THE VARIOUS ROCKS AS SOIL-FORMERS.
_Rock-weathering in arid and humid Climates._—From what has been
said in the preceding chapters of the physical and chemical agencies
concerned in rock-weathering, it is obvious that climatic differences
may materially influence the character of the soils formed from one
and the same kind of rock. Since kaolinization is also a process of
hydration, the presence of water must greatly influence its intensity,
and especially the subsequent formation of colloidal clay; so that
rocks forming clay soils in the region of summer rains may in the arid
regions form merely pulverulent soil materials. Many striking examples
of these differences may be observed, _e. g._, in comparing the outcome
of the weathering of granitic rocks in the southern Alleghenies with
that of the same rocks in the Rocky Mountains and westward, especially
in California and Arizona. The sharpness of the ridges of the Sierra
Madre, and the roughness of the hard granitic surfaces, contrasts
sharply with the rounded ranges formed by the “rotten” granites of the
Atlantic slope, where sound, unaltered rock can sometimes not be found
at a less depth than forty feet; while at the foot of the Sierra Madre
ridges, thick beds of sharp, fresh granitic sand, too open and pervious
to serve as soils, cover the upper slopes and the “washes” of the
streams, causing the latter to sink out of sight. A general discussion
of the kinds of soils formed from the various rocks must, therefore,
take these differences into due consideration.
GENERAL CLASSIFICATION OF ROCKS.
Rocks may be broadly classified into three categories, viz:
1. _Sedimentary_ rocks, formed by deposition in water and hence more or
less distinctly stratified.
2. _Metamorphic_ rocks, formed from rocks originally sedimentary, by
subterranean heat in presence of water. Usually crystalline, that is,
composed of more or less distinct (large or minute) crystals of one or
several of the minerals mentioned above.
3. _Eruptive_ rocks, ejected in the molten state from volcanoes or
fissures; crystalline or not, according to slow or rapid cooling.
_Sedimentary Rocks._—Sedimentary rocks are forming to-day by deposition
from either sea or fresh water, precisely as they were in the most
remote geological times; the oldest clearly sedimentary rocks being
sometimes undistinguishable in their nature and composition from the
very latest immediately preceding our present time. They may for the
purposes of the present work be simply classified as follows:
1. _Limestones_, formed in comparatively shallow seas, or fresh water
basins, from the calcareous shells or skeletons of various organisms.
2. _Sandstones_, and conglomerates (sometimes called pudding-stones)
formed from the debris of pre-existing rocks disintegrated by the
agencies described above, (chap. 1-2), re-cemented by means of
solutions of one or several substances, such as silex, carbonate of
lime, ferric hydrate and others. Loose sands and gravels are the
initial stages of such rock formation as well as the results of their
disintegration.
3. _Clays_, _Claystones and Clay shales_, consisting of clay substance
with more or less sand, and soft or hard according to the nature of the
waters or solutions that may have acted upon them, with or without the
aid of heat. These rocks can only be formed in comparatively quiet or
“back” waters, since clay would not ordinarily be deposited in moving
water.
_Metamorphic Rocks._—The effects of subterranean heat or metamorphism
upon the sedimentary rocks may be roughly stated as follows:
_Limestones_ are transformed into marbles of various degrees of purity,
according to the nature of the original rocks.
_Sandstones_ when cemented by silex are transformed into quartzite, of
greater or less purity according to the nature of the “sand” entering
into its composition. When cemented by materials other than quartz,
these also will be segregated in the form of various minerals in the
body of the rock.
The _clay rocks_ form the most varied products under the influence of
(aqueo-igneous) metamorphism; granites, gneiss, syenite and hornblendic
schist are among the most common. The great variations in the
composition of clayey materials account for the correspondingly great
variations in the nature of the resultant metamorphic rocks.
_Igneous or Eruptive Rocks._—These are usually divided into two groups;
the one characterized by a large proportion of free quartz (silicic
acid), and hence designated as _acidic_, and usually of a light tint;
the other the _basic_, containing little or no free quartz, and
commonly of a dark tint caused by the presence of a large amount of
iron (contained in pyroxene, more rarely in hornblende).
Of the latter class are the dark “basaltic” rocks
constituting the mass of the enormous eruptive sheet
of the Pacific Northwest, covering the greater part of
Washington, Oregon and northeastern California. The lavas
of the Hawaiian islands are of the same class and even more
basic; while the eruptives of Nevada, middle and southern
California, and eastward to the Rocky Mountains, are mostly
of the light-colored, acidic type. The same is largely true
of the rocks of the Andes of Central and South America, the
gray “Andesites,” also represented in the Caucasus.
As one and the same eruptive material may, according to the greater or
less rapidity of cooling, appear as a glassy mass (obsidian, pumice,
volcanic ash, tuff, etc.,) or as a crystalline rock resembling coarse
granite in structure, it is not easy to identify them in all their
various forms. This can frequently be done only by ascertaining their
component minerals by the microscope, or by chemical analysis. The
same is sometimes true of metamorphic rocks; and as in the latter, the
several feldspars and quartz, with pyroxene instead of hornblende,
constitute the predominant soil-forming minerals. More rarely, garnet,
chrysolite, leucite and other silicates require consideration.
_Generalities regarding the Soils derived from various Rocks._
It is hardly necessary to insist that as in the case of the rocks
composed of single minerals, already referred to above, the predominant
mineral or minerals of compound rocks determine the facility of
weathering, as well as the quality of the soil resulting therefrom.
Since rocks are named essentially in accordance with the _kinds_ of
minerals that constitute their regular mass, the _proportion_ in which
the several constituents stand to each other may vary greatly. Thus a
_granite_ may consist, over considerable areas, mainly of a mixture of
potash feldspar and quartz; in others, mainly of quartz and mica with
little feldspar. Very frequently, hornblende replaces mica partially
or wholly. The latter will weather much more slowly than feldspar
or hornblende, and will produce an inferior soil when decomposed.
Allowing for such variations, a fairly approximate general estimate
of the quality and peculiarities of soils from crystalline rocks may
nevertheless be made. To some extent such estimates must make allowance
not only for the chief ingredients, but also for those which are
called “accessory” or characteristic, and which while not present in
large amount, may nevertheless exert a considerable influence upon the
quality of the soil.
_Soils from granitic and crystalline rocks._—In the case of the
(potash-feldspar) granite soils it is generally admissible to expect
that they will be fairly supplied with phosphoric acid, because in
the great majority of cases, minute crystals of apatite (phosphate of
lime) are more or less abundantly scattered through it. From the potash
feldspar present, granite soils may always be relied on for a good
supply of potash for plant use; on the other hand, unless hornblende be
present, they are pretty certain to be deficient in lime, since neither
lime, feldspar nor calcite are probable accessory ingredients of this
rock.
Granite is exceedingly apt to weather by mechanical disintegration
far in advance of its chemical decomposition. It is therefore common
to find in sedentary soils overlying granite, a gradual increase of
grains of its component crystalline minerals as we descend in the
subsoil; until finally the latter grades off into rock almost unchanged
save in lacking coherence. This is seen strikingly in the southern
Appalachians, as well as in the Sierra Nevada and Sierra Madre of
California; at Cintra in Portugal, at Heidelberg in Germany, and
elsewhere.
But of the rocks that resemble granite and are popularly so called, a
good many are not “true to name” and therefore form soils differing
materially from the type just mentioned.
Thus the so-called granite areas of the Sierra Nevada of
California are largely occupied by a rock containing,
besides quartz, chiefly soda-lime feldspar and some
hornblende, and scarcely any mica. It is more properly
a diorite (grano-diorite); the soils formed from it are
rather poor in potash, not strongly calcareous, and quite
poor in phosphoric acid. On account of the small proportion
of hornblende (unusual in diorites), these soils are
light-colored (not “red”), and bear a growth of small pine
instead of the usual oak growth of the lower Sierra slopes.
What is said of granite soils is also generally true of those formed
from _Gneiss_, which is composed of the same minerals as granite,
but has a slaty cleavage and on that account when upturned on edge,
weathers rather more rapidly than most granites. Owing to the frequent
occurrence of lenticular masses of quartz in gneiss, its soils are
more commonly of a siliceous nature than are those of the true granite
regions, and not as “strong” as the latter. This is the more true
since gneiss often passes gradually into _mica schist_, which, being a
mixture of quartz and mica only, not only weathers very slowly but also
supplies but little of any importance to plants, to the soils formed
from it. Such soils would mostly be absolutely barren but for the
frequent occurrence in the rock, of accessory minerals that yield some
substance to the soil. Yet it remains true that inasmuch as gneiss and
mica-schists are among the rocks in which mineral veins most commonly
occur, the proverbial barrenness of mining districts is very frequently
traceable to these rocks. The same may be said of some of the related
rocks, such as gabbro, minette and others.
Normal _diorite_ consists of hornblende and soda-feldspar, with more or
less quartz.
The soils derived from certain diorites of the Sierra Nevada of
California have just been referred to. But these granite-like diorites
are on the whole exceptional; it should be added that the (diabasic)
“greenstones” of the Eastern United States and of the Old World, which
are usually much finer-grained, do not form the mass of fine, angular
debris constituting the subsoil in the Sierra Nevada, but weather into
rounded masses and fine-grained soils possessing, on the whole, a fair
fertility, though liable to contain an excessive proportion of silex in
various forms.
Of the _eruptive rocks_ as a class it is often said that they form
very productive soils; yet, as these rocks differ widely from each
other in composition, this statement must be taken with a great deal of
allowance. Very many of them decompose with extreme slowness on account
of their glassy nature; this is particularly true of obsidian, pumice
stone, and the “volcanic ash” derived from its pulverization, and which
is found unchanged, in sharp scales, among the decayed minerals of
other rocks in complex soils. Other volcanic ash, however, being formed
by the pulverization of crystalline or of basic lavas, weathers rather
readily, as already stated; so that certain plants take possession in
the course of a few years. The general classification into basic and
acidic rocks, given above, is of importance in connection with soil
formation from eruptive masses; for the basic rocks are much more
easily attacked by the atmospheric agencies than the acidic class.
A broad distinction must, however, be made between the
basic rocks of the basaltic class, which contain black
pyroxene as a prominent ingredient, and those which, like
many trachytes, are rich in feldspathic minerals. The
latter are naturally rich in alkalies (potash and soda)
which they impart to the corresponding light-colored soils;
while the black basaltic rocks and lavas weather into “red”
soils, sometimes containing extraordinary amounts of iron
(ferric hydrate) and (from the lime-feldspars they contain)
a fair supply of lime, but oftentimes very little potash.
Experience seems to prove that the red basalt soils are
mostly rather rich in phosphoric acid; this is especially
true of the country covered by the great eruptive sheet of
the Pacific Northwest, in the rocks of which the microscope
readily detects the presence of numerous needles of
apatite (lime phosphate). The same is true of the highly
iron-bearing soils from the black basaltic lavas of the
Hawaiian islands, even though they have been leached of
all but traces of lime and potash. All these soils are
physically “light” and easily workable, since the rocks
in question contain but little alumina from which to form
clay; they are sometimes extremely rich in iron, even to the
extent of being capable of serving as iron ores.
The soils derived from _trachytes_ and trachytic lavas are generally
light-colored and light in texture; the latter from the presence of
a large proportion of volcanic glass, together with undecomposed
crystalline minerals. These are usually rich in potash, but poor in
lime and phosphates. The high quality of the wines of the lower Rhine
has been ascribed to these soils, which however vary greatly within
the areal limits of the production of the high-grade wines, not only
from gray trachytes to dark colored, highly augitic basalt, but also to
acidic quartz porphyries or rhyolites, and clay-slates.
The rhyolites on the whole yield the poorest soils among the eruptive
rocks; they are slow to weather at best, and the soils produced are
poor and unsubstantial, largely from the predominance of quartz and
undecomposable, glassy material; of which the phonolites are the
extreme type, resisting the influence of the atmospheric agencies just
as would so much artificial glass. Soils consisting largely of volcanic
glass may be found covering considerable areas in the Sierra Nevada of
California. Such “volcanic ash” soils are usually very unthrifty, and
bear a growth of small pines.
_Soils from sedimentary rocks._—_Limestones_, when pure and hard,
are very slow to disintegrate, and are also very slowly attacked by
carbonated water (see chap. 3, page 41). Soft impure and vesicular
limestones are, however, very rapidly attacked, especially when
underlying a surface clothed with the luxuriant vegetation that usually
flourishes on soils rich in lime. The popular adage that “a limestone
country is a rich country,” is of almost universal application and
stamps lime, from the purely practical standpoint, as one of the most
important soil ingredients.
_Residual Limestone Soils._—Striking examples of the formation of
large, fertile soil areas by the leaching out of limestones are found
in the States of Alabama, Mississippi, Louisiana and Texas, where the
fertile black prairies have been largely thus formed. The “blue-grass”
country of Central Kentucky is another case in point.
The following table shows a representative example of the relative
composition of the (cretaceous) “Rotten Limestone” of Mississippi, and
the “residual” soil-stratum derived from it. The average thickness of
the layer of residual clay above the limestone is about eight feet,
but ranges from seven to ten; the upper layers of the limestone are
somewhat softened, but the rock is always fresh at twelve feet, from
which depth the sample analyzed was taken, in a cistern adjoining
the field from which the soil and subsoil were procured. The black
soil varies in depth from 8 to 15 inches; then there is a change to
a brownish subsoil, reaching down to about two feet, and in drying
cleaving into prismatic fragments. The black soil has here in the
highest degree the peculiarity of crumbling in drying from its
water-soaked condition, so that it may be plowed when wet without
injury, although in the roads it works up into the toughest kind
of mud. The prairie is sparsely timbered with compact, fair-sized
black-jack oak, accompanied originally by red cedar.
The limestone derives its popular name of “rotten” from its being
usually soft enough to be cut with a knife or hatchet, and is therefore
somewhat used for building, and for burning lime.
COMPOSITION OF LIMESTONE, AND RESIDUAL SOIL AND SUBSOIL,
FROM BLACK PRAIRIE, MONROE CO., MISSISSIPPI.
================================+============+=========+========
| “ROTTEN | SUBSOIL | SOIL
|LIMESTONE.” |(YELLOW).|(BLACK).
--------------------------------+------------+---------+--------
FINE EARTH. |Depth 12 ft.| 2-3 ft. |15 ins.
Chemical analysis of fine earth.| | |
--------------------------------+------------+---------+--------
Insoluble matter | 10.90 | 71.54 | 78.29
Soluble silica | | |
Potash (K₂O) | .25 | .54 | .33
Soda (Na₂O) | .32 | .23 | .08
Lime (CaO) | 45.79 | 1.08 | 1.37
Magnesia (MgO) | .88 | .77 | .36
Br. Ox., of Manganese (Mn₃O₄) | | .05 | .14
Peroxide of Iron (FeO) | 1.42 | 5.42 | }
Alumina (Al₂O₃) | 1.96 | 13.15 | } 14.22
Phosphoric Acid (P₂O₅) | | .05 | .10
Sulfuric Acid (SO₃) | | .04 | .03
Carbonic Acid (CO₂) | 35.73 | |
Water and Organic matter | 2.84 | 6.99 | 5.75
| ------ | ----- | ------
Total | 100.09 | 99.86 | 100.67
| | |
Humus | | | 1.93
“ Ash | | | 4.38
Hygroscopic moisture | | 10.35 | 12.82
absorbed at °C | | 19° | 19°
--------------------------------+------------+---------+--------
It appears from the above table that in the change from the original
limestone to the soil mass as found at three feet depth, 81.5% of the
lime carbonate has been eliminated by leaching, leaving behind somewhat
less than one fifth of the original mass. Taking the average depth of
the soil mass at 8 feet, this thickness of material has required about
45 feet of the rotten limestone. Considering that notwithstanding the
tenacity of the clay soil, some of it must in the course of time have
been washed away, we may safely assume that the original rock surface
was from 50 to 60 feet higher than at present.
_Sandstone Soils._—The indefiniteness of the nature of “sandstones” as
such renders generalizations in regard to the soils formed from them
rather difficult, save as to their physical qualities, which in the
nature of the case are always “light.” In the Old World and in the
humid region generally, sandstone and sandy soils are usually spoken
of as being poor, because there the sand almost always consists of
quartz grains only, and hence the fine portions alone can be looked to
for plant nutrition. Consequently, the more sand is seen in a soil,
the poorer it is usually presumed to be. But this presumption would be
wholly erroneous in the arid regions. (See chapt. 6, p. 86).
Clearly, the nature of the soils produced by the weathering of
sandstones depends upon two points: first, the nature of the cement
binding the sand grains, and second the character of the latter
themselves.
_Varieties of Sandstones._—As has been stated above, the cements may
be roughly classified into five kinds, and their intermixtures, to
wit: _quartzose_ or siliceous, _calcareous_, _ferruginous_, _aluminous
or clayey_, and _zeolitic_. As regards the first, it is obvious
that siliceous sandstones will disintegrate with great difficulty,
since neither the cement nor the grains are susceptible of material
change by weathering. Such sandstones frequently pass insensibly
into quartz rock, and the light, unsubstantial soils they produce
are of the poorest, containing often mere traces of the plant-food
ingredients. This of course, is true, not only of the soils formed by
the actual weathering of sandstones, but equally of those consisting of
quartz-sand deposited by water or drifted by winds.
Of this character are the pine-forest soils of the coast
region of the Gulf of Mexico, particularly the “Sand
hammocks” of the immediate Gulf border, from Mississippi
Sound to Charlotte Harbor, Florida; the sandy lands of the
Grand Traverse region of Michigan, and many other minor
areas in the United States, usually characterized by a pine
growth, often more or less stunted, according to the nature
of the sand grains.
_Calcareous sandstones_ usually form a very much better class of soils,
partly for the intrinsic reason given above as regards limestones as
soil-formers. The calcareous cement is very rarely pure calcite; in
most cases it is very impure, as, most commonly, is also the “sand”
itself. This is explained from the fact that such rocks (mostly soft
and often quite unconsolidated) are, like limestones themselves, the
result of deposition in shallow seas or lakes, receiving deposits from
the land drainage, and enriched by the animal and vegetable life of
such waters. Not uncommonly they contain, disseminated through them,
grains of the mineral glauconite (a hydrous silicate of iron and
potash), which readily supplies available potash; while the remnants
of animals and plants furnish more or less of available phosphates.
Thus the general presumption regarding calcareous sandstones is that
the derived soils are of good quality, frequently of the very best. The
same, however, does not appear to be true of sandstones cemented by
dolomite; the soils derived from magnesian sandstones are in many cases
noted for their unproductiveness. (See chapt. 3, p. 42).
_Ferruginous Sandstones_ manifestly derive no important soil
ingredients from their cement when the latter is measurably pure ferric
hydrate; and when in addition the sand itself is purely siliceous, the
soils resulting from the disintegration of the rocks are very poor.
Such are, e.g., the soils derived from the ferruginous
sandstones of the Lafayette formation in a part of northern
Mississippi and adjacent portions of Tennessee and Alabama,
characterized by small scrubby oak or dwarfed pine. On the
whole, however, such purely ferruginous quartz sandstones
are exceptional, and should not detract from the favorable
inferences usually to be drawn from the iron-rust tint of
soils (see chapter 15).
Sandstones with purely _zeolitic_ cement are on the whole not of
frequent occurrence, the zeolites forming, more commonly, the hard
portion of a clay-sandstone cement, which disintegrates by their
weathering-out.
In regions where the tufaceous rocks of eruptives prevail,
we not uncommonly find the “volcanic ash” solidly cemented
by a zeolitic mass, which is then usually apparent in
cavities or crevices in the form of crusts or crystals. Such
tuffs are commonly rich in alkalies and lime, but mostly
poor in phosphates, and in disintegration form soils of a
corresponding nature. They are largely represented in the
valleys off Puget Sound, as well as in portions of central
Montana, and northward.
_Clay-Sandstones_ (argillaceous sandstones) when soft, as is mostly
the case, form as a rule desirable loam soils, of a generalized
composition, difficult to predict. It is here that the composition of
the sand grains themselves most frequently comes into play in modifying
the soil quality. From clay-sandstones to claystones of various degrees
of sandiness there is, of course, every grade of transition, the soils
ranging correspondingly in the scale of lightness or clayeyness.
As a general rule, the potash contents of such soils are sensibly
proportioned to the clayey ingredient, at least in the humid regions.
_Claystones_ (i. e., clays hardened by some one or more of the cements
mentioned in connection with sandstones), will in the nature of the
case, when disintegrated from the condition in which they lie in the
geological formations, make correspondingly clayey, heavy soils, which
as experience shows are usually rich in the ingredients of plant food,
but frequently too heavy and intractable in tillage to be readily
utilized.
There are, of course, exceptions; such as soils formed
from pipe-clays, in which little if any mineral plant-food
remains, and which are best used for other purposes than
agriculture, unless under special conditions it may be worth
while to reclaim them by fertilization.
_Natural Clays._—Clays occur in nature in a great variety of
modifications that have received designations known in common life.
Such are porcelain clay, pipe-clay, fire-clay, potters’ clay,
brick-clay, and many others of more or less local use only. As these
materials practically concern the farmer in very many cases, they may
properly find a brief discussion here.
The variety-names enumerated above in the order of the actual contents
of the materials in true clay substance (“colloidal clay”), are partly
based upon that fact, partly upon the degree of plasticity attained
by that substance, and essentially upon the nature and amount of
foreign admixtures associated with it. Thus, porcelain clay is chalky
kaolinite, sometimes associated with enough of pure white plastic
clay to render it workable in the potter’s lathe, but more commonly
requiring to be molded in porous molds; it is very refractory to heat.
Pipe-clay is also white, but more plastic and usually less refactory.
Fire-clay is a refractory pipe-clay commingled with some coarse
infusible material, such as quartz sand (or the same clay burnt and
crushed), in order to prevent excessive contraction and change of shape
in drying and burning. Potters’ clay is a much less pure, and from that
cause more fusible clay, which when burnt forms at a moderate heat
a semi-fused, more or less hard mass, such as crockery and pottery
ware. Brick-clay is a still more impure clay, or loam, containing
considerable sand and usually iron oxid, and largely falls already
within the limits of tillable soils or subsoils, rendered fusible by
the presence of relatively considerable amounts of iron, magnesia and
lime.
_Iron_ colors natural clays either red, yellow, green
or blue; the latter two colors turning to yellow or red on
exposure to the air, and to red on burning. Black color is
usually due to carbon, such clays often turning white on
heating.
Clays containing much lime are usually of a gray or whitish
tint, and like the soft crumbly limestones are often called
marls, and are used as such for land improvement. But it
should be understood that the colors of clays, mostly
derived from some iron compound, have little to do with
their uses in the arts, except that no deeply colored clay
(black excepted) is refractory in the fire.
_“Colloidal” Clay._[17]
In connection with soils, clay may be defined, in the most general
terms, as being the substance which imparts plasticity and adhesiveness
to soils when wetted and kneaded, and which, when heated to redness,
loses this property completely and permanently, becoming hard and
coherent in proportion to the degree of heat to which it is exposed.
[17] This term was first employed by Th. Schloesing, in communications
to the French Academy of Sciences, and reported in the Comptes Rendus
of that body; first in 1870. Unaware of Schloesing’s work, the writer
began a full investigation of the subject of mechanical soil analysis
in 1871, and published the results in 1873 (Am. Jour. Sci., Oct. 1873).
Up to that time the limited resources of the library of the University
of Mississippi had not given him an opportunity to see Schloesing’s
publication. The two independent investigations, though conducted on
somewhat different lines, gave of course practically the same results,
and complement each other.
In common life, however, the name is applied to the whole of any
naturally occurring earth which on wetting and kneading assumes a
reasonable degree of plasticity and adhesiveness. When the latter
property becomes nearly or quite insensible, the earth is designated as
a “loam,” more or less “clayey” according to the amount of the pure,
plastic and adhesive material associated with the mineral powders and
sand that form the bulk of most soils.
Chemically, the pure clay substance[18] probably consists (as has been
stated above) of silica and alumina in the proportion of nearly 46 to
40, the rest (14%) being water of hydration, which is lost on burning
the clayey material. But while it is true that such is the composition
of the plastic substance of clays, plasticity and adhesiveness are by
no means invariable properties of this compound. In its purest state,
as kaolinite, it is readily mistaken for chalk, (and is sometimes
used as such), being powdery to the touch and entirely devoid of
plasticity[19] when wetted and kneaded. The microscope shows this
chalky kaolinite to consist of minute, mostly rounded, originally
six-sided, thin plates, which when pure resemble to the touch powdered
talc (soapstone) or even black-lead, rather than any clay known to
common life. But being exceedingly soft, the kaolinite substance
is easily ground or triturated into an extremely fine powder; and
Johnson and Blake[20] succeeded in producing sensible plasticity and
adhesiveness by long-continued trituration of kaolinite with water in a
mortar. A similar process, but continued much longer by the mechanical
agencies concerned in soil-formation (see chapt. I), is unquestionably
the chief factor concerned in the formation of natural plastic clays;
but whether this is the _only_ process by which the powdery kaolinite
may be transformed into plastic clay, is a question not definitely
settled. It is at least possible that repeated freezing and thawing, as
well as the action of hot water, may take a part in the transformation,
beyond that by which they destroy the crumbly (flocculated) structure
of soils and clays, and render them plastic; as is done in the maturing
of clays by potters.
[18] There is still some discussion as to the chemical identity of
colloidal clay with Kaolinite; but the objections are not convincing.
[19] It has of late been attempted to extend the meaning of this
word to the behavior of all powders when wetted with water. But the
adhesive plasticity of clay stands almost alone, in that (aside
from contraction) it preserves in drying the form into which it may
have been molded while wet, even when struck, whereas other powdery
substances similarly treated at once collapse back into the original
powder. The exclusive use of clay in modeling offers the typical
example of plasticity as generally understood. The addition of any
powdery substance, however fine, diminishes the plasticity of clay.
[20] American Journal of Science, 2d Ser., Vol. 43, p. 357.
_Causes of Plasticity._—In any case the property of plasticity and
adhesiveness is restricted to the particles so fine that they fail to
settle, in the course of 24 hours, through a column of pure water eight
inches (200 m) high, while some are so extremely minute that they will
not settle for many months, and even for several years.[21] Such turbid
“clay water” may sometimes be found existing in nature, in moist,
secluded places, for weeks after the subsidence of the overflows of
rivers whose water is exceptionally free from dissolved mineral matter.
[21] Williams (Forsch. Agr. Phys. Vol. 18, p. 225 ff.) claims that
the diameter of the minutest clay particles is one-thousandth of
a millimeter, their form being that of scales showing continual
(Brownian) motion in water. He maintains that the plasticity of clay
is due to this minute size, and this view has gained wide acceptance
in late works on the subject. But this assumption cannot be maintained
in the face of the fact that nothing like the adhesive plasticity of
clay can be attained even by the finest powders of other substances,
least of all by those having the closest mineralogical resemblance to
kaolinite, such as graphite and talc. Above all, the most persistent
trituration with water utterly falls to restore plasticity to clay once
baked so as to expel its water of hydration, although the fineness
of the particles is thereby not only not diminished, but actually
increased, by contraction in heating. No powders however fine can
replace the functions of clay in soils, _viz._ the maintenance of
floccules, and tilth dependent thereupon; and they distinctly impair
the plasticity of clay. The fine “slickens” of quartz mills merely
render soils containing them more close and impervious, and more
difficult to flocculate. Even gelatinous masses like hydrated ferric
and aluminic oxids fail to replace clay in its adhesive functions.
_Separation of Colloidal Clay._—This property of
the plastic clay substance, of diffusing in pure water,
furnishes the means of separating from it the coarser, sandy
and silty portions of soils and natural clays, and observing
its characteristic properties, so far as the almost
unavoidable admixture of some other substances, presently to
be considered, permits.
In natural soils the clay particles usually incrust the
powdery ingredients, cementing them together; or themselves
form complex aggregates (floccules) of large numbers
of individual particles. These may be loosened from
their adhesion or cohesion either by prolonged, gentle
kneading of the wet clay, or by more or less prolonged
digestion (soaking) in hot water, or more expeditiously,
by lively boiling with water. The boiling should not,
however, be prolonged beyond the time actually required
for disintegration, since (as Osborne[22] has shown)
long-protracted boiling tends to render the clay permanently
less diffusible.
From the turbid clay-water the diffused clay may be obtained
either by evaporating the water (which as the bulk is very
large, is usually inconvenient), or, more conveniently,
by throwing it down from its suspension by the action of
certain substances which possess the property of curdling
(coagulating) the clay substance into flocculent masses
that settle quickly. Of all known substances, lime, in the
form of lime-water, acts most energetically in producing
this change; but other solutions of lime, as well as most
salts and mineral acids, produce the same effects when
used in sufficient quantity. Common salt is among the most
convenient, because it can most readily be leached out of
the clay precipitate thus thrown down. This when white,
resembles boiled starch, but being usually colored by iron
might be easily mistaken for the mixed precipitate of ferric
hydrate and alumina so commonly obtained by chemists in
soil analysis. When separated from the water and dried,
the jelly-like substance (“colloidal clay”) shrinks as
extravagantly as would so much boiled starch, into hard,
shiny crusts or flakes, which when struck in mass are
sometimes even resonant, and bear more resemblance to glue
than to the clay of everyday life. Like glue, too, but much
more quickly and tenaciously, the dried colloidal clay
adheres to the tongue, so as to render the separation
painful; when wetted it quickly bulges with great energy,
and in a short time resumes its former jelly-like condition.
When moistened with less water it assumes a highly plastic
and adhesive condition, so that it is difficult to handle
and almost as sure to soil the operator’s hands as so much
pitch.
[22] Rep. Conn. Agr. Expt. Stn., 1886, 1887.
_Effects of Alkali Carbonates upon Clay._—The carbonate of potash and
soda, when in very dilute solution (.01 to .05%) exert upon diffused
clay an effect the reverse of the acids and neutral salts. They destroy
the flocculent aggregates formed by precipitation with these, or
naturally existing in the soil, and tend to puddle the clay so as to
render it impervious to water. It is thus that in the alkali lands of
the arid regions we often find the soil or subsoil consolidated into
a very refractory “hardpan,” difficult to break even with a sledge
hammer and impossible to reduce to tilth until the alkali carbonate is
destroyed by means of a lime salt, such as gypsum. (See chapt. 23).
Ammonia water also helps to cause the diffusion of clay in water, but
its effect of course disappears upon drying. It is probable that this
property of sodic carbonate can be utilized in rendering earth dams
firmer and more secure against the penetration of water.