus while a solution of potassic chlorid may be made to expel almost completely the sodium present in analcite, subsequent treatment with sodic chlorid solution will again almost completely displace the potash before taken up. The same happens when the natural mineral potash leucite, (see p. 32) of frequent occurrence in certain lavas, is pulverized and treated with a sodic solution; resulting finally in the production of a mass corresponding to natural analcite, the sodium mineral corresponding to leucite.

In other words, in any zeolitic powder the alkaline or alkaline earth bases present may be partially or wholly displaced by digestion with an excess of solution of any of these, varying according to the amount of solution employed, and the length of time and temperature of action.

This characteristic behavior of zeolites is exactly reproduced in soils. Few soils permit any saline solution to pass through them unchanged; solutions of alkaline chlorids filtered through soils almost invariably cause the passing through of calcium and magnesium chlorids, while a part of the alkaline base is retained; and as a matter of fact, we find that this absorbing power of soils for alkaline bases is more or less directly proportional to the amount of matter which may be dissolved or decomposed with elimination of silica, by means of acids.

This absorption of bases from solutions by chemical fixation will be farther discussed later on; but it should be mentioned here that both naturally and artificially, rock-masses are very commonly cemented, wholly or in part, by zeolitic material. Hydraulic concretes may be considered as sandstones or conglomerates whose grains are cemented by a zeolitic cement consisting of silica, lime and alumina, with usually some potash or soda, and of course containing the basic water; hence unaffected by the farther action of the latter substance after the time of setting has expired, which varies somewhat according to the nature of the material used. That similar cements should occur in natural sandstones is to be expected; thus we find not unfrequently that certain sandstones are materially softened, and their resistance destroyed, by treatment with even moderately dilute acid, while silica and the usual zeolite bases pass into solution. It is not often, however, that zeolitic material alone cements the sandstone; it is most frequently associated with siliceous, calcareous and sometimes even with ferruginous cementing material.

A zeolitic mass, at first gelatinous and then becoming granular-crystalline is frequently observed oozing from the lower surface of newly made concrete reservoir dams: just as we find similar oozes consolidated into natrolite crusts in the crevices of natural sandstones.

CALCITE AND LIMESTONES.

Calcite or calcareous spar is one of the minerals most commonly known in the crystallized form, and is readily recognized by its perfect cleavage in three directions, producing cleavage forms with smooth, rhomb-shaped faces (rhombohedrons); these are sometimes colorless and perfectly transparent, and laid on printed paper show the letters double. But it may be whitish-opaque, and of various colors, which may also be imparted to the limestones formed from it. It is readily distinguished from quartz, which it sometimes resembles, by its cleavage, its inferior hardness, being easily scratched with a knife; and by its effervescence with acids, the latter being the crucial test when other marks are unavailable, as when it forms soft granular masses or “marls.” In all cases it can be recognized by its crystalline form under the microscope, even when the substance containing it has been pulverized in a mortar. The great importance of this compound—calcic carbonate—from the agricultural point of view renders it desirable that it, as well as limestones as such, should be recognized, when seen, by every farmer.

In mass the pure mineral constitutes white marble; colored or variegated marbles are more or less impure from the presence of other minerals. Some compact limestones also are nearly pure; and as supplying only a single ingredient of plant food these would not be much better soil-formers than quartz or serpentine. But it is quite otherwise with common limestones; the mass of which, it is true, is formed of calc-spar, but owing to its origin, is in the great majority of cases so far commingled with other matters of various character, that limestones are popularly reputed to form the very best soils. “A limestone country is a rich country” is a popular axiom to which there are, on the whole, but few exceptions.

Origin.—Actual observation of what is happening at the present time, as well as the examination of the rock as anciently formed, prove conclusively that with insignificant exceptions, all limestones have been formed from the framework and shells, and to some extent from the bones, of marine and fresh-water organisms, ranging in size from the extinct giants of the lizard relationship to those recognizable only by the microscope. Owing to the solubility of lime carbonate in carbonated water, the organic forms have often (in crystalline limestones) been almost completely obliterated in some portions, but in others are so preserved as to prove undeniably the similarity of origin of the whole, and that they have been formed in relatively shallow water, as they are to-day.

Impure Limestones as Soil-formers.—From what has been said regarding the composition of sea-water, it will readily be inferred that a pure deposit of any one kind cannot easily be formed in it; moreover, the matter held in mechanical suspension everywhere near the coasts must very commonly be included within the calcareous deposits formed off-shore. Hence few limestones dissolve in acids without leaving a residue of sand, clay and various other substances, usually even some organic matter not fully decomposed; sometimes less than half of the mass is really lime carbonate. It is obvious that when the solvent action of carbonated water is exerted upon such impure limestones, a loose residue of earthy matters will remain behind. It is by this process that a considerable proportion of the richest soils in the world have been formed, which have given rise to the popular maxim above quoted. They are emphatically “residual” soils; sometimes, it is true, somewhat removed, by washing-away, from their point of origin, but in many cases forming a compact soil-layer on top of the unchanged rock, into which there exists every shade of transition. Striking examples of such residual soils in place are seen in the black prairies of the southwestern United States; they are mostly rather “stiff” (clayey), and hence has arisen a local popular error, to the effect that clay or “heavy” soils are always calcareous. On the other hand, the blue-grass region of Kentucky, and most of the lands of the arid regions are prominent examples of “light” calcareous soils.

Caves, Sinkholes, Stalactites.—Perhaps the most striking exemplification of the solvent power of carbonated water is seen in the formation of limestone caves. As a matter of fact, the vast majority of all existing caves is found in limestone formations; and such formations, as will be more fully discussed hereafter, nearly always bear a luxuriant vegetation. The water filtering through the vegetable mold, in which carbonic acid is constantly being formed, becomes charged with it, and on reaching the underlying rock, dissolves to a corresponding extent the lime carbonate of which this rock wholly or chiefly consists. When penetrating crevices it soon enlarges these, to an extent proportioned to the length of time and the strength of the solvent; and thus gradually subterranean passages or caves are formed, which at first are almost always the bed of a stream, the mechanical action of which accelerates the process of enlargement, until after some time the water is perhaps drained off through some crevice to a lower level, where the same process is repeated.

Sometimes the ceiling gives way, forming the funnel-shaped “sinkholes” or “lime-sinks” so familiar in some of the Mississippi Valley States. Sometimes the lime solution on reaching the ceiling of the cave, instead of dropping down, evaporates there and eventually forms icicle-like “stalactites” out of the dissolved substance; while when dropping on the floor and thus growing upwards, the corresponding formation is called “stalagmite.” These caves, subterranean rivers, sinkholes, natural bridges and tunnels, etc., mostly owe their origin to this solvent action of carbonated water on limestone formations.

T. M. Reade (in his treatise on Chemical Denudation in Relation to Geological Time) calculates that 143.5 tons of lime carbonate are annually removed by solution from each square mile of land in England and Wales, and that the average amount thus removed annually from each square mile of the earth’s surface is about fifty tons.

The same occurs on a small scale, when calcareous land is underdrained; the lime carbonate dissolved from the soil is partially deposited in the drain pipes, which it frequently obstructs. Similarly, an impure, porous deposit of calcareous tufa is frequently formed on the surface, at the foot or in rills of calcareous hills. When “hard” water, being usually such as contains lime carbonate dissolved in carbonic acid, is boiled, or long exposed to the air, carbonic gas escapes and the lime salt is deposited partly on the walls of the kettle, partly forming a pellicle on the surface of the water.

Dolomite, or bitter spar, greatly resembles calcite in its aspect and properties, although containing nearly half its weight (47.6%) of magnesic, together with calcic carbonate. It is, however, nearly always whitish-opaque; its crystalline and cleavage surfaces are usually somewhat curved; and its effervescence with acids is much less lively than in the case of calcite. Like the latter it often forms pure granular rock deposits, frequently used instead of marble and limestone, and under that designation. The dolomite rocks, however, are much more subject to weathering than the non-magnesian limestones, and it is a curious fact that in contradistinction to the limestone regions proper, those having strongly magnesian limestones or dolomites as their country rock are frequently remarkably sterile. In some portions of Europe dolomite areas are sandy deserts, whose sand consists of weathered dolomite, so pure as to offer no adequate supply of mineral food to plants. In the United States, magnesian limestones underlie the “barrens” of several States and thus seem to justify their European reputation of being poor soil-formers. The exact cause of this difference is not fully understood, for at first sight it is not clear why the presence of the magnesian carbonate should interfere with the well-known beneficial effects of the lime compound. O. Loew and May and others have, however, shown that a certain excess of lime over magnesia in the soil is necessary to prevent the injurious effects exerted by magnesic compounds on plant nutrition, in the absence of an adequate supply of lime. This point will be discussed more in detail farther on.

Bull. No. 1, U. S. Dept. Agr. Veg. Path. and Physiol. Investig.

Selenite or Gypsum, sulfate of lime with about 14 per cent of water, though not as abundant in nature as the carbonate or limestone, is a very widely disseminated mineral and often occurs in large masses over considerable areas. These are undoubtedly in most cases the result of evaporation of sea water (see p. 26), more rarely of the transformation of limestone. In mass it frequently resembles the latter, but is readily distinguished by its softness; it does not grit between the teeth, is readily cut with a knife and does not effervesce with acids. Very commonly it occurs in crystals, which are easily split into thin plates. The crystals are very frequently found imbedded in gray or bluish, tough clays, in rosettes, or flat sheets which mostly show characteristic incurrent angles (caused by twinning), and are hence known as “swallowtail” crystals. Such sheets of selenite are popularly called “isinglass,” which name however is equally applied to the mineral mica (see p. 35).

Gypsum is only exceptionally an abundant ingredient of soils; yet such soils prevail quite extensively on the upper Rio Grande, in New Mexico and adjacent portions of Chihuahua, Coahuila, and on the Staked Plains of Texas. Here whole ranges of hills are sometimes composed of gypseous sand, bear a scanty, peculiar vegetation, and are ill adapted to agricultural use. It may be said in general that few naturally gypseous soils are very productive. This is largely because of the very heavy clays which commonly accompany it, as the compound itself is not only not hostile to plant life but is in extended use as a valuable fertilizer (“land plaster”) for special purposes. From causes not fully understood as yet, it particularly promotes the growth of leguminous plants, notably the clovers; and as stated in chapter 9, it also specially favors nitrification in soils. In the arid region it renders important service in the neutralization of “black alkali” or carbonate of soda in alkali soils. Being soluble in 400 parts of water, it easily penetrates downward in most soils, and in doing so effects changes in the zeolitic portions, setting free potash from silicates and thus indirectly supplying plants with this essential ingredient in a soluble form. About 200 pounds per acre is an ordinary dose.

For agricultural use the rock gypsum is ground in mills so as to be easily distributed, and dissolved by the soil water. Frequently, however, it occurs in the soft granular form (gypseous marl) requiring only light crushing; thus in the hills bordering the Great Valley of California, and in parts of New Mexico and Texas.

Iron Minerals.—In connection with calcite and dolomite, the several minerals constituting the common iron ores require mention. One of these is:

Iron Spar or siderite; carbonate of iron, corresponds in composition to calcite and dolomite and crystallizes in the same form. It sometimes occurs in large masses and is an important iron ore, brownish-white in color, and when compact resists the attack of atmospheric oxygen remarkably well. Like the carbonates of lime and magnesia, it is soluble in carbonated water, and its deposits are undoubtedly formed from such solutions. The latter are copiously formed wherever fermenting or decaying organic matter is in contact with iron-bearing materials, such as rust-colored sands or clays; and if the solution so formed can percolate without coming in contact with air, iron spar is formed. But whenever the solution comes in contact with air, it absorbs oxygen and the ferrous carbonate is converted into ferric hydrate or rust, mineralogically known as:

Limonite or brown iron ore. This ore is frequently found deposited on the upper surface of clay layers traversing sandy strata, the clay having arrested the carbonate solution and thus given time to the air to effect the change. Sometimes such deposits form great masses in rock-caves, fissure-veins, or crevices; and like siderite, it is an important iron ore, though frequently quite impure, as in the case of bog ore, which is formed in ill-drained subsoils. It is also sometimes found as the residue from the weathering of rocks rich in hornblende or pyroxene, and in this, as well as in other cases, is pulverulent, constituting yellow ochre. It makes a rust-colored streak on biscuit porcelain or unglazed queensware. It is the coloring material of all yellow or “red” soils and clays, as well as of brown sandstones, which are cemented by it.

As is well known, such clays and sandstones become dark red by heating or “burning,” as in the case of common brick clays; the brown or yellow ferric hydrate losing its water and becoming red ferric oxid. The latter sometimes occurs in nature in the impure, pulverulent condition, constituting “red ochre”; but more commonly and abundantly it is found in the form of

Hematite or red iron ore, which is sometimes formed in nature by limonite losing its water, but more commonly in different ways. It is but rarely found in soils and is of no special interest in that connection.

A fourth form of iron ore, quite common in the soils of some regions, is

Magnetite or magnetic iron ore, also known as lodestone. This mineral, the oxygen-compound of iron corresponding to “blacksmith’s scale,” also occurs in large masses and is an important and usually a very pure iron ore. It occurs very commonly disseminated through certain rocks, and in their weathering it remains unattacked and thus passes unchanged into the soils and sands, constituting the “black sand” so well known to gold miners and almost universally present in the alluvial soils of the Pacific coast. These black grains are of course attracted by the magnet and can thus be easily recognized and extracted. In soils they are simply inert, like quartz sand.

But while the ore is of little interest to the farmer, it is quite otherwise with the compound of this oxid with water, the ferroso-ferric hydrate; intermediate in composition between the white ferrous and the brown ferric hydrates. As mentioned above, the black silicate minerals, such as hornblende and pyroxene, are bottle-green when seen in thin sections. Nearly the same color, with modifications running toward blue and bright green, is often seen in natural clays and rocks, and is almost always caused by the ferroso-ferric hydrate. Such materials always become red or reddish when heated by the formation of red ferric oxid; while when exposed to damp air, they assume the rust color of ferric hydrate.

Reduction of ferric hydrate in ill-drained soils.—When such oxidized, rust-colored clays or soils are exposed to the action of fermenting organic matter, the first effect observed is the change of color from rusty to bluish or greenish, by the reduction of the ferric to ferroso-ferric hydrate. Afterward, if the action is continued, the solution of ferrous carbonate (see above) may be formed, and the greenish or bluish color may disappear.

The importance of this reaction to farming practice lies in the fact that the blue or green tint, wherever it occurs, indicates a lack of aeration, usually by the stagnation of water, in consequence of imperfect drainage. Such a condition, always injurious to plants, becomes doubly so when it is associated with the formation of a metallic solution, such as ferrous carbonate, and promptly results in the languishing or death of plants in consequence of the poisoning of their roots. In the presence of sulfates such as gypsum, the formation of iron pyrite (ferric bisulfid) and sulfuretted hydrogen, is likely to take place. Moreover, under the same conditions the phosphoric acid of the soil may be concentrated into ferrous or ferric phosphate, which pass into deposits of bog ore in the subsoil.