out unmistakably the effects of this operation. If ebullition for eight
or nine days reduces clay from ten to two per cent, increasing the
0.05–0.01 millimeter diameter grades by six per cent, it is evident that
boiling for one day or a shorter time becomes a questionable treatment.
Further experiments[158] made by boiling clay in a platinum vessel with
a platinum condenser showed that this precipitation of the clay was
largely if not wholly due to the salts extracted from the soil.
When the clay has once been converted into the granular condition,
considerable difficulty is experienced in restoring it to the state in
which it is capable of prolonged suspension in water.
The results of the studies herewith reported may be summed up as
follows:
1. The Berlin-Schöne method of elutriation gives fairly correct
separations with sandy soils containing little clay or matters finer
than 0.01 millimeter diameter, but on soils of fine texture, as loams
rich in humus and clays, it gives results which are grossly inaccurate,
the error on single grades amounting to from eight to fourteen per cent.
2. In respect of rapidity, economy of time, and ease of operation, the
Schöne elutriation has no advantage over the beaker method.
3. Schloesing’s method on its mechanical side makes no satisfactory
separations, and the chemical treatment it employs is liable to alter
seriously the texture of the soil.
4. The determination of clay from a single subsidence from any
conventional depth or volume of water, or for any conventional time, is
not a process certain to effect even a roughly approximate separation of
the finest quartz grains from true clay.
5. Boiling with water must be rejected as a treatment preliminary to
mechanical analysis, because it not only abrades and reduces the coarser
sediments, but may dehydrate and coagulate the true clay and thus alter
essentially the texture and grain of the soil.
=239. General Conclusions.=—The methods of Hilgard and Osborne have been
given in detail and largely in the descriptive language used by the
authors. The other methods of elutriation in use in other countries have
also been described. For practical use the methods of Hilgard and
Osborne are to be preferred to all others. For simplicity and speed the
Osborne method has the preference over the Hilgard. For rigid control of
the work the Hilgard method is to be preferred. The effect of long
boiling on clay pointed out by Osborne would suggest that the boiling
process preliminary to the Hilgard method be made as short as possible.
It would seem that the churn attrition in the Hilgard method might well
be regarded as a substitute for the soft pestling of the Osborne
process, and any prolonged boiling in the former method might be safely
omitted. When carefully carried out, the results of the Hilgard and
Osborne method are fairly comparable.
=240. Distribution of Soil Ingredients.=—The determination of the
distribution of the soil ingredients in the sediments obtained in silt
analysis is illustrated by the following table:[159]
────────────╥────────────╥───────────╥────────────╥────────────
Hydraulic ║ ║ ║ ║
value ║ Clay ║ <0.25mm ║ 0.25mm. ║ 0.5mm.
Per cent in ║ ║ ║ ║
soil ║ 21.64 ║ 23.56 ║ 12.54 ║ 13.67
════════════╬══════╤═════╬═════╤═════╬══════╤═════╬══════╤═════
║ │ ║ │ ║ │ ║ │
║ =A= │ =B= ║ =A= │ =B= ║ =A= │ =B= ║ =A= │ =B=
║ │ ║ │ ║ │ ║ │
Insoluble ║ │ ║ │ ║ │ ║ │
residue ║ 15.96│ 4.35║73.17│17.29║ 87.96│11.03║ 94.13│12.72
Soluble ║ │ ║ │ ║ │ ║ │
silica ║ 33.10│ 7.17║ 9.95│ 2.34║ 4.27│ 0.53║ 2.35│ 0.32
Potash ║ 1.47│ 0.32║ 0.53│ 0.12║ 0.29│ 0.04║ 0.12│ 0.01
Soda ║(1.70)│ ║ 0.24│ 0.06║ 0.28│ 0.04║ 0.21│ 0.02
Lime ║ 0.09│ 0.03║ 0.13│ 0.03║ 0.18│ 0.02║ 0.09│ 0.01
Magnesia ║ 1.33│ 0.29║ 0.46│ 0.11║ 0.26│ 0.03║ 0.10│ 0.01
Manganese ║ 0.30│ 0.06║ 0.00│ 0.00║ 0.00│ 0.00║ 0.00│ 0.00
Ferric oxid ║ 18.76│ 4.06║ 4.76│ 1.11║ 2.34│ 0.29║ 1.03│ 0.14
Alumina ║ 18.19│ 3.97║ 4.32│ 1.04║ 2.64│ 0.33║ 1.21│ 0.17
Phosphoric ║ │ ║ │ ║ │ ║ │
acid ║ 0.18│ 0.04║ 0.11│ 0.02║ 0.03│ 0.00║ 0.02│ 0.00
Sulfuric ║ │ ║ │ ║ │ ║ │
acid ║ 0.06│ 0.01║ 0.02│ 0.01║ 0.03│ 0.00║ 0.03│ 0.00
Volatile ║ │ ║ │ ║ │ ║ │
matter ║ 9.00│ 1.33║ 5.61│ 1.43║ 1.72│ 0.23║ 0.92│ 0.29
────────────╫──────┼─────╫─────┼─────╫──────┼─────╫──────┼─────
Total ║100.14│21.64║99.30│23.56║100.00│12.54║100.21│13.67
Total ║ │ ║ │ ║ │ ║ │
soluble ║ │ ║ │ ║ │ ║ │
matter. ║ 75.18│ ║20.52│ ║ 10.32│ ║ 5.16│
„ „ ║ │ ║ │ ║ │ ║ │
bases ║ 41.84│ ║10.44│ ║ 5.99│ ║ 2.76│
Soluble ║ │ ║ │ ║ │ ║ │
silica in ║ │ ║ │ ║ │ ║ │
crude ║ │ ║ │ ║ │ ║ │
substance.║ 0.38│ 0.01║ │ ║ │ ║ │
────────────╨──────┴─────╨─────┴─────╨──────┴─────╨──────┴─────
────────────╥───────────╥──────────┬──────────┬────────
Hydraulic ║ ║ │ │
value ║ 1.0mm. ║ │ │
Per cent in ║ ║ Other │ Total │Original
soil ║ 13.11 ║sediments.│sediments.│ soil.
════════════╬═════╤═════╬══════════╪══════════╪════════
║ │ ║ │ │
║ =A= │ =B= ║ │ │
║ │ ║ │ │
Insoluble ║ │ ║ │ │
residue ║96.52│12.74║ 13.76│ 71.89│ 70.53
Soluble ║ │ ║ │ │
silica ║ │ 0.36║ │ 10.36│ 12.30
Potash ║ │ „ ║ │ 0.49│ 0.63
Soda ║ │ „ ║ │ 0.12│ 0.09
Lime ║ │ „ ║ │ 0.09│ 0.27
Magnesia ║ │ „ ║ │ 0.44│ 0.45
Manganese ║ │ „ ║ │ 0.06│ 0.06
Ferric oxid ║ │ „ ║ │ 5.60│ 5.11
Alumina ║ │ „ ║ │ 5.51│ 8.09
Phosphoric ║ │ ║ │ │
acid ║ │ „ ║ │ 0.06│ 0.21
Sulfuric ║ │ ║ │ │
acid ║ │ „ ║ │ 0.02│ 0.02
Volatile ║ │ ║ │ │
matter ║ │ „ ║ │ 3.64│ 3.14
────────────╫─────┼─────╫──────────┼──────────┼────────
Total ║ │13.10║ │ 98.28│ 100.63
Total ║ │ ║ │ │
soluble ║ │ ║ │ │
matter. ║ │ ║ │ │
„ „ ║ │ ║ │ │
bases ║ │ ║ │ │
Soluble ║ │ ║ │ │
silica in ║ │ ║ │ │
crude ║ │ ║ │ │
substance.║ │ ║ │ │ 0.19
────────────╨─────┴─────╨──────────┴──────────┴────────
=A.= Calculated on the amount of sediment.
=B.= Calculated on the amount of soil.
It is seen from the above analyses that the clay is by far the richest
in mineral constituents, of all the ingredients separated in silt
analysis, the amount in the clay being more than twice that of all the
others combined. Its volatile matter is also the largest. The large
amount of soda, however, is probably in part due to the sodium chlorid
used in the precipitation of the diffused clay. The following points in
regard to the distribution of the different ingredients are instructive:
1. The iron and alumina exist in almost identical relative proportions
in each sediment, making it probable that they are in some way
definitely correlated.
2. Potash and magnesia also exist in almost the same quantities, and
their ratio to each other in all the sediments being almost constant
seems to indicate that they occur combined, perhaps in some zeolitic
silicate which may be a source of supply to plants.
3. Manganese exists only in the clay, a mere trace being found in the
next sediment.
4. The lime appears to have disappeared in the clay, having probably
been largely dissolved in the form of carbonate by the large quantity of
water used in elutriation. Its increase in the coarser portions may be
owing to its existence in a crystallized form not so readily soluble.
5. In a summary of the ingredients, it is seen that there is a loss in
potash, magnesia and lime in the sediments as compared with the original
soil; and this loss is doubtless due to the solution of these bodies in
the water of elutriation.
A noteworthy fact shown in this table is the rapid decrease of
acid-soluble matter in the coarser sediments; even what is dissolved
from so fine a sediment as 1.0 millimeter hydraulic value, equal to a
diameter of 0.04 millimeter, is in this case a negligible quantity. This
suggests forcibly the inutility of introducing into chemical soil
analysis, grains of as large a size as will pass a sieve of one
millimeter aperture. The hydraulic value of these grains would be
somewhere between 150 and 200 millimeters per second. While the exact
results of the above analysis may not be applicable to all soils, yet
the range is so wide that the systematic exclusion from chemical
analysis of inert material, by means of preliminary mechanical
separation, seems likely to lead to important improvements in the
interpretation of the results.
=241. Percentage of Silt Classes in Different Soils.=—The adaptation of
a soil to different crops depends largely on the sizes of the particles
composing it and consequently on the relative percentages of the silt
classes.
The following table gives the mechanical analysis of some markedly
different types of subsoils:[160]
Diameter, Conventional Truck
millimeters. names. and Grass
Early small and
truck. fruit. Tobacco. Wheat. wheat. Limestone.
1 2 3 4 5 6
2–1 Fine gravel 0.49 0.04 1.53 0.00 0.00 1.34
1–0.5 Coarse sand 4.96 1.97 5.67 0.40 0.23 0.33
0.5–0.25 Medium sand 40.19 28.64 13.25 0.57 1.29 1.08
0.25–0.1 Fine sand 27.59 39.68 8.39 22.64 4.03 1.02
0.1–0.05 Very fine
sand 12.10 11.43 14.95 30.55 11.57 6.94
0.05–0.01 Silt 7.74 4.95 28.86 13.98 38.97 29.05
0.01–0.005 Fine silt 2.23 2.02 7.84 4.08 8.84 11.03
0.005–0.0001 Clay 4.40 8.79 14.55 21.98 32.70 43.44
————— ————— ————— ————— ————— —————
99.70 97.52 95.04 94.20 97.63 94.23
Org. matter, water and
loss 0.30 2.48 4.96 5.80 2.37 5.77
=242. Description of the Soils.=—Number one represents the very early
truck lands of southern Maryland. It is a light yellow sand, belonging
to the Columbia terrace formation. Under an intense system of
cultivation and heavy manuring with organic matter, good crops of garden
vegetables are produced which mature very early, at least ten days or
two weeks before the crops from any other part of the state. Under the
prevailing meteorological and cultural conditions this soil maintains
about five or six per cent of moisture, while a heavier wheat and grass
soil maintains from twelve to twenty per cent. The truck soil is so
loose and open in texture that the rain-fall passes through it very
readily, and it is undoubtedly owing to this drier soil that the plant
is forced to the early maturity which secures it from competition from
other parts of the State and insures a good market price.
Number two represents the later truck and fruit lands of southern
Maryland. These lands contain rather more clay than those just
described; they are somewhat heavier and closer in texture, and are
rather more retentive of moisture. This land gives a larger yield per
acre than the one just described, and in every way crops make a more
vigorous growth and development, but the crop is about a week or ten
days later in maturing, and for this reason it brings a lower price in
the market. It is much better land than number one for small fruit and
peaches. These lands are altogether too light in texture for the
profitable production of wheat, and it would cost altogether too much to
improve them so that even a moderate yield of wheat could be obtained.
Number three is a tobacco land of southern Maryland. The finest tobacco
lands of this locality come between the truck and wheat lands in
texture, and contain from ten to twenty per cent of clay. The lighter
the texture of the soil and the less clay it contains, the less tobacco
it will yield per acre, but the finer the texture of the leaf. The
tobacco yields more per acre on the heavier wheat soils, but the leaf is
coarse and sappy and cures green and does not take on color. It brings a
very low price in the market and does not pay for cultivation. The crop
on the lighter lands is of much finer quality; there is a smaller yield
per acre but the leaf takes on a fine color in curing, and brings a much
better price per pound. Wheat is commonly raised on these tobacco lands
to get advantage of the high manuring, and because the rotation is
better for the land than where tobacco is grown continuously on the same
soil. The finest tobacco lands are, however, too light in texture for
the profitable production of wheat. These lands belong to the neocene
formation.
Number four is a type of the wheat lands of southern Maryland. These
lands represent soil of about the lightest texture upon which wheat can
be economically produced under the climatic conditions which there
prevail. They contain from eighteen to twenty-five per cent of clay, and
are much more retentive of moisture than the best tobacco lands. This
type is about the limit of profitable wheat production. These soils will
maintain about twelve per cent of water during the dry season. Garden
truck is so late in maturing on these lands that there is often a glut
in the market when the crop matures, and the crops often do not pay the
cost of transportation. The lands are too light in texture for a
permanent grass sod. They belong to the neocene formation.
Number five represents the heavier wheat lands of southern Maryland,
belonging probably to a different horizon of the neocene formation and
containing about thirty per cent of clay. This soil is much more
retentive of moisture and produces very much larger crops of wheat than
the last sample. It is strong enough and sufficiently retentive of
moisture to make good grass lands. It is too close in texture and too
retentive of moisture for the production of a high grade of tobacco, or
to be profitable for market truck.
Number six is from a heavy limestone soil of lower Helderberg formation.
It is a strong and fertile wheat and grass land.
=243. Interpretation of Silt Analysis.=—The primary conceptions upon
which the interpretation of the mechanical analysis is based may be
briefly stated as follows:[161] The circulation of water in the soil is
due to gravity, or the weight of water, acting with a constant force to
pull the water downward, and also to surface tension, or the contracting
power of the free surface of water (water-air surface), which tends to
move the water either up or down, or in any direction, according to
circumstances. There is a large amount of space between the grains in
all soils in which water may be held, ranging from about thirty per cent
in light sandy lands to sixty-five or seventy per cent in stiff clay
soils. The relative rate of movement of water through a given depth of
soil will depend upon how much space there is in the soil; upon how much
this space is divided up, _i. e._, upon how many grains there are per
unit volume of soil; upon the arrangement of the grains of sand and
clay; and upon how this skeleton structure is filled in and modified
with organic matter. It also appears that the ordinary manures and
fertilizers change this surface tension, or pulling power of water; that
they also change the arrangement of the grains, and consequently the
texture or structure of the soil may be changed and the relation of the
soil to water, through the effect of the ordinary manures and
fertilizers in causing flocculation or the reverse.
=244. Number of Particles in a Given Weight of Soil.=—The approximate
number of particles in the soil can be calculated from the results of
the mechanical analysis by the following formula:[162]
(_a_/((π(_d_)³ω)/6)) ÷ A
Where _a_ is the weight of each group of particles, _d_ the mean
diameter of the particles in the several groups in centimeters, ω is the
specific gravity of the soil, and A is the total weight of soil. For the
specific gravity of ordinary soils, the constant 2.65 may be used.
In using the formula the per cents are expressed as grams. Thus, if
there were twenty per cent of silt, this would be taken as twenty grams,
and if the results of the analysis added up ninety-seven per cent the
whole weight of soil would be taken as ninety-seven grams. The diameter
_d_ is taken as the mean for the extreme diameters taken for any group,
for instance, for the silt this would be 0.003 centimeter, which is
assumed to be the diameter of the particles in that group. This formula
can only give approximate values, as the number of separations in a silt
analysis must necessarily be small, amounting usually to not more than
eight or ten grades, on account of the time and labor required for
closer separations. There is relatively rather a wide range in the
diameters of grains within any one of these grades, and absolute values
could not be expected without a vast number of separations, so that all
the grains in each group would be almost exactly of the same size.
The clay group has relatively the widest limits, which is unfortunate,
as this is the most important of all the groups on account of the
exceedingly small size of the particles. The figure 0.0001 millimeter is
taken as the lowest limit of the diameter of the clay particles. These
particles have been heretofore assumed to be ultra-microscopic, but by
the use of a microscope of high power with oil-immersion objective and
staining fluids, it has been possible to define the clay particles in a
turbid liquid which has stood so long as to be only faintly opalescent.
Pending more exact measurements, the figure 0.00255 millimeter has been
used as the diameter of the average sized particle in the clay group.
The following table gives the approximate number of grains per gram in
the different types of subsoils calculated from the mechanical analysis
of the typical soils already given:
NUMBER OF PARTICLES OF EACH CLASS IN ONE GRAM OF SOIL.
─────────────────┬─────────────────┬─────────────────┬─────────────────
Silt classes. │ No. 1 │ No. 2│ No. 3
Diameter (_d_) in│ Early truck. │ Truck and small│ Tobacco.
centimeters. │ │ fruit.│
─────────────────┼─────────────────┼─────────────────┼─────────────────
0.15 │ 0│ 0│ 3
0.075 │ 85│ 34│ 102
0.0375 │ 5,511│ 4,011│ 1,900
0.0175 │ 37,230│ 54,610│ 11,890
0.0075 │ 207,500│ 199,700│ 267,900
0.003 │ 2,073,000│ 1,355,000│ 8,092,000
0.00075 │ 38,210,000│ 35,360,000│ 140,900,000
0.000255 │ 1,915,000,000│ 3,918,000,000│ 6,637,000,000
│ —————————————│ —————————————│ —————————————
│ 1,955,000,000│ 3,954,973,355│ 6,786,273,795
─────────────────┼─────────────────┼─────────────────┼─────────────────
Silt classes. │ No. 4 │ No. 5 │ No. 6
Diameter (_d_) in│ Wheat. │Grass and wheat. │ Limestone.
centimeters. │ │ │
─────────────────┼─────────────────┼─────────────────┼─────────────────
0.15 │ 0│ 0│ 12
0.075 │ 726│ 4│ 60
0.0375 │ 8,273│ 181│ 157
0.0175 │ 32,340│ 5,556│ 1,456
0.0075 │ 554,100│ 202,600│ 125,900
0.003 │ 3,962,000│ 10,670,000│ 8,231,000
0.00075 │ 73,990,000│ 154,900,000│ 199,900,000
0.000255 │ 10,150,000,000│ 14,570,000,000│ 19,430,000,000
│ ——————————————│ ——————————————│ ——————————————
│ 10,228,547,439│ 14,735,778,341│ 19,638,258,585
─────────────────┴─────────────────┴─────────────────┴─────────────────
=245. Estimation of the Surface Area of Soil Particles.=—The approximate
extent of surface area of the soil grains in one gram of soil can be
calculated from the foregoing by the following formula:[163]
π(_d_)²_n_
in which _d_ is the mean of the diameters of any group in centimeters,
and _n_ is the number of particles in the group.
The following table gives the approximate extent of surface area of the
particles in one gram of soil calculated from the preceding table:
APPROXIMATE EXTENT IN SQUARE CENTIMETERS, OF SURFACE AREA IN ONE GRAM
OF SOIL.
Soil number.
————— —————— —————— —————— —————— ——————
Diameter, millimeters. 1 2 3 4 5 6
1.5 0.0 0.0 0.4 0.0 0.0 0.1
0.75 1.8 0.6 1.8 12.8 0.1 0.1
0.375 24.3 17.7 8.4 36.5 31.0 0.7
0.175 35.8 52.6 11.4 31.1 5.3 1.4
0.075 21.3 35.3 47.3 97.9 35.8 22.2
0.03 218.8 38.3 228.9 112.0 301.4 232.7
0.0075 67.4 62.5 248.9 130.8 273.5 353.4
0.00255 390.8 800.5 1355.0 2072.0 2976.0 3965.0
————— —————— —————— —————— —————— ——————
Total 760.2 1007.5 1902.1 2493.1 3593.1 4575.3
=246. Logarithmic Constants.=—The following logarithmic constants have
been used in the calculation of the approximate number of grains per
gram and of the surface area, using 2.65 in all cases as the specific
gravity of the soil.
Diameter. (_d_) Approximate number of grains. Surface area.
log.(π(_d_)³_w_)/(6) log.(_d_)²π
0.15 centimeters \̅3.6703 \̅2.8493
0.075 „ \̅4.7674 \̅2.2473
0.0375 „ \̅5.8641 \̅3.6451
0.0175 „ \̅6.8711 \̅4.9831
0.0075 „ \̅7.7674 \̅4.2473
0.003 „ \̅8.5734 \̅5.4513
0.00075 „ \̅1̅0.7674 \̅6.2473
0.000255 „ \̅1̅1.3616 \̅7.3101
=247. Mineralogical Examination of the Particles of Soil Obtained by
Mechanical Analysis.=—The principal object of the mechanical analysis of
soils as has already been set forth is the separation of the soil into
portions, the particles of which have the same hydraulic value. It is
evident without illustration that particles of the same hydraulic value
do not necessarily have the same size. The rate of flow of a liquid
carrying certain definite particles does not imply that these particles
are of the same dimensions. Of two particles of the same size and shape,
that one which has the lower specific gravity, will be carried off at
the lower rate of flow. At the end of the operation, therefore, the
several portions of the soil obtained will be found composed of
particles of sizes varying within certain limits, and of these particles
the larger ones will tend to be composed of minerals of lower specific
gravity, and the smaller ones of minerals of higher specific gravity. Of
the same mineral substance, the particles which are most irregular,
exposing for a given weight the largest surface will be found to pass
over at a lower velocity than those of a more nearly spherical shape.
The same law holds good for particles falling through a liquid at rest,
_i. e._, the heavier and more spherical particles, weight for weight,
will sooner reach the bottom of the containing vessel. To complete the
value of a mechanical analysis, it becomes necessary to submit the
several portions of soil obtained not only to a chemical but also to a
mineralogical examination. Only the outlines of the methods of examining
silt separates for mineral constituents can be given here and special
works in petrography must be consulted for greater details.[164]
It is evident that the methods of separation and examination from a
mineralogical point of view about to be described can only be applied to
silts of the largest size. The finer silts can not be separated into
portions of different specific gravities by separating liquids of
varying densities on account of the slowness with which they subside,
thus tending to adhere to the sides of the separating vessels and to
form floccules which are not all composed of the same kind of mineral
particles. While, therefore, these processes are more appropriately
described in connection with the silts obtained by hydraulic
elutriation, they can be applied with greater success to the fine
particles passing the different sieves used in the preparation of the
soil for analysis or to the finely pulverized soil as a whole.
The minerals which have contributed to soil formation, moreover, are
better preserved in the larger silt particles and therefore more easily
identified. While the desirability of securing like determinations in
the finer silts is not to be denied, in the present state of the art the
analyst must be content with the examination of the larger particles.
=248. Methods of Investigation.=—The chief points to be observed in the
examination of the fine particles of soil are the following: (1) the
size and shape of the particles; (2) measurement of crystal angles; (3)
separation into classes of approximately the same specific gravity; (4)
separation by means of the magnet; (5) determination of color and
transparency; (6) determination of refractive index; (7) examination
with polarized light; (8) examination after coloring; (9) chemical
separation. For many of the optical studies above noted, it is first
necessary to prepare thin laminae of the mineral particles and properly
mount them for examination. For the purposes of this manual only those
processes will be described which are essentially connected with a
proper understanding of the nature of the soil particles. For the more
elaborate methods of research the analyst will consult the standard
works on mineralogy and petrography.