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.[14]
[14] 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.[15]
[15] 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[16] 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.
[16] 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.