THE DENSITY AND VOLUME-WEIGHT OF SOILS.
Aside from the humus-substances the specific gravity of the common
soil constituents, taken individually, do not vary widely; kaolinite
being the lightest (2.60), feldspar next (2.62); then quartz (2.65),
calcite (2.72). Mica and hornblende range (according to their iron
contents) from 2.72 to over 3.0. The average specific gravity of soils
of ordinary humus content only will thus range between 2.55 and 2.75;
sandy soils approaching very closely to that of quartz alone.
_Volume-Weight._—The specific gravity of the soil is, however, of
little practical consequence compared with the “_volume-weight_,” _i.
e._, the weight of the natural soil as compared with an equal bulk of
water. A cubic foot of water weighs 62½ pounds; a similar volume of
soil usually weighs more, but in the case of peaty lands may actually
(when dry) weigh less. The extreme range is from 110 pounds for
calcareous, and somewhat less for siliceous sand, to as little as 30 to
50 pounds in the case of peaty and swamp soils. It may be conveniently
remembered that while average arable loams range from 80 to about 95
pounds per cubic foot, “heavy” clay soils range from 75 pounds down
to 69, observed by the writer in the case of certain alluvial soils,
poor in humus,[29] of the Sacramento river, California. Manured garden
soils, and the mold surface soil of deciduous forests, generally
contain so much humus as to depress their weight considerably, varying
according to their state of tilth from 66 to 70 pounds per cubic foot.
[29] This remarkable soil seems to have been derived from the finest
“slickens” of the hydraulic gold mines.
_Weight per acre-foot._—As for practical purposes and calculations it
is often desirable to know approximately the weight in pounds of an
acre (43,560 square feet) one foot deep, it is convenient to remember
that in the case of sandy land, this weight (per “acre-foot”) may be
assumed at four millions of pounds; for loams, at 3½ millions; for
clay lands, 3¼ millions; for humus or garden land and woods earth,
about 3 millions of pounds; for reedy swamp and peaty lands, 2 to 2½
millions.
The loose tilth and humus-content of the surface soil will
in general cause it to weigh less, bulk for bulk, than the
underlying subsoil, even when the latter is more clayey;
moreover, the continuous pressure from above will tend to
consolidate the subsoil and substrata. Warington (Phys.
Properties of Soils, pp. 46, 47) gives interesting data on
this point from the Rothamstead fields, as follows:
Old pasture, first nine inches 71.3 pounds per cub. ft.
Same, fourth “ “ 102.3 “ “ “ “
Arable land, first “ “ 89.4 “ “ “ “
Same, fourth “ “ 101.4 “ “ “ “
The influence of humus and unhumified organic matter, as
well as of tillage, in diminishing the volume-weight of
soils is here strikingly shown.
_Air-space in Natural Soils._—The difference between the specific
gravity as usually determined, and the volume-weight of soils, is of
course caused by the large amount of air contained in them when dry,
but which in wetting them is partially or wholly replaced by water.
[Illustration: FIG. 10.—Various possible arrangements of soil
particles.]
Theoretically, assuming all soil grains to be globular, and packed as
closely as possible (in oblique order), the space not filled by them
would be the same for all sizes, whether that of marbles, or so minute
as to be hardly felt between the fingers; and would be 25.95 per cent
of the soil volume.[30] If the same globular particles were packed
as loosely as possible, _i. e._, in square instead of oblique order
(see figures 10 and 11), the vacant space would be 47.64 per cent If
however we imagine each sphere to be itself composed of a number of
smaller ones, the empty space will obviously be greatly increased, to
an extent proportionate to the diminution of solid mass thus brought
about. The pore-space might in that case, with the oblique arrangement
of the globules as shown in Fig. 10, be as high as 74.05 per cent But
since the soil particles may be of all shapes and sizes within the same
soil, and usually fit much more closely than would globular grains,
the empty space rarely approaches (only in certain alluvial soils and
in loose mulches) to the figure last named. In sandy soils it may fall
as low as 20%, and in coarse gravelly soils even as low as 10%. Most
cultivated soils range between 35 and 50% of empty space.
[30] King, Physics of Agriculture, p. 116, ff.
_Effects of Tillage._—That these figures can be only approximations is
obvious from the consideration that one and the same soil will vary
materially in its volume-weight according to its temporary condition
of greater or less compactness. After land has been beaten by winter
rains, its volume-weight will be found to have materially increased
from the well-tilled condition brought about by thorough cultivation.
This difference is strikingly seen when, in plowing, the height of
the ground on the land side is compared with that of the turned
furrow-slice in well conditioned loamy land. This loose condition is
called _tilth_, and it results from the formation of relatively large,
complex crumbs[31] or floccules, between which there are large air
spaces that were wholly absent in the untilled land; the floccules
themselves being also more loosely aggregated than was the case before
tillage.
[31] The word crumbs, which is generally understood as meaning a
relatively large, loose aggregate, seems preferable to the word
kernels, suggested for the same by King (Physics of the Soil, p.
110). Kernels are understood to be bodies rather more solid than the
surrounding mass, and do not convey the idea of loose aggregates. The
word “Krümelstructur” (crumb-structure), adopted by Wollny for this
phenomenon, has both fitness and priority in its favor.
[Illustration: FIG. 11.—Land before and after plowing. The compactness
of the soil is indicated by the density of dotting. Before plowing
there is a compact surface crust (s), below which the soil becomes less
and less compact as we go deeper. After plowing we find the soil (fs,
furrow-slice) converted into a loose mass of crumbs (floccules), with
increase of bulk. Compacted plowsole at pl.]
[Illustration: FIG. 12.—A soil-crumb, magnified to show the particles
of which it is composed. The particles are held together by the
water-menisci, just as are the hairs of a brush when wetted. The white
spaces between the particles represent air.]
_Crumb or Flocculated structure._—Figure 11 illustrates the difference
between the unplowed land, consolidated especially on the surface by
winter rains, and in its upper portion consisting largely of single
grains; while the plowed land, toward which the furrow-slices have
been turned, is greatly increased in height and volume and consists
almost wholly of variously-shaped and-sized aggregates or floccules,
loosely piled upon one another and separated by large interspaces. The
increase in volume from consolidated clay to crumb-structure is given
by Wollny (Forsch., vol. 20, p. 13, 1897) at 41.9%, to powder as 33%.
On moistening dry clay increased 36.9%, quartz powder 8.01%. When land
is plowed in the proper moisture-condition the crumbs of floccules are
held together by the surface tension of the capillary films (menisci)
of water at the points of contact. In the case of sands, the crumbs
will collapse into single grains whenever the water-films evaporate,
unless some cementing substance was dissolved or suspended in the
water. (See figure 12). Lime carbonate is one of the substances most
commonly found permanently cementing the floccules; hence the ready
tillage of most calcareous soils, and especially the loose texture of
the “loess” of the western United States, and of Europe and Asia. In
these deposits we find sandy and silt aggregates or concretions ranging
from ten or more inches in length (loess puppets) to microscopic size,
held together by lime carbonate, but collapsing into silt and sand when
the material is treated with acid so as to dissolve the cement. The
rough surfaces of these aggregates, gripping into each other, explain
the stability of the steep loess cliffs in the United States, as well
as in northeastern China, as observed by Von Richthofen and Pumpelly.
_Clay_ is most frequently the substance which imparts at least
temporary stability to the crumbs and crumb-structure; this is one of
its most important functions in soils, as it serves to _maintain_ tilth
once imparted by cultivation, even after the land dries out. Beating
rains, and cultivation while too wet, will in this case of course
destroy the crumbs and the loose tilth.
Other substances which greatly aid the maintenance of tilth are the
several humates (of lime, magnesia, iron), which when fresh are
colloidal (jelly-like) like clay itself, but unlike the latter, when
once dried do not resume their plastic form by wetting (Schloesing).
The crumbs thus formed are therefore quite permanent and contribute to
the looseness of soils rich in humus. One part of lime humate is said
by Schloesing to be equal in cementing power to eleven parts of clay.
Silica, silicates and ferric hydrate are sometimes found cementing soil
crumbs, wholly or in part.
The importance of the ready penetration of air, water and roots thus
rendered possible is obvious; and the question arises how it happens
that wild plants are able to do without tillage.
_How Nature Tills._—When we examine the undisturbed soil of woods or
prairie in the humid region, we will as a rule find the natural surface
soil in a very good condition of tilth; the obvious cause being the
presence in it of an abundant network of surface roots and rootlets of
grasses and herbs, which in connection with the fallen foliage prevent
the beating and compacting of the soil surface; which can be seen to
happen before the observer’s eyes whenever a heavy rain falls on a bare
land surface, however well tilled.
_Crusting of Soils._—In some soils, especially of the Gulf States, the
beating of rain followed by warm sunshine so effectually compacts the
surface that in the case of taprooted plants like cotton, it becomes
necessary to cultivate after each rain, so as to break the crust that
would otherwise not only prevent the proper circulation of air, but
would also serve to waste the moisture of the land. The same land in
the wild condition suffered no such change, being protected by the
native vegetation, and by fallen leaves. (See chapt. 8).
_Soils of the arid region._—In the regions of deficient rainfall the
conditions are modified in several respects. Grass sward rarely exists,
nearly all grasses assuming the habit of growing in tufts or bunches
some distance (a foot or two) apart; hence the name of “bunch grass”
commonly used, which however means not any one definite kind of grass,
but serves to distinguish the grasses of the uplands from those of the
moist lowlands, where true sward may be found. Between these bunches
of grass the soil is fully exposed, and being free from roots and
leaf-covering is compacted, _unless its nature is such that the usually
gentle rains do not produce a serious crusting of the surface_.
That such is actually the predominant nature of the soils formed under
arid influences has already been stated; and thus the hard-baked
soil-surface so often seen in the Eastern United States in unplowed
bare land, or during the prevalence of a drought, is rarely seen in
the arid region. The clay lands that do exist are usually sufficiently
calcareous to possess the property of “slaking” into crumbs whenever
wetted after drying. But where this is not the case, the stony hardness
brought about by the long dry and warm season is long in being removed
by the winter rains.
_Changes of soil-volume on wetting and drying._—The behavior of
colloidal clay in the above respects has already been described above
(see chapt. 4, page 59). It is obvious that whenever soils contain a
large proportion of such clay, their behavior on wetting and drying
will approximate to those of the pure clay. This is exemplified in the
heavy clay, or so-called “prairie soils” of the United States, which
when thoroughly wetted in spring will, during a dry summer, form wide,
gaping cracks. These in the long summers of the arid region may extend
to the depth of several feet, with a width of as much as three and
more inches at the surface of the ground. This, of course, contributes
greatly to the drying-out of the soil to the same depth, and results
as well in the mechanical tearing of the root-system of growing
plants; sometimes causing the total destruction of vegetation. In some
clay soils it happens that after a rain or irrigation, the shrinkage
occurring upon the advent of warm sunshine will cause the surface crust
to so contract around the stem, _e. g._, of grain, as to constrict and
_injure the bark_, causing serious injury to the crop. In soils of
this character very _thorough tillage in preparing for a crop_, and
the maintenance of _a loose surface during its growth, are of course
extremely essential_.
In the arid region it will frequently happen that such soils when not
tilled to a sufficient depth, will during the later part of the summer
so shrink and crack beneath the shallow-tilled surface layer that the
latter will bodily fall into the cracks, exposing the roots to all the
deleterious influences of mechanical lesion and drying-out. It is thus
obvious that the cultivation of such soils should not be undertaken at
all by those not naturally able and willing to bestow upon them, to
the fullest extent, the deep and thorough tillage which is absolutely
essential in the utilization of their usually high productive power.
_Extent of Shrinkage._—The extent of this shrinkage
in drying, and subsequent expansion in wetting, have been
measured by the writer by the use of the sieve cylinder
described below (chapt. 11, p. 209), as serving for the
determination of the water capacity of soils. When a soil of
the kind above referred to is placed in the sieve cylinder
in the tilled (flocculated) condition, then allowed to
absorb its maximum of water and then dried at 100 degrees
C., the contraction in drying can be very strikingly seen,
and its amount measured by filling up the empty space with
mercury; then measuring the latter after expelling the
surplus by means of a ground glass plate laid on top. The
contraction of several heavy clay soils, thus measured,
has been found by the writer to range from 28 to as much
as 40 per cent of the original bulk.[32] The soil thus
contracted, when again wetted, does not return altogether to
its original bulk, but remains in a more or less compacted
condition, like that of a soil which has been rained upon.
[32] Wollny (Forsch. Vol. 20, p. 13 ff., 1897) records similarly high
shrinkages in his experiments.
The expansion and contraction of a heavy clay soil on wetting and
drying are well illustrated in the figure below, in which the soils are
shown in the shallow cylinder which serves for the determination of
water-holding power (see chapt. 11, p. 209). The middle figure shows
in profile the expansion of a dry, pulverized “black adobe,” struck
level, when allowed to absorb its maximum of water; it rises above
the rim of the sieve-box to nearly the half height of the latter. The
outside figure to the right shows the same soil after drying; that to
the left, a red clay soil similarly treated. It is easily seen that
these variations in volume may bring about very marked results in the
fields; the surface of which, apart from the cracks usually formed,
may be several inches lower in the dry season than during wet weather.
[Illustration: RED CLAY SOIL. BLACK “ADOBE” CLAY SOIL.
FIG. 13.—Expansion on Wetting and Contraction on Drying of heavy clay
soils.]
_Contraction on Wetting._—In the case of alkali soils containing much
carbonate of soda, a very notable _contraction_ occurs in _wetting_
the loose, dry soil. The cause is here obviously the collapse of the
crumbs, formed in dry tillage or crushing, into single grains, closely
packed. The same result is observed in the naturally depressed “alkali
spots” (see chapt. 22).
“_Hog-wallows._”—In the _field_ the wetting of cracked clay soils
produces some very curious effects. The effect of the first light rains
usually is to crumble off the edges or angles near the surface, the
materials thus loosened falling into the lower portion of the cracks.
This is repeated at each successive shower followed by sunshine,
the crevices thus becoming partly filled with surface soil. When,
subsequently, the heavier and more continuous rains wet the land fully,
also causing the consolidated mass in the crevices to expand, the
latter cannot close on account of the surplus material having fallen
into them; the result being that the intermediate portions of the
soil are compelled to bulge upward, sometimes for six or more inches,
creating a very uneven, humpy surface, well-known in the southwestern
United States as “hog-wallows.”[33]
[33] A totally different kind of “hog-wallows,” occurring in California
and the arid region generally, have been described in a previous
chapter under the head of Aeolian soils (See chapt. 1, p. 9).
Such a surface is always therefore an indication of an _extremely heavy
soil, difficult to cultivate_; yet embracing some of the most highly
and permanently productive lands known in the United States, and in
India, where the “regur” lands of the Deccan are of this character;
they have been cultivated without fertilization for thousands of years.
The subjoined physical analyses of lands of such extreme character as
to be almost uncultivatable will serve to exemplify their physical
composition.
PHYSICAL ANALYSES OF HEAVIEST CLAY SOILS.
===================================================================
|No 242 Miss.|No. 643 Cal.
+------------+------------
|Hog-wallows |Black Adobe.
| soil. |Contra Costa
| Jasper Co. | Co.
|Mississippi.|California.
-----------------------------------------+------------+------------
Weight of gravel over 1.2 mm. diameter } | .83 |
“ “ between 1.2 and 1 mm } | |
“ “ between 1 and 0.6 mm | 1.19 |
Fine earth | 97.98 | 100.00
+------------+------------
| 100.00 | 100.00
FINE EARTH. | |
| |
Hydr. Value. | Diameter. | |
-------------------------+---------------+ |
Clay <.0023 mm | ? | 48.00 | 45.96
| | |
{ <0.25 mm | .010 | } 35.18 | 37.64
{ 0.25 mm | .016 | } |
Silt { 0.5 mm | .025 | 5.50 | 2.74
{ 1.0 mm | .036 | 3.74 | 3.31
{ 2.0 mm | .047 | 2.54 | 2.95
{ 4.0 mm | .072 | .20 | 2.39
| | |
{ 8.0 mm | .120 | .27 | 1.68
{ 16.0 mm | .160 | .90 | .79
Sand { 32.0 mm | .30 | 1.67 | 2.36
{ 64.0 mm | .50 | 2.00 |
| +------------+------------
| | 100.00 | 100.00
-------------------------+---------------+------------+------------
It will be noted that in both these extremely heavy soils the sum of
the clay and finest sediments is a little over 83%.
It should be stated that both these soils after being thoroughly
wetted become so adhesive that it is almost impossible to travel over
the tracts occupied by them, and that they are practically almost
untillable, being too adhesive when wet; yet if allowed to dry to a
certain extent (varying within very narrow limits) they turn up by
the plow in large clods, which after a few hours of sunshine become
of stony hardness and will resist all efforts at pulverization or the
production of tilth.[34]
[34] In driving a light carriage over the land represented by No. 643
above, after a light rain, the wheels gathered up so much soil within
a hundred yards as to render it necessary to stop and chop it off the
tires by means of a hatchet. This is a common experience in the black
prairie lands of Texas.
_Calcareous Clay Soils crumble on drying._—The heavy clay soils of some
of the calcareous prairies of the Southwest, instead of contracting
into a stony mass on drying, on the contrary resolve into a mass of
crumbs, thus producing excellent tilth. This occurs even though the
land may have been plowed when wet, and of course is a great advantage.
The most striking exemplification of this peculiarity occurs in the
heavy but profusely fertile “buckshot” clay lands of the Yazoo bottom,
in Mississippi, where it is usual to plant corn and sweet potatoes in
the semi-fluid mud left after an overflow, after turning a shallow
furrow, then covering by turning another. To the onlooker it seems
impossible that such plantings could be successful; but within a short
time the muddy surface becomes a bed of crumbs (“buckshot”), forming a
seed-bed not readily excelled by any made by artificial means. Hence,
largely, the almost invariable success of crops in the Yazoo region.
_Port Hudson Bluff._—The same clay produces a most unpleasant result
at the foot of the Port Hudson bluff, where it crops out some feet
above low water. When after a freshet the water level falls below this
stratum, on drying the clay disintegrates into crumbs just as does the
Yazoo buckshot soil; with the result that at the next rise, the loose
mass subsides into the river as a flood of mud. Thus the foot of the
bluff is being constantly undermined, and the falling of the bluff
scarp has obliged the town above to recede many hundreds of feet from
its original historic site.
The exact proportions of lime carbonate necessary to produce this
phenomenon, and its necessary relations to clay substance and other
physical soil ingredients, yet remain to be investigated.[35]
[35] Schübler (Grundsätzed. Agrikulturchemie, 1838) ascribes the
crumbling of calcareous clay soils to the difference in the contraction
of calcareous sand and the clay substance. But it is doubtless more
directly connected with the flocculation of the latter by lime.
_Loamy and Sandy Soils._—It is largely the _absence_ of these extreme
changes of volume that renders the cultivation of loamy or even sandy
lands so much more easy, and the success of crops so much more safe,
than is the case in clay soils. Whenever the content of colloidal clay
diminishes below 15%, the shrinkage in drying from the wet condition
becomes so slight as to cause no inconvenience; while in sandy soils
properly speaking, no perceptible change in volume occurs.
Peaty soils, however, and all those containing a relatively large
amount of humus, are also liable to visible shrinkage when passing from
the wet to the dry condition. But on account of their looseness and
porosity such shrinkage does not usually result in the formation of
cracks or rupture of the roots, as is the case in heavy clay lands. The
entire mass of the soil then shrinks downwards, but rarely forms cracks
on the surface. Hence the introduction of humus into “heavy” soils is
among the best means of improving their tilling qualities.
_Formation of Surface Crusts._—Some soils, especially those of a
clay-loam character, are very liable to the formation of hard surface
crusts from the beating of rains, and from surface irrigation; owing,
doubtless, to the ready deflocculation of their clay substance. It is
not easy to define the precise physical composition conducive to this
crust formation; but the subjoined physical analyses show examples
of soils in which this tendency is very prominent and is frequently
annoying, in that when they occur in the regions of frequent summer
rains, it becomes necessary after each one to till the surface in hoed
crops (_e. g._, in cotton-fields) in order to prevent the injurious
effects of such consolidation of the surface. It may, of course, be
prevented by mulching, or on the large scale by green-manuring, to such
extent as to prevent contraction.
The subjoined physical analyses of two soils from the
Brown-Loam region of Northern Mississippi (see chap. 24),
shows the composition of lands excellent in every respect
other than the tendency to crust after each rain:
PHYSICAL ANALYSES OF CRUST-FORMING SOILS.
===========================================================
|Diameter.|Hydr. Value.|No. 219.|No. 197.
------------------+---------+------------+--------+--------
Coarse materials | 1-3 mm. | | } |
| | | } .23 |
{ |.5-1 “ | | } |
{ | .50 | 64 mm. | 1.47 |
Sand { | .30 | 32 “ | 2.33 | .79
{ | .16 | 16 “ | 1.17 |
{ | .12 | 8 “ | .78 | .18
| | | |
{ | .072 | 4 “ | .76 | .78
{ | .047 | 2 “ | 9.79 | 3.56
Silt { | .036 | 1 “ | 7.20 | 13.12
{ | .025 | .50 | 13.11 | 16.64
{ | .016 | .25 | 15.07 | 27.28
{ | .010 | <.25 | 26.36 | 18.87
| | | |
Clay | ? | <.0023 | 19.10 | 17.23
------------------+---------+------------+--------+--------
These soils agree in having a sufficient amount of clay (17
to 19%) to characterize them as clayey loams, associated
with a very large proportion of the grain-sizes of less than
.025 mm., or .5 mm. hydraulic value. A higher proportion of
clay, even though associated with a similarly high or even
larger proportion of these fine sediments, seems to prevent
crusting, probably because the swelling of the clayey
ingredient on wetting and its extravagant contraction in
drying breaks up the continuity of the surface. The heaviest
clay soils, such as those shown on a preceding page, neither
crust nor crumble on drying after wetting, but contract into
lumps of stony hardness, _as a whole_.
The burning-out of the humus from well-tilled surface soils during the
extended heat and dryness of rainless summers, brings about such a
contraction or packing of the surface soil of orchards in California
as to greatly reduce their productiveness, and to render necessary
diligent green-manuring as the only practical remedy. In many cases,
liming of the surface also serves well to prevent this injurious
effect, which to some extent of course follows surface irrigation as
well as rains.
In most soils, repeated alternate wetting and drying _in place_
produces a loose, flocculated texture, so long as no deflocculation is
brought about by mechanical causes, such as beating rains or running
water.
_Effects of Frost on the Soil._—The expansion suffered by water in
freezing necessarily tends to separate the soil particles previously
held together by the surface tension of the capillary water, or
otherwise flocculated or cemented. Freezing of the soil is therefore of
material assistance in _disintegrating_ cloddy, ill-conditioned soils,
leaving them in loose, crumbly condition after the ice has melted and
the surplus water drained off; so as to materially facilitate tillage
and root penetration. When, however, soils thus circumstanced are
tilled or trodden while too wet, they quickly become puddled, being
practically reduced to single-grain structure. (See this chapt. p.
110). Hence the injury caused by allowing cattle to range in winter on
cultivated land subject to freezing and thawing, which it sometimes
takes years to correct.
A disagreeable effect often produced by the freezing and thawing of
wet lands is the “heaving-cut” of grain, resulting from the upward
expansion of the surface soil in freezing, that may readily rupture the
roots; while on thawing, the soil surrounding the upheaved stool is apt
to settle down, especially in case of a rain, leaving the stool and
roots exposed either to drying or freezing, as the case may be. Hence
the desire of grain farmers in northern climates, for a sufficient
covering of snow to protect the fall-sown grain, rather than an “open
winter,” during which the grain is exposed to alternate freezes and
thaws, or extreme cold.
In certain soils, notably in those liable to crusting (p. 117), instead
of heaving the soil, the water in freezing emerges bodily from small
cracks, in foliated or wire-like forms (“ice-flowers”) resembling those
of native silver, and formed substantially in the same way, by a kind
of “wire-drawing” process, aided by crystallization.
Small ice-crystals formed on the surface of small crevices
filled with water cause others to be formed at their lower
ends, and the expansion occurring in freezing, forces the
ice upward; the process repeating itself under favorable
conditions, until the stalks or sheets of ribbed ice grow to
a height of several inches. This phenomenon is especially
frequent in the middle cotton States—Arkansas, Tennessee,
northern Mississippi, etc., where frequent changes from
rainstorms or thaws to cold northwest winds occur in winter.