PHYSICAL COMPOSITION OF THE SOILS.
As has already been stated (chapt. 1, p. 10), the general physical
constituents of soils are _rock powder or sand_ and _silt_, more or
less decomposed according to the nature of the original rocks; _clay_,
the product of the decomposition of feldspars and some other silicates;
_humus_, the complex product of the decomposition of vegetable and
animal matters on and in the soil mass; as well as vegetable matter
not yet humified. Each of these several constituents must now be
considered more in detail. Since clay is the substance whose functions
and quantitative proportions influence most strikingly the agricultural
qualities of land, it should be first discussed.
_Clay as a Soil Ingredient._
The plasticity and adhesiveness of clay, together with the extreme
fineness of its ultimate particles (said to reach the 1-25000 of an
inch), explains its great importance as a physical soil ingredient.
It serves to hold together and impart stability to the flocculent
aggregates of soil particles that compose a well-tilled soil; for
without clay the sand would collapse into close-packed single grains
so soon as dried, and loose tilth would be impossible. Sand drifts
illustrate this condition.
On the other hand, the fineness of the particles serves to render clay
very retentive of moisture as well as of gases and of solids dissolved
in water, imparting these important properties to soils containing it;
while coarse sandy soils are oftentimes so deficient in them as to
render them unadapted to any useful culture, despite the presence of an
adequate supply of plant-food.
When to these essential physical properties of clay, there is added
the fact that usually the clay-substance as it exists in soils
contains the most finely pulverized and most highly decomposed
portions of the other soil-minerals, and therefore the main part of
the available mineral plant-food, it is easy to understand why soils
containing a good supply of clay should be called and considered
“strong” land by the farmers of all countries. “Poor” clay soils are
exceptional; but sometimes the clay content reaches such a figure that
the difficulties of tillage render them too uncertain of production for
profitable occupation.
_Amount of Colloidal Clay in Soils._—Any and all of the kinds of clay
mentioned (p. 57) as occurring naturally may, of course, enter into and
form part of soils. But as the amount of true, plastic clay substance
contained in them is very indefinite, it becomes necessary, in order
to classify soils in respect to their tillableness, to ascertain more
definitely the amount of pure, or nearly pure, colloidal clay substance
contained in the several classes of soils ordinarily recognized and
mentioned in farming practice. That this determination can at best be
only approximate, is obvious from the fact mentioned above (chapt. 4,
p. 59), that pure kaolinite itself is not plastic, and only becomes
so by the indefinite comminution and hydration it experiences in the
processes of soil-formation. As the progress of this process is also
indefinite, the same soil containing particles ranging from the finest
to the chalky scales of pure kaolinite, the drawing of a line must be
more or less arbitrary and empirical.
From numerous experiments and comparisons made, the writer
has been led to place the limits of “plastic clay” at and
below such grain sizes as will remain suspended (afloat) in
a water column eight inches high, during 24 hours. To go
beyond this point in the examination of soils for practical
purposes, would render such examinations so laborious and
hence so rare, that this kind of work would be practically
excluded from ordinary practice. According to this view
the following percentages of such “clay” correspond
approximately to the designations placed opposite:
Very sandy soils .5 to 3% clay
Ordinary sandy lands 3.0 to 10% “
Sandy loams 10.0 to 15% “
Clay loams 15.0 to 25% “
Clay soils 25.0 to 35% “
Heavy clay soils 35.0 to 45% and over.
It must be distinctly understood, however, that these
figures make no claim to accuracy or invariability. For,
the tilling qualities of a soil containing one and the
same amount of such “clay” may be very materially modified
according to the kind and amount of each of the several
grain-sizes of rock powder or sand they contain.
_Influence of fine powders on plasticity and adhesiveness._—An
admixture of a large amount of fine powders diminishes materially the
adhesiveness of a clay soil, even though it may render it even more
“heavy” in tillage; while the admixture of coarse sand, even in very
considerable proportions, does not greatly influence the adhesiveness
of the clay. The latter alone cannot therefore serve as a proper guide
or basis for the classification of soils in respect to tillage; we
must also take into consideration the nature and amount of the several
granular sediments mixed with it.
Moreover, the nature and especially the adhesiveness of the clay
substance as obtained by analysis may vary considerably in the presence
of a very large amount of the finest grain-sizes; among which ferric
hydrate or iron rust is especially apt to accumulate predominantly
in the clay, considerably increasing its apparent weight and greatly
diminishing its adhesiveness.[24] In strongly ferruginous soils,
therefore, it becomes necessary to take into special consideration the
amount of the ferric hydrate or rust which accumulates in the clay
substance. The presence of large amounts of humus or vegetable mold
also influences materially the adhesiveness and physical properties
of the clay obtained by the method described, although most of it
remains with the finer powdery sediments or grain-sizes. There are,
besides, other colloidal or at least amorphous substances present in
all soils, such as silicic, aluminic and zeolitic hydrates, which are
all non-plastic, and yet sufficiently fine to form part of the “clay”
obtained as above specified.
[24] This fact emphasizes the impossibility of explaining the
plasticity and adhesiveness of clay simply as a function of fineness of
grain.
Despite these imperfections, (which however can in a measure be
taken into consideration in judging of a soil’s tilling qualities by
its clay content), the figures given in the above table approximate
much more nearly to a tangible basis for such estimate, than the
utterly indefinite mixtures which under the older methods of analysis
have been, and still are to some extent, used as a basis for soil
classification by writers on agriculture.
_Rock Powder; Sand, Silt and Dust._
The powdery (sandy and silty) constituents of soils usually constitute
the greater part of their mass; and the proportions present of the
several grades of fineness exert a most decisive influence upon their
cultural qualities, and very commonly upon their agricultural value
also. It is needless to add that the kind of mineral of which they
consist or from which they were formed, is also of great importance in
determining the quality of soils from the standpoint of the chemist,
with respect to their content of mineral plant-food.
WEATHERING IN HUMID AND ARID REGIONS.
_Sands of the Humid Regions._—As has already been stated, “sand” is
usually understood to be, in the main, quartz more or less finely
pulverized, generally intermingled with a few grains of other minerals.
With this understanding, since quartz is practically inert with respect
to plant nutrition, it follows that soils consisting mainly of this
substance contain but little plant-food; hence the common expression
“poor, sandy land,” the outcome of the experience had in Europe and
in the Eastern United States, and which until recently has been held
to be of general application. The “sands of the desert” have, both in
ordinary life and in poetry, always stood as the symbol of sterility.
Thus the sandy lands (“sand hammocks”) of Florida, the
(long-leaf) pine lands of the Gulf States, the “pine
barrens” of New Jersey and of Michigan, are noted both
for their sandy soils and their sterility after brief
cultivation; necessitating fertilization within a few
years from the time of occupation. In Europe, the “Heide”
(heather) soils of northeastern Germany are of the same
cultural character.
_Sands of the Arid Regions._—The experience of arid countries however,
has long ago shown that some very sandy lands—_e. g._, such as form
the oases of the north African deserts—may be extremely productive
when irrigated, and also of considerable durability. Actual experience
and close investigation given this subject in the arid regions of the
United States has fully demonstrated that lands appearing to the
casual observer to be hopelessly sterile sandy deserts, very commonly
prove to be even more productive than the more clayey lands of the same
regions. Examination of the sand shows, in these cases, that instead
of mere grains of quartz, the minerals of the parent rock, partially
decomposed, themselves constitute a large proportion of the sandy mass.
But in the regions of deficient rainfall, as has already been stated,
(p. 47) the formation of clay (kaolinization) is exceedingly slow;
hence the decomposition of the rock powder results in the production
of predominantly pulverulent instead of clayey soils. But the mineral
plant-food is not on that account less available, provided other
physical conditions necessary for the success of plant growth are
fulfilled. Among these moisture stands foremost; hence the relative
proportions of the several grain-sizes are of vital importance, since
upon this depends to a great extent the proper supply and distribution
of moisture, without which no amount of plant-food will avail.
Moreover, the finest and most highly decomposed powder is the portion
from which the roots draw their chief food-supplies.
The point last mentioned is well shown in the results
obtained by Dr. R. H. Loughridge, from the analysis of each
of the several grain-sizes into which he had resolved a very
generalized soil of the State of Mississippi, representing a
very large land area in that State as well as in Tennessee
and Louisiana. The details of this investigation are given
farther on; but summarily it may be stated that he found
practically the whole of the acid-soluble mineral plant-food
accumulated within the portion of the soil the fineness of
whose grains was below .025 millimeters (one-thousandth of
an inch); ingredients so fine as to be wholly impalpable
between the fingers. Moreover, two-thirds of the total
amount was found in the portion described above as “clay.”
It is thus readily understood why clay soils are in the
regions of summer rains commonly designated as “strong”
lands.
The corresponding later investigations of Rudzinski (Ann.
Agr. Inst. Moscow, Vol. 9, No. 2, pp. 172-234; Exp. Sta.
Record, Dec. 1904, p. 245) and of Mazurenko (Jour. Exp.
Landw. 1904, pp. 73-75; Exp’t Stn. Record, Dec. 1904, p.
344) fully corroborate Loughridge’s conclusions, for typical
soils of European Russia.
In the arid or irrigation regions, however, the case is different, for
the reason that much of the decomposed rock-substance remains adherent
to the surface of the larger grains, and plastic clay is formed to a
much less extent. Much available plant-food may therefore, in arid
lands, be present even in rather coarsely sandy soils almost devoid
of clay; such as in humid climates would be likely to be found wholly
barren. (See chapt. 19).
PHYSICAL ANALYSIS OF SOILS.
_Use of Sieves._—Down to a certain point the separation of the soil
into its several grain-sizes may be accomplished by means of sieves.
We may thus separate coarse gravel from fine gravel and from sand;
and the latter may itself be separated into several sizes by the same
means. This presupposes, of course, that the soil has been previously
prepared for the purpose by crushing the lumps consisting of aggregates
of finer particles, that in the operation of tillage would again be
resolved into their fine constituents, or be penetrated by roots. But
this preparation of the soil for sifting must not be carried beyond
the point mentioned, for a grain consisting of particles somewhat
firmly cemented together will under ordinary conditions play in the
soil precisely the same part as a solid sand-grain, and must not
therefore be broken up, if the soil is to be examined in its natural
condition. The pressure of the fingers or of a rubber pestle is as
far as trituration should go. The disintegration of these compound
particles by means of acids, as prescribed and practiced by the French
soil chemists, may wholly change the physical nature of the soil by
the breaking-up of mechanical aggregations which in the usual course
of tillage would remain intact. This is especially true of strongly
calcareous soils, and particularly those containing calcareous sand.
The sieves used for this purpose should not be ordinary wire
sieves, but should have bottoms of sheet brass perforated
by _round_ holes of the various diameters desired, of
fractions of inches, or preferably of millimeters. For the
finer grain sizes, silk bolting cloth is used by the U. S.
Bureau of Soils.
In the sifting process it will be found that so soon as the
finer grain-sizes of the sand are approached, the sieve
fails to act satisfactorily; the more so, the more clay was
originally contained in the material. The fine particles
flock together, forming little pellets, which refuse to be
separated by the sieve. This difficulty can, of course, be
partly overcome by previously separating the clay from the
sand by means of water, as detailed above; but even then
it will be found that so soon as the grain-sizes fall much
below ¹/₅₀ of an inch (½ millimeter) the same difficulty is
experienced, so long as the sand is dry. By playing a small
stream of water upon the sieve, however, all the particles
beyond the ¹/₅₀₀ of an inch may be successfully separated
from the coarser portion; and for many practical purposes
the separation need be carried no farther.
_Use of Water for Separating Finest Grain-Sizes._—The
scientific investigator, however, must of necessity proceed
to separate the finer grain-sizes from each other, since,
as will presently be shown, they influence the tilling
qualities of the soil to a much greater degree than do
the coarser particles. Such farther separation can be
accomplished only by the aid of water.
_Subsidence Method._—When a small amount of soil is stirred up in
water, and is afterward allowed to stand for some time, the different
grain-sizes will settle consecutively in accordance with their
sizes (or weights); the smallest ones settling latest, and the clay
only remaining suspended, as stated above. So long, however, as any
considerable amount remains suspended in the water, the latter is not
only denser but especially more viscid than if the clay were absent.
In order therefore to obtain correct results by any method involving
the use of water, it is necessary to remove the clay before proceeding
to the separation of the granular sediments. This, as has been already
stated, is approximately accomplished by allowing the soil, when
diffused in water after proper disintegration, to settle for 24 hours
from a column of water 200 mm. high, whereby all grain-sizes, of
and above .01 mm. diameter are removed from the turbid liquid. This
sedimentation is then repeated until after 24 hours the water becomes
clear. The clay is then determined in the “clay water” by evaporation
or precipitation; the granular sediments may then be successfully
separated by sedimentation.
The U. S. Bureau of Soils uses for the separation of clay, instead of
subsidence for 24 hours, the more expeditious process of centrifuging
the turbid soil water in appropriate glass cylinders, by the aid of
an electric motor; and thus in a relatively short time obtains “clay”
in which the upper limit of size is one-half of that mentioned above,
viz., .005 mm. But for the costliness of the appliances required,
including the entire time of an operator, this method of separating the
clay would undoubtedly be preferable to the elimination by subsidence;
the more as a more minute grain-size for the clay group is thus secured.
[Illustration: FIG. 6.—Schöne’s Elutriator.]
The separation of the clay having been accomplished, the various sizes
of silt and sand may be separated by again suspending them in water;
and interrupting the settling process at stated times, the grain-sizes
corresponding to definite velocities in settling may be segregated and
weighed. When this process of settling and decanting is carefully and
repeatedly carried out, very good results are obtained.
_Hydraulic Elutriation._—The sedimentation (or “beaker”) method,
long practiced in the arts is, however, quite tedious, requiring the
constant close attention of a skilled observer. The desired results
may, in the writer’s judgment, be more conveniently obtained by the
hydraulic method, whenever no very large volume of work of this kind is
required to be done at once.
When instead of allowing the soil to settle in quiet water, the latter
is used as an ascending current of regularly graded velocities, it
is clear that the soil particles will be carried off by this current
in exact conformity with their several sizes (or strictly speaking,
volume-weights); and when maintained in such a current for a sufficient
length of time, the entire quantity of the sediment corresponding to
the prevailing velocity will be carried away. It is of course easy
to ascertain to what grain-sizes certain velocities of the upward
current (regulated by a stopcock with arm moving on a graduated scale)
correspond, and to regulate accordingly the intervals between the
different velocities to greater or less detail, as may be desired. A
number of instruments have been devised for this purpose.
_Schöne’s Elutriator_ is the one commonly used in Europe; in it the
upward current ascends in a conical glass tube, (see figure 6) entering
through a narrow, curved inlet tube, in which the soil sample is kept
agitated by the current itself. The objection to this plan is twofold:
first, the narrow, curved inlet-tube is readily clogged by the soil
mass at the lower velocities, which are thereby changed, so that,
unless a very small amount of soil only is employed, the whole mass is
not kept properly stirred; second, the circulating currents brought
about by the conical shape of the tube cause the sediment-particles to
coalesce into complex, larger ones (floccules), which will then settle
down and fail to pass over at the current-velocity corresponding to
their individual component parts.
[Illustration: FIG. 7.—The Churn Elutriator (Hilgard’s) for the
physical analysis of soils.]
_Churn Elutriator with Cylindrical Tube._—The errors just alluded
to are obviated by an arrangement devised by the writer, in which
a rapidly revolving stirrer, placed at the base of a cylindrical
tube in which the washing process is conducted and which eliminates
counter-currents, continually disintegrates these compound particles,
and thus enables the entire quantity of the sediment corresponding to
the prevailing current-velocity to pass off with a comparatively slight
expenditure of time on the part of the operator (see figure 7). A wire
screen interposed between the churn and cylindrical glass tube prevents
communication of the whirling motion to the column. As the apparatus
works automatically, the analyst has only to observe from time to time
whether or not the turbidity near the top of the tube has disappeared;
and as the sediment accumulates at the bottom of the tall receiver
bottle,[25] no harm is done if the attendant should neglect to change
the velocity in time, except that water will run to waste.
[25] The figure given of this elutriator in Bulletin No. 24, on
physical soil analysis, published by the U. S. Bureau of Soils, shows
as the receiver a bottle entirely too low to insure the complete
retention of the sediments by settling. The receiving bottle should not
be less than twelve inches high and five inches wide.
The conical relay glass below the churn serves to retain the coarser
grades of sediments which are not concerned in the velocities employed
in the elutriator tube, and thus prevents injurious attrition. But
these sediments can at any time be stirred up by the incoming current
and brought into the washing tube if desired. In the same manner the
passing-off of the finer sediments can be materially accelerated by
running off rapidly about two-thirds of the turbid column of water
every twenty minutes.
It should be fully understood that prior to attempting such separation,
the “colloidal clay” must first be removed by the subsidence or
centrifugal method, since otherwise much larger grain-sizes may be
carried off at a given velocity.
_Yoder’s Centrifugal Elutriator._—A very ingenious instrument which
combines the elutriation and sedimentation processes into one, has
been devised by P. A. Yoder, of the Utah Expt. Station. The elutriator
bottle is placed in a centrifuge driven by an electric motor; it is
closed by a glass stopper carrying a delivery tube to a short distance
above the bottom of the elutriator bottle, as well as an outflow tube
ending at the base of the stopper; the latter also carries a funnel
coinciding with the center of rotation. Into this funnel flows
gradually the muddy water containing the soil in suspension; and the
rate of its flow, together with the velocity of rotation, determines
the size of the sediment-granules that will be deposited in the
slack-water below the mouth of the delivery tube. The muddy soil-water
is kept agitated in a funnel-shaped reservoir by air-bubbles from a
constant-pressure chamber.
While the principle of this instrument is good, it is quite complicated
and the results obtainable from it in practice have not as yet been
made public. The inventor claims that an analysis may by its means be
completed in less than three hours.
In all hydraulic elutriators a provision for constant pressure in
the reservoir supplying the current of water is needed; although in
Schöne’s and some other instruments a gradually decreasing pressure in
a plain reservoir is employed. A large glass bottle or carboy fitted
with the proper tubes so as to constitute a Mariotte’s bottle (in which
the air enters near the bottom of the vessel), is a very convenient
arrangement.
_Number of Sediments._—The number of grain-sizes or sediments into
which the soil mass is to be segregated is of course entirely within
the option of the operator. Experience has shown that it is unnecessary
to discriminate very closely between the several sizes of the coarser
portion of the sand, such as those lying between one-fourth and
one-half of a millimeter. But below this point, and especially between
one-tenth of a millimeter and the clay, a proper discrimination becomes
very important. The series first devised by the writer in 1872 is based
upon a consecutive doubling of the velocities of the current from
a quarter of a millimeter per second to thirty-two millimeters per
second; the sediment of sixty-four millimeter-velocity corresponding to
a diameter of one-half of a millimeter, will remain in the elutriator.
Above this, as before remarked, the sieve (especially when aided by a
jet of water) effects a satisfactory segregation.
The table below shows the elements of these series both as regards
current-velocities and maximum quartz-grain diameters carried off by
each. In a great many cases, however, it is altogether unnecessary to
go into such detail, and a subdivision into six or seven divisions
is quite sufficient. Such a subdivision, based upon the doubling of
grain-sizes instead of current-velocities, has been adopted by Prof.
Milton Whitney, of the U. S. Department of Agriculture, and others.
TABLE OF DIAMETERS AND HYDRAULIC VALUES OF SEDIMENTS.
===============================================================
|Velocity per second,|Maximum diameter
Designation of materials.| or hydraulic | of quartz
| value. | grains.
-------------------------+--------------------+----------------
| _Mm._ | _Mm._
| |
Grit | (?) | 1-3
{ | (?) | .5-1
{ | 32-64 | .50
Sand { | 16-32 | .30
{ | 8-16 | .16
{ | 4- 8 | .12
{ | 2- 4 | .072
{ | 1.0- 2 | .047
Silt { | .5- 1 | .036
{ | .25-0.5 | .025
{ | 0.25 | .016
{ | < 0.25 | .010
| |
Clay | < 0.0023 |
-------------------------+--------------------+----------------
_Results of such analyses._—A tabular presentation of the results of
analyses made in accordance with the above plan will give a good idea
of the differences between the various grades of soils recognized
in farm practice, to any one accustomed to the study of figures.
But a much more satisfactory showing is made by placing the several
grain-sizes segregated, into small vials or tubes of identical diameter
and placing them in parallel series alongside of each other.[26] The
curves formed by the surfaces of the several sediment-columns in each
series show to the eye very strikingly the relations of the several
grades of soils to each other, and suggest at once that while gentle
slopes or gently undulating curves belong to soils of intermediate,
loamy character, steep grades and zigzags show soils of extreme types.
This is exemplified in the subjoined Figures:
[26] Convenient stands for this purpose, used by the writer since 1872,
may be cut from L-shaped moldings of wood, such as can be readily
ordered from any planing mill. The vials can be cemented, wired or
tied.
[Illustration: FIG. 8.—Illustration of Results of Hydraulic
Elutriation, showing extremes of soil texture, and intermediate loam.]
[Illustration: FIG. 9.—Illustration of Results of Hydraulic
Elutriation, showing Alluvial Silts and Pine-Woods Soil.]
_Physical composition corresponding to popular designations of Soil
quality._—The subjoined table illustrates the physical composition
of a number of soils from the State of Mississippi, selected for
their representative character, in order to deduce therefrom
approximate definitions of physical character corresponding to popular
designations. This table, published in 1873 in accordance with results
obtained during the two preceding years, does not require any material
modification on account of subsequent investigations. It lacks,
however, a characteristic representative of the predominant soils of
the arid region, viz., the silty soils so prevalent in dry climates,
only approximately represented by No. 165 of the table; hence two such,
from California, exemplifying respectively the valley deposits of the
Sacramento and Colorado rivers, have been added to the list.
It must not, however, be understood that these typical soils
necessarily represent correctly the physical constitution of all
soils falling under the same popular designation; for we are far from
being able as yet to predict accurately in every case the tilling
qualities of a soil material from its physical composition. To do this
it would be necessary not only to know with some degree of precision
the several physical coefficients of each of the several grain-sizes,
and perhaps of many more intermediate ones; but we would also have to
construct a formula according to which each could be given its proper
weight when present in varying proportions, and of varying shapes,
surface condition, and material. For this our present knowledge is
wholly inadequate, if indeed the problem is not beyond the limits of
mathematical computation. We must for the present at least be satisfied
with the empirical approximations afforded us by the constantly
increasing number of such analyses, correlated with farming experience.
Since the finest grain-sizes above those classed as “clay”
do not tend to “lighten” soils, but even to render them more
intractable (“putty soils”), while coarser ones gradually
change the dense clay-texture into the “loamy,” it is clear
that in between there must be a neutral point, some grain
sizes which by themselves do not influence soil texture
either way. Discussion of numerous physical analyses, and
some direct experiments, have led the writer to conclude
that this theoretically neutral grain-size lies at or
near the diameter of .025 mm., or .5 mm. hydraulic value.
In correlating the results of analysis with the tilling
qualities of the soil as to “heaviness and lightness,”
therefore, that grain-size may usually be left out of
consideration.
PHYSICAL ANALYSES OF SOILS AND SUBSOILS.
(A) = 238 White Pipe Clay. Tishomingo Co.
(Z) = Hygroscopic Moisture (+7 = to +21°C)
===========+==============+================+=======
| Diameter | Velocity |
|(Millimeters) | (Hydr. V.) |
Designation| | |
of +--------------+----------------+-------
Materials. | | |
| | Millimeters | (A)
| | per sec. |
-----------+--------------+----------------+-------
Grit | 1-3 | |
| | |
{ | .5-1 | |
{ | .50 | 64 |}
Sand { | .30 | 32 |} 0.06
{ | .16 | 16 |}
{ | .12 | 8 | 0.08
| | |
{ | .072 | 4 | 0.02
{ | .047 | 2 | 0.04
Silt { | .036 | 1 | 0.08
{ | .025 | 0.5 | 0.08
{ | .016 | 0.25 | 2.00
{ | .010 | <0.25 | 21.15
| | |
Clay | ? | <0.0023 | 74.65
+-------
| 98.16
|
(Z) | 9.09
Ferric Oxid | 0.13
-------------------------------------------+-------
(B) = 248 Tallahoma Subsoil. Jasper Co.
(C) = 165 Flatwoods Soil. Chickasaw Co.
(D) = 206 Pine Hill Soil. Smith Co.
(E) = 209 Pine Hill Subsoil. Smith Co.
(F) = 397 Oxford Subsoil. Lafayette Co.
(G) = 219 Table Lands Subsoil. Benton Co.
(H) = 173 Prairie Subsoil. Monroe Co.
(I) = 230 H’y Flatwoods Soil. Pontotoc Co.
(J) = 246 Red Hills Subsoil. Attala Co.
(K) = 196 Hog-wallow Subsoil. Jasper Co.
(Z) = Hygroscopic Moisture (+7 = to +21°C)
==========+===============================================================
| MISSISSIPPI UPLANDS.
+------------------+------------------+-------------------------
Designation| Sandy. | Loam. | Clay.
of +-----+------+-----+-----+-----+------+-----+------+-----+------
Materials.| | | | | | | | | |
| (B) | (C) | (D) | (E) | (F) | (G) | (H) | (I) | (J) | (K)
| | | | | | | | | |
----------+-----+------+-----+-----+-----+------+-----+------+-----+------
Grit | 6.94| 2.90 | 0.36| 0.36| |} |} | 0.33 |} | 0.83
| | | | | |} 0.23|}2.10| |}1.97|
{|17.65| 6.96 | 2.98| 0.83| |} |} | 0.35 |} | 1.19
{|18.81| 2.81 | 6.62| 6.21|} | 1.47|} | | 0.72| 1.96
Sand {|10.16| 4.41 | 7.75| 3.38|}0.79| 2.33|}0.62| | 2.32| 1.64
{| 2.66| 3.13 | 3.01| 3.85|} | 1.17|} | | 2.09| 0.88
{| 1.66| 2.02 | 1.59| 1.49| 0.18| 0.78| 0.20| 0.23 | 0.70| 0.26
| | | | | | | | | |
{| 1.02| 2.23 | 1.19| 0.64| 0.78| 0.76| 1.26| 0.18 | 1.29| 0.19
{| 0.88| 5.06 | 3.56| 2.63| 3.56| 9.79| 2.92| 1.61 | 1.80| 2.49
Silt {| 1.96| 9.67 | 6.50| 5.40|13.12| 7.26| 7.36| 2.66 | 3.60| 3.67
{| 7.89|14.18 |13.97| 7.77|16.64| 13.14| 8.81| 9.13 | 2.73| 5.39
{| 8.40|22.03 |14.20|16.65|27.28| 15.07| 7.85|26.64 | 3.30| 10.31
{|15.53|15.62 |29.36|37.75|18.87| 26.50|35.22|32.35 |25.33| 24.18
| | | | | | | | | |
Clay | 8.63| 7.86 | 4.58|10.70|17.23| 19.19|33.16|25.48 |40.25| 47.03
+-----+------+-----+-----+-----+------+-----+------+-----+------
|99.28|98.68 |95.67|97.77|98.35| 97.65|99.50|97.87 |96.11|100.00
| | | | | | | | | |
(Z) | 1.80| 3.36 | 2.48| 7.69| 8.79| 7.24|11.35| 9.33 |18.60| 14.48
Ferric Oxid| 1.10|(1.45)| 1.25| 4.45| 2.53| 5.11| 5.42|(5.90)|10.50| 4.00
----------+-----+------+-----+-----+-----+------+-----+------+-----+------
(L) = 390 Buckshot Soil. Issaquena Co.
(M) = 237 Loess. Claiborne Co.
(N) = 365 Tallahatchie All. Soil. Panola Co.
(O) = 377 Frontland Subsoil. Sunflower Co.
(P) = 395 Dogw. Ridge Soil. Coahoma Co
(Q) = Southwest Pass. Plaquemine Par.
(R) = Southwest Mud-lump. Plaquemine Par. La.
(Z) = Hygroscopic Moisture (+7 = to +21°C)
==========+=======================================================
| MISSISSIPPI RIVER BOTTOM.
+-------+-------+---------------------------------------
Designation| Swamp.| River.| River Deposit. Delta
of +-------+-------+-------+-------+-------+-------+-------
Materials.| | | | | | |
| (L) | (M) | (N) | (O) | (P) | (Q) | (R)
| | | | | | |
----------+-------+-------+-------+-------+-------+-------+-------
Grit | 0.09 |} |} | | | |
| |} 0.24 |} 0.09 | | | |
{| 0.05 |} |} | | | |
{| | 0.37 | 0.04 |} 0.32 | 0.15 |} 0.18 |} 0.10
Sand {| 0.36 | 0.61 | 0.05 |} | |} |}
{| | 0.93 | 0.21 | 2.79 | | 0.47 | 5.02
{| 0.31 | 1.65 | 1.30 | 2.41 | 3.75 | 7.03 | 3.68
| | | | | | |
{| 0.27 | 1.95 | 2.68 | 16.90 | 21.46 | 12.38 | 5.34
{| 1.56 | 14.25 | 9.38 | 19.97 | 21.83 | 13.27 | 10.09
Silt {| 2.23 | 16.20 | 9.88 | 13.90 | 14.01 | 15.87 | 5.58
{| 3.68 | 20.08 | 20.37 | 4.27 | 9.93 | 8.25 | 9.54
{| 8.97 | 5.59 | 19.79 | 1.89 | 9.58 | 7.26 | 8.01
{| 38.19 | 33.38 | 25.30 | 30.08 | 8.65 | 19.67 | 34.46
| | | | | | |
Clay | 44.30 | 2.51 | 9.64 | 5.51 | 10.35 | 12.20 | 18.18
+-------+-------+-------+-------+-------+-------+------
|100.01 | 97.74 | 98.73 | 98.04 | 99.72 | 96.58 |100.00
| | | | | | |
(Z) | 14.31 | 4.18 | 6.12 | 5.68 | 3.95 | |
Ferric Oxid|(5.82) | 3.27 | 2.58 | 2.31 | 2.69 | |
----------+-------+-------+-------+-------+-------+-------+-------
(S) = 10 Sacramento. Sacramento Co.
(T) = 506 Gila. San Diego Co.
(Z) = Hygroscopic Moisture (+7 = to +21°C)
===========+==================
| CALIFORNIA.
+------------------
Designation | River Deposit.
of +---------+--------
Materials. | |
| (S) | (T)
| |
-----------+---------+--------
Grit | |
| |
{ | |
{ | | .13
Sand { | | .15
{ | | .11
{ | .32 | .75
| |
{ | 3.16 | 2.51
{ | 10.27 | 8.32
Silt { | 13.67 | 12.64
{ | 13.11 | 11.28
{ | } 43.61 | } 31.79
{ | } | }
| |
Clay | 12.06 | 23.97
+---------+--------
| 96.20 | 91.71
| |
(Z) | 9.18 | 9.26
Ferric Oxid | |
-----------+---------+--------
_Number of soil grains per gram._—It is of some interest to consider
the number of grains of different sizes that may be contained in, _e.
g._, a gram of soil. If for this purpose we assume all the soil grains
to be spherical, we shall obtain the minimum figures, for most other
shapes will pack more closely. King (Physics of Agriculture, p. 117)
calculates such figures for different grain-sizes, assuming the density
to be that of quartz (2.65), with the result that while with a diameter
of one millimeter (1-25 inch) the number of grains would be 720, and
with one-tenth of a mm. 720,000; if made of the finest particles only,
viz., one thousandth of a mm., the number would be 720,000 billions.
Probably few of the clayey soils we ordinarily deal with are of this
order; it is doubtless approached in certain fine plastic clays.
_Surface afforded by various grain-sizes._—The amount of _surface_
afforded by a similar amount of soil must naturally be considered in
this connection, since upon it depends not only the amount of moisture
which the soil may hold in the form of superficial films, but also the
extent of surface upon which the weathering agencies as well as the
root hairs of plants may act. Quoting again from King’s work, we find
on the same premises given above for the _number_ of grains, that their
_surface_ would in the case of grains of one mm. diameter be eleven
square feet per pound (about half a pint) of material; while in the
case of the finest grade we should have 110,538 square feet, or more
than two and a half acres.
From actual experiments made with the flow of air through various
soils, King calculates that while in ordinary loam soils the total
surface is about an acre per cubic foot, in fine clay soils it rises to
as much as four acres. If we imagine this large surface to be covered
with even a very thin film of water, it is readily seen how large an
amount may be present in a cubic foot of moist soil.
E. A. Mitscherlich (Bodenkunds für Land-und-Forstwirthe;
Berlin, 1905) attributes to the surface offered by the
soil particles supreme importance in determining the
productiveness of soils. According to him the internal
soil-surface determines directly the ease with which roots
can penetrate the soil; and he proposes the determination
of this factor by means of the heat produced in wetting
the soil (“Benetzungswärme”), measured in a calorimeter,
as a substitute for all methods of physical soil analysis,
which are vitiated by the varying shapes and densities
of the particles; while his method gives directly the
actual surface. To the consumption of energy required
by difficult penetration he attributes most of the
differences in production, and hence refers to the internal
soil-surface as governing nearly all the other physical
factors. The introduction of many arbitrary assumptions,
and the failure to show that the admitted inaccuracy of
the ordinary mechanical soil analyses are of any practical
importance, greatly detract from the cogency of the rigorous
mathematical discussion carried through his work by
Mitscherlich.
_Influence of the several grain-sizes on soil texture._—Undoubtedly
the most potent of all the sediments appearing in the above table in
influencing soil texture, is the “clay.” That the materials included
under this empirical designation may vary considerably in different
soils, has already been sufficiently insisted on; and it is doubtful
that in the present imperfect state of our knowledge of the functions
of the several physical grain-sizes, we would be much wiser were we to
go to the extreme advocated by Williams (Forsch. Agr. Phys., vol. 18,
p. 225, ff.), of determining with precision the actual amount of such
extremely fine clay particles as cease altogether to obey the law of
gravity when once suspended in water. It is at least doubtful that the
essential property of adhesive plasticity belongs only to these, for
this property doubtless increases gradually as the size diminishes,
although unquestionably not a mere function of the latter, since it
belongs only to the hydrated silicate of alumina.
_Ferric Hydrate._—Probably the body which most
commonly modifies materially the adhesive and contractile
properties of the clay substance, is ferric hydrate;
the more as on account of its high density it tends to
exaggerate materially, in many cases, the apparent content
of true clay, and the estimate of the soil’s plasticity
based upon it. A good example in point is the case of soil
No. 246 (Miss.) of the above table. This is a heavy clay
soil, yet not excessively adhesive; scarcely as much so as
No. 230 (Miss.), the heavy gray “flatwoods” soil, and not
nearly as “sticky” when wet as No. 173 (Miss.), the prairie
subsoil, although containing apparently 15% more clay than
the former, and 7% more than the latter. But No. 246 is a
highly ferruginous clay, in which the ferric hydrate is in
a very finely divided condition, and materially influences
the physical qualities of the clay substance. Were it all
accumulated in the “clay,” it would diminish the percentage
of true clay by 11.75%, reducing the clay-percentage to
28.5% which accords more nearly with the soil’s only
moderate adhesiveness, and not excessively heavy tillage.
But it must be remembered that the iron oxid shown in the analysis is
not nearly always in this finely diffused condition. Frequently it
incrusts the sand grains; quite commonly it forms small concretions of
limonite, which themselves act as sand grains; and again, it may be
present in the form of “black sand” or magnetic oxid, as is commonly
the case in California and on the Pacific slope generally. To take
this point properly into account, therefore, it would be necessary to
determine the amount of ferric hydrate actually present in the “clay”
as separated by subsidence of the granular constituents.
_Other substances._—This circumstance as well as the inevitable
presence of other modifying substances, clearly shows the desirability
of being enabled to examine the physical properties of this “clay”
directly, by collecting its entire amount as obtained in analysis,
instead of merely determining it by weighing fractional portions. When
this is done the analysis is much more valuable as indicating the true
tilling qualities of the land. The increase of bulk suffered by this
substance after wetting, is a very fair index of its content of true
clay, and is preferable to the chemical analysis proposed by some
investigators. For it is quite impossible to distinguish the silica and
alumina derived from the kaolinitic substance proper, from that which
is due to the decomposition of zeolites.
It is possible, however, to determine the possible _maximum_ of the
kaolinite ingredient by taking into consideration the quantitative
ratio according to which silica and alumina combine to form it, viz.,
approximately 46% of the former to 40 of the latter, the rest being
water. By using this calculation we can often demonstrate clearly the
presence in the “clay” of considerable amounts (up to 33%) of _aluminic
hydrate_; since no zeolitic mass can contain as much alumina as does
kaolinite. Whether the aluminic hydrate be in the form of gibbsite,
bauxite, diaspore,[27] or in the gelatinous state, the nature of the
soils containing it proves that it is totally destitute of plasticity
and adhesiveness; and this consideration will often serve to explain
the fact that soils showing in their chemical analysis high percentages
of alumina, nevertheless show quite low degrees of plasticity,
adhesiveness and water absorption. What part it may take in modifying
the physical properties of the soil we can thus far only conjecture.
[27] Bauxite is not only the most abundant of the three hydrates of
alumina known to occur naturally, but also stands nearly midway between
the two others in its water content, viz., a little over 25%; that of
diaspore being nearly 15%, gibbsite about 35%.
_Influence of the granular sediments upon the tilling qualities of
Soils._—Considering the granular sediments by themselves, in the
absence of clay, it may be stated in a general way that while in a
moist condition they flocculate sufficiently to produce a fair tilth,
they will nevertheless on drying collapse into a close arrangement
resulting from the single-grain structure. The form of the grains
being angular instead of rounded, they are apt to form a very closely
packed mass far from suitable to vegetable growth; as will be seen by
an example taken from one of the culture stations of the University of
California, from a piece of land which on the surface would be called
a very sandy loam, but after we descend increases in its content of
fine grains until at a depth varying from eighteen inches to three
feet we find what appears to be a hardpan, which is equally impervious
to roots and water and causes the water to stagnate to such an extent
that after heavy rains the land becomes so boggy as to render plowing
almost impossible without endangering the team. A close examination of
this hardpan shows that, unlike others, it is devoid of any cement, and
when taken out can be readily crushed between the fingers, and softens
in water, but does not become plastic. Its imperviousness is therefore
due solely to the close packing of the sand grains, for it contains
practically no plastic clay, and under the microscope the grains
are seen to be angular-wedge-shaped and composed of the remnants of
granite. The physical analysis shows the following result:
MECHANICAL ANALYSIS OF HARDPAN.
==================================
Designation.|Diameter.|Percentage.
------------+---------+-----------
{ | .50 mm. | 10.93
Sand { | .30 “ | 21.23
{ | .12 “ | 7.27
| |
{ | .072 “ | 9.63
{ | .047 “ | 12.00
Silt { | .036 “ | 7.19
{ | .025 “ | 1.25
{ | .016 “ | 14.20
| |
“Clay” | ? | 8.64
------------+---------+-----------
It is doubtful whether this condition of things can be remedied by the
usual measure of breaking up the hardpan either by hand or by means
of giant-powder blasting. Experience seems to show that the effect
is only temporary, and that in the course of time, by the action of
the percolating waters, the particles settle back into their original
impervious condition. It is just possible, however, that if once
penetrated by roots, the intervention of these would permanently
destroy the close structure, so as to make this a fair subsoil for the
growth of trees and other plants. The writer is not aware that this
kind of purely physical hardpan without cement has ever been observed
elsewhere.
This physical condition is doubtless responsible for two other
phenomena, viz., the “putty soils,” and also certain difficulties
experienced in irrigation.
“_Putty Soils_” is the name popularly given in the Cotton States, and
probably elsewhere, to soils usually occurring in low ground and also
known as “cray-fishy.” They consist of very uniform, powdery sediment,
with little or no coarse sand and still less of clay to render them
coherent. When wet these soils behave precisely as would glazier’s
putty, adhering to the surface of even the best-polished plowshare, so
that no furrow-slice can be turned and the plow is soon dragged out
of the ground. At a very closely limited condition of moisture such
lands may plow fairly well; but when this limit is passed in the least
(as sometimes happens in the course of a single day), it turns up only
hard clods, which in a few hours of sunshine become so hard that no
instrument of tillage short of a sledgehammer will make any impression
upon them. The physical analysis of these usually gray soils shows that
they contain only a trifling amount of clay; perhaps 1 or 2%, playing
the part of linseed oil in making putty out of whiting. Even the
addition of lime does not help such soils much, because there is little
or no clay to flocculate. They are, as a matter of fact, among the most
refractory lands the farmer has to deal with. A soil showing similar
behavior, though not quite as extreme as in the case of the Gulf or
Cotton States’ soils in question, occurs at the culture substation at
Paso Robles, California, and is probably closely correlated to the
physical hardpan referred to above. The physical analysis of this soil
yielded the following result:
MECHANICAL ANALYSIS OF SOIL.
==================================
Designation.|Diameter.|Percentage.
------------+---------+-----------
{ | .50 mm. | 14.24
{ | .30 “ | 15.17
Sand { | .16 “ | 8.88
{ | .12 “ | 5.60
| |
{ | .072 “ | 6.75
{ | .047 “ | 8.35
Silt { | .036 “ | 8.55
{ | .025 “ | 6.03
{ | .016 “ | 17.77
| |
“Clay” | ? | 7.50
------------+---------+-----------
It would seem the best and almost only remedy to be applied to
such soils as these is the introduction of vegetable matter or
green-manuring, by which their texture is loosened: for the hauling of
mere clay upon the land would hardly accomplish the purpose intended,
within the limits of farm economy.
_Dust Soils_, which during the dry season are even in their natural
condition so loose as to rise in clouds and render travel very
uncomfortable, are not uncommon in arid countries, _e. g._, in
Washington and adjacent parts of Oregon, on the uplands bordering the
Columbia, Yakima and Snake rivers. The physical analyses of three of
such soils, given in the table below, will convey some idea of their
peculiarities in this respect.
PHYSICAL ANALYSIS OF DUST SOILS.
=========================================================
|Hydr. Value.|Diameter. |No 17.|No. 37.|No. 79.
----------+------------+----------+------+-------+-------
Clay |<.0023. mm. |<.10--? | .93| 3.59 | 1.27
{ |<.25 mm. | .010 | 30.93| 13.06 | 32.29
Silt { | .25 to .5| .016 | 3.20| 5.82 | 12.75
{ | .5 to 2.0| .025-.047| 7.18| 27.37 | 37.51
{ |2.0 to 8.0| .047-.120| 21.88| 43.78 | 10.92
Sand |8.0 to 64.0| .12- .50 | 32.39| 49.57 | 3.97
| | +------+-------+-------
Total | | | 96.57| 98.18 | 98.72
----------+------------+----------+------+-------+-------
_Slow penetration of Water._—Soils of this class are wetted with
extreme slowness by irrigation water; so that when first taken under
cultivation it sometimes takes twenty-four hours to soak the land for
twelve inches in each direction. Irrigation furrows must be placed
very close together and in large numbers, in order to ensure the
wetting of the soil so that the crop shall not suffer from lack of
moisture at a distance of two or not more than three feet. Where the
irrigation furrows are drawn farther apart a fine stand of grain may
be seen within eighteen inches of the same, while farther away the
crops may be dying from lack of moisture. This difficulty is by no
means infrequent in the arid region, and is difficult to overcome
except by frequent and thorough tillage, which gradually increases the
rapidity of water-penetration; as has been shown in the soils of the
alluvial prairies of the Yakima country in the State of Washington.
It is necessary, however, to take care that they shall always contain
an adequate amount of humus or vegetable matter, in order to prevent
re-consolidation by the burning-out of the humus during the warm,
rainless season.
There is an unmistakable resemblance between these dust soils of the
Northwest and the “putty” soils mentioned above; both showing a very
low percentage of clay with a relatively large amount of the finest
sediments, with a sudden downward break of the curve before the coarser
grain-sizes are reached. It would seem as though the absence of these
intermediate grains favors the close packing of the fine sediments
in the interstices of the coarse ones, thus bringing about the
imperviousness, which is the chief obstacle to their cultivation.
_Effects of coarse Sand._—Coarse sand intermingled with heavy clay
soils has but little effect in improving the tilling qualities, unless
carried to such excess as renders it financiallyimpracticable. In
actual practice it is frequently possible to improve such soils by
properly distributing upon them the washings of the adjacent hills,
which will always carry sands of many grades; and when it is intended
to improve garden land by hauling sand it is important to choose the
latter so as to complement the deficient grain-sizes of the soil.
The sand of wind drifts or dunes is generally well adapted to such
improvement, being, as Udden[28] has shown, of a fairly definite
composition of sufficiently wide range of grain-sizes for the purpose.
The effects of humus in modifying soil texture are discussed farther on.
[28] The Mechanical Composition of Wind Deposits, Bull. No. 1,
Augustana Library Publications; 1898.